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Cyanobacteria can form blooms and in these situations they dominate the phytoplanktonic community, reaching extremely high densities. In the domain Bacteria, high population densities can stimulate a phenomenon known as quorum sensing, which may produce several modifications in the cell physiology. Very little is known about quorum sensing in Cyanobacteria. Because of their planktonic way of life, quorum sensing should be more evident during a bloom event. In this work, we tested whether cell density could shape the production of bioactive compounds produced by Cyanobacteria. The experiments consisted of two treatments, where cultures of Cyanobacteria were maintained at low and high cellular densities through a semi-continuous set-up. Analyses were performed by HPLC-PDA and MALDI-TOF MS. Seventeen peptides were detected and 14 identified, including microcystins, aeruginosins, cyanopeptolins and microviridins. The results showed that cellular density seems to have a significant effect on the peptides production. Most of the compounds had significantly higher cellular quotas in the higher-density treatment, although microviridins and an unknown peptide were produced only at low density. These results may hint at a possible role for quorum sensing in triggering the production of several cyanobacterial peptides.

Cyanobacteria produce several peptides whose functions and regulation mechanisms are not well understood. Iron availability has previously been suggested as a potential factor controlling production of microcystin, the best studied of these peptides, although results were not always consistent. Here, we examined how production of several peptides by 3 cyanobacterial strains changed in response to iron starvation or limitation. Experiments were run with 2 strains of Radiocystis fernandoii (28 and 86) and one of Planktothrix agardhii (27). The peptide spectra were analyzed and the gene expression of 2 peptides was assessed. Under iron starvation (no iron) all microcystin variants and one cyanopeptolin were significantly reduced in R. fernandoii 28 and P. agardhii 27. Only one compound, anabaenopeptin in P. agardhii 27, increased significantly. The strain R. fernandoii 86 did not show any significant modification in response to iron deprivation. Under moderate limitation, most peptides also decreased, although less abruptly. Conversely, changes in gene expression showed that genes coding for microcystin and aeruginosin were differentially expressed and were significantly higher under iron deprivation. This is consistent with previous findings that suggested that microcystin is produced to overcome stress. The lack of microcystin production despite the increased gene transcription is discussed in light of previous findings. This result might be due to either modifications of the cell metabolic state as a result of shortage of some important enzymes under iron deprivation or to microcystin loss as a result of its binding to proteins or other potential sink sources in the cell.

Summary Understanding how ecological traits have changed over evolutionary time is a fundamental question in biology. Specifically, the extent to which more closely related organisms share similar ecological preferences due to phylogenetic conservation – or if they are forced apart by competition – is still debated. Here, we explored the co‐occurrence patterns of freshwater cyanobacteria at the sub‐genus level to investigate whether more closely related taxa share more similar niches and to what extent these niches were defined by abiotic or biotic variables. We used deep 16S rRNA gene amplicon sequencing and measured several abiotic environmental parameters (nutrients, temperature, etc.) in water samples collected over time and space in Furnas Reservoir, Brazil. We found that relatively more closely related Synechococcus (in the continuous range of 93%–100% nucleotide identity in 16S) had an increased tendency to co‐occur with one another (i.e. had similar realized niches). This tendency could not be easily explained by shared preferences for measured abiotic niche dimensions. Thus, commonly measured abiotic parameters might not be sufficient to characterize, nor to predict community assembly or dynamics. Rather, co‐occurrence between Synechococcus and the surrounding community (whether or not they represent true biological interactions) may be a more sensitive measure of realized niches. Overall, our results suggest that realized niches are phylogenetically conserved, at least at the sub‐genus level and at the resolution of the 16S marker. Determining how these results generalize to other genera and at finer genetic resolution merits further investigation.