The genome of a freshwater planktonic Synechococcus strain reveals the potentiality to fix nitrogen

The non-heterocystous diazotrophic (N-fixing) unicellular cyanobacteria of small size have been recognized to be important in marine system and can contribute to the oceanic nitrogen fixation substantially. Certain Synechococcus strains showed nitrogenase activity only under anaerobic conditions, others are active also under microaerobic conditions, like the Synechococcus SF1, capable of photosynthesis and N fixation. The Synechococcus marine strains Miami BG43511 and BG43522 developed a kind of temporal regulation: N2 fixation is performed during dark and photosynthesis during light. Nevertheless, there are no evidences of the existence of planktonic freshwater Synechococcus with the potentiality of N2 fixation. We selected one Synechococcus strain isolated from a volcanic lake in central Italy, previously sequenced for the 16S DNA gene, we sequenced the entire genome focussing on the nifHDK operon genes. The presence of these genes indicate the potentiality to perform the nitrogen fixation. From the GC% content difference between the nif operon and the other genes it is possible to hypothesize a HGT for the gene acquisition. To evaluate if the genes could be active we carried out a laboratory experiment growing the strain in media with and without nitrogen and following its growth and RNA expression. Synechococcus LL was able to grow in both conditions but without fixing nitrogen, as nifHDK were not expressed. As the cultures without N bleached, losting their pigments, we hypothesize that Synechococcus could have used internal nitrogen reserve, instead of activating nitrogen fixation genes.