Spatial-temporal study of cluster 5 picocyanobacteria and exopolymeric microgels in Lake Maggiore

In the oligotrophic Lake Maggiore, the majority of picocyanobacteria are phycoerythrin-bearing Synechococcus-type belonging to cluster 5 (Pcy-5). Their distribution was followed in a seven-year study along a depth gradient from the surface down to 350 m in relation to Transparent Exopolymer Particles (TEP), Total Organic Carbon (TOC), Chlorophyll-a (Chl-a) and water temperature. Pcy-5 abundances exhibit pronounced inter-annual variability, showing years of high numerical abundances as well as years with low numbers. In the upper 20 m, Pcy-5 peaks at around 10-15 m and then progressively decreases. Here, the Pcy-5 presence has been outlined for the first time in the deep layers of a deep lake, thus opening an interesting discussion on these organisms’ survival mechanisms in the absence of the light needed to perform photosynthesis. The relation of Pcy-5 with extracellular microgels was significant in autumn, when peaks of both variables were observed. In the other seasons, TEP was correlated with temperature and Chl-a, indicating the autochthonous origin of this fraction.