ORIGINAL RESEARCH
published: 10 July 2018
doi: 10.3389/fmars.2018.00235
Spatial and Temporal Dynamics of
Inorganic Phosphate and
Adenosine-5′ -Triphosphate in the
North Pacific Ocean
Karin M. Björkman 1*, Solange Duhamel 2 , Matthew J. Church 3 and David M. Karl 1
1
Oceanography, University of Hawaii, Honolulu, HI, United States, 2 Lamont Doherty Earth Observatory, Palisades, NY,
United States, 3 Flathead Lake Biological Station, University of Montana, Lake County, IL, United States
Edited by:
Javier Arístegui,
Universidad de Las Palmas de Gran
Canaria, Spain
Reviewed by:
Marta Sebastian,
Instituto de Oceanografía y Cambio
Global, Spain
Sarah Elizabeth Reynolds,
University of Portsmouth,
United Kingdom
*Correspondence:
Karin M. Björkman
bjorkman@hawaii.edu
Specialty section:
This article was submitted to
Marine Biogeochemistry,
a section of the journal
Frontiers in Marine Science
Received: 24 January 2018
Accepted: 18 June 2018
Published: 10 July 2018
Citation:
Björkman KM, Duhamel S, Church MJ
and Karl DM (2018) Spatial and
Temporal Dynamics of Inorganic
Phosphate and
Adenosine-5′ -Triphosphate in the
North Pacific Ocean.
Front. Mar. Sci. 5:235.
doi: 10.3389/fmars.2018.00235
Temporal variability in dissolved inorganic, organic phosphate (Pi, DOP) and particulate
phosphorus (PPO4 ) concentrations, and microbial utilization of Pi and dissolved
adenosine-5′ -triphosphate (DATP) was studied at Station ALOHA (22.75◦ N, 158◦ W) in
the North Pacific Subtropical Gyre (NPSG) over a multi-year period. Spatial variability
of the same properties was investigated along two transects, to and from Hawaii,
that traversed the NPSG boundaries to the east (2014) and north (2016). Radiotracer
techniques were employed to measure the turnover time of Pi and DATP pools to
calculate Pi uptake rates and the Pi hydrolysis rates of DATP. Pi concentrations were
more variable, both in time and space, than DOP, ranging two orders of magnitude
compared to a factor of two for DOP. The DATP pool, while constituting on average
<0.15% of the total DOP-P, was as dynamic as Pi (∼1–200 pmol l−1 ), with lowest
concentrations coinciding with Pi depletion. The Pi turnover times ranged from a few
hours to several weeks, and were correlated with measured Pi concentrations (r = 0.9;
Station ALOHA, n = 28; 2014, n = 14; 2016, n = 12). Pi uptake rates averaged 3.6
± 1.3 nmol-P l−1 d−1 (n = 28: Station ALOHA), 9.2 ± 4.7 nmol-P l−1 d−1 , (n = 15;
2014) and 5.1 ± 2.5 nmol-P l−1 d−1 , (n = 12; 2016). The turnover time of the DATP
pool was typically substantially shorter (0.4–5 days) than for the Pi-pool, and uptake rates
ranged from 1 to 115 pmol l−1 d−1 . However, at very low Pi and ATP concentrations, ATP
turnover was longer than Pi turnover and ATP uptake rates lower. Total ATP hydrolysis
was high along both transects, exceeding the ATP taken up by the microbial community,
resulting in a net release of Pi into the ambient seawater. This net release was positively
correlated to Pi concentration. The relative contribution by microbial size classes to total
P-uptake depended on whether P was derived from ambient Pi or from DATP, with the
<0.6>0.2 µm size class dominating the DATP uptake. Our results indicate that during
Pi limiting conditions, regenerated P is rapidly consumed, and that Pi limitation occurs
locally and transiently but does not appear to be the predominant condition in the upper
water column of the NPSG.
Keywords: phosphorus cycling, North Pacific subtropical gyre, phosphate, ATP, station ALOHA
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INTRODUCTION
Time-series (HOT), or Center for Microbial Oceanography:
Research and Education (C-MORE) expeditions during an 11year time-period from 2005 to 2015. In the late summer/early
fall season of 2014 (18 Aug−16 Sep, 2014), a northeast to
southwest (hereafter; zonal) transect cruise was conducted
onboard the R/V New Horizon (NH1417: Nitrogen Effects on
MicroOrganisms—NEMO) originating in San Diego, CA and
terminating in Honolulu, HI. This transect cruise occupied 128
stations with near daily samplings for P concentrations, Putilization experiments and primary productivity (n = 26, 20, and
25 respectively; Table 1,Figure 1), providing an opportunity to
assess spatial variability of the inorganic and organic P dynamics
over a larger area to complement the temporal investigations
at or around Station ALOHA. In the spring of 2016 (April
19–May 4), a meridional cruise onboard the R/V Kaimikai-OKanaloa (KOK 1606: SCOPE—Gradients Cruise I), originating
and ending in Honolulu, HI, occupied 14 stations between 23◦ N
and 38◦ N along longitude 158◦ W, from within the NPSG into
the transition zone toward sub-polar waters. Twelve stations were
sampled during the meridional transect and comprised the same
suite of measurements as the NH1417 cruise but investigated
a different region and season than the earlier zonal transect
(Table 1,Figure 1).
Phosphorus (P) is essential for all life and is a key component
of nucleic acids, cell membrane lipids and in biological energetic
processes via e.g., adenosine-5′ -triphosphate (ATP). Inorganic
phosphate (Pi) is frequently at very low concentrations in aquatic
environments and although the bioavailability of nitrogen (N)
appears to be a proximately limiting resource for primary
producers in marine ecosystems, P is believed to be the ultimately
limiting macronutrient over geological time scales (Falkowski,
1997; Tyrell, 1999). Within the North Pacific Subtropical Gyre
(NPSG) inorganic pools of P and N are generally at much lower
concentrations than their respective dissolved organic pools
(DOP, DON; Karl et al., 2001b), indicative of their preferential
exploitation by the microbial community. The utilization of
the DOP pool, at least during times of Pi-stress or limitation,
should constitute a reservoir or buffer for P. Several studies have
shown that microorganisms in marine environments do utilize
the DOP both during P stress and under P-replete conditions
in nature (Björkman and Karl, 2003; Mather et al., 2008; Lomas
et al., 2010; Duhamel et al., 2011, 2017). However, the DOP
pool is chemically diverse and still only partially characterized,
but consists predominantly of phosphate esters and a substantial
amount of phosphonates (Clark et al., 1998; Kolowith et al.,
2001; Karl and Björkman, 2015; Repeta et al., 2016). Among
the most commonly studied DOP compounds is ATP. ATP has
several advantages as a “model” compound, among them that
it is available in different radiolabeled forms. This makes it
possible to discern preferential uptake of subcomponents of the
molecule, and importantly, it is possible to measure its ambient
particulate and dissolved concentrations (Bossard and Karl, 1986;
Ammerman and Azam, 1991a; Björkman and Karl, 2001) and to
follow its utilization by the microbial community (Casey et al.,
2009; Björkman et al., 2012). Although ATP may only be a small
portion, and not necessarily representative of the average DOP
pool constituents, getting a clearer assessment of its flux through
the different P-pools will aid in elucidating the behavior of the
bioavailable DOP.
In this study we compare temporal and spatial variabilities
in the utilization of Pi and ATP by the surface ocean microbial
community, as a function of Pi, DOP and dissolved ATP (DATP)
concentrations. Radiotracer techniques were used to determine
the turnover times of the Pi or ATP pools respectively, as well as
the Pi hydrolysis rates from the DATP pool. The temporal study
was conducted at or near Station ALOHA (22.75◦ N, 158◦ W)
in the North Pacific Subtropical Gyre (NPSG) over a multiyear period (2005–2015). The spatial variability in these same
properties was investigated during an August-September 2014
zonal transect from California to Hawaii, and an April–May 2016
meridional transect from Hawaii to approximately 36◦ N along
longitude 158◦ W. This is, to our knowledge, the first time that
all the above P-pools and accompanying rates of utilization have
been measured in concert within the oligotrophic oceans.
Hydrography and Sample Collections
Water samples were collected using polyvinyl chloride (PVC)
Niskin R -type bottles mounted on a 24-place rosette frame,
equipped with conductivity, temperature and pressure (CTD)
sensors. In addition to the parameters measured with the
environmental sensors, discrete seawater samples were also
collected. These included samples for soluble reactive phosphate
(hereafter called inorganic phosphate [Pi]), total dissolved
phosphorus (TDP), particulate P (PPO4 ), DATP and chlorophyll
a. With the exception of DATP samples, which were prefiltered through a 0.2 µm polycarbonate filter, the sample
collections and subsequent analyses were performed according
to the HOT standard protocols for the temporal data collected
at HOT (hahana.soest.hawaii.edu/hot/methods/) with slight
modifications during the transect cruises. In brief, for chlorophyll
a determinations, seawater (150 ml to 2 l) was filtered through
a glass fiber filter (GF/F; Whatman) and the filter extracted
in 5 ml of 100% acetone. The samples were extracted for 1–
7 days at −20◦ C in the dark prior to fluorometric analysis
(Turner Designs; model 10-AU, or TD700). Samples for nutrient
concentrations were collected into acid washed, deionized water
and sample rinsed, high-density polyethylene bottles, stored
upright and frozen (−20◦ C) until analyzed (Dore et al., 1996).
Sample Analyses
The Pi concentrations were measured using the MAGnesium
Induced Co-precipitation technique (MAGIC; Karl and Tien,
1992), followed by standard colorimetric assays (Murphy and
Riley, 1962) using a 10 cm cuvette cell (Beckman DU 640
spectrophotometer). The Pi samples were treated to reduce
arsenate to arsenite to eliminate cross reactivity with the
molybdenum blue complex (Johnson, 1971). The analytical
precision of this method is ± 1 nmol l−1 with a detection
limit (DL) of ≤ 3 nmol l−1 , using the definition of DL = 3 x
MATERIALS AND METHODS
Station Locations
Sampling for rate measurements was conducted at, or near,
Station ALOHA (22.75◦ N, 158.00◦ W) on several Hawaii Ocean
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TABLE 1 | Station number, date sampled, location co-ordinates (latitude [Lat], longitude [Long]), temperature (T; ◦ C), salinity (PSS), chlorophyll a concentrations (ng l−1 ),
and 14 C-primary production (µg C l−1 d−1 ), at 25 m during the 2014 zonal transect and 15 m during the 2016 meridional transect.
Date
Lat (◦ N)
Long (◦ W)
T (◦ C)
Salinity
Chlorophyll a (ng l−1 )
Primary production (µg C l−1 d−1 )
2
19 Aug 2014
33.118
120.064
19.04
4
20 Aug 2014
33.789
123.022
18.20
33.57
420 ± 18
32.4 ± 3.4
33.28
848 ± 4
10
21 Aug 2014
34.604
126.226
33.5 ± 4.6
19.89
33.12
93 ± 4
20
22 Aug 2014
34.909
7.3 ± 0.4
127.259
19.78
33.10
92 ± 8
37
23 Aug 2014
8.1 ± 0.3
33.870
128.799
21.25
33.61
99 ± 9
39
8.2 ± 0.3
24 Aug 2014
33.124
129.954
20.92
33.37
65 ± 6
5.6 ± 0.4
43
25 Aug 2014
31.640
132.509
22.19
34.03
99 ± 4
7.9 ± 0.2
46
27 Aug 2014
26.543
138.509
23.58
35.33
78 ± 1
6.8 ± 0.3
50
28 Aug 2014
25.875
138.514
24.36
35.26
77 ± 2
6.1 ± 0.1
53
29 Aug 2014
27.960
140.167
24.44
35.48
75 ± 4
6.4 ± 0.6
61
30 Aug 2014
29.584
140.783
24.25
35.30
89 ± 9
8.2 ± 0.9
69
31 Aug 2014
28.818
141.984
24.68
33.44
249 ± 3
14.3 ± 0.9
76
1 Sep 2014
28.813
143.984
24.85
35.45
222 ± 14
14.4 ± 1.3
81
2 Sep 2014
29.300
146.909
25.42
35.62
66 ± 3
6.4 ± 0.2
84
3 Sep 2014
29.610
147.704
25.57
35.62
80 ± 4
6.1 ± 0.2
88
4 Sep 2014
30.328
150.240
25.88
35.40
136 ± 7
8.0 ± 0.2
98
5 Sep 2014
30.550
150.434
26.00
35.39
104 ± 9
7.3 ± 0.2
100–5
6 Sep 2014
30.320
150.368
26.07
35.33
126 ± 15
9.0 ± 0.3
100–11
7 Sep 2014
30.322
150.370
26.08
35.39
133 ± 1
10.0 ± 0.3
100–12
7 Sep 2014
30.321
150.367
26.07
35.44
No data
No data
102
8 Sep 2014
29.400
150.222
26.03
35.52
65 ± 2
6.2 ± 0.2
106
9 Sep 2014
26.221
150.837
26.62
35.38
113 ± 1
12.7 ± 0.2
110
10 Sep 2014
23.667
151.316
26.60
35.15
148 ± 3
11.5 ± 1.1
122
11 Sep 2014
24.283
152.550
27.23
35.40
169 ± 7
12.0 ± 0.1
127–3
12 Sep 2014
24.484
152.947
27.21
35.40
112 ± 9
9.2 ± 0.7
127–9
13 Sep 2014
24.484
152.951
27.39
35.38
173 ± 10
6.5 ± 0.2
2
20 Apr 2016
23.497
158.00
24.04
35.22
80 ± 2
3.5 ± 0.9
4
22 Apr 2106
28.143
158.00
20.00
35.15
80 ± 5
3.0 ± 0.6
5
23 Apr 2016
29.452
158.00
19.80
35.21
98
2.0 ± 0.2
6
24 Apr 2016
32.583
158.00
16.17
34.60
264
9.1 ± 0.2
7
25 Apr 2016
34.058
158.00
14.82
34.42
331
7.5 ± 0.6
8
26 Apr 2016
37.302
158.00
11.40
34.18
776
9.9 ± 0.7
9
27 Apr 2016
36.570
158.00
12.01
34.13
214
5.5 ± 0.4
10
28 Apr 2016
35.463
158.00
13.33
34.05
706
11.3 ± 0.8
UW
29 Apr 2016
36.053
158.00
14.22
34.12
No data
13.7 ± 1.1
12
30 Apr 2016
33.092
158.00
16.07
34.48
604
26.6 ± 2.0
13
1 May 2016
29.700
158.00
20.91
35.15
040
5.9 ± 0.2
14
2 May 2016
26.283
158.00
23.05
35.31
047
7.2 ± 0.8
22.750
158.00
–
–
–
–
23.4 ± 0.5
35.1 ± 0.2
Station
NH1417-
KOK1606-
Station ALOHA
–
–
93 ± 17
6.4 ± 0.9
2005-2015
Apr
Aug–Sep
–
–
26.2 ± 0.6
35.2 ± 0.2
80 ± 7
7.4 ± 1.8
2014 (HOT 265)
Sep
–
–
27.23
34.95
60 ± 0
7.8 ± 0.5
2016 (HOT 283)
Apr
–
–
24.25
34.80
69 ± 7
6.1 ± 0.1
Station ALOHA data are the mean values at the 25 m horizon for Aug–Sep and April from 2005 to 2015 (n = 20 and 10, respectively), and Sep 2014 and April 2016.
the standard deviation (s.d.) of the analytical precision. TDP
was analyzed by the wet persulfate/high temperature oxidation
method (Menzel and Corwin, 1965) followed by MAGIC and
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the standard Pi colorimetric assay. TDP measurements have an
analytical precision of ± 5 nmol l−1 . DOP was calculated as the
difference between TDP and Pi concentrations in paired samples.
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North Pacific Ocean Phosphorus Dynamics
FIGURE 1 | Map over study area in the North Pacific Ocean showing a multiyear composite of chlorophyll a concentrations in the surface waters (OCI algorithm,
MODIS; Aqua, 9 km resolution) and the station locations for the zonal transect from San Diego, CA to Honolulu, HI in August-September of 2014, and the meridional
transect from Honolulu to approximately 37◦ N along the 158◦ W longitude and back in April-May 2016. Station ALOHA (white circle) located at 22.75◦ N, 158.00◦ W
and is the site for the Hawaii Ocean Time-series (HOT). The zonal transect occupied 127 stations (yellow bullets) of which 20 stations where phosphorus (Pi, ATP) rate
experiments were conducted, including two additional stations for 14 C-primary production (red triangles; yellow station numbers). Circled is the area along the 30◦ N
meridian where Pi was depleted during the 2014 cruise. Note that Stations 88, 98, and 100 overlap in the figure, but were separated by 8–18 nmiles; the stations
closest in space were separated in time by 3 days. The meridional transect (red triangles; black station numbers) occupied 14 stations of which 12 were sampled for P
experiments and 14 C-primary production. (Zonal transect: California Current System [CCS] – Stns 1–4; Transition zone [TZ] – Stns 5–43; North Pacific Subtropical
Gyre [NPSG] – Stns 46–127: Meridional transect NPSG – Stns 2–5 and 13–14; TZ – Stns 6–12).
presented as the mean ± s.d., (n = x) where x is the number
of observations. Station ALOHA core data presented here were
obtained from the publicly available HOT data archive; HOT
Data Organization and Graphical System (HOT-DOGS: http://
hahana.soest.hawaii.edu/hot/hot-dogs/).
DATP concentrations were determined as described in Björkman
and Karl (2001) which is based on the co-precipitation of ATP
with brucite, based on the same principal as the MAGIC method
used for Pi, allowing DATP to be concentrated from seawater
before analysis using firefly bioluminescence. Modifications
from the original protocol were made to sample volume and
concentration factors as follow; triplicate 50 ml subsamples from
each field sample were amended with 250 µl 1 N NaOH, mixed
and centrifuged at 1,000 × g for 60 min. The supernatant was
carefully aspirated and the pellet dissolved with 50 µl 2.5 N HCl,
followed by the addition of 250 µl of deionized water. Prior to
analyses the sample was mixed with an equal volume of Tris
buffer (pH 7.4, Sigma-Aldrich T7693). Samples for the calibration
curve were made in surface seawater with the additions of known
amounts of ATP and treated as samples. Blanks were made from
surface seawater samples without ATP additions and treated with
apyrase to hydrolyze any ATP in the sample (Sigma-Aldrich
#A6132; stock 10 units ml−1 , 10 µl per sample:Tris mix). The
DL for this assay is 9 pmol DATP l−1, with the precision of
± 3 pmol l−1 . Data averaged from multiple observations are
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Incubation Experiments
Seawater was collected into acid washed, deionized water and
sample rinsed, clear polycarbonate (PC) incubation bottles
(Nalgene: 75–250 ml). All data presented in this study for Station
ALOHA and NH1417 were collected from 25 m, and from
15 m during KOK1606. The samples were spiked with tracer
amounts of either 32 P or 33 P orthophosphate (MP Biomedicals #
064014L, Perkin-Elmer #NEZ08000; carrier free) or ATP labeled
at the gamma position with either 32 P or 33 P (MP Biomedicals
# 01350200, Perkin-Elmer NEG302H00; specific activity 111
TBq mmol−1 ). The final radioactivity of the samples typically
ranged from 0.1 to 1 MBq l−1 depending on experiment.
The bottles were incubated in on-deck incubators, cooled with
running, surface seawater and shielded with blue plexiglass
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North Pacific Ocean Phosphorus Dynamics
response to increasing concentrations of Pi. Very short Pipool turnover times were measured at this location and were
presumed to be due to low ambient concentrations of Pi (this
was later confirmed by chemical analysis). Seawater samples were
amended with non-radioactive Pi to target additions of 0, 5,
10, 25, 50, 75, 100, and 200 nmol Pi l−1 . A subsample from
each concentration step was placed into PC incubation bottles,
spiked with 33 P-Pi, incubated and sampled as described above.
A separate set of incubation bottles, amended with 0, 25, 50,
and 100 nmol Pi l−1 , was spiked with 33 P-ATP. The remaining
seawater was stored frozen (−20◦ C) for later analysis of Pi
concentrations. Time-course sampling was conducted for the
three lowest P concentrations at 5, 25, 75, and 150 min of spiking
with 33 P-Pi. Incubations receiving 25 nmol Pi l−1 or above, and
the 33 P-ATP spiked samples, were sub-sampled in triplicate after
approximately 3 h of incubation.
(Arkema #2069) to achieve the approximate light level and
spectral quality of the depth from which the sample water
had been collected. The incubation time varied depending
on location and expectancy of the rates, but all incubations
were conducted during daytime. Experiments were typically
performed as time-course incubations with 3–5 sampling events
over a 3–6 h period. A subsample (0.1–0.5 ml) was collected
from each incubation bottle to measure the total radioactivity
added. Particulate activity was determined by filtering a 5–10 ml
aliquot through a 0.2 µm pore size polycarbonate membrane
filter (Nuclepore). Polycarbonate filters of 0.6 and 2 µm pore
sizes were also included on occasion at Station ALOHA, and
at all stations sampled along the two transects, to assess the
relative size class contribution to community Pi or DATP
pool turnover. The filters were rinsed with 3 × 2 ml of
filtered seawater to remove unincorporated radioactivity, placed
into a 7 ml plastic scintillation vial (Simport) and 4 ml of
scintillation cocktail (Ultima Gold LLT, Perkin Elmer) added.
Radioactivity was measured using a liquid scintillation counter
(Perkin-Elmer, LSC 2910TR). Data were corrected for activity
loss during the counting process due to decay of the shortlived isotopes. Measurements for primary productivity were
performed in conjunction with the P-incubation experiments
on both transect cruises. Primary productivity was determined
by the 14 C-bicarbonate assay (14 C-PP) using 75 ml PC-bottles,
spiked with 14 C-bicarbonate (MP Biomedicals 117441H) to a
final radioactivity of approximately 7.4 MBq l−1 .
Calculations of P Uptake Rates and Kinetic
Parameters
The Pi or ATP uptake rates and turnover times in days (TOT,
d) were calculated as follows: TOT(d) = t/r where t is the
total radioactivity added (Bq l−1 ) and r is the rate of radiolabel
uptake into the particulate fraction (Bq l−1 d−1 ). The rate was
determined from linear regression of the incubation time and
radioactivity of the filters from the time course experiments. This
calculation assumes that the specific activity of the substrate pool
is constant during the incubation period. In our experiments
<10% of the radiolabel was taken up during the incubation time,
with the exception of the samples from stations 88 (30.33◦ N,
150.24◦ W) and 100 (30.32◦ N, 150.38◦ W) along the 2014 transect.
At station 88; >80% of the radiolabel was taken up by 80 min and
at station 100; ∼40% was captured on the filters after 75 min. At
such high proportions of the radiolabel taken up, the calculated
uptake rate will be biased as a result of recycling, and may lead
to underestimates of the actual rate of uptake. However, the finer
time-course resolution and additions of Pi at station 100 allowed
for the determination of rates at this station (see above Kinetic
Pi rate experiment). In this experiment we also calculated the
kinetic parameters Vmax and Km for the maximum uptake rate
and half saturation constant, respectively, using Hanes-Woolf
linear transformation of the data. This transformation uses the
substrate concentration (S) and uptake velocity (V) to derive
Vmax and Km as follows: S/V = (1/Vmax ) × S +Vmax /Km . Here
S/V is the measured TOT in our incubations, the slope of the
linear regression is 1/Vmax , and the y-intercept is Vmax /Km .
The rate of Pi or PATP uptake, expressed as nmol l−1 d−1
or pmol l−1 d−1 , was calculated from the turnover time of the
respective radioactive tracers and the measured concentration of
Pi or DATP of the samples.
ATP Hydrolysis Rates
During the 2014 and 2016 transect cruises, and at Station
ALOHA in 2013, the total hydrolysis rates of DATP were assessed
from the 33 P-ATP incubation experiments. The protocol used
was modified from Ammerman and Azam (1991b). The filtrate
from the 0.2 µm filter fraction was collected during the time
course incubations. Triplicate 1 ml aliquots from each sample
were placed into micro-centrifuge tubes and mixed with 0.2 ml
activated charcoal slurry (20 mg charcoal ml−1 in 0.03 N sulfuric
acid: The activated charcoal selectively binds organic molecules,
such as DATP, while Pi will remain in solution. This allows for the
separation of hydrolyzed 33 P-Pi from 33 P-DATP). The samples
were thoroughly mixed by vortex and centrifuged for 15 min
at 20,800 × g. A subsample (0.75 ml) of the supernatant was
placed into a scintillation vial for radioactivity counting. DATP
hydrolysis was measured from the increase of radioactivity over
time in the supernatant, indicating that Pi had been cleaved
from ATP and released into the ambient seawater, i.e., Pi
regenerated from DATP. Total DATP hydrolysis was calculated
as the sum of the particulate activity retained on the 0.2 µm
filters (i.e., P from DATP taken up by microbes, hereafter; PATP)
and the regenerated, non-incorporated 33 Pi from 33 P-DATP in
the charcoal extractions. All rates were based on time course
sampling and the rates determined through linear regression over
time.
RESULTS
Transect Characterizations
The zonal transect in 2014 traversed the California Current
System (CCS) for the first few stations (Stations 1–4),
characterized by relatively low sea-surface temperatures (SST),
high nutrient concentration, high chlorophyll a and 14 C-PP
Kinetic Pi Rate Experiment
During the 2014 transect at station 100 (30.32◦ N, 150.38◦ W),
an additional experiment was conducted to assess the kinetic
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North Pacific Ocean Phosphorus Dynamics
(Table 1, Figure 2B). Stations 5–43 comprised the transition
zone between the CCS and the NPSG (Figure 1). This region
showed high eddy activity as detected by satellite estimates
of sea surface height anomalies (Shilova et al., 2017), with
variable chlorophyll a and declining Pi concentrations. Stations
44–127 were all within the NPSG with low eddy activity
and unusually high SST, reaching temperatures above 27◦ C
nearing the Hawaiian Islands. Time-series data from Station
ALOHA have only rarely recorded such high temperatures, and
in September 2014 was the first time in a decade to do so.
The meridional transect in 2016 reached the transition zone
between the NPSG and the subpolar front just south of 32◦ N
(Station 6). This region demonstrated high spatial variability in
a variety of parameters (e.g., chlorophyll a, salinity) likely due to
entrainment and filaments of mixing water masses belonging to
the oligotrophic gyre and more nutrient-enriched water in the
transition zone.
Chlorophyll a and 14 C-Primary Productivity
Over the past decade at Station ALOHA chlorophyll a
concentration at the 25 m horizon varied by approximately a
factor of two with the lowest concentrations observed during the
summer months (72 ± 16 ng l−1 ; n = 28) and the highest during
winter (119 ± 31 ng l−1 ; n = 26) with an average concentration
of 93 ± 17 ng l−1 (Table 1). Chlorophyll a concentrations
along the 2014 zonal transect were highest within the CCS,
but even within the NPSG the concentrations varied over 3fold (Station 81; 66 ± 3 ng l−1 , Station 69; 249 ± 3 ng l−1 :
Table 1), with the highest chlorophyll a found in a phytoplankton
bloom (Figure 1,Table 1). During the spring 2016 meridional
transect, chlorophyll a varied two-fold within the NPSG and
at the northernmost stations reached concentrations similar to
those observed in the CCS. Station ALOHA 14 C-PP, at the 25 m
horizon, ranged from 4.5 to 11.8 µg C l−1 d−1 , with a mean for
August-September of 6.4 ± 0.9 µg C l−1 d−1 (n = 20) and 7.4 ±
1.8 µg C l−1 d−1 (n = 10) for April. Along the two transects 14 CPP varied by approximately a factor of 2–3 within the NPSG, but
were much higher in the nutrient-enriched regions of the CCS
and the northernmost stations of the 2016 transect (Table 1).
During the zonal transect, within the NPSG, a peak in 14 C-PP
was observed at stations 69 and 76 (∼ 14 µg C l−1 d−1 ). These
two stations were also associated with the highest chlorophyll a
measured during that transect within the NPSG (Table 1).
FIGURE 2 | Temporal and spatial variability in inorganic phosphate (Pi),
dissolved organic phosphate (DOP) and particulate organic phosphate (PPO4 )
concentrations (µmol l−1 ) at (A) Station ALOHA (2005–2016), (B) along the
zonal transect in the fall 2014 and (C) along the meridional transect in the
spring 2016. Dissolved adenosine-5′ -triphosphate (DATP) concentrations are
shown in nmol l−1 for the two transects in (B,C). The shaded triangles in (B)
are below the stipulated detection limit, and presented only as a comparison
to Pi. Arrows in (B,C) indicates the stations within the CCS, TZ, and NPSG as
described in Figure 1. Note that the scale is logarithmic.
P Concentrations at Station ALOHA and
Along Transects
Variability in Pi concentrations at 25 m at Station ALOHA ranged
approximately 25-fold over the past decade (2005–2015; 7–195
nmol P l−1 ; Figure 2A), with an average concentration of 70
± 39 nmol l−1 (n = 107), and median of 62 nmol l−1 . It
should be noted that Pi inventories over this period displayed
a rapid increase beginning in mid-2012, persisting through
the end of 2013 before slowly subsiding into 2014 and 2015
(Figure 2A). The upper ocean inventories during this 1.5–year
period were significantly higher than in the preceding 7–year
period (integrated 0–100 m, [2005–2011, n = 67] 4.8 ± 2.4
mmol m−2 ; [2012–2013, n = 20] 11.3 ± 3.5 mmol m−2 ) and
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this skewed the mean Pi concentrations at the 25 m horizon
upwards for the period by approximately 30% from the longer
term mean for Station ALOHA. Pi concentrations during the
2014 zonal transect varied 60-fold driven by the high values
found within the CCS, but were still variable by a factor of
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approximately 20 for stations within the NPSG (Figure 2B;
Stations 46–127; range 7–141 nmol l−1 ). The stations occupied
within the region along 29.3–30.5◦ N and between 140 and 150◦ W
(Stations 61–102), were characterized by low Pi concentrations
(range 7–35 nmol l−1 , mean 15 ± 1 nmol l−1 , n = 10), and
within the more intensely sampled region around 29.3–30.5◦ N,
∼150.3◦ W (stations 88–102) the mean Pi concentration was
even lower at 9 ± 2 nmol l−1 (n = 6). During the spring
2016 meridional transect, Pi concentrations within the NPSG
were 42 ± 8 nmol l−1 , and increased though the transition
zone into the subpolar front to approximately 500 nmol l−1 at
37.2◦ N (Figure 2C). The concentrations of DOP were much less
variable both in space and time and ranged about a factor of
two with concentrations at Station ALOHA averaging 185 ± 31
nmol P l−1 (n = 85; range 109–248 nmol P l−1 ; Figure 2A),
and 2014 transect stations within the NPSG at 182 ± 34 nmol
P l−1 (n = 19; range 119–232 nmol P l−1 ; Figure 2B). The
DOP concentrations were significantly lower (t-test, p < 0.001)
in the region encompassing stations 61–102 compared to the
other stations within the NPSG. The meridional transect in the
spring of 2016 showed a more uniform distribution even into
the subpolar regions, ranging from a high of 170 nmol P l−1
at the southernmost site to a low of 114 nmol P l−1 toward
the subpolar front (Figure 2C). The relative contribution of
DOP to the total dissolved P (TDP) pool at Station ALOHA
varied from 45% to near 100%, with an average of 72 ± 11%
(n = 85). Along the zonal transect DOP contributed 41–95%
of the total pool, when the CCS stations were included, and
62–95% (mean 84 ± 11%, n = 19) for stations sampled within
the NPSG. DOP concentrations were relatively low during the
meridional transect, but the DOP contribution to the TDP pool
within the NPSG was in the range of the long-term mean for
Station ALOHA (73 ± 3%, n = 5). The PPO4 concentrations
varied approximately 3-fold during the 10-year period at Station
ALOHA (Figure 2A; mean 12 ± 4 nmol l−1 , n = 106; range
6–25 nmol l−1 ). The mean PPO4 during 2011–2015 was 16 ±
3 nmol l−1 (n = 41). During the zonal transect in 2014, PPO4
peaked within the CCS (Figure 2B), whereas concentrations
within the NPSG varied approximately 2-fold (mean 21 ± 4 nmol
l−1 , n = 19; range 16–33 nmol l−1 ). Similarly, the meridional
transect in 2016 showed the highest PPO4 concentrations within
the transition zone and toward the subpolar front (Figure 2C),
while concentrations within the NPSG showed small variations
averaging 20 ± 2 nmol l−1 (n = 5). DATP concentrations
along the zonal transect ranged from ∼1 to 180 pmol DATP
l−1 , with the lowest concentrations coinciding with the lowest
Pi pool and shortest Pi-, and ATP-pool turnover times. The
highest concentrations were found within the CCS. However,
the DATP concentrations were less dynamic than the range in
concentrations imply, and were typically ∼100 pmol l−1 for the
majority of stations sampled (91 ± 15 pmol l−1 , n = 11; excluding
stations where Pi < 15 nmol l−1 ; Figure 2B). The meridional
transect DATP concentrations ranged from 33 ± 3 to 209 ±
20 pmol l−1 with the highest values found within the transition
zone. The average DATP concentration within the NPSG for
the meridional transect 2016 was 60 ± 27 pmol l−1 (n = 5:
Figure 2C).
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FIGURE 3 | Turnover time (days) of the Pi-pool and dissolved and particulate
ATP-pools (DATP and PATP, respectively) as measured by 33 P radiotracer
techniques. The Pi and PATP turnover times are based on the P uptake by the
microbial community, whereas the DATP turnover time is the sum of the PATP
and the net release of Pi originating from the hydrolysis of DATP. (A) zonal
transect in the fall of 2014, and (B) meridional transect in the spring 2016.
Error bars are derived from the error estimate of the regression of the
time-course sampling used to calculate the turnover time. Values for Station
ALOHA are mean ± 1 s.d. (Pi n = 30; ATP n = 7). Arrows indicate the stations
within the CCS, TZ, and NPSG as described in Figure 1. Note that the scales
differ among panels.
Pi Rate Measurements
Pi TOT at Station ALOHA varied from a few days to several
weeks over a multi-year study with a long term mean of 18
± 11 days (n = 28) and an uptake rate of 3.6 ± 1.3 nmol
l−1 d−1 (range 0.6–7.1 nmol l−1 d−1 ; median 3.3 nmol l−1
d−1 ). The range observed for the two transects were from
a few hours to 3 weeks within the NPSG, and reached 2–3
months in the CCS and subpolar transition zone respectively
(Figures 3A,B). Very short turnover times (hours to 3 days) were
found along the zonal transect around 30◦ N (Stations 61–100).
Some of the stations (69–76) in this region were associated with a
high phytoplankton biomass and enhanced primary productivity
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FIGURE 4 | (A,B) Pi-uptake rates (nmol l−1 d−1 ; columns) and the corresponding Pi concentrations (µmol l−1 ; bullets) and (C,D) the relative contribution from
different microbial size classes to the Pi-uptake measured. The zonal transect is shown in (A,C); the meridional transect in (B,D). Station ALOHA values for Pi uptake
rates and Pi concentrations are shown to the left in each panel (mean ± 1 s.d; n = 28). Arrows indicate the stations within the CCS, TZ and NPSG as described in
Figure 1. Note that the scales differ among panels.
(chlorophyll a >0.2 µg l−1 , 14 C-PP ∼ 14 µg C l−1 d−1 ; Table 1).
The Pi-uptake rate along the zonal transect ranged from a
few nmol l−1 d−1 to approximately 15 nmol−1 d−1 within the
NPSG, and reached nearly 30 nmol l−1 d−1 within the CCS
(Figure 4A). However, at the stations where we recorded the
lowest Pi concentrations, and associated short turnover times,
uptake rates were much enhanced (Figure 4A). The experiment
to assess the kinetic response to increasing amounts of Pi was
conducted at station 100, where there were indications of very
low ambient Pi. Although the turnover time grew longer with
additional Pi, as would be expected, the calculated rates for the
whole water community did not show a clear kinetic response,
as the calculated rates were similar for all concentrations tested
(Table 2). A linear regression analysis gave an uptake rate
estimate of 39 ± 1 nmol l−1 d−1 (n = 8, r2 = 0.991) for
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the whole water community. However, the >2 µm size class
did show a kinetic response with increased Pi. The Pi uptake
rates in this size class tripled between 10 and 75 nmol l−1 Pi,
and although the relative contribution of the >2 µm size class
to the whole water community was small, it increased from
∼2 to 8% at concentrations above 75 nmol l−1 Pi, indicating
that cells >2 µm were P-limited at ambient concentrations at
this station. The kinetic analysis indicated that the Km for the
>2 µm size class was 36 ± 7 nmol−1 (Table 2), which was
much above the measured ambient Pi concentration of 5 ± 1
nmol l−1 . Pi-uptake rates along the meridional transect were
comparable to the long term mean of Station ALOHA within
the NPSG (mean 2.8 ± 0.9 nmol l−1 d−1 ; n = 5), with higher
rates throughout the transition zone (range 5.5–12 nmol l−1 d−1 ;
Figure 4B).
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TABLE 2 | Kinetic experiment carried out at Station 100 during the east-west transect in 2014. Seawater was amended with increasing concentrations of inorganic
phosphate (Pi) to determine the turnover times (TOT) of the Pi- pool and Pi-uptake rates in different size fractions of the microbial community.
Pi (nmol l−1 )
Pi-uptake rate (nmol l−1 d−1 )
TOT (days)
>0.2 µm
>0.6 µm
>2µm
>0.2 µm
>0.6 µm
>2 µm
5±1
0.10 ± 0.00
0.61
8.4
54 ± 11
8.8
0.6
10 ± 0
0.23 ± 0.01
0.42
10.6
42 ± 3
23.7
0.9
17 ± 1
0.33 ± 0.01
0.49
15.6
56 ± 4
35.2
1.1
31 ± 1
0.70 ± 0.03
1.39
22.2
45 ± 2
22.6
1.4
55 ± 1
1.38 ± 0.10
2.37
35.5
40 ± 3
23.4
1.6
77 ± 1
2.19 ± 0.09
3.88
28.6
35 ± 2
19.8
2.7
112 ± 1
2.55 ± 0.08
4.60
49.7
44 ± 2
24.4
2.3
189 ± 1
4.95 ± 0.11
5.50
54.1
38 ± 1
34.4
3.5
Vmax
nmol l−1 d−1
–
–
39 ± 1
20 ± 1
4±1
nmol l−1
–
–
n.d
n.d
36 ± 7
Km
The half-saturation constant (Km ) and maximum uptake rate (Vmax ) were calculated from Hanes-Woolf linear transformation of the data. N.d, not determined, due to lack of kinetic
response to added Pi.
for the meridional transect was 1.43 ± 0.47 days (n = 12:
Figures 3A,B).
The total DATP hydrolysis rates ranged nearly two orders
of magnitude (3–230 pmol l−1 d−1 ) along the zonal transect,
and approximately 10-fold along the meridional transect
(Figures 5A,B). The very low rates were again in the region along
30◦ N where both Pi and DATP pools were depleted. During the
zonal transect, excluding the stations where Pi and ATP were
depleted, an average of 28 ± 9% (n = 8) of the total DATP
hydrolyzed was not incorporated into cells, but regenerated as Pi
within the NPSG, and an even higher proportion was regenerated
within the transition zone (53 ± 4%, n = 5) and CCS (64 ±
0%, n = 2). The net Pi regenerated from DATP was even greater
during the spring meridional transect with 53 ± 8% (n = 5)
and 81 ± 8% (n = 7) within the NPSG and transition-sub
polar regions, respectively (Figure 5B). The proportion (%) of Pi
regenerated from DATP to total DATP hydrolysis correlated with
ambient Pi concentrations, with an increasing proportion of the
total not taken up with increasing Pi concentrations (Figure 6).
Below 30 nmol Pi l−1 the regeneration diminished rapidly until Pi
release from DATP was no longer measurable at ∼ 5 nmol Pi l−1
(Figure 6).
The relative contribution by different microbial size classes to
the total community ATP-uptake, showed a different distribution
than that of Pi. A great majority of P derived from ATP
was incorporated into the smallest size class (<0.6>0.2 µm)
with 80.1 ± 2.7% and 74.0 ± 2.0% of the total particulate
uptake along the zonal and meridional transects respectively
(Figures 5C,D). The >2 µm size class contributed <5% of the
total uptake. The exceptions were at the Pi depleted stations (88,
100) and the bloom station (69) during the zonal cruise where the
relative uptake by the larger size classes was considerably greater
(Figure 5C). Furthermore, comparing the TOT of Pi and DATP
for the different size classes, in paired experiments, revealed that
there was no correlation between the two for the whole water
community (>0.2 µm; Figure 7A), whereas the TOT for both
Pi and DATP was similar in the >2 µm size class (Figure 7B,
The relative contribution by different microbial size classes
to the total community Pi-uptake, although variable along
the two transects, showed a clear dominance by cells <2 µm
with an average of 88% of Pi taken up by this size class
within the NPSG and through the transition zone, but was
lower in the CCS where cells >2 µm represented ∼ 30% of
the total Pi taken up (Figures 4C,D). Cells <0.6 µm became
progressively more important to the total Pi uptake when
transitioning from high Pi to low Pi environments. During
the zonal transect from the CCS, transition zone to NPSG,
cells <0.6 µm contributed 43 ± 2, 54 ± 12 to 67 ± 18%
respectively to the total Pi uptake. The same pattern, although
less pronounced, was seen during the meridional transect with
32 ± 13% at the transition zone and 42 ± 12% within the
NPSG. At the stations along 30◦ N where Pi concentrations
were <15 nmol l−1 (Stations 61, 81, 88, 100) the contributions
from the >2 µm size class were at their lowest with <5%
of the total. However, at station 69, where both chlorophyll
a and 14 C-PP were enhanced, indicative of a phytoplankton
bloom, the >2 µm size class represented ∼ 45% of the total
Pi-uptake.
ATP Rate Measurements
The TOT of ATP, based on the rate of uptake by the microbial
community, was typically much shorter than that measured
for the Pi-pool (Figures 3A,B) at 1–2 days in most cases
(zonal transect range; 0.44 ± 0.05 to 2.17 ± 0.08 days;
mean 1.6 ± 0.6 days, n = 20: meridional range; 1.02 ±
0.06 to 11.3 ± 1.9 days; mean 5.17 ± 3.04 days, n = 12:
Figures 3A,B). Only at the stations along the zonal transect
where Pi concentrations were below 10 nmol l−1 Pi (Stations
61, 88, 100), was the ATP pool turnover longer than that for
Pi (Figure 3A). The total DATP pool TOT (i.e., the sum of
the ATP taken up by microbes and Pi regenerated from the
hydrolysis of DATP and released into the ambient seawater)
was less than 1 day on average along the zonal transect
(0.95 ± 0.30 days: n = 20). The mean DATP turnover time
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FIGURE 5 | (A,B) Total DATP hydrolysis rates (pmol l−1 d−1 ) and the corresponding DATP concentrations (pmol l−1 ; bullets). The total DATP hydrolysis rate is
partitioned into rate of microbial uptake of P from ATP (gray), and the net release rate of Pi from DATP into the surrounding seawater (white). (C,D) Show the relative
contribution from different microbial size classes to the microbial ATP-uptake measured. The zonal transect is shown in (A,C); the meridional transect in (B,D). Station
ALOHA data for Pi uptake rates and Pi concentrations are shown to the left in each panel as the mean ± 1 s.d. (n = 7). Arrows indicate the stations within the CCS,
TZ and NPSG as described in Figure 1. Note that the scales differ among panels.
meridional transect r = 0.9 (n = 14; Figure 8). However, stations
within the NPSG during the meridional transect showed weaker
correlation at r = 0.7 (n = 5). The relationship between TOT and
Pi concentrations showed a better fit to a power function than a
linear regression and it is noteworthy that below 20 nmol l−1 Pi,
the slope of a linear regression is comparatively low, indicating
that any additional Pi does not have an appreciable effect on
TOT, hence additional Pi leads to an increase in uptake rate.
Above ∼50 nmol l−1 Pi the TOT to Pi relationship stabilizes,
i.e., additional Pi does not change uptake rate, so TOT instead
increases. The same was not true for the TOT of DATP vs.
DATP concentration, which was poorly correlated, in fact, the
TOT of the DATP pool was better correlated to the Pi pool
size than the DATP pool size (r = 0.7 vs. r = 0.5; data not
shown).
r = 0.9). In the Pi addition experiments at Station 100, where ATP
had longer TOT than Pi at ambient concentrations of Pi, the TOT
did not change significantly with additional Pi in the smallest size
class (<0.6>0.2 µm) at ∼ 1.3 days. However, at Pi additions of
50 and 100 nmol l−1 , the TOT for DATP was faster than for Pi.
The TOT of DATP in the two larger size classes was more similar
to Pi with additional Pi.
Correlation Between TOT, Pi, and DATP
Pool Concentrations
The TOT of Pi correlated well with Pi concentrations within the
NPSG (Figure 8). Correlation analysis of paired measurements
of Pi concentrations and TOT at Station ALOHA had an r = 0.9
(n = 28); the zonal transect 2014 r = 0.9 (n = 13) and the
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DISCUSSION
(zonal, 2014; meridional, 2016), presented an opportunity
to compare the temporal variability in the various P-pools
and microbial uptake rates of Pi and DATP observed at
Station ALOHA, with spatial variability in the NPSG as well
as areas within the transition zones into the CCS to the
east, and toward the subpolar waters to the north of the
North Pacific Ocean. The transition zones showed strong
trends in both salinity and temperature with salinities and
temperatures increasing from the boundaries to the east and
north into the NPSG, as well as higher inorganic nutrients,
chlorophyll and primary productivity compared to those of the
NPSG.
Characterization of Study Areas
The majority of the work presented here was conducted
within the NPSG, an environment characterized by its
oligotrophic nature with perennially low inorganic nutrients,
low standing stocks of chlorophyll and biomass, and typically
low primary productivity, with Station ALOHA serving as a
representative for the biome (Karl and Lukas, 1996; Karl and
Church, 2014). This ecosystem is microbial based, recycling
intensive, with picophytoplankton (<2 µm), specifically
the cyanobacterium Prochlorococcus, typically the largest
contributors to phytoplankton biomass and primary production
(Karl, 1999; Karl et al., 2001a). The two transect cruises
P Concentrations at Station ALOHA and
Along Transects
Although our understanding and knowledge of the cycling
of P in the oligotrophic oceans have increased over the past
decades, it is clear that these ecosystems are highly dynamic and
not readily predictable. In particular the chemical composition
and microbial utilization of the DOP pool, and its relative
bioavailability remains an ongoing field of exploration (Karl and
Björkman, 2015). Many studies have examined the bioavailability
of DOP either by its susceptibility to alkaline phosphatase
treatment (Moutin et al., 2008; Suzumura et al., 2012), through
bioassays using known DOP compounds (Berman, 1988;
Björkman and Karl, 1994; Björkman et al., 2000; Duhamel et al.,
2017), or directly via specific ATP pool labeling (Karl and
Bossard, 1985; Bossard and Karl, 1986; Björkman and Karl, 2005).
In this study, we show the Pi-pool to be highly variable within
the NPSG. Nevertheless, the range in concentrations at Station
ALOHA and along the transects was similar. This suggests
a mosaic upper ocean, with wide, yet limited dynamic range
in terms of Pi-availability both temporally and across regions
within the NPSG. In contrast, the DOP pool showed much
less variability, and interestingly, the PPO4 pool was relatively
invariant within the NPSG, although always at relatively low
FIGURE 6 | Proportion (%) of the total DATP hydrolysis released into the
surrounding seawater as dissolved Pi, versus the ambient inorganic phosphate
(Pi) concentration. Zonal transect (black bullets), meridional transect (white
bullets) and data from BiG RAPA transect cruise in 2010 within the South
Pacific Subtropical Gyre (Duhamel et al., 2017).
FIGURE 7 | Inorganic phosphate (Pi) turnover time (TOT; days) vs. ATP TOT (days) within the North Pacific Subtropical Gyre (NPSG) for (A) the whole water
community (>0.2 µm), along the two transects and at Station ALOHA, and (B) the >2 µm size class. The dashed line indicates the 1:1 relationship.
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further utilization of DOP. For example, in the subtropical North
Atlantic, zinc has been implied as a limiting factor for the activity
of alkaline phosphatase (Mahaffey et al., 2014), indicating that
essential microelements may impact DOP degradation processes.
The study region for the zonal transect around 30◦ N is known
to consistently, if not predictably, harbor large, summertime
phytoplankton blooms (Wilson, 2003, 2011; Villareal et al.,
2012) and satellite imagery showed enhanced chlorophyll in this
same area, consistent with such blooms, lasting from mid-July
through September 2014. In a previous study within this same
region Duhamel et al. (2010) found areas where the microbial
community was under P-stress, as implied by the comparatively
high alkaline phosphatase activity measured. The 2014 zonal
transect coincided with late bloom stages, and areas most likely
in post-bloom conditions as indicated by the vanishingly low
concentrations of Pi in the surface waters.
The concentration of the DOP pool constituent DATP was
quite uniform along the two transects within the NPSG, except
where Pi was depleted and where the DATP pool also was
drawn down to near the detection limit for our analysis. The
concentrations were higher during the fall than spring transect
possibly due to seasonal variability in DATP as previously
observed at Station ALOHA (Björkman and Karl, 2001).
Although DATP-phosphate is but a small fraction (∼0.15%) of
the total DOP pool, it appears to be highly bioavailable.
FIGURE 8 | Inorganic phosphate (Pi) concentrations vs. Pi-pool turnover times
(TOT; days) within the North Pacific Subtropical Gyre (NPSG). Station ALOHA
(black bullets), stations near Hawaii (white bullets), the zonal transect (gray
triangles) and the meridional transect (white triangles). Correlation coefficients
for these data were r = 0.9 for Hawaii (n = 45, mixed layer) and the zonal
transect (n = 13), r = 0.8 for stations near Hawaii (n = 31), and r = 0.7 for the
meridional transect (n = 5).
P Turnover Times and Uptake Rates
concentrations. This may indicate that even at very low Piinventories, the particulate P pool is buffered by increased
utilization of the DOP pool. However, a reduction of the DOP
pool in response to very low Pi is not evident at Station ALOHA,
but may not be readily resolved given the analytical precision
of the measurement (i.e., ability to detect a change in DOP of
±5%, or 10–15 nmol l−1 with sufficient certainty). Yet, during
the 2014 zonal transect, in the region around 30◦ N where Pi
was depleted, the DOP pool was significantly reduced relative
to other stations along the transect, as well as compared to
the long term mean at Station ALOHA. This lower DOP pool
size may reflect an increased utilization of DOP due to Plimiting conditions. Nevertheless, the DOP concentrations in this
region remained above 100 nM-P, indicating that a substantial
fraction of the DOP pool may not be readily available to the
microbial community in the surface ocean. Other studies have
reached similar conclusions that only a relatively small fraction
of the DOP pool appears to be bioavailable, e.g., Moutin et al.
(2008) found that only 10–20% of the in situ DOP pool in
the South Pacific Subtropical Gyre was hydrolyzable by alkaline
phosphatase, similar to what Suzumura et al. (2012) reported
for stations in the North Pacific near Japan (22–39%), and in
the tropical and subtropical North Atlantic phosphomonoesters,
substrates for alkaline phosphatase, constituted 19–37% of the
DOP pool (Reynolds et al., 2014). This is also consistent with
the degradation of selected DOP compounds at Station ALOHA,
as well as the assessment of DOP utilization though bioassays
(Björkman and Karl, 1994, 2003). An alternative explanation
for high residual DOP during Pi-deplete conditions may be
that other resources, such as available nitrogen or iron, are
limiting or co-limiting production, and these limitations hamper
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The Pi pool turnover times observed in this study were within
the range previously reported from the NPSG (Perry and Eppley,
1981; Björkman et al., 2000), of a few hours to several weeks,
again highlighting the variability in Pi-pool dynamics within the
NPSG. The Pi-pool turnover time was strongly correlated with
measured Pi pool concentrations, but did show concentration
dependent kinetics, i.e., increasing uptake rate with increasing
Pi concentration, at low ambient Pi concentrations, as described
previously for Station ALOHA (Björkman et al., 2012).
The turnover time of the DATP pool was typically
substantially shorter than for the Pi-pool, reflecting both
its small pool size and that the microbial hydrolysis and
uptake of P from DATP proceeds separately from that of the
Pi pool (Ammerman and Azam, 1985; Björkman et al., 2012;
Duhamel et al., 2017). Total DATP hydrolysis was high along
both transects, exceeding the uptake of Pi from DATP by the
microbial community, resulting in a net release of regenerated
Pi into the ambient seawater. There was a positive relationship
between Pi concentrations and the proportion of the DATP
hydrolyzed, i.e., net regeneration of Pi from DATP, similar
to that observed by Ammerman and Azam (1991a), with a
higher fraction of the total hydrolyzed P taken up at lower Pi
concentrations. In the South Pacific Subtropical Gyre, where Pi
concentrations are perennially higher compared to the NPSG
(Moutin et al., 2008), the proportion of regenerated Pi from
ATP was also higher than observed in this study (Figure 6)
emphasizing the decoupling of DATP hydrolysis and Pi uptake
at high Pi concentrations (Duhamel et al., 2017). The steepest
increase in the proportion of DATP regenerated as Pi was seen
at Pi concentrations <100 nmol l−1 Pi, indicating that the
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North Pacific Ocean Phosphorus Dynamics
DOP and PPO4 pools were less variable; the particulate pool
potentially buffered by the much larger reservoir provided by the
DOP pool. Although Pi-stress, or limitation, occurred locally or
periodically, P sufficiency appears to be the prevailing condition
for the dominant components of the microbial community
within the NPSG. DATP was typically more rapidly turned over
than the PATP pool, which turned over faster than the Pipool. Microbial uptake of P was dominated by the <2 µm size
class; however, the turnover time for Pi or ATP in the >2 µm
fraction were similar, suggesting that microbial utilization of
these two phosphate pools are independent of one another
within the NPSG and that DATP predominantly is processed
by the smallest microbial components of this ecosystem. DATP
hydrolysis commonly resulted in the net release of Pi into the
ambient seawater and may increasingly contribute to P-nutrition
during low Pi conditions.
newly regenerated Pi is of increasing importance to P-nutrition
the smaller the Pi pool becomes, and the coupling between
DATP hydrolysis and uptake becomes increasingly tighter,
i.e., the recycling of P intensifies, as Pi concentrations drop
below ∼100 nmol l−1 . Although the smallest components of
the microbial community appear to have the highest affinity
for Pi (Thingstad et al., 1993; Cañellas et al., 2000; Vadstein,
2000), and typically do not appear to be P-limited, the larger
cells can capitalize at higher Pi concentrations with increased
Pi-uptake rates as well as capturing a larger proportion of the
total community P taken up (Björkman et al., 2012). This was
observed during the two transect cruises, and demonstrated in
the Pi addition experiment at station 100 where a range of Pi
concentrations was used. In this experiment microorganisms
in the >2 µm fraction increased both Pi uptake rate as well as
the relative contribution of the total uptake with increasing Pi.
The uptake of P originating from DATP was greatly dominated
by the smallest size class (<0.6>0.2 µm) and likely driven by
heterotrophic bacterial hydrolysis by the enzyme 5′ -nucleotidase
and subsequent uptake (Azam and Hodson, 1977; BengisGarber and Kushner, 1982; Ammerman and Azam, 1991a,b).
Interestingly, at the Pi depleted stations along 30◦ N during the
zonal transect, Pi from ATP was transferred to a greater extent
into the two larger size classes, although the rates were very
low. However, this partitioning was not seen in the Pi-uptake,
where the largest size classes contributed the least to overall
Pi uptake. Taken together, this indicates that this P depleted
region was poised for rapid P assimilation, should P become
available.
AUTHOR CONTRIBUTIONS
KB and SD designed the experiments, performed the fieldand laboratory work. KB wrote the manuscript. SD, DK,
MC contributed significantly to the intellectual content of the
manuscript. DK and MC provided funding.
ACKNOWLEDGMENTS
We thank the Captains and crew of the R/V Kilo Moana,
R/V Kaimikai-O-Kanaloa, R/V New Horizon and R/V Knorr.
We thank Lance Fujieki and Benedetto Barone for graphical
assistance. Funding was provided by the National Science
Foundation for the Hawaii Ocean Time-series program (OCE1260164, MC, DK) and the Center for Microbial Oceanography:
Research and Education (C-MORE, DBI-0424599, DK) and
Dimensions in Biodiversity (OCE-124221, MC). Additional
support was provided by the Gordon and Betty Moore
Foundation: Marine Microbiology Initiative (3794, DK) and the
Simons Foundation (SCOPE project ID 329108: DK and MC and
Gradients project ID 426570SP: V. Armbrust; U. Washington,
with subcontract to DK).
CONCLUSION
To gain a better appreciation for the flux of essential nutrients,
such as phosphate, through biogeochemical cycles in the vast
open ocean biomes, it is necessary to assess the dynamics of the Ppool and how its bioavailability may impact spatial and temporal
variability in productivity and community composition. Our
study showed that Pi concentrations in the surface waters within
the NPSG were highly dynamic both in time and space whereas
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