RESEARCH ARTICLE
Allelopathic activity among Cyanobacteria and microalgae isolated
from Florida freshwater habitats
Miroslav Gantar1, John P. Berry2, Serge Thomas2, Minglei Wang3, Roberto Perez3 & Kathleen S. Rein3
1
Department of Biological Sciences, Florida International University, Miami, FL, USA; 2Southeast Environmental Research Center, Florida International
University, Miami, FL, USA; and 3Department of Chemistry and Biochemistry, Florida International University, Miami, FL, USA
Received 21 March 2007; revised 23 September
2007; accepted 2 December 2007.
First published online 8 February 2008.
DOI:10.1111/j.1574-6941.2008.00439.x
Editor: Gary King
Keywords
allelopathy; Cyanobacteria ; green algae;
photosynthesis inhibition; Fischerella ; indole
alkaloid.
Abstract
We evaluated allelopathic interactions between strains of Cyanobacteria and green
algae isolated from south and central Florida. Allelopathy, including inhibition or
stimulation of growth, was assessed by cocultivation of each of the isolated strains,
as well as by evaluation of extracts prepared from the isolates. All of the strains of
Cyanobacteria, and four of the six isolates of green algae, showed some allelopathic
activity (i.e. inhibition or stimulation of the growth of other strains). Of these, the
most pronounced activity was observed for the cyanobacterial isolate Fischerella
sp. strain 52-1. In the cocultivation experiments this cyanobacterium inhibited the
growth of all tested green algae and Cyanobacteria. The crude lipophilic extracts
from Fischerella sp. strain 52-1 isolated from both the biomass and the culture
liquid inhibited photosynthesis of the green alga Chlamydomonas sp. in a
concentration- and time-dependent manner and caused extensive loss of ultrastructural cell organization. Preliminary chemical characterization of compounds
extracted from Fischerella sp. strain 52-1 indicated the presence of indole alkaloids,
and further characterization has confirmed that these compounds belong to the
hapalindoles previously isolated from other species of Fischerella and related
genera. Further chemical characterization of these compounds, and further
investigation of their apparent role in allelopathy is ongoing.
Introduction
The composition and dynamics of algal communities is
influenced not only by physical and chemical environmental
factors, but also by interactions between members of these
communities. In particular, the complex chemical signaling,
referred to as allelopathy, includes an array of secondary
metabolites that can act either as positive or negative
regulators of the growth of sympatric species. As such,
allelopathy can be considered as an adaptation to achieve a
competitive advantage over other members within the same
microbial community (Legrand et al., 2003).
Allelopathic activity of Cyanobacteria has attracted the
attention of researchers for two primary reasons. First,
understanding allelopathy can provide closer insight into
successions in natural algal communities. In light of increased concern regarding harmful algal blooms (HAB), in
particular, allelopathy is also seen as a possible factor in the
dominance of toxin-producing species in a habitat (RengeFEMS Microbiol Ecol 64 (2008) 55–64
fors & Legrand, 2001). Second, the active compounds are
seen as environment-friendly herbicides or biocontrol
agents that might have a potential commercial significance.
A number of authors have described antibiotic metabolites from microalgae, including compounds that inhibit
bacteria (Falch et al., 1995; Chetsumon et al., 1998), fungi
(Piccardi et al., 2000) and viruses (Loya et al., 1998), as well
as Cyanobacteria and other microalgae (Flores & Wolk,
1986; Schlegel et al., 1999; Smith & Doan, 1999). Studies
screening Cyanobacteria for their antialgal activity have
shown that about 10% of strains exhibit this biological
activity (Schlegel et al., 1999).
The primary aim in the present study was the assessment
of the interactions between individual cultured strains of
Cyanobacteria and green algae that were isolated from
southern and central Florida. The mutual interactions of
these potentially cohabitating organisms were studied in
laboratory-designed experiments in which the strains were
either cocultivated or grown in the presence of the extracts
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Correspondence: Miroslav Gantar, Florida
International University, Department of
Biological Sciences, University Park Campus,
Miami, FL 33199, USA. Tel.: 11 305 348
4030; fax: 11 305 348 1986;
e-mail: gantarm@fiu.edu
56
M. Gantar et al.
of other organisms. The most active species was identified
and its compound isolated and partially characterized.
Materials and methods
cultures were isolated by standard microbiological procedures, and maintained on BG11 medium (Rippka et al.,
1979) in a light incubator at 24 1C under continuous
fluorescent white light at an intensity of 30 mE m 2 s 1.
Sequencing of the rRNA gene
Organisms and cultivation
For DNA isolation and sequencing, filaments of Fischerella
sp. strain 52-1 from pure culture were first pelleted by
centrifugation, washed and DNA isolated using the Bio101
Fast DNA isolation kit as per the manufacturer’s instructions. A 675-bp 16S rRNA gene fragment was amplified by
PCR using the primer set CYA106F/CYA781R (Nubel et al.,
1997). The gene fragment was cloned using the Invitrogen
TOPO TA cloning kit and sequenced through MWG biotech. A 663-bp fragment was sequenced from positions 85 to
747 in both directions. The result of the BLAST query showed
99.6% similarity to the sequence from Fischerella sp. strain
1711 (accession number AJ544076).
Cocultivation of Cyanobacteria and
chlorophytes
The isolated strains were inoculated onto BG11 agar,
and incubated in a light incubator under fluorescent white
light (30 mE m 2 s 1) for 10 days or until uniform growth
appeared. Agar disks were excised using a sterile glass
Table 1. Origin and morphological characteristics of cyanobacteria and chlorophytes
Strain
Source
Characteristics
Cyanobacteria
Fischerella sp. 52-1
Lake Tennessee, central Florida
Branched filaments, ellipsoid cells in main filament – 10 mm and cylindrical cells in
the side filaments 5 mm in diameter
Filaments 20 mm in diameter
Lyngbya sp. 15-2
Floating periphyton mat, C111 canal,
southern Florida
Nostoc sp. 23-2
Lake Istokpoga, central Florida
Nostoc sp. Ev-1
Shark Valley, Everglades, southern Florida
Nostoc sp. 37-7
Crescent Lake, central Florida
Nostoc sp. 58-2
Lake Istokpoga central Florida
Pseudanabaena
Storm Water Treatment Area (STA 1),
sp. 21-9-3
southern Florida
Scytonema sp. 26-1 Periphyton mat C-111 canal,
southern Florida
Chlorophyta
Ankistrodesmus
Lake Howard, central Florida
sp. 45-2
Chlamydomonas
Shark Valley, Everglades southern Florida
sp. Ev-29
Excentrosphaera
Lake William Roe Park, central Florida
sp. 46-4
Chlorella sp. 2-4
Everglades, Shark Valley, southern Florida
Selenastrum
Periphyton mat, C-111 canal, southern
sp. 34-4
Florida
Rhizoclonium
Shark Valley, Everglades, southern Florida
sp. Ev-17
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Straight filaments, cylindrical cells 2.5 mm 5.0 mm
Straight filaments, spherical cells 5 mm in diameter
Contorted filaments, cells spherical or cylindrical, diameter 3 mm
Straight filaments, cells oval 2.5 mm 5.0 mm
Straight filaments, cells isodiametric, diameter 2.5 mm
Branched, sheathed filaments, cylindrical, diameter 12 mm, cells
Cells narrow contorted, 25–30 mm long
Cells ovoid 7.5–10.0 mm in diameter
Spherical cells 10–60 mm and spores 3 mm in diameter
Cells spherical or ellipsoidal, 7–8 mm in diameter
Lunate cells, 5–7 mm long
Unbranched filaments, cylindrical cells 5 mm in diameter
FEMS Microbiol Ecol 64 (2008) 55–64
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The organisms investigated include eight strains of Cyanobacteria, and six strains of green algae (Chlorophyta) (Table
1). None of the cultures was axenic. Taxonomic identification of the strains to the genus level was based on morphological criteria given in Komarek & Anagnostidis (1986,
1989), Prescott (1962) and Whitford & Schumacher (1984).
Due to the ambiguity in distinguishing Fischerella from
Hapalosiphon, the taxonomic identification of strain 52-1
was supplemented by 16S rRNA gene sequencing followed
by a BLAST search, which indeed showed that this strain
belongs within the genus Fischerella.
Except for Fischerella sp. strain 52-1 and Lyngbya sp.
strain 15-2, which were known to be toxic from our previous
work (Berry et al., 2004a, b) all other strains were randomly
selected from our culture collection. The strains were
isolated from various freshwater habitats in south and
central Florida. To our knowledge none of the strains used
in this work was part of HABs. The source of the isolates and
their morphological characteristics are given in Table 1. The
57
Allelopathy among Cyanobacteria
Effect of extracts
Cellular extracts of Cyanobacteria and green algae were
prepared from biomass that was obtained by growing
the cultures in 4 L BG11 medium with aeration and constant
illumination of 30 mE m 2 s 1. The biomass was harvested
after 4 weeks by centrifugation. Freeze-dried biomass
(100 mg) was extracted first with 10 mL of chloroform to
yield a lipophilic extract, and then with the same volume of
30% ethanol to obtain a hydrophilic extract. Both extracts
were evaporated to dryness, and resuspended in absolute
ethanol and 30% ethanol, respectively, to give a final
concentration of 1 mg dry residue mL 1 of solvent.
In the case of the cyanobacterium Fischerella sp. 52-1, the
culture liquid was additionally extracted. After removing the
biomass by centrifugation, the culture liquid, which contained a visible whitish floating precipitate, was extracted
with chloroform, evaporated and resuspended in ethyl
alcohol to a final concentration of 1 mg mL 1.
To test for inhibition of the growth by the extracts, lawns
of each strain on BG11 agar plates were incubated in dim
light (10 mE m 2 s 1) for 24 h. Subsequently, wells in the agar
were made with a sterile glass tube (7 mm in diameter), and
filled with 100 mL of extract. The plates were dried in a
laminar flow cabinet. Each extract was tested in triplicate.
The control plates contained the solvent only. In none of the
cases did the solvent have any effect on the growth of the test
organisms. The plates were incubated for 10 days after
which the size of the inhibition zone was observed and
measured. The data presented are the means of three
replicate measurements.
strain Ev-29 was followed in a time-course experiment. The
crude lipophilic extract from the culture liquid of Fischerella
sp. strain 52-1 was added at different concentrations (5, 12,
25, 50, 500 mg mL 1) to the wells of 96-well plates. Each
concentration was analyzed in three replicates. The control
wells contained only the solvent. The extracts or the solvent
in the case of controls were evaporated from the wells
in vacuo. A suspension of 200 mL of Chlamydomonas cells in
BG11 medium at a concentration of c. 104 mL 1 was added
in each well. The plates were incubated at a light intensity of
30 mE m 2 s 1 for 8 days. Plates were read on an HP plate
reader at a wavelength of 600 nm. The first reading was taken
immediately after inoculation, and readings were subsequently taken 2, 4, 6 and 8 days following inoculation.
During the growth period, wells were mixed with a pipette
tip once a day. After 4 days, an aliquot of fresh BG11
medium (100 mL) was added to each well to compensate for
evaporation.
Effect of Fischerella extract on the
photosynthetic activity of Chlamydomonas
The effect of the crude lipophilic extracts from the culture
medium of Fischerella sp. strain 52-1 on the rate of photosynthesis in Chlamydomonas sp. strain Ev-29 was assessed by
the sensitive and selective fluorimetric measurement of
the chlorophyll fluorescence of photosystem II (PSII) after
pulse excitations at low (1 mmol quanta m 2 s 1, fluorescence
F0,) and saturated actinic irradiance (3000–4000 mmol
quanta m 2 s 1, fluorescence Fm) using the Phyto-Pam ED
(Schreiber et al., 1986). Various volumes of the crude extract
solution from 0 to 1.5 mL were mixed with Chlamydomonas
culture (50 mL, final culture volume), which had a cell
concentration of about 15 mg Chla L 1. In total, nine concentrations of crude extract were tested: 0 (= control), 0.1, 1, 3, 5,
7.5, 10, 20 and 30 mg mL 1. These test cultures were continuously stirred at low revolution with a magnetic stirrer under
artificial light delivering 150–200 mmol m 2 s 1. Fluorimetric
photosynthetic measurements were taken at 0, 10, 30 and
60 min after initial exposure to the crude extract on a 3-mL
sample of the algal suspension, which was placed in a quartz
cuvette. The photosynthetic capacity of the sample, or the
maximum energy conversion efficiency (Y), was calculated as
(Fm F)/Fm. The percentage inhibition was determined over
time by calculating: (Yt,0 Yt,c)/Yt,0; where at time t, Yt,0 is Y
in the control culture and Yt,c is Y in the culture exposed to
the concentration of crude extract of concentration c. The
data presented are representative results obtained from two
separate experiments.
Effect of Fischerella crude extract on the growth
of Chlamydomonas
Electron microscopy
The effect of the cyanobacterial strain Fischerella sp. strain
52-1 on the growth of the green alga Chlamydomonas sp.
Chlamydomonas Ev-29 was grown in an aerated culture for
5 days under standard conditions. Extracts of Fischerella sp.
FEMS Microbiol Ecol 64 (2008) 55–64
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tube with a diameter of 7 mm and transferred to an
empty Petri plate. The Petri plate containing three excised
disks was overlayed with BG11 agar cooled to 45 1C.
Plates were incubated in dim light for 24 h to allow diffusion
of metabolites from the disk into the surrounding medium.
Subsequently, this agar was overlayed with one of the
test strains suspended in BG11 ‘soft agar’ (45 1C) to form a
lawn. The same procedure was repeated for each combination of isolated strains. Plates were incubated at 24 1C and
illumination of 30 mE m 2 s 1. The inhibition or stimulation
zone around the disks was recorded after 10 days of
cocultivation. Each cocultivation combination was carried
out in triplicate. Agar disks containing algal or cyanobacterial growth that were overlayed with the same strain were
used as controls. In none of the cases was any effect
produced.
58
Chemical characterization of Fischerella sp.
strain 52-1
In the present study, preliminary chemical characterization
of metabolites from Fischerella sp. strain 52-1 was conducted. Specifically, chloroform extracts were evaluated by
thin-layer chromatography (TLC; silica gel, 1 : 1 hexane ethyl: acetate) visualized with the Ehrlich’s reagent that is
specific for indole alkaloids, including those that have been
previously identified as biologically active metabolites from
Fischerella and related genera. Indole alkaloids identified in
this way have been subsequently isolated, and characterized
by mass spectrometry and nuclear magnetic resonance
(NMR; unpublished data).
Statistical analysis
The data presented are the means of three replicate measurements. SEs were calculated using STAT-100 (Biosoft) software.
Growth inhibition of Chlamydomonas sp. strain Ev-29
subsequent to six exposures to crude extract of Fischerella
sp. strain 52-1 was compared over time using the multiple
pairwise Tukey’s honestly significant difference (HSD) test
conducted at the 95% confidence level. All tests were
performed using SPSS 14.0 after the homoscedasticity of the
variance was checked.
Results
In the cocultivation experiments each strain was cultivated
in the presence of each of the 13 other cyanobacterial and
green algal strains. Of 182 total combinations, some effect,
either inhibitory or stimulatory, was recorded in 37 cases
(Table 2). Most of these cases (28) showed growth inhibition, while growth stimulation was recorded in nine cases.
The only strain that was inhibitory to all others was the
cyanobacterium Fischerella sp. strain 52-1 (Table 2). The
strain that showed the greatest stimulatory effect towards
the other strains was the green alga Chlamydomonas sp.
c 2008 Federation of European Microbiological Societies
Published by Blackwell Publishing Ltd. All rights reserved
strain Ev-29. The cyanobacterium Nostoc sp. strain 23-2
showed a stimulatory effect but only towards the green algae
(Table 2). The strain that was most susceptible to an
inhibitory effect was the cyanobacterium Nostoc sp. strain
Ev-1, which was inhibited by two green algae and three
cyanobacterial isolates.
In addition to cocultivation experiments, the growth of
each organism was also tested in the presence of the extracts
from the each of the other strains (Table 3). The extract that
showed an inhibitory effect on the largest number of tested
Cyanobacteria and green algae was a lipophilic extract
obtained from the cyanobacterium Fischerella sp. strain
52-1 (Table 3). In particular, the most potent extract of this
cyanobacterium was that obtained from the culture liquid.
The strain that was most susceptible to inhibition by other
green algae and Cyanobacteria was the green alga Chlamydomonas sp. strain Ev-29, and the organism that was most
frequently stimulated was the cyanobacterium Nostoc sp.
strain 23-2.
In order to determine the IC50 of the crude extract from
Fischerella sp. strain 52-1, cells of Chlamydomonas sp. strain
Ev-29 were grown in the presence of a range of concentrations of the extract that was obtained from the culture
liquid. Growth of Chlamydomonas sp. strain Ev-29 was
concentration-dependent (Fig. 1). Total lysis of cells was
achieved at a concentration of 500 mg mL 1 of crude extract.
Based on curve calculations, the IC50 for the crude extract
was estimated to be 25 mg mL 1.
Fischerella crude extract showed inhibition of photosynthesis at a concentration as low as 1 mg mL 1. The
inhibition kinetics described a sigmoidal curve over time
that was concentration-dependent (Fig. 2). Inhibition of
photosynthesis reached at least 50% after 1 h of incubation
at concentrations of 5 mg mL 1 and above. Inhibition of
photosynthesis of more than 80% was reached with concentrations above 10 mg mL 1.
By using the multiple pairwise Tukey’s HSD statistical
test, it was shown that growth inhibition vs. the control was
present at all crude extract concentrations as early as on day
2. On day 2, growth inhibition among the various treatments was similar for concentrations of 12 and 25 mg mL 1,
25 and 50 mg mL 1, and 50 and 500 mg mL 1. Thereafter,
all pairwise comparisons conducted were significantly
different.
After being exposed to the crude extract from the culture
liquid of Fischerella for 24 h, cells of Chlamydomonas sp.
strain Ev-29 showed distinctive morphological and structural changes. Light microscopy revealed bleaching and extensive vacuolization of the cells. Electron microscopy revealed
degeneration of thylakoids into a system of irregularly
arranged, sometimes swollen, membranes. In addition,
disappearance of other cell structures including the nucleus
was apparent (Fig. 3).
FEMS Microbiol Ecol 64 (2008) 55–64
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strain 52-1 were added to test tubes, and the solvent
evaporated. Subsequently, aliquots of Chlamydomonas cells
were transferred to the test tubes, such that the final
concentration of the crude extract was 1 mg mL 1. The tubes
were incubated in the light incubator for 24 h. Control cells,
without extract, were treated in the same fashion. Both
control and extract-treated cells were harvested by centrifugation and embedded in 1.5% agar. The agar blocks were
fixed in 2.5% (w/v) glutaraldehyde, 1% (w/v) OsO4 and 1%
(w/v) KMnO4. The fixed cells were dehydrated in an
increasing alcohol series, and infiltrated with Spurr’s resin
(Spurr, 1969). Sections were stained with uranyl acetate
and lead citrate and observed under a Philips electron
microscope.
M. Gantar et al.
c
2008 Federation of European Microbiological Societies
Published by Blackwell Publishing Ltd. All rights reserved
Lyngbya
15-2
1
/1
Nostoc
58-2
1/
1
S
S
1
1
1/
Nostoc
Ev-1
1
Nostoc
23-2
/1
1/
Nostoc
37-7
S
1
1/
1/
/1
/1
1/
1
1/
S
1/
1
S
/1
1/
S
1/
1/
S
S
1
S
Fischerella Pseudanabaena Scytonema Rhizoclonium Chlamydomonas Excentrosphaera Chlorella Ankistrodesmus Selenastrum
52-1
29-3-1
26-1
Ev-17
Ev-29
45-3
2-4
45-2
34-4
Agar disks containing growth of an effector organism were overlayered with the test organism and the zone of inhibition or stimulation was observed.[]
1, inhibition zone 4 15 mm; 1/ , weak inhibition zone 10–15 mm; /1, very weak inhibition zone o 10 mm; , no effect; S, stimulation.
Lyngbya 15-2
Nostoc 58-2
Nostoc 23-2
Nostoc Ev-1
Nostoc 37-7
1/
Fischerella 52-1
1
Pseudanabaena
29-3-1
Scytonema 26-1
Rhizoclonium Ev-17
Chlamydomonas
Ev-29
Excentrosphaera
1/
45-3
Chlorella 2-4
Ankistrodesmus
45-2
Selenastrum 34-4
Effector
organism
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FEMS Microbiol Ecol 64 (2008) 55–64
Test organism
Table 2. Different types of interrelationships during cocultivation of cyanobacteria and green algae
Allelopathy among Cyanobacteria
59
a
c
a
c
a
c
a
c
a
c
a
c
e
a
c
a
c
a
c
a
c
a
c
a
c
a
c
a
c
Lyngbya 15-2
FEMS Microbiol Ecol 64 (2008) 55–64
1/
1
1
1/
Nostoc
58-2
S
1/
1/
1/
1/
1/
1
1/
1/
1/
Nostoc
Ev-1
S
1/
1/
1/
S
S
S
S
S
1
1
Nostoc
23-2
1/
1/
1/
Nostoc
37-7
Fischerella
52-1
1/
1/
1/
Pseudanabaena Scytonema
29-3-1 26-1
The symbols represent a range of inhibition zones obtained as means from three replicates.
1, inhibition zone 4 15 mm; 1/ , weak inhibition zone 10–15 mm; /1, very weak inhibition zone o 10 mm;
Ankistrodesmus
45-2
Selenastrum
34-4
Rhizoclonium
Ev-17
Chlamydomonas
Ev-29
Excentrosphaera
45-3
Chlorella 2-4
Pseudanabaena
29-3-1
Scytonema 26-1
Fischerella 52-1
Nostoc 37-7
Lyngbya
15-2
1/
1/
1/
1
1/
1
1
1
1/
Chlamydomonas
Ev-29
, no effect; S, stimulation.
1/
S
1/
S
1/
Rhizoclonium
Ev-17
S
1/
1/
1/
1/
1/
1
1/
S
/1
1/
S
S
-
1/
10
1/
1
1
ExcentroAnkistrosphaera Chlorella desmus Selenastrum
45-3
2-4
45-2
34-4
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Nostoc Ev-1
Nostoc 23-2
Nostoc 58-2
Extract
type
Effector
organism
Test organism
Table 3. Effect of the aqueous (a) and chloroform (c) extracts from the biomass and the extract from the culture liquid (e) on the growth of test organisms
60
M. Gantar et al.
61
Allelopathy among Cyanobacteria
Fig. 3. (a) Electron micrographs showing normal cells of Chlamydomonas sp. strain Ev-29 and (b) disintegration of cell structures after 24 h of
exposure to the crude toxin extract from the culture liquid of Fischerella
sp. strain 52-1. N, nucleus; T, tylakoids; P, pyrenoid. Scale bar, 1 mm.
Fig. 2. Inhibition of photosynthesis in Chlamydomonas sp. strain Ev-29
by the lipophilic extract of Fischerella sp. strain 52-1. Inhibition was
dependent on extract concentration and time. The crude extract was
obtained from the culture liquid and dry residue dissolved in ethyl
alcohol. The following concentrations of Fischerella crude extract were
used: 30 (n), 20 ( ), 10 (B), 7.5 (&), 5 (m), 3 (’), 1 ( ) and
0.1 mg mL 1 (^).
Lipophilic (CHCl3) extracts prepared from both biomass
and culture medium of Fischerella sp. strain 52-1 were
evaluated by TLC. Visualization with Ehrlich’s reagent
indicated the presence of indole compounds, specifically by
the appearance of diagnostic purple-colored ‘spots’ on TLC
plates. Indole alkaloids identified in this way have been
subsequently isolated and chemically characterized by mass
spectrometry and NMR and shown to belong to the
hapalindoles (unpublished data).
Discussion
Early studies of allelopathy among algae were limited to field
observations by monitoring algal successions in natural
FEMS Microbiol Ecol 64 (2008) 55–64
environments (Keating, 1977). Later, diverse laboratorybased studies were directed to biochemical and ecological
effects (Legrand et al., 2003). In the present study, we
applied two methods to assess allelopathic interactions
between Cyanobacteria and green algae – specifically, cocultivation of isolated strains and evaluation of stimulatory or
inhibitory effects of cell extracts. These methods, as expected, did not produce identical results (Tables 2 and 3).
Growth inhibition was recorded more frequently when
extracts, rather than the cocultivation method, were used.
This may be explained by the fact that some biologically
active compounds are not released into the external environment until the cells are lysed, and therefore this kind of
relationship cannot be considered to be truly allelopathic. In
the case of Fischerella sp. strain 52-1, the cocultivation was
inhibitory for all the tested Cyanobacteria and green algae
(Table 2) while the crude extract was not effective against
Pseudanabaena 29-3-1 and Scytonema 26-1 (Table 3). It is
possible that the inhibitory effect is a result of a synergistic
interaction of two or more compounds, some of which
might have been lost in the process of extraction.
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Fig. 1. Concentration-dependent growth inhibition of Chlamydomonas
sp. strain Ev-29 by the crude extract of Fischerella sp. strain 52-1 from the
culture liquid. Chlamydomonas cells were grown in the presence of the
following concentrations of Fischerella extract: 500 (n), 50 (m), 25 ( ),
12 ( ) and 5 mg mL 1 (^); control (B). The controls contained the solvent
only. Vertical bars are SEs generated from the mean of three replicates.
62
c 2008 Federation of European Microbiological Societies
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domonas sp. strain Ev-29. Inhibition of photosynthesis by
the Fischerella crude extract was relatively rapid but not
instantaneous, becoming apparent only 10 min after exposure. The inhibition depended both on the concentration of
the allelopathic compound and on the time of exposure
(Fig. 3). A rate of inhibition of photosynthesis of 50% was
achieved at a concentration of 5 mg mL 1 after 1 h of
exposure. One-month-old cultures of Fischerella accumulated in the medium 4 mg mL 1 of crude extractable toxin, a
concentration that would be sufficient to inhibit the photosynthesis of other algae. It should be mentioned that we used
the PHYTO-PAM ED fluorometer, which measures the
electron transfer rate between PSII and PSI, and therefore
at this point it is not known whether this inhibition is a
direct result of a change in the electron transfer rate, or
whether it occurred indirectly through the dark reaction
pathway (i.e. Calvin cycle).
Cyanobacteria are known to harbor in their mucilaginous
sheaths bacteria that are sometimes impossible to remove.
In this work we used nonaxenic cultures, and therefore the
potential effect of accompanying bacteria cannot be completely excluded. Not only might these bacteria degrade the
biologically active compounds produced by Cyanobacteria
(Jones et al., 1994), but there is also evidence that they may
contribute to toxin production (Gallacher et al., 1997).
Ultrastructural study has revealed that 24 h after exposure
to the crude extract, cells of Chlamydomonas sp. underwent
dramatic changes primarily causing disintegration of the
thylakoid system. Being soluble in organic solvents, and
therefore presumably lipophilic, it is possible that the toxin
primarily targets the lipid-rich thylakoid membranes and
eventually leads to disruption of the electron transport
system.
Algicidal compounds have been identified in different
cyanobacterial genera (Smith & Doan, 1999) such as Hapalosiphon (Moore et al., 1984), Oscillatoria (Bagchi et al.,
1990), Scytonema (Pignatello et al., 1983) and Nostoc
(Todorova & Jüttner, 1995). It appears that species of
Fischerella are the most typical producers of algicidal
compounds among Cyanobacteria. The algicidal fischerellin
was isolated from Fisherella muscicola (Gross et al., 1991;
Hagmann & Jüttner, 1999) and was identified as an inhibitor
of the oxygenic photosynthetic pathway. Etchegaray et al.
(2004) identified two allelochemicals, an aminoacylpolyketide, fisherellin A, and an alkaloid, 12-epi-hapalindole F,
from Fischerella sp. CENA 19. However, it appears that
indole alkaloids are not characteristic of just the genus
Fischerella. Norharmane [9H-pyrido(3,4-b)indol] was isolated from culture medium of Nodularia harveyana and had
a strong algicidal activity against the cyanobacterium
Arthrospira laxissima (Volk, 2005).
Based on our preliminary data, production of indole
alkaloids by Fischerella species may indeed be a characteristic
FEMS Microbiol Ecol 64 (2008) 55–64
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With the exception of Fischerella sp. strain 52-1, the
inhibitory activity of individual strains was specific and it
was observed only for one or a few test organisms. It should
be also pointed out that, in some cases, only weak inhibition
(inhibition zone o 10 mm) was observed (Table 3). Antialgal activity is reported mostly for Cyanobacteria, although
green algae can also produce allelopathic compounds such
as the fatty acids described for Botryococcus braunii (Chiang
et al., 2004). In our investigation, the green algal strains,
with the exception of Rhizoclonium sp. strain Ev-17 and
Selenastrum sp. strain 34-4, also showed some although low
levels of inhibitory activity against other algae and Cyanobacteria.
The strain that was most susceptible to the inhibitory effect
of the extracts was the green alga Chlamydomonas sp. strain
Ev-29. At the same time, this alga had a pronounced
stimulatory effect when cocultivated both with Cyanobacteria
and with green algae (Table 2). The stimulatory effect of
Chlamydomonas reinhardtii towards the cyanobacterium
Anabaena flos-aquae was described and explained as a
nutrifying effect of the extracelullar products of the green
alga (Kearns & Hunter, 2000). We did not make any attempt
to determine the nature of this stimulation, but it appears
that the stimulatory compounds were excreted during active
growth and were not present in the extracts.
The only organism that showed inhibition against all the
tested Cyanobacteria and green algae was the cyanobacterium Fischerella sp. strain 52-1. This is in agreement with
previous reports that allelopathic activity is found mostly
among filamentous and nitrogen-fixing Cyanobacteria
(Schlegel et al., 1999; Smith & Doan, 1999), including
Fischerella (Doan et al., 2000) and the related genus Hapalosiphon (More, 1984). A larger zone of inhibition was
observed in most of the cases when the crude extract was
prepared from the culture liquid, rather than the cellular
biomass (Table 3). As both extracts were prepared at
1 mg mL 1 concentration this indicates that the relative
abundance of the active compound is higher in the extract
from the culture liquid than from the cellular extracts.
Although there are previous reports on algicidal compounds from Fischerella (Smith & Doan, 1999) there is no
information on possible ecological implications. Given that
Fischerella is a benthic sessile organism, we can only speculate that production of allelopathic compounds by Fischerella provides a competitive advantage and plays an
important role in deterring other organisms from colonizing
its filaments.
The mode of action of allelopathic substances includes
inhibition of growth (Keating, 1978; Schlegel et al., 1999),
inhibition of PSII (Hagmann & Juttner, 1996) and inhibition of cellular motility (Kearns & Hunter, 2001). We have
shown that the Fischerella crude extract from the culture
medium inhibited photosynthesis in the green alga Chlamy-
M. Gantar et al.
63
Allelopathy among Cyanobacteria
Acknowledgements
We would like to thank the National Institutes of Environmental Health Sciences (NIEHS) for financial support of
this research through NIEHS ARCH grant S11 ES11181.
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