Limnol. Oceanogr., 54(6, part 2), 2009, 2448-2459
© 2009, by the American Society of Limnology and Oceanography, Inc.
Elevated C02 increases sensitivity to ultraviolet radiation in lacustrine
phytoplankton assemblages
Cristina Sobrino,^1-* P. J. Neale,a J. D. Phillips-Kress,a R. E. Moeller,b and J. A. Porter0-2
a
Smithsonian Environmental Research Center, Edgewater, Maryland
Miami University, Oxford, Ohio
c
Lehigh University, Bethlehem, Pennsylvania
b
Abstract
This study tests the effects of elevated C02 and ultraviolet radiation (UVR) on phytoplankton photosynthesis
through in situ incubations in Lake Giles, Pennsylvania. In a first experiment, C02 was supplied from a tank to
simulate atmospheric C02 concentrations predicted in scenarios of future global change. In a second experiment,
elevated C02 conditions were obtained by the mineralization of added colored dissolved organic matter (CDOM)
of terrestrial origin (400 /anol L_1 final concentration). The results demonstrated that for natural assemblages
from Lake Giles, atmospheric C02 concentrations similar to those predicted for the end of the century can
increase primary productivity up to 23% in the absence of photoinhibition. However, elevated C02
concentrations also increased sensitivity of phytoplankton to UVR, making cells more susceptible and increasing
photoinhibition. Increased sensitivity was observed in samples incubated with the artificial C02 supply as well as
with the CDOM addition, the latter resulting in C02 partial pressures close to three times present atmospheric
levels. Photosynthetic rate modeled for elevated C02 and midday solar exposure just below the lake surface was
17% of potential production compared with 21% under usual C02 levels in the lake, resulting in similar rates
between phytoplankton assemblages grown under high and low C02 levels. Understanding the effect on primary
productivity of the interaction between factors that may be affected by global change is essential to predict future
changes in ecosystems and climate.
The increase in atmospheric C02 caused by anthropogenic production since 1970 has been propagated to the
aquatic systems through the surface layer (Sabine et al.
2004), reaching values 1.4 times higher than the preindustrial concentrations. On the other hand, global change
affects solar ultraviolet B (UVB, 280-320 nm) and ultraviolet A (UVA, 320^100 nm) penetration in surface waters
through changes in the stratospheric ozone concentration,
cloud cover, and changes in the colored part of dissolved
organic matter (CDOM) (for review see Zepp et al. 2007).
The latter becomes especially relevant in freshwater
ecosystems and coastal waters. CDOM concentration and
composition controls the penetration of UV radiation
(UVR) into water bodies, but CDOM is also photodegraded by solar UVR (Zepp et al. 1995; Moran and Zepp
1997; Moran et al. 2000), resulting in loss of color
(photobleaching) and the enhancement of UVR exposure
to aquatic organisms. In addition, UVR-induced photomineralization of CDOM releases C02 and increases the
biological availability of DOM, which can be used as an
energy source for microbial respiration (Moran et al. 2000;
De Lange et al. 2003; Lennon 2004). All these findings
indicate that UVR penetration in aquatic environments can
reinforce global climate change directly, by increasing
atmospheric C02, reducing phytoplankton carbon fixation,
and increasing CDOM photomineralization, and indirectly,
* Corresponding author: sobrinoc@uvigo.es
Present address:
1
University of Vigo, Vigo, Spain
2
University of the Sciences in Philadelphia, Philadelphia,
Pennsylvania
through the production of biolabile compounds for
microbial respiration.
The interaction between elevated C02 concentrations
and incident UVR (280-400 nm) affects phytoplankton
metabolism, producing different responses from those
expected from the addition of the individual effects
(Sobrino et al. 2005, 2008). Nevertheless, the in situ
response of natural assemblages is still unknown. In
general, it is expected that long-term elevation of C02 in
the absence of photoinhibition would increase phytoplankton growth and carbon fixation (Hein and Sand-Jensen
1997; Ibelings and Maberly 1998; Riebesell et al. 2000), but
natural samples have not always shown a significant
response (Tortell and Morel 2002). Conversely, UVRcaused decrease in areal primary productivity can be up to
30% and varies with the sensitivity of phytoplankton to
UVR (Pienitz and Vincent 2000; Helbling et al. 2001; Neale
et al. 2001). The sensitivity of phytoplankton photosynthesis to UVR, i.e., the change in photosynthesis per unit UV
exposure, is related to the balance between the damage and
repair capacity of the cells. It varies among species but it
can also be affected by natural environmental factors such
as temperature (Sobrino and Neale 2007), light acclimation
history (Villafane et al. 2004), and nutrient and C02
availability, among others (Litchman et al. 2002; Sobrino et
al. 2008). Furthermore, the increase or decrease in
sensitivity of photosynthesis to UVR produced by some
external conditions is not always directly related to the
increase or decrease in photosynthesis under nonphotoinhibitory exposures. For example, in Nannochloropsis
gaditana, a picoplanktonic marine species, increased C02
did not increase carbon fixation under nonphotoinhibitory
2448
CO2 and UVR photoinhibition in lakes
photosynthetically active radiation (PAR, 400-700 nm)
exposures but decreased sensitivity to UVR when compared with cells grown under atmospheric levels of C02
(i.e., cells became more resistant to UVR) (Sobrino et al.
2005). In contrast, elevated C02 concentrations increased
carbon fixation in the estuarine diatom Thalassiosira
pseudonana, but C02 elevation also increased sensitivity
to UVR (i.e., cells became less resistant to UVR) (Sobrino
et al. 2008). The opposite responses observed between these
species regarding sensitivity to UVR was mainly attributed
to differences in the degree of acclimation to elevated C02
conditions and it is explained in detail in Sobrino et al.
(2008). In this sense freshwater ecosystems with consistent
C02 supersaturation of the water column provide phytoplankton assemblages that are long-term acclimated to high
C02 conditions and are an interesting tool to test future
scenarios of elevated C02 concentrations (Cole et al. 1994;
Graneli et al. 1996). Even so, the resulting primary
production under high C02 and UVR photoinhibitory
exposures will ultimately depend on the degree of the
maximum photosynthetic rates attained under nonphotoinhibitory exposures, the degree of sensitivity to UVR,
and the irradiance and spectral composition of the light
reaching the organisms.
In this study we analyze the interacting effects of UVR
and high C02 concentrations on natural phytoplankton
assemblages through in situ incubations performed during
summer 2005 and 2006 in Lake Giles, Pennsylvania. Unlike
the previous experiments with cultures, where the extra
carbon pool was always supplied from a C02 tank, in this
case we studied the effect on phytoplankton photoinhibition of C02 concentrations that were both artificially
enhanced as well as naturally enhanced by photomineralization of added CDOM. These experimental approaches
were intended to simulate future scenarios of climate
change with elevated atmospheric C02 concentrations or
increases in allochthonous inputs of organic matter,
respectively. The latter might occur as climate warming
promotes the soil development associated with altitudinal
increase in treeline and vegetation, and as changes in
precipitation regimes increase episodic storm events (Hauer
et al. 1997; Hinton et al. 1997). We calculated photosynthetic rates in the presence and absence of UVR and
estimated the biological weighting functions (BWFs) for
the photoinhibition of lake phytoplankton (reviewed by
Cullen and Neale 1997; Neale 2000).
Methods
Location and characteristics of Lake Giles—Lake Giles is
a small (0.48 km2) postglacial lake formed in carbonatepoor Devonian sandstones and shales of the Pocono
Plateau of northeastern Pennsylvania (41.4°N, 75.1°W,
elevation 428 m above sea level). The water is slightly acidic
(pH ca. 6.0 in 2005-2006) with low dissolved inorganic
carbon (DIC, 25-33 umol L_1), as well as low dissolved
organic carbon (DOC, ca. 100 /imol L_1). The summer
mixed layer (ca. 6 m) of the 24-m-deep lake is relatively
UVR transparent and low in algal biomass (ca. 1 «g L_1
chlorophyll a [Chi a]). The lake is considered oligotrophic,
2449
and mixed-layer phytoplankton are typically colimited by
nitrogen and phosphorus in summer (R. E. Moeller
unpubl.).
Setup—In summer 2005, a water sample was collected
from 3-m depth in the lake at dusk using a pump at low
pressure. After transportation to the laboratory (less than
1 h), the water was screened through 48-/im mesh to
remove macrozooplankton grazers, and kept overnight in
20-liter microcosms with or without aeration (ca. 0.6 L
min-i) with 101.3 Pa C02 in air (0.1% C02 = 1000 parts
per million by volume C02) until the start of the in situ
incubation the next morning. Microcosms were translucent
flexible polyethylene UVR-transparent containers (Nalgene® I-Chem Certified• Series 300 low-density polyethylene Cubitainers• ). Transmittance (7) into the cubitainers decreases progressively from 95%T at 700 nm to
84% T at 320 nm and 78% T at 290 nm. The cubitainers
were previously incubated with lake water for 7 d and
washed with 10% HC1. The air-C02-mixture gas tank was
provided by Airgas. High-C02 and low-C02 (i.e., at lake
C02 concentrations) microcosms were moored at 3-m
depth for 6 d, tightly closed, and completely filled with
sample to minimize air space. At this depth the incubated
phytoplankton received approximately 10% of the incident
UVR. An additional cubitainer, filled with 0.2-/im filtered
lake water and aerated overnight similarly to the high-C02
samples, was also incubated in situ to quantify C02
diffusive loss from the high-C02 cubitainers. The experiment was performed in duplicate, with the duplicates
starting on consecutive days (20 July 2005 and 21 July
2005). For each treatment, subsamples were collected at the
beginning (T0), near the middle, and after 6 d.
In summer 2006, water samples were again collected at
3 m but incubated near the surface (10 cm average depth)
with and without an addition of CDOM for 6 d. The
CDOM source was water from Beaver Pond, a small
(0.09 km2), 12-m maximum depth, dystrophic pond located
approximately 2 km northeast of Lake Giles, in an
adjoining watershed. Beaver Pond is highly colored, with
DOC concentrations ranging from 400 to 1200 umol L_1
(varying seasonally) and a DOC-specific absorbance at
320 nm (a320 [DOC]"1) of 4.15 m2 g C~K Beaver Pond
water was filtered through a 5-um prefilter (Ace Hardware)
and concentrated using a reverse osmosis unit built in the
Morris Laboratory at Lehigh University according to Sun
et al. (1995). Approximately 5% CDOM was added to Lake
Giles water for a final DOC concentration of 400 /anol L_1
and #320 [DOC]-1 of 3.35 m2 g C_1. Because of its
chromophoric properties and proximity to Lake Giles,
Beaver Lake water concentrated by reverse osmosis serves
as a realistic approximation of the allochthonous CDOM
input to Lake Giles and as an appropriate CDOM source
for the experiments.
Lake water was passed through a 48-/im mesh to remove
macrozooplankton as it was collected. A portion of this
water, with and without added CDOM, was incubated in
2-liter, UV-transparent Teflon• fluorinated ethylenepropylene bottles to assess the changes in phytoplankton
sensitivity to UVR. In addition, bottles were covered with
2450
Sobrino et al.
UVR-opaque or UVR-transparent film to obtain two
different UVR treatments with similar PAR: -UVR
bottles were covered with Courtgard• clear UV-absorbing film (CPFilm) that removes most of the UVR (50%
cutoff at 400 nm) but allows the transmission of PAR, and
+UVR bottles covered with Aclar clear film (Aclar
fluoropolymer, Honeywell) that allows the transmission
of full-spectrum PAR+UVR (98% of UVB 295-319 nm,
99% UVA 320-399 nm). PAR exposure in the two
treatments was the same since both Aclar and Courtgard
reflect about 5% at all wavelengths when immersed. A
second portion of the water was further filtered through a
1-^m pore size polycarbonate (Nucleopore) filter to remove
algae. This water, with and without added CDOM, was
incubated in 250-mL Teflon bottles also with either
Courtgard or Aclar.
Atmospheric conditions were quite constant during both
years, with water temperature of 25.7°C ± 0.1°C at 3 m in
2005 (mean ± SEM, n = 4) and 21.2°C ± 1.4°C at the
surface in 2006 (mean ± SEM, n = 3), and mainly clear
days, excepting a stormy weather episode at the end of 2006
incubations.
DIC and C02 concentrations—Samples were collected in
precleaned glass bottles, overflowing the bottle with the
sample. They were kept in the dark at room temperature
until analysis the same day using the method of Stainton
(1973), in which a 25-mL sample is acidified and
equilibrated with a N2 headspace, then analyzed by gas
chromatography (Shimadzu GC 8A chromatograph with
PoraPak Q column). Water samples were analyzed in
triplicate using Na2C03 solutions as standards. An Orion
pH meter with Ross• combination electrode was used to
measure pH. Samples were analyzed at room temperature
after mixing in 1% by volume of pHisa• ionic strength
adjuster (•Thermo Fisher Scientific). C02 concentrations
in the samples were calculated from total DIC concentrations, pH, and temperature using the program csys.m from
Zeebe and Wolf-Gladrow (2001), applying dissociation
constants for freshwater.
Chi a concentrations—Samples for Chi a and photosynthesis measurements were collected using a 1-liter polycarbonate bottle. Water samples (50 mL) were filtered in
triplicate onto 25-mm glass fiber filters (GEE Whatman)
for Chi a analysis. Filters were kept frozen until extraction
following Pechar (1987). After 24 h at -10°C, fluorescence
was measured (Sequoia-Turner model 112 or Turner 10AU fluorometer) and calibrated with Chi a determined
spectrophotometrically.
Taxonomic composition—Samples for phytoplankton
enumeration were preserved with acid Lugol's solution,
settled in Utermohl chambers, and counted at 480 X
magnification using an Olympus 1X71 microscope.
Photosynthetic responses to PAR—Photosynthesis-irradiance (P-E) curves for PAR-only exposure were obtained
in a "photosynthetron" incubator using a modification of
the protocol described by Lewis and Smith (1983). The
temperature-regulated incubator uses a halogen lamp to
assess the photosynthetic response to PAR (n = 10-12
intensities) as the conversion of inorganic H*4CO^ (NEN
sodium bicarbonate solution, 8.4 /iCi 14C /miol-1 C, Perkin
Elmer) into organic (acid-stable) compounds over a 1-h
incubation. Photosynthetic parameters, Pf and Es, were
estimated using nonlinear regression fitting of the equation:
Pf( 1-exp
EpAR
(1)
where PB is photosynthesis normalized to Chi a content
(g C [g Chi]"1 h-i), Pf is the light-saturated rate of
photosynthesis, Es is the saturation irradiance for PAR
(fimol photons m-2 s_1), and £PAR is the PAR irradiance.
Samples were incubated in 7-mL glass scintillation vials
previously washed in 10% HC1 and extensively rinsed. Each
vial containing 6 mL of sample was inoculated with
H14CO^" (ca. 0.02 /iCi mL-1) and capped immediately to
avoid C02 exchange with the atmosphere during the
incubation. Since each vial was inoculated individually
(cf. Lewis and Smith 1983), total 14C activity was
determined by averaging the activity measured in four
100-^L subsamples taken from selected vials near the end
of the incubation. PAR scalar irradiance was measured in
vials filled with filtered lake water using 4-n sensor (QSL2101, Biospherical Instruments).
Photosynthetic response to UVR: BWF and BWFTI
P-E model—Simultaneously with the photosynthetron
incubations and using similar inoculation procedure and
incubation time, the photosynthetic response to UVR was
assessed using a special polychromatic incubator, the
"photoinhibitron" (Cullen et al. 1992). A detailed description of the photoinhibitron incubator is given in Sobrino et
al. (2008). Spectral irradiance in each cuvette was measured
with a custom-designed spectroradiometer as previously
described (Neale and Fritz 2001). The cuvettes were filled
with 7 mL of sample, inoculated with H14CO^ (ca.
0.02 /iCi mL-1), and capped during exposure.
The BWF/P-E models are based on a PAR-only P-E
curve equation, adding a term that represents inhibition of
photosynthesis by UVR and PAR (Eq. 2). The inhibition
term relates the effect of UVR to "weighted" irradiance,
E*nh (i.e., spectral irradiance corrected by the biological
effect or weight [Eq. 3]), and includes the BWF. The BWFs
appear as a set of weighting coefficients, e(X), that measure
the strength of UVR action at each wavelength, therefore
providing an estimation of the sensitivity to UVR exposure
in relation to wavelength (reviewed by Neale 2000). For a
step-by-step protocol for calculation of the BWFs for this
study see Cullen and Neale (1997) and Sobrino et al. (2008).
The BWF TlP-E model (also called T model) with a
saturating exponential P-E term was selected to predict
photosynthesis in the presence of PAR and UVR exposures
because it showed the highest R2 and least bias in residuals.
It is shown as:
PB =if (1-exp
EpAR
min 1
1
' E
(2)
CO2 and UVR photoinhibition in lakes
400
£ e(A) x E(X) x Al + epAR x
£RAR
(3)
A = 280
where min denotes the minimum function, E*nh is a
dimensionless index for the biologically effective or
weighted irradiance, e(A) is biological weight ([mW
m-2]-1) at wavelength A, E(X) is spectral irradiance
(mW m~2 nm-1) at A, and AA is the wavelength resolution,
1 nm. Inhibition by PAR is included using a single broad
band weight, £PAR ([W m-2]-1), for PAR irradiance (Cullen
et al. 1992; Cullen and Neale 1997). BWFs were estimated
from the measured rates of photosynthesis using nonlinear
regression and principal component analysis (PCA) for
samples from summer 2005 (n = 2, m = 80 spectral
treatments per incubation), and the Rundel method for
samples from summer 2006 (n = 1-3, m = 40 spectral
treatments per incubation) (Rundel 1983; Cullen and Neale
1997). The Rundel fits with the smaller data sets produce
BWFs with simpler shapes. Tests of fits using both the PCA
method on m = 80 data set and Rundel method on a m =
40 subset of the same data showed that both methods gave
similar trends in the variation of sensitivity between
treatments. The BWFs shown in the manuscript correspond to the average BWF for each treatment with the
SEM for each wavelength calculated from individual error
estimates by propagation of errors.
Weighted irradiance in the in situ incubations (Teflon
bottles) was calculated as
400
^] E(A)xf(A)xA(A)xe- *X;)z
(4)
;. = 280
+ 8PARXEpARXf-%*«\
whereas average weighed irradiance in the lake was
400
c
inh_
£
[e(A) x f (A) x A(A) x (l -g-*^)/^):
X = 280
+ ePARx£,pAR(l-e — ^PAR- /^PARZ
N
with z (m) representing the depth to the center of
submerged bottles in Eq. 4 and the average mixed layer
depth of the lake in Eq. 5, and Kd{X), K¥AR (m_1) the
attenuation coefficients in the UV and PAR, respectively.
KJs for the lake water column were estimated using a
profiling UV radiometer (Biospherical Instruments BIC or
PUV-500) and an exponential relationship between measured KJs (305, 320, 340, 380 nm) and wavelength.
Average Kd values during the 2005 experiments varied
from Kd (305 nm) = 1.61 to Kd (400 nm) = 0.30, and in
2006 from Kd (305 nm) = 2.43 to Kd (400 nm) = 0.36. For
samples incubated with a CDOM addition KJs were
estimated by adjusting the exponential decay of light
absorption to the measurements assessed in situ using
actinometers centered inside the Teflon bottles. Actinometry involved nitrite (UVA) and nitrate (UVB) solutions
prepared and assayed as described by Jankowski et al.
(1999). The solutions were exposed for a 6-h period
2451
centered on solar noon in 1-cm-diameter borosilicate glass
vials mounted in the center of the bottles.
Light measurements—Incident solar UVB radiation at
the surface was measured by a Smithsonian SRI8 UVB
spectroradiometer, which has 18 channels in the wavelength range of 290 to 324 nm. Then, UVA and PAR
spectral irradiances were calculated using a radiative
transfer model (Ruggaber et al. 1994) implemented by the
STAR software package (H. Schwander University of
Munich) and adjusted for measured UVB as described by
Fritz et al. (2008). UVB and UVA unweighted irradiances
for a clear-day spectra at midday (21 July 2005) were 2.6 W
m~2 and 50.9 W m~2, respectively. UVR weighted
irradiance calculated using the BWF for the inhibition of
photosynthesis in the diatoms Phaeodactylum tricornutum
(Cullen et al. 1992) and T, pseudonana (Sobrino et al. 2008),
and the action spectra for deoxyribonucleic acid damage of
Setlow normalized to 300 nm (Setlow 1974) were 1.44
(dimensionless), 3.04 (dimensionless), and 0.11 W m~2,
respectively.
Results
In experiments performed in summer 2005, where the
C02 enrichment was supplied from a tank and algae
received small UV exposure during the incubation, DIC in
low C02 microcosms declined from average 28.3 /imol L_1
to 22.4 /rniol L_1 after 6 d of incubation, and from average
64.1 /imol L_1 to 43.5 /imol L_1 in the high-C02 microcosms. Analysis of the carbon forms contributing to total
DIC showed that at pHs of 5.98 ± 0.04 and 5.71 ± 0.04, for
low C02 and high C02 (mean ± SEM, n = 6), respectively,
C02 was predominant, with average partial pressures
declining from 58.9 Pa to 46.8 Pa in the low-C02
microcosms, and from 160.9 Pa to 104.3 Pa in the highC02 microcosms (Fig. 1A). Diffusive loss of C02 from the
high-C02 cubitainer was 2.9 Pa d_1 (data not shown).
Although the rate of C02 decrease (after correction for
C02 leakage) was higher in the high-C02 than in the lowC02 microcosms, the increase in phytoplankton biomass,
measured as bulk Chi a accumulation, was larger in the
low-C02 than in the high-C02 microcosms, reaching
average values of 3.4 \ig L_1 and 2.8 tig L_1, respectively
(Fig. IB) (repeated-measures ANOVA [RMANOVA],/; =
0.046, n = 2, m = 3). At the start of the experiment, the
cyanobacterium Merismopedia tenuissima made up 18% of
the counted biovolume, and the chlorophytes Sphaerocystis
schroeteri and Elakatothrix gelatinosa comprised respectively 74% and 8% of the total biovolume. Numerous much
rarer taxa were not considered. Elevated C02 increased
Merismopedia abundance 77% compared with low C02
conditions, whereas no significant differences were observed in Sphaerocystis or Elakatothrix with regard to C02
concentrations.
P-E curves in the absence of UVR showed that Chi aspecific carbon fixation increased significantly from the
start to the end of the experiment (RMANOVA, p =
0.0376, n = 2, m = 10), and was higher in the high-C02
microcosms (Fig. 2) (RMANOVA, p = 0.0002, n = 2,m =
Sobrino et al.
2452
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Time (days)
Fig. 1. Changes in (A) C02 partial pressure (Pa), and (B)
chlorophyll accumulation, Chi a (fig L_1) in Lake Giles microcosms incubated at 3 m in 2005 with and without added CO?
(mean ± SEM). Open symbols and dashed lines correspond to
samples preaerated with 0.1% C02 (101.3 Pa, high C02) and
closed symbols and solid lines correspond to samples without
added C02 (low C02). The circles represent the experiment
starting 20 July and the squares those from the experiment
starting 21 July. Error bars that are not shown (i.e., closed
symbols) correspond to values smaller than the symbols.
10). High C02 concentrations produced an increase in Pf
of 23% compared with low C02 concentrations (Table 1).
The C02 level also affected the light saturation parameter
Es, and the initial slope of the photosynthesis irradiance
curves, a (=Pf/Es). The latter parameter increased from
0.014 to 0.019 under high C02 concentrations, indicating a
36% increase in photosynthetic efficiency (i.e., assimilated
carbon:PAR) under these conditions (Table 1).
1
200
1
i
i
i
i
i
400 600 800 1000 1200 1400
Irradiance (umol photons m" s")
Fig. 2. Photosynthesis-irradiance (P-E) curves for PAR-only
exposure (photosynthetron) obtained for Lake Giles microcosms
from 2005 experiments. Open symbols and dashed lines correspond to samples aerated with 0.1% C02 (101.3 Pa, high C02)
and closed symbols and solid lines correspond to samples without
added C02 (low C02). Crossed symbols represent Lake Giles
water samples at the start of the incubation. The circles and thin
lines correspond to the values from the experiment starting 20 July
and the squares and thick lines to those from the experiment
starting 21 July. Nonlinear regression was obtained by fitting
values to Eq. 1.
P-E curves obtained from PAR plus UVR exposures in
the photoinhibitron showed that carbon fixation in the lake
assemblages decreased both at higher irradiance and with
the inclusion of shorter wavelengths in the spectra (Figs. 3,
4). Figure 3 shows only the photosynthetic rates obtained
using the long-pass filters LG 370 and WG 320 for
simplicity as an example of the responses observed in the
photoinhibitron. Although photosynthetic rates were always higher under high C02, the relative decrease of the
photosynthetic rates under photoinhibitory irradiances was
more pronounced under high-C02 than under low-C02
conditions. In addition, photoinhibition was more severe
when the spectra included shorter wavelengths, i.e., by
using the WG 320 long-pass filter compared with the LG
370. The analysis of the carbon fixation rates for all the
spectral treatments (n = 80) using the BWFj-AP-is model
(Eq. 2) allowed the estimation of the sensitivity coefficients
(or "weights") for each wavelength in both high- and lowC02 conditions. The set of weighting coefficients, e(/l),
formed the BWFs for the inhibition of photosynthesis
(Fig. 4A). Similar to the trends observed previously with
the P-E curves using the LG 370 and WG 320 long-pass
filters, the BWFs showed that sensitivity of photosynthesis
to UVR increased for shorter wavelengths in both highand low-C02 microcosms (Fig. 4A). However, sensitivity
to UVR was higher under high-C02 concentrations, as
shown by the higher values of E(1) in the high-C02
microcosms. Significant differences in sensitivity to UVR
CO2 and UVR photoinhibition in lakes
Table 1. Photosynthetic parameters of Lake Giles water
samples incubated for 6 d in July 2005 at 3-m depth with a 0.1%
C02 enrichment (101.3 Pa, high C02) and without the C02
enrichment (low C02). Lake Giles T0 (LG(r0)) corresponds to
values obtained from Lake Giles water samples at the start of the
in situ incubation. The parameters were estimated using nonlinear
regression fitting of Eq. 1 to the values obtained from 14C
incubations in the photosynthetron (i.e., only PAR). Values (mean
± SEM) are the light-saturated rate of photosynthesis, P f (g C [g
Chi a]-1 h_1), the saturation irradiance, Es (/rniol photons
m-2 s_1), and the initial slope of the photosynthesis-irradiance
curves, a, (g C [g Chi a]-1 h_1 [/miol photons m-2 s-1]-1). R2 is
the coefficient of determination.
pf
Es
a
R2
LG(r„)
Low CO?
High C02
2.47±0.05
166.0±14.6
0.015±0.001
099
2.96±0.06
213.0±16.26
0.014±0.001
0.99
3.65±0.09
192.5± 17.07
0.019±0.002
049
were observed between 335 and 370 nm, corresponding to
the UVA region of the spectra. Further analysis weighting
solar spectral irradiance at the surface by the obtained
BWFs showed that for clear-sky exposures at midday,
differences in weighted irradiance became significant at
longer wavelengths (1 > 315 nm) due to the greater weights
for high-C02 treatments at those wavelengths (Fig. 4B).
The weighted irradiance spectra (E*(X)) also showed a peak
at 330 nm, demonstrating the strong biological effect of
this wavelength region. Analyzing e(330) values as a
sensitivity index of photosynthesis to UVR in each
replicate, it was observed that sensitivity to UVR increased
from the start to the end of the incubation, with samples
collected on 21 July having the largest differences between
2453
high- and low-C02 conditions (Fig. 5). Photosynthetic
rates under ambient irradiance at the surface including
both PAR and UVR are predicted (Eq. 2) to be strongly
inhibited to 17% of potential (uninhibited) photosynthesis
under high-C02 levels compared with 21% under usual
C02 levels in the lake, producing similar photosynthetic
rates (i.e., PB) under high- and low-C02 conditions
(Table 2).
In experiments performed in summer 2006, Sphaerocystis
was again the dominant alga, comprising 80% of the total
biovolume, whereas Merismopedia and Elakatothrix represented less than 1%. Numerous other taxa made up the
other 19% of biovolume. Algae were incubated near the
lake surface, so that there was significant exposure to UVR,
and C02 enrichment occurred because of the progressive
degradation of CDOM by both microbial and photochemical processes. CDOM addition changed the DIC composition and reduced UVR penetration within the sample.
Initially, UVA irradiance that reached the interior of the
UVR-exposed Teflon bottles was 49% of the incident
subsurface irradiance, and UVB was 37%. After 6 d of UV
exposure, light attenuation decreased because of photobleaching of CDOM, and UVA and UVB in the Teflon
bottle reached values of 68% and 53% of the incident
irradiance, respectively. By comparison, UVR exposure of
the Lake Giles phytoplankton assemblage (average over
4.3-m surface mixed layer) was 28% and 10% of incident
UVA and UVB. DIC also changed after 6 d of incubation
from 25.6 /imol L_1 in the lake to averages (including both
-UVR and +UVR) of 51.9 /imol L"1 and 32.3 /anol L"1
in bottles with and without CDOM addition, respectively
(Fig. 6A). No significant differences in DIC were observed
among samples without CDOM, independent of the
spectral treatment or the inclusion of algae as opposed to
1
1
1
1
i
i
i
3.5
WG320
3.0
o
'
/
25 \
\
\
Q
\
- / /~\ \
Lr
\\
'7
\\
" '/
V
2.0 O
era
n
1.5 £
a,
1.0 tr
rv
Irradiance (/jmo\ photons m~'
200 400 600 800 100012001400
Irradiance (/anol photons m" s" )
0.5
0.0
Fig. 3. Photosynthesis-irradiance (P-E) curves for PAR+UVR exposures obtained from
exposures in the polychromatic incubator (photoinhibitron) using Lake Giles water samples
incubated for 6 d at 3 m with and without C02 enrichment. Open symbols and dashed lines
correspond to phytoplankton aerated with 0.1% C02 (101.3 Pa, high C02) and closed symbols
and solid lines correspond to phytoplankton without added C02 (low C02). Irradiance is shown
only as PAR for simplicity, but LG 370 long-pass filter allowed the transmission of X > 370 nm
and WG 320 I > 320 nm. Associated curves show the trends in each data set (these are not
BWF predictions).
2454
Sobrino et al.
0.001
1
•
1
1
1
1
1
1
1
1
1
i
i
|
i
i
i
0.00020
|
A:
High CO,
-
0.00015
•
%.^
LI
B
^v*
£ 0.0001
?c
R
^>sNy//\S^V N -•-.
0.00010
o
m
m
Low CO2
0.00005
0.00001
0.10
1
'
0.08
—-- E"(X) High
E (A) Low
E<
1000
w 0.06
§
| 0.04
•jf 0.02
0.00
300
320
340
360
380
Wavelength (nm)
Fig. 4. (A) Average biological weighting functions (BWFs,
[mW m~2]-1) for the inhibition of photosynthesis in Lake
Giles water samples incubated for 6 d at 3 m with and without
C02 enrichment. Thin lines represent the SEM calculated from
error estimates of individual BWFs. (B) UVR spectrum for solar
exposures (E(X), mWm~2nm_1) from measurements at Lake
Giles during summer at midday combined with radiative transfer
model calculations (thin line; see Methods), and weighted spectral
irradiances (E*(l), nm-1) calculated for the same solar spectrum
from the average BWFs estimated for Lake Giles phytoplankton
incubated with and without C02 enrichment (thick lines). The
irradiance spectrum was smoothed to aid comparison of
E*(A) spectra.
E(2),
only bacteria. However, significant differences related to
light treatment were observed in the samples with CDOM;
when UVR was imposed, DIC reached maximum values of
62.2 ^mol L_1. In samples with CDOM, pH declined
slightly from 6.02 to average 5.94 as C02 partial pressure
increased from 47.2 Pa to an average of 101.9 Pa (Fig. 6B).
Maximum C02 concentrations reached by the UVRexposed samples were 116.2 Pa in the sample including
both phyto- and bacterioplankton, and 126.3 Pa in the
sample containing only bacteria (Fig. 6B).
The photosynthetic response in the absence of UVR
differed from that observed in the 2005 experiments.
0.00000
T
Low
High
Fig. 5. Sensitivity to UVR measured as the weighting
coefficient e(330) (mW m~2)-1 obtained from Lake Giles water
samples from 2005 experiments. T0 corresponds to values
obtained from the Lake Giles water sample at the start of the in
situ incubation. High C02 corresponds to phytoplankton aerated
with 0.1% C02 (101.3 Pa) and incubated for 6 d at 3 m and low
C02 to phytoplankton incubated without added C02. Open bars
are the values from the experiment starting 20 July and the closed
bars those from the experiment starting 21 July.
Elevated C02 concentrations produced by the CDOM
addition after 6 d of incubation did not increase the
photosynthetic rates (Table 3); however, differences were
observed between treatments exposed to full solar spectrum
and those where UVR was excluded. Chi a, Pf, and Es
increased 38%, 47%, and 27%, respectively, in phytoplankton assemblages exposed to UVR compared with those
without UVR (Table 3).
In contrast, when photosynthesis was assessed in the
presence of PAR plus UVR in the photoinhibitron, results
were similar to those obtained the previous summer.
Phytoplankton from higher C02 concentrations showed
higher sensitivity to UVR than the phytoplankton from the
lake (Fig. 7A). High-C02 assemblages acclimated to UVR
during the in situ incubation showed lower sensitivity to
UVR photoinhibition than high-C02 phytoplankton that
had been protected from UVR. For phytoplankton
incubated with CDOM, comparison of the detailed BWFs
instead of a specific value at 330 nm showed that spectral
differences between phytoplankton exposed to UVR and
those protected from UVR during the incubation were
found in the UVA region of the spectrum, consistent with
the greater relative transmission of UVA by CDOM
(Fig. 7B). Therefore, differences in the spectral sensitivity
of photosynthesis arose from higher UVA irradiance
inducing acclimation to UVA in the assemblages incubated
with CDOM and exposed to the whole solar spectrum
compared with those protected from UVR. Similar results
were obtained when we estimated the weighted irradiances
for a clear day at midday (Table 2). For these conditions,
the results also showed that CDOM additions did not
2455
CO2 and UVR photoinhibition in lakes
Table 2. Photosynthetic parameters of Lake Giles water samples under UVR. Average weighted irradiance for UVR, expressed as a
percentage of that at the surface, associated with the lake surface layer and each incubation (%£*inh(UVR)), and, for a clear summer day
at midday in surface (10 cm depth), weighted irradiances, photosynthetic rates, and percentage photoinhibition for full-spectrum
irradiance (i.e., PAR and UVR) estimated for Lake Giles phytoplankton. E*inh stands for the average weighted irradiance
(dimensionless), PB (g C [g Chi a]-1 h_1) corresponds to estimated photosynthetic rates at the surface under solar exposure, PB:PR is the
ratio of solar-inhibited to potential photosynthesis obtained from incubations in the photoinhibitron, and %inh is the percentage
inhibition of those samples at surface. LG corresponds to freshly collected samples from Lake Giles (average depth of the mixed layer was
5.1 m and 4.3 m for July 2005 and June 2006, respectively). The symbols + or — stand for the presence or absence of CDOM addition (C)
and UV radiation (UV) in samples that were incubated under these conditions for 6 d.
Treatment
Calculated for midday irradiance at surface
%2Tw,(UVR)
£*i„h
pB
f*:f^
%inh
2005
LG
Low COz
High C02
23.1
9.0
10.0
4.29
4.76
5.88
0.58
0.62
0.63
023
0.21
0.17
76.7
79.0
828
2006
LG
+C+UV
+C -UV
17.6
55.1
1.7
5.52
926
11.07
0.46
026
0.19
0.18
0.11
009
81.6
89.2
91.0
70
Lake
+ CDOM
•CDOM
+ Algae
60
-o
O
5
-UV
+ Algae
50
-UV
+UV
- Algae
-UV
+UV
40
JL
30
2(%
0
125
n
i n I
•
I I
•
-+fiJY
A.
- Algat
-UV
+JV|
i
100 k
75
X
50
25
0
Fig. 6. Dissolved inorganic carbon (DIC, /miol L_1) and C02 partial pressures (C02, Pa)
measured in samples collected from Lake Giles and incubated in Teflon bottles at the surface
(10 cm) for 6 d for experiments carried out in 2006 (mean ± SEM). Lake denotes freshly collected
Lake Giles water samples. The symbols + or - denote the presence or absence of added CDOM,
algae (by filtering through 1-^m filter), and UV radiation (UV). The dashed line in the lower plot
indicates saturated dissolved C02 partial pressure for surface water under the 2005-2006
atmosphere (pC02 = 38.50 Pa).
Sobrino et al.
2456
Table 3. Chi a content (/ig L_1), C02 partial pressure />C02 (Pa), and photosynthetic parameters {see Table 1 legend) of Lake Giles
water samples from June 2006 experiment. LG corresponds to values obtained from freshly collected samples from Lake Giles. The
symbols + or - stand for the presence or absence of CDOM addition (C) and UV radiation (UV) in the Teflon bottles incubated in
surface (10 cm average depth) for 6 d.
LG
-c+uv
-c -UV
+C+UV
+C -UV
Chi a
fCOz
Pf
Es
a
R2
0.63±0.01
1.17±0.11
0.73±0.04
0.89±0.11
0.76±0.03
47.2±0.8
53.8±2.5
61.3±1.9
116.2±18.5
72.0±7.6
2.51±0.07
2.11±0.06
1.60±0.04
2.36±0.06
1.57±0.03
136±13
136±13
109±13
131±12
102±9
0.0185
0.0155
0.0146
0.0180
0.0153
047
046
045
047
0.97
increase photosynthetic rates independently of the presence
of UVR or despite the high C02 concentrations (Table 2).
Photoinhibition by midday surface irradiance varied from
82% to 91% of maximum photosynthesis obtained in the
absence of damage (Table 2), and with PAR responsible for
only 5% of total photoinhibition (data not shown).
Discussion
This study demonstrates that a rise in atmospheric C02
concentrations similar to that predicted for the end of the
century (IPCC 2001) can increase primary productivity in
natural lake waters up to 23% in the absence of
photoinhibition. However, cells acclimated to high C02
concentrations become more sensitive to photoinhibition
than those under present atmospheric levels of C02,
reducing the observed increase in primary productivity
when cells are exposed to photoinhibitory UVR. These
findings are similar to the results obtained from the
estuarine diatom T. pseudonana grown in the laboratory
under controlled conditions (Sobrino et al. 2008), but this is
the first report of this type of response for natural
phytoplankton assemblages incubated in situ. Furthermore, the increase in sensitivity to UVR under high-C02
conditions occurred in two different scenarios, when C02
was enhanced both by aeration with a C02 air mixture and
under high C02 levels produced by the mineralization of
terrestrial-derived CDOM. In both cases carbon fixation
was assessed after allowing at least 4 d for physiological
adjustment to a step increase in C02, after which cells
reached a steady state comparable with that expected under
a progressive increase in C02 (Sobrino et al. 2008).
The experiments performed in 2005, where the only
experimental factor contributing to changes in the photosynthetic metabolism was the increase in the external C02
concentrations, demonstrated that the increase in the
maximum photosynthetic rates under high C02 levels
compared with those under low C02 levels was accompanied by a significant increase in photosynthetic efficiency
(a). This response has been previously related to a decrease
in the activity of the C02-concentrating mechanisms of
phytoplankton, which reduces the energy required to fix
carbon (Berman-Frank et al. 1998). Decreases in cellular
Chi a content, carbonic anhydrase and ribulose-l,5-bisphosphate carboxylase/oxygenase activity, and the size of
the intracellular pools of photosynthetic intermediates also
have been described for phytoplankton after acclimation to
elevated C02 conditions (Aizawa and Miyachi 1986;
Berman-Frank et al. 1998; Sobrino et al. 2008). These
cellular changes seem to indicate that high C02 levels
promote the downregulation of the photosynthetic machinery in phytoplankton, increasing the resource use
efficiency, as proposed in previous theoretical studies
(Raven 1991). In fact, the lower accumulation of chlorophyll in the lake samples from 2005 incubated with added
C02 compared with the samples without a C02 addition
also seems to support this same contention. Nevertheless,
changes in taxonomical composition caused by the elevated
C02 conditions could also contribute to the response
observed for the Chi a. Among phytoplankton taxa,
growth of the cyanobacterium Merismopedia was promoted
by high C02 conditions compared with other phytoplanktonic species. These findings agree with previous studies
that show an increased predominance of cyanobacteria in
freshwater lakes under predicted scenarios of global change
(Johnk et al. 2008). It is possible that a downregulated
metabolism would have relatively lower amounts of
enzymes involved in the repair process of UVR-caused
damage or lower activation state of the general defense
mechanism (e.g., lower concentrations per cell of superoxide dismutase and ascorbate peroxidase) (Lesser 1996).
Reduced amount or activity of the enzymes involved in
cellular repair would increase susceptibility to UVR,
resulting in more photoinhibition when UVR stress is
imposed than would occur in cells with normal metabolic
activity (Litchman et al. 2002). Additionally, high-C02
conditions could enhance the contribution of UVRsensitive cellular components to photosynthesis, resulting
in more susceptible phytoplankton than under low-C02
conditions (Beardall et al. 2002).
Even though the exact mechanisms resulting in the
increase in sensitivity under high-C02 atmospheric levels
have not been elucidated thus far, our results showed that
the increase in sensitivity to UVR also can be produced at
present atmospheric C02 levels by the interaction of
natural environmental factors with DOM. Comparable
with other, similar lakes, Lake Giles water was supersaturated in C02 (47.2 Pa) as a result of the high heterotrophic activity and DOM photomineralization by solar
radiation (Cole et al. 1994; Graneli et al. 1996). Remarkably, concentrations of C02 increased (to average 59.7 Pa;
Fig. 6B) in containers incubated without added CDOM
probably because of higher light exposure at the surface
and bacterial decomposition of native CDOM. Exclusion
CO2 and UVR photoinhibition in lakes
0.0005
+C+UV
I
'
1
1
1
1
1
1
1
1
1
1
1
1
•
•
-
B -
.
;v
-
0.001
-
0.0001 -
0.00001 r
k
--• +C +UV
•+c -U V
, ,
i
.
300
320
-
V. ^^s
X, ^
•>\
V
"
\"^ ,
340
i
360
,
,
I
1
,
I
X
380
Wavelength (nm)
Fig. 7. Sensitivity to UVR measured as (A) the weighting
coefficient e(330) (mWrn-2)-1 and (B) the biological weighting
functions (BWFs, &{l), [mW m-2]-1) for the inhibition of
photosynthesis estimated for phytoplankton assemblages from
Lake Giles incubated at the surface in UVR-transparent Teflon
bottles (2006 experiments). "Lake" stands for freshly collected
Lake Giles water samples. The symbols + or - stand for the
presence or absence of added CDOM and UVR. Thin lines
represent the SEM calculated from error estimates of
individual BWFs.
of UVR did not strongly affect C02 production from
native CDOM; therefore heterotrophic activity probably
accounted for most of the buildup. However, much higher
C02 production was observed when UVR was included in
the spectral exposure of the CDOM-enriched bottles,
reaching C02 average partial pressures of 116.2 Pa,
approximately three times present atmospheric C02 levels
(38.5 Pa). Our results for the production of C02 from
mineralization of CDOM are in the range predicted by
previous studies (Sobek et al. 2003) but do not quantify to
2457
what extent biomineralization or photodegradation contributed to C02 accumulation in our experiments. Photomineralization already has been identified as an important
contributor to C02 production from the degradation of
terrigenous CDOM (Graneli et al. 1996; Moran et al. 2000).
Reports show that it can account for the removal of 1050% of total DOC compared with 10-27% removed by
biomineralization (Moran et al. 2000; Lennon 2004;
Obernosterer and Benner 2004).
Interestingly, the higher C02 concentrations produced
by the CDOM addition were not enough to enhance
carbon fixation in phytoplankton assemblages where UVR
was excluded from the in situ incubation. On the contrary,
phytoplankton incubated in the presence of UVR had
maximum photosynthetic rates (measured in the absence of
UVR) that were 47% higher than the photosynthetic rates
of in situ phytoplankton incubated in the absence of UVR
(Table 3). Previous studies have demonstrated that UVR
can drive the phototransformation of CDOM to bioreactive compounds, increasing the availability of limiting
substrates for primary production (Moran and Zepp 1997;
Vinebrooke and Leavitt 1998). In Lake Giles, nitrogen and
phosphorus tend to colimit epilimnetic phytoplankton Chi
a levels in summer, a general finding from multiday
nutrient enrichment bioassays, under noninhibitory PAR,
including June 2005 and July 2006 (R.E. Moeller unpubl.).
So we would expect this community to respond positively
to UVR-driven nutrient regeneration.
When in situ photosynthesis was estimated for a clear sky
at midday, UVR was calculated to reduce photosynthesis by
an average of 78% relative to uninhibited photosynthesis.
The contribution of PAR to photoinhibition was much
smaller, with average values for both years close to 6%
(Table 2). The robustness of the BWF T/P-E model used in
this study to predict UV- and PAR-dependent photosynthetic responses is supported by average R2 values close to
0.97, obtained when comparing the observed and the
predicted photosynthetic rates. Similar values of UVR
photoinhibition have been also reported for other lakes and
estuaries (Helbling et al. 2001; Neale et al. 2001).
Analyzing the causes of the changes in sensitivity to
UVR in Lake Giles phytoplankton assemblages in situ was
complicated because of the interaction among the different
environmental variables. The phytoplankton assemblage
from 2005 was less sensitive to UVR than that for 2006,
possibly due to lower nutrient concentrations in 2006
inducing lower repair capacity and increasing sensitivity of
phytoplankton to UVR (Litchman et al. 2002), and
different taxonomic composition. Regarding this, cell
counts of the cyanobacteria Merismopedia showed an
increase under high C02 conditions in 2005 incubations.
Merismopedia was also relatively more abundant in 2005
than in 2006 on the basis of both cell counts and
biovolume. Previous studies have shown that UVR
exposure promotes phytoplankton photoacclimation to
UVR, decreasing sensitivity to UVR (Fritz et al. 2008;
Sobrino et al. 2008), whereas an artificial addition of C02
increases sensitivity (Sobrino et al. 2008). Additionally,
CDOM addition can increase the sensitivity to UVR both
through reduction of UVR photoacclimation and irradi-
2458
Sobrino et al.
ance in general (Villafane et al. 2004), as well as the
production of C02 from photobleaching (this study).
Because of this, we observed the highest sensitivity to UV
in the treatment with added CDOM and no UVR exposure.
This was despite an even higher C02 concentration in the
UVR exposed treatment.
In conclusion, the results from this study show that high
C02 concentrations can increase primary productivity in
lake phytoplankton assemblages even in the presence of a
presumed nutrient limitation. However, elevated C02
conditions, caused by changes in atmospheric concentrations or by UVR photodegradation of CDOM, increase
sensitivity to UVR, making cells more susceptible to
photoinhibition than those under present or nonelevated
C02 levels. The overall effect on a water column basis will
depend on the relative proportion between a near-surface
photoactive zone where UVR can drive increased availability of nutrients and C02 but lower productivity, and a
deeper zone where UVR is absent but PAR can drive
higher rates of photosynthesis in response to the nutrient
and C02 enhancements. In addition, the confirmation of
these results in other systems can imply the presence of
more sensitive phytoplankton, independently of the source
of damage, in future scenarios of global climate change and
in waters presently enriched with high C02 concentrations.
Future efforts should be made to understand the physiological mechanisms of this increase in sensitivity, estimate
its effect on integral water column production, and assess
interactions with other environmental stressors under
elevated C02 conditions. Finally, the effect of elevated
atmospheric C02 concentrations on waters with significant
inputs of organic matter still needs to be studied.
A ckno wledgmen ts
We thank the Lacawac Sanctuary for use of its facilities, and
the Blooming Grove Hunting and Fishing Club for access to Lake
Giles. Donald Morris (Lehigh) devised the CDOM concentrating
protocol, and Craig Williamson (Miami) provided light and UV
attenuation data for Lake Giles. We also thank two anonymous
reviewers for their suggestions on the previous version of this
manuscript.
This research was supported by postdoctoral fellowships to
C.S. from the Spanish Ministry of Education and Science and
from the Smithsonian Institution, and by National Science
Foundation grant DEB-IRCEB-0552283.
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SOBRINO,
Associate editors: Warwick F. Vincent and John P. Smol
.Received 0J Ocfo&r 200&
Accepted: 13 March 2009
Amended: 10 May 2009