Phenotypic plasticity in freshwater picocyanobacteria

Abstract

Picocyanobacteria can occur as single-cell (Pcy) or as colonies (CPcy). Published evidence suggests that some Pcy strains have the capability to aggregate under certain culture conditions, however this has not been demonstrated to occur in natural environments. We investigated whether the Pcy and CPcy belong to the same species (i.e. phylotype), and the factors that determine their morphological and genetic variability in a hypertrophic shallow lake dominated by picocyanobacteria. Six main different morphologies and >30 phylotypes were observed. All sequences retrieved belonged to the 'Anathece + Cyanobium' clade (Synechococcales) that are known to have the capability of aggregation/disaggregation. The temporal variation of picocyanobacteria morphotype composition was weakly correlated with the DGGE temporal pattern, and could be explained by the composition of the zooplankton assemblage. Laboratory experiments confirmed that the small cladoceran Bosmina favoured the dominance of CPcy, i.e. Cyanodictyon doubled the size of the colonies when present, most likely through the aggregation of single-cell picocyanobacteria into colonies. Flow cytometry cell sorting and 16S rRNA + ITS sequencing of the Pcy and CPcy cytometrically-defined populations revealed that some phylotypes could be found in both sorted populations, suggesting phenotypic plasticity in which various Synechococcales phylotypes could be found in situ either as single-cells or as colonies.

Identification of Cyanobacteria in a Eutrophic Coastal Lagoon on the Southern Baltic Coast

“…The separation of Anathece from Cyanobium was based on morphology (Komárek et al, 2011), whereas there is no evidence for this in the phylogeny. Moreover, Huber et al (2017) demonstrated the variability of this feature. The strains that we isolated belonged to the same Cyanobium lineages, but consisted of solitary cells under our grazer-free culture conditions.…”

“…The position of the thylakoids cannot be determined with light microscopy, so the only remaining easily visible difference is the formation of colonies. For example, Jezberová and Komárková (2007a,b) and Huber et al (2017) demonstrated the variability of this feature under different culture conditions. The formation of colonies depended on the presence of a grazer.…”

“…Morphological and molecular analyses in situ and in vitro gave partly contrasting results. These differences can be explained by culturing biases, misleading determination features, ambiguous database entries, and the plasticity of Cyanobium as shown by Jezberová and Komárková (2007a,b) and Huber et al (2017). The focus was on the most conserved and reliable feature, the 16S rRNA gene.…”

“…The formation of colonies depended on the presence of a grazer. Jezberová and Komárková (2007a,b) and Huber et al (2017) conducted experiments using strains of the Synechococcus–Prochlorococcus–Cyanobium clade (SPC-clade), which were phylogenetically different Cyanobium lineages. These strains were determined as Anathece by Jezberová and Komárková (2007a), because of their ability to form colonies.…”

“…For example, in picocyanobacteria, unicellular specimens smaller than 2–3 μm were assigned as species of Synechococcus or Cyanobium , whereas specimens in the same size range forming colonies and mucilage were identified as species of Aphanothece, Aphanocapsa or Anathece . Jezberová and Komárková (2007a) and Huber et al (2017) demonstrated that Cyanobium species can form colonies under grazing pressure, which then morphologically resemble Anathece . Komárek et al (2011) confirmed that these two genera are closely related, based on 16S rRNA phylogeny.…”