Single culture and co-culture of two Synechococcus phylotypes respond differently to nanoflagellate grazing

Cyanobacteria belonging to the genus Synechococcus are found in lake waters typically as planktonic single cells and monospecific microcolonies. In oligotrophic lakes, single cells dominate in spring, while microcolonies are mostly found in late summer-autumn when the large colonial cyanobacteria increase in number. Since grazing activity is known as one of the major factors inducing microbial phenotypical changes, the formation of Synechococcus microcolonies was proposed as an efficient defence strategy against sizeselective predators. To better understand this ecological interaction, we explored the effect of grazing by the mixotrophic nanoflagellate Poteriochromonas sp. on the aggregation of two freshwater Synechococcus strains belonging to different phylogenetic clades (phycoerythrin-rich cells, PE, Group A; phycocyanin-rich cells, PC, Group I). During four days of incubation, we followed the dynamics of single-cells, microcolonies, and flagellates in semicontinuous cultures under different treatments (single culture and co-culture, with and without predators) by flow cytometry, epifluorescence microscopy and PhytoPAM. In single culture with the addition of Poteriochromonas, we observed the formation of grazing-induced monoclonal PE microcolonies, conversely limited in PC. In co-culture, there was an interaction between PE and PC, with an active microcolony formation by both PE and PC, and an increase of PC photosynthetic fitness (Fv/Fm). In co-culture, the microenvironment, generated by the formation of PE microcolonies, PC cells, bacteria and Poteriochromonas, can be the site of a beneficial "communication signalling" among Synechococcus cells for attaining the best spatial distribution for the fitness of the group