Phylogenetic diversity of freshwater picocyanobacteria

Picocyanobacteria are photosynthetic prokaryotes, coccoid or rod-shaped, with a cell diameter < 2 µm. They are common in lakes and oceans, and abundant across a wide spectrum of trophic conditions (Callieri et al 2012). The dominant genus of freshwater picocyanobacteria is Synechococcus. Analysis of 16S rRNA gene of freshwater Synechococcus showed its polyphyletic origin, requiring better insights in the present classification of the genus and possibly a revision. We isolated more than 40 picocyanobacterial strains from lakes geographically located in Italy, Mexico and Argentina, and characterized by different origin, trophic conditions and salinity. The strains originated from different water depths and had different pigmentation: phycoerythrin-containing (PE) or phycoerythrin-lacking strains (PC). A fragment of each isolated strain's 16S rRNA gene was amplified (primers 16S5'F and B23S5'R, Lepere 2000). Sequences of about 750bp were aligned to selected reference sequences belonging to the most representative Synechococcus Groups and used to design a Neighbour-Joining (NJ) phylogenetic tree. Maximal Photosystem II (PSII) quantum yield of dark adapted cultures, Fv/Fm, and photosynthetic parameters from PhytoPAM Rapid Light Curve (RLC) at 14 light intensities (from 4 to 1984 µE m-2 s-1) were measured for all the strains. The majority of Italian isolates, both PC and PE, assembled within the Cyanobium gracile cluster Group A (Crosbie et al 2003). Lake Maggiore strains convened in Group B together with many of our isolates. Other strains displayed distinct biogeographic differentiation: e.g. those isolated from high altitude tropical athalassohaline lakes in Mexico formed a well distinguished group of halotolerant strains phylogenetically close to the Marine Subcluster 5.2 (Synechococcus WH5701) and to Marine Subcluster 5.3. Notably, Synechococcus strains from tropical lakes had the highest Fv/Fm, typical of ecotypes with high photochemical efficiency of PSII. Synechocccus strains isolated from the ultraoligotrophic, large and deep Lake Nahuel Huapi (North Patagonia, Argentina) resulted as closely related to the Lake Constance Subalpine Group II (Synechococcus BO8805 and BO0014) and to the Marine Subcluster 5.2, with high Fv/Fm. The phylogenetic analysis of our new isolates provides more clear evidence of the non-marine nature of Group A as well as of the phylogenetic relationship between marine and freshwater halotolerant Synechococcus strains.