PRIMARY RESEARCH PAPER
Selectivity and detrimental effects of epiphyticPseudanabaena on Microcystis colonies
Ramsy Agha . Marıa del Mar Labrador .
Asuncion de los Rıos . Antonio Quesada
Received: 4 February 2016 / Revised: 5 April 2016 / Accepted: 9 April 2016 / Published online: 26 April 2016
� Springer International Publishing Switzerland 2016
Abstract The cyanobacterium Microcystis aggre-
gates into colonies with a mucilaginous sheath that
constitutes a special microhabitat for many microor-
ganisms that associate to it. Here, we examine the
notorious, yet scarcely studied case of epiphytic asso-
ciation by the cyanobacterium Pseudanabaena sp. to
colonialMicrocystis. Co-cultivation of Pseudanabaena
with different Microcystis strains evidenced strong
specificity in the interaction, with dramatically different
outcomes in each case, including (1) inability of
Pseudanabaena to access the slime of Microcystis, (2)
neutral co-existence of epiphytic Pseudanabaena and
Microcystis, and (3) rapid epiphytic proliferation of
Pseudanabaena, followed by lysis and rapid decay of
Microcystis cells. Whereas strain-specific oligopeptide
production could not explain the observed specificity,
differences in slime microstructures amongMicrocystis
strains revealed by low-temperature scanning electron
microscopy suggest that slime structural features might
initially determine the ability of Pseudanabaena to
colonizeMicrocystis, subsequently driving the outcome
of the interaction. Furthermore, even under ‘‘neutral’’
co-existence, Pseudanabaena proliferation results in an
increase indensity that leads tocolony settling, implying
potential selective losses under natural conditions. Both
the selective and antagonistic characters of the interac-
tion indicate that epiphytic Pseudanabaena have the
potential to contribute to the dynamics of strains in
naturalMicrocystis communities.
Keywords Phycosphere � Epiphytic interaction �Microcystis � Pseudanabaena
Introduction
Cyanobacteria are the dominant component of phyto-
plankton in many freshwater and marine environments
where they may form nuisance blooms. Cyanobacte-
rial blooms disrupt ecosystem functioning by increas-
ing turbidity and inducing hypolimnetic anoxia (e.g.,
Bartram & Chorus, 1999). Among bloom-forming
cyanobacteria, genusMicrocystis represents one of the
Handling editor: Judit Padisak
Electronic supplementary material The online version ofthis article (doi:10.1007/s10750-016-2773-z) contains supple-mentary material, which is available to authorized users.
R. Agha
Department of Ecosystem Research, Leibniz Institute of
Freshwater Ecology and Inland Fisheries, Berlin,
Germany
M. del Mar Labrador � A. Quesada (&)
Department of Biology, Universidad Autonoma de
Madrid, C. Darwin 2, 28049 Cantoblanco, Spain
e-mail: [email protected]
A. de los Rıos
National Museum of Natural Sciences, Spanish Council
for Scientific Research (CSIC), Serrano 115,
28006 Madrid, Spain
123
Hydrobiologia (2016) 777:139–148
DOI 10.1007/s10750-016-2773-z
most successful taxa worldwide (Sivonen & Jones,
1999), whose blooms raise public health concerns due
to its ability to produce microcystins, a group of
hepatotoxic metabolites with tumor promoting activ-
ity (Falconer & Humpage, 2005). Morphologically,
genus Microcystis is characterized by forming macro-
scopic colonies of coccoid cells that are able to
regulate their buoyancy with the aid of gas vesicles.
Colonies are embedded in a mucilaginous matrix
mainly consisting of complex heteropolysaccharides
(Plude et al., 1991; Forni et al., 1997; Pereira et al.,
2009). This matrix commonly constitutes a microhab-
itat that harbors heterotrophic bacteria, archaea,
flagellates, and other microorganisms (e.g., Shia
et al., 2010; Dziallas & Grossart, 2011). Association
to the so-called phycosphere provides colonizing
microorganisms with shelter against grazing, along
with a rich source of nutrients and organic carbon
(Worm & Søndergaard, 1998; Jiang et al., 2007).
Similarly, association to Microcystis colonies grants
access to otherwise unreachable depths with increased
light and/or nutrients availability, thanks to buoyancy
regulation and daily vertical migration of colonies in
the water column. Likely due to the fact that Micro-
cystis rapidly looses colonial morphology upon labo-
ratory isolation (Reynolds et al., 1981), studies on
epiphytic interactions on Microcystis using colonial
laboratory isolates are unavailable. Instead, epiphytic
interactions with Microcystis have been generally
addressed by analyzing colonies directly from their
environment. This approach showed that microbial
epiphytic associations can exert both positive and
negative effects on Microcystis, such as mutually
beneficial nutrient exchanges (Jiang et al., 2007; Shen
et al., 2011), or, conversely, induced physiological
stress and cell lysis (Caiola et al., 1991; Gumbo &
Cloete, 2013). The notion that epiphytes can have a
profound effect on the fitness of Microcystis led to
recognize epiphytic interactions as a potential factor
driving Microcystis bloom dynamics and/or decay
(e.g., Manage et al., 2001). Beside heterotrophic
organisms, the cyanobacterium Pseudanabaena muci-
cola, formerly referred to as Phormidium mucicola
(Komarek & Kastovsky, 2003), can also often be
found associated to Microcystis colonies (Sedmak &
Kosi, 1997; Vasconcelos et al., 2011; Yarmoshenko
et al., 2013). Genus Pseudanabaena is a scarcely
studied group of filamentous, non-heterocystous
cyanobacteria characterized by simple trichomes with
a width below 4 lm that can present polar aerotopes,
complementary chromatic adaptation, gliding motil-
ity, and anaerobic N2 fixation in some strains
(Komarek, 2005; Acinas et al., 2009). The interaction
between Microcystis and Pseudanabaena embodies a
notorious and rather unique example of epiphytic
association between members of the phylum
Cyanobacteria that remains virtually unexplored in
the literature. To fill this gap, we herein examine
phycosphere colonization by Pseudanabaena sp. on
colonialMicrocystis isolates in order to shed light into
the nature of the interaction, as well as inferring its
potential contribution to the dynamics of conspecific
Microcystis strains in the field. In fact, this work was
stimulated by observations from a previous field study,
where morphologically identical Microcystis colonies
with and without the presence of epiphytic Pseudan-
abaena were found coexisting in the water column
(see Online Resource 1). Moreover, in that study,
sedimentation traps used to collected settling seston
presented abundant colony-like aggregates over the
season that were fully colonized by filaments of
Pseudanabaena (i.e., free ofMicrocystis cells). These
observations prompted the idea that epiphytic associ-
ation between Pseudanabaena and Microcystis might
operate on a strain-selective basis and can potentially
inflict selective sedimentation or lytic losses to
specific Microcystis colonies. In this regard, recent
studies have claimed that focusing on the species as
the lowest taxonomic unit may be insufficient to
understand the complex dynamics and ecology of
planktonic cyanobacteria. Instead, intraspecific poly-
morphic strains with regard, for example, to gas
vesicles properties (Beard et al., 2000), niche parti-
tioning (Johnson et al., 2006), or oligopeptide pro-
duction (Agha et al., 2014) are proposed to
encapsulate ecologically functional lineages that rep-
resent the basis on which different biological pro-
cesses operate, including loss processes (Agha &
Quesada, 2014). Considering such intraspecific
dimension, we employed different recently isolated
Microcystis spp. strains still conserving their colonial
morphology to address three main questions. First, we
seek to confirm the existence of specificity in the
interaction between Pseudanabaena and Microcystis
suggested by previous field observations or, con-
versely, show that the interaction occurs indifferently
among Microcystis strains. Second, we examine two
intraspecific polymorphic traits in Microcystis that
140 Hydrobiologia (2016) 777:139–148
123
could act as potential drivers of such specificity, in
particular, slime microstructures and cellular
oligopeptide compositions. Lastly, we seek evidence
for impacts in the buoyancy of Microcystis colonies
upon Pseudanabaena association, which may imply
Microcystis loss processes in the water column under
natural conditions.
Materials and methods
Co-cultures
In order to test for selectivity in the interaction
between Pseudanabaena and Microcystis, several
recently isolated, non-axenic strains were used
(Table 1). Strains are stored as unicellular cultures in
the collection of cyanobacterial strains of the Univer-
sidad Autonoma de Madrid. Colonies of each Micro-
cystis strain were transferred to Erlenmeyer flasks with
50-ml BG11 medium to achieve cellular concentra-
tions of 5 9 105 cells ml-1 and then inoculated with
104 filaments of Pseudanabaena strain UAM-700
(600 cells ml-1). Co-cultures were kept at 29�C under
continuous white fluorescent light at 50 lmol photons
m-2 s-1. Individual co-cultures were microscopically
inspected and Pseudanabaena proliferation within the
colonies was monitored on a daily basis using an
Olympus BH2 microscope equipped with a BH2-
RFCA epifluorescence system (Olympus). Individual
Microcystis colonies were collected before the addi-
tion of Pseudanabaena, and 4 and 8 days after
inoculation. Colonies were stored at -80�C for LT-
SEM examination.
LT-SEM examination
Slime microstructures of the different Microcystis
strains were examined by Low-Temperature Scanning
Electron Microscopy (LT-SEM). Individual Micro-
cystis colonies of each strain were prepared for
observation in a cryotransfer system (Oxford
CT1500) following De los Rıos et al. (2015). Cry-
ofractured samples were gold sputter coated in prepa-
ration unit and observed under a DSM960 Zeiss SEM
microscope at -135�C.
Analysis of oligopeptide compositions
For each strain, individual Microcystis colonies or
Pseudanabaena filament suspensions were collected
for oligopeptide extraction after Agha et al. (2013).
Oligopeptide analysis was performed by Matrix-
Assisted Laser Desorption Ionization—Time of Flight
Mass Spectrometry (MALDI-TOFMS) using a Bruker
Reflex MALDI mass spectrometer equipped with a
TOF (Time of Flight) detector. MALDI-TOF MS data
acquisition, oligopeptide identification, and spectral
data processing are described in detail elsewhere
(Agha et al., 2012).
Buoyancy experiments
In order to evaluate losses of buoyancy upon Pseu-
danabaena epiphytic growth, colonies of the Micro-
cystis strain UAM-2C1B were transferred to sterile,
25-cm high tubes filled with liquid BG11 up to a
20 cmmark. The lower 15 cm of the tubes height were
wrapped in opaque plastic to reduce light availability
in the bottom of the tubes. Three tubes were inoculated
with 104 filaments of Pseudanabaena sp. UAM-700,
whereas three tubes containing only UAM-2C1B
colonies served as controls. Culture conditions were
identical as described above. Three days after the
addition of Pseudanabaena, differences in buoyancy
among treatment and control tubes were visually
evident. However, to unequivocally show differences
in density resulting from epiphytic inhabitation,
Table 1 Cyanobacterial strains used in this study
Strain name Species Origin Date of isolation
UAM-KIN M. aeruginosa See of Galilee (Israel) Summer 2011
UAM2C1B M. novacekii Cazalegas reservoir (Central Spain) Spring 2011
UAM2C1F M. aeruginosa Cazalegas reservoir (Central Spain) Spring 2011
UAM-700 Pseudanabaena sp. Valmayor Reservoir (Central Spain) Summer 2010
All strains were isolated and maintained as monoclonal non-axenic cultures at 28�C in liquid BG11 medium under continuous white
fluorescent light of 50 lmol photons m-2 s-1. All Microcystis strains presented colonial morphology
Hydrobiologia (2016) 777:139–148 141
123
colonies from both control and treatment tubes were
carefully collected and transferred to centrifuge tubes
containing a previously generated Percoll (Life
Sciences) density gradient solution (1.23% Isotonic
Percoll solution in NaCl 1.5 M, generating an isopic-
nic layer in the middle of the tubes of 1.008 g cm-3).
The tubes were then centrifuged at 5009g during
15 min and colonies migrated to their respective
isopicnic layer.
Results
Selectivity experiments with clonal isolates
Microscopic examinations of the co-cultures at 0, 4,
and 8 days after the addition of Pseudanabaena
UAM-700 revealed different outcomes for each co-
culture combination, evidencing dissimilar suscepti-
bility of Microcystis strains to epiphytic colonization:
Pseudanabaena did not access the slime of the M.
aeruginosa strain UAM-2C1F and colonies remained
unaffected during the whole culture period (Fig. 1). In
contrast, Pseudanabaena sp. filaments rapidly colo-
nized the slime of theM. novacekii strain UAM-2C1B
and proliferated within the colony slime (Fig. 2).
However, microscopic inspection did not reveal any
evident changes in the vitality (i.e., autofluorescence)
and subsequent growth of Microcystis UAM-2C1B or
Pseudanabaena. Strikingly, Pseudanabaena filaments
rapidly colonized the slime of M. aeruginosa strain
UAM-KIN and strongly proliferated, coincident with
rapid declines in the density ofMicrocystis cells in the
colonies over time. After a period of 8 days, the slime
was completely overrun by Pseudanabaena, while
Microcystis cells were almost absent (Fig. 3). The
marked differences observed among co-cultures evi-
denced strong selectivity in the interaction among both
taxa.
Exploration of factors driving selectivity
In a previous study, observations of Microcystis
colonies with epiphytic Pseudanabaena as settled
seston chronologically matched with the disappear-
ance of a particular Microcystis oligopeptide chemo-
type from the water column (Agha et al., 2014).
Therefore, we explored the possibility that the inter-
action betweenMicrocystis and Pseudanabaenamight
occur on a chemotype selective basis, i.e., driven by
differences in oligopeptide compositions among coex-
isting strains, in analogy to recently identified selec-
tive antagonistic interactions of cyanobacteria with
other organisms (Sønstebø & Rohrlack, 2011).
Oligopeptide profiles of the different strains used
were hence analyzed by MALDI-TOF MS and
compared. However, analyses did not show any
differentiating oligopeptide among strains that could
explain the observed differences in susceptibility
among Microcystis strains. Furthermore, Pseudan-
abaena sp. UAM-700 presented its own set of
intracellular oligopeptides (Table S1). Co-culture
samples presented no additional peptides compared
to those detected when analyzing the two respective
strains alone (data not shown).
In addition to oligopeptide analyses, individual
Microcystis colonies were examined by Low-Temper-
ature ScanningElectronMicroscopy (LT-SEM) inorder
to assess whether differences in slime microstructure of
colonies could determine the success ofPseudanabaena
Fig. 1 Micrographs of Microcystis aeruginosa strain
UAM2C1F before addition (a), 4 days (b), and 8 days
(c) after addition of Pseudanabaena sp. strain UAM-700. The
slime showed to be inaccessible for Pseudanabaena filaments
and colonies remained unaffected
142 Hydrobiologia (2016) 777:139–148
123
to colonize the phycosphere and, eventually, influence
subsequent effects on Microcystis. Interestingly, LT-
SEM examination revealed notorious differences
among strains: Colonies of the M. aeruginosa strain
UAM-2C1F, which showed to be inaccessible to
Pseudanabaena, presented a massive, ‘‘concrete-like’’
mucilaginous slime that densely surrounded individual
cells (Fig. 4). In contrast, M. aeruginosa strain UAM-
KIN (susceptible toUAM-700 in co-culture) displayed a
slime envelope with a remarkably more diffuse struc-
ture, plenty of irregularities, and cavities. Upon colo-
nization, trichomes of Pseudanabaena could be
observed embedded within the slime (Fig. 5b, c). The
diffuse structure of the slime showed to be a genuine
feature of this strain that could be also observed in
colonies before Pseudanabaena inoculation (Fig. 5a).
Lastly, colonies of theM. novacekii strain UAM-2C1B
(seemingly unaffected by epiphytic Pseudanabaena)
showed for themost part amucilaginous envelopewith a
diffuse structure, resembling that of UAM-KIN. How-
ever, cells appeared closely packed together and, in
these areas, slime with greater consistency densely
surrounded groups of cells (Fig. 6).
Buoyancy loss assessment
Despite the possibility of neutral co-existence of
Pseudanabaena and Microcystis in co-culture (e.g.,
with strain UAM-2C1B), changes in colony density
upon Pseudanabaena colonization were evaluated.
Increases in colony density upon Pseudanabaena
colonization and growth can have ecological implica-
tions, especially if it leads to effective loss of buoyancy
and, subsequently, to selective settling of inhabited
colonies. Under natural conditions, strain-selective
sedimentation of Microcystis colonies, even when not
causing their decay, exerts a direct effect on the pelagic
composition of strains. To evaluate this, the buoyancy
ofMicrocystis strain UAM-2C1B with and without the
presence of epiphytic Pseudanabaena was compared.
Three days after the addition of Pseudanabaena sp.
UAM-700, differences in buoyancy were visually
Fig. 2 Micrographs of Microcystis aeruginosa strain
UAM2C1B before addition (a), 4 days (b), and 8 days
(c) after addition of Pseudanabaena sp. strain UAM-700.
Pseudanabaena filaments rapidly colonized the mucilage and
proliferated, but no apparent effects on Microcystis cells were
evident
Fig. 3 Micrographs of Microcystis aeruginosa strain UAM-
KIN before addition (a), 4 days (b), and 8 days (c) after additionof Pseudanabaena sp. strain UAM-700. Pseudanabaena fila-
ments rapidly accessed the slime and proliferated, while
Microcystis cell density within the colony sharply declined. At
day 8, Pseudanabaena filaments densely occupied most of the
colony and only marginal amounts ofMicrocystis cells could be
observed
Hydrobiologia (2016) 777:139–148 143
123
evident. Colonies inhabited by Pseudanabaena accu-
mulated in the bottom of all replicate tubes, while
control colonies maintained positive buoyancy. The
increase in overall density upon Pseudanabaena colo-
nization was further confirmed by Percoll density
gradient centrifugation, with inhabited colonies migrat-
ing to the bottom layers (d[ 1.008 g cm-3) of the
density gradient solution (Fig. 7).
Discussion
Whereas selective losses in cyanobacterial popula-
tions have been associated to grazing (Czarnecki et al.,
2006), parasitism (Sønstebø & Rohrlack, 2011), or
programed cell death (Sigee et al., 2007), epibiotic
interactions in the phycosphere have rarely been
addressed. In the case of the epiphytic interaction
between Pseudanabaena and Microcystis, the few
existing studies dating from the 1980s already
attributed negative effects to the interaction. These
were based on observations that Pseudanabaena
grows very aggressive in culture, often destroying
Microcystis cells in short times (Gorham et al., 1982),
which led to describing epiphytic Pseudanabaena as a
parasite (Chang, 1985). However, these studies did not
explore the epiphytic interaction directly, possibly due
to the problems in maintainingMicrocystis in colonial
morphology under culture conditions (Reynolds et al.,
1981; Bolch & Blackburn, 1996). Here, the interaction
Fig. 4 LT-SEM images of Microcystis aeruginosa strain
UAM2C1F before addition (a), 4 days (b), and 8 days
(c) after addition of Pseudanabaena sp. strain UAM-700, taken
in back-scattered electron mode (a, b) and secondary electron
mode (c). Cells are embedded in a dense, massive slime which
covers the whole colony. Green arrows indicate Microcystis
cells
Fig. 5 LT-SEM images in back-scattered electron mode of
Microcystis aeruginosa strain UAM2C1B before addition (a),4 days (b), and 8 days (c) after addition of Pseudanabaena sp.
strain UAM-700. Colonies present a diffuse slime structure with
numerous cavities. Epiphytic Pseudanabaena filaments are
indicated by red arrows.Green arrows indicateMicrocystis cells
144 Hydrobiologia (2016) 777:139–148
123
was addressed using colonialMicrocystis isolates. Our
findings support the antagonistic nature of interaction
reported previously and evidence detrimental effects
on Microcystis upon Pseudanabaena association.
These effects manifest either directly by cell lysis or,
more subtly, by colony sedimentation. However, the
mechanisms by which Pseudanabaena induces Mi-
crocystis lysis remain unknown and deserve further
research. Possible causes include hypersensitive
response (Sigee et al., 2007) or induced lysis (Caiola
& Pellegrini, 1984) or activation of lysogenic viral
cycles (Sedmak et al., 2008).
Besides its antagonistic nature, the interaction
displayed a high degree of specificity. High specificity
is consistent with prior field observations reporting the
co-existence of colonies with and without epiphytic
Pseudanabaena (Ilhe, 2008). However, in that study,
the occurrence of epiphytic Pseudanabaena could not
be related to morphospecies affiliation, microcystin
production, or cell quota. Here, we explored a broader
intraspecific chemical polymorphism as possible
drivers of specificity, namely, the differential produc-
tion of bioactive oligopeptides (Agha & Quesada,
2014). While oligopeptides remain largely within the
producing cells, they are also localized in the mucilage
and can therefore affect the chemical environment
around the colony (Young et al., 2005). However, our
analyses did not reveal any distinctive oligopeptide
among strains that could explain the observed speci-
ficity patterns. Instead, examination of colonies by
LT-SEM revealed marked differences in colony slime
microstructure, suggesting that the topology and
consistency of the mucilage may be an important
feature driving selectivity. The characteristics of the
substratum, roughness, and microtopological features
Fig. 6 LT-SEM images in back-scattered electron mode of
Microcystis aeruginosa strain UAM-KIN before addition (a),4 days (b), and 8 days (c) after addition of Pseudanabaena sp.
strain UAM-700. Microcystis cells (green arrows) are densely
packed together in small subcolonies surrounded by dense
slime, while slime between subcolonies displays a diffuse,
loosely bound structure
Fig. 7 Partitioning of colonies of Microcystis strain UAM-
2C1B after Percoll centrifugation. (1.23% isotonic Percoll
solution in NaCl 1.5 M, generating a 1.008 g cm-3 isopicnic
plain in the middle of the tubes). Left Control colonies in the
absence of Pseudanabaena. Right Colonies inoculated with
Pseudanabaena UAM-700
Hydrobiologia (2016) 777:139–148 145
123
are determinant factors for epibiotic colonization
(Donlan, 2001; Bers & Wahl, 2004) and might
significantly affect the attachment and proliferation
of epiphytic Pseudanabaena onMicrocystis slimes. In
our study, readily accessible slimes for Pseudan-
abaena (strains UAM-KIN and UAM-2C1B) showed
a diffuse structure, dominated by loosely bound EPS,
presenting numerous cavities and irregularities.
Instead, strain UAM-2C1F presented a massive,
tightly bound slime that was inaccessible for Pseu-
danabaena. Whereas a chemical characterization of
these differences would be desirable, a rapid loss of
colonial morphology in culture prevented undertaking
this analysis. Our observations suggest that differ-
ences in structural organization of the mucilaginous
envelope in colonies of Microcystis might be crucial
for an effective association, acting as a first barrier to
epiphytic colonization by Pseudanabaena.
Our observations indicate that access to themucilage
by Pseudanabaena does not necessarily lead to direct
detrimental effects for Microcystis cells, as evidenced
by co-cultures in the case of strain UAM-2C1B.
However, we show that Pseudanabaena proliferation
causes increased density of hosting colonies, leading to
effective buoyancy loss. This phenomenon has evident
implications in the field, as epiphytic growth of
Pseudanabaena in natural systems, even when not
causing their lysis, may lead to colony sedimentation
and thereby to selective loss processes. In light of, first,
the existing selectivity in the epiphytic association and,
second, its impacts on buoyancy and viability of
inhabited colonies, epiphytic association by Pseudan-
abaena likely represents a biological process exerting
selective losses to particular Microcystis strains. Much
like in the case of other parasites (e.g., Sønstebø &
Rohrlack, 2011), strain-selective interactions poten-
tially contribute to the composition and dynamics of
coexisting strains in the water column. Hence, further
quantitative field studies addressing this interaction are
needed to uncover its relative importance to overall
Microcystis losses in the water column.
A priori, deep pelagic habitats do not seem suit-
able for Pseudanabaena spp., as access to photic depths
is strongly restricted due to its lack of gas vesicles and
inability to regulate their buoyancy. In pelagic environ-
ments, Pseudanabaena likely profits from attaching to
Microcystis colonies by accessing depths with adequate
light conditions in an otherwise aphotic environment.
Ilhe (2008) could observe that the abundance of M.
aeruginosa and M. novacekii colonies presenting epi-
phytic Pseudanabaena increased with time, being
lowest at the onset of the season, steadily increasing
during summer pelagic growth, and reaching maxima
toward autumnal sedimentation periods. These obser-
vations support the hypothesis that Pseudanabaena
associates to Microcystis colonies as a strategy to
colonize pelagic habitats in deep systems: Much like in
the case ofMicrocystis and other planktonic cyanobac-
teria (Cires et al., 2013), the recruitment phase likely
constitutes for Pseudanabaena the initial inoculum into
the water column, followed by summer pelagic growth
and maximum epiphytic development toward autumnal
sedimentation. Maximum proliferation toward the end
of the season would maximize population size to resist
benthic overwintering, which arguably represents a
stage of latency associated to substantial population
losses for Pseudanabaena. This is consistent with
previous studies showing Pseudanabaena abundances
to be lowest when re-entering the water column at the
onset of the season (Ilhe, 2008). Whereas this study
represents a first exploration of the overlooked interac-
tion between cyanobacteria of the genera Pseudan-
abaena and Microcystis, further research is definitely
needed todescribe the notorious life style and ecologyof
genus Pseudanabaena, as well as unraveling the
mechanisms underlying the selectivity and effects
resulting from its association to Microcystis, which
arguably constitutes an additional biotic interaction
contributing to the complex successional patterns of
Microcystis strains in natural populations.
Acknowledgments This article is dedicated to our colleague
Fernando Pinto, who sadly passed away during the preparation
of this manuscript and whose assistance operating the LT-SEM
made this work possible. Prof. Assaf Sukenik is acknowledged
for kindly providing bloom samples from Lake Kinneret. RA
was supported by a Postdoctoral Fellowship from the Alexander
von Humboldt Foundation during the writing process of this
manuscript. The authors also acknowledge the European Co-
Operation in Science and Technology COST Action ES1105
‘CYANOCOST’ for networking and knowledge-transfer
support. LT-SEM analyses were supported by the grant
CTM2012-3822-C02-02.
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