We studied the phylogenetic diversity of nonmarine picocyanobacteria broadening the sequence data set with 43 new sequences of the 16S rRNA gene. The

Cyanobacteria are among the oldest photoautotrophic organisms on Earth, and have contributed to shaping the planet's biogeochemistry with their significant biomass and key metabolic activities. Synechococcus, the focus of this review, is one of the prevalent genera in the order Chroococcales, common in oceans and lakes and characterized by a coccoid unicellular or microcolony morphology. The evolution of its phycobilisomes is the key of the adaptation of this tiny photosynthetic cell to different light regimes and environmental conditions. Furthermore, Synechococcus strains are widely distributed from the equator to the poles, showing an extreme adaptability to high and low temperatures. Because of their structural plasticity and ecological adaptability, these cyanobacteria are particularly interesting in the current condition of fast climate change. Moreover, picocyanobacteria of the Synechococcus genus have a potentially vast impact on global cycles thanks to their significant role in the biogeochemical cycles of aquatic ecosystems. As increasing abundances are predicted for this genus worldwide, and in light of the connection between cyanobacteria and global change events, a better characterization of these organisms promises important and timely ecological insights. Here, I will summarize the morphological and genetic characteristics of Synechococcus strains and their distribution in freshwater lakes, also considering its marine counterpart.

Global warming has been shown to strongly influence inland water systems, producing noticeable increases in water temperatures. Rising temperatures, especially when combined with widespread nutrient pollution, directly favour the growth of toxic cyanobacteria. Climate changes have also altered natural water level fluctuations increasing the probability of extreme events as dry periods followed by heavy rains. The massive appearance of Dolichospermum lemmermannii ( = planktonic Anabaena), a toxic species absent from the pelagic zone of the subalpine oligotrophic Lake Maggiore before 2005, could be a consequence of the unusual fluctuations of lake level in recent years. We hypothesized that these fluctuations may favour the cyanobacterium as result of nutrient pulses from the biofilms formed in the littoral zone when the lake level is high. To help verify this, we exposed artificial substrates in the lake, and evaluated their nutrient enrichment and release after desiccation, together with measurements of fluctuations in lake level, precipitation and D.lemmermannii population. The highest percentage of P release and the lowest C∶P molar ratio of released nutrients coincided with the summer appearance of the D.lemmermannii bloom. The P pulse indicates that fluctuations in level counteract nutrient limitation in this lake and it is suggested that this may apply more widely to other oligotrophic lakes. In view of the predicted increase in water level fluctuations due to climate change, it is important to try to minimize such fluctuations in order to mitigate the occurrence of cyanobacterial blooms.

Single cells and microcolonies of freshwater picocyanobacteria: a common ecology