Picocyanobacterial community structure and space-time dynamics in the subalpine Lake Maggiore (N. Italy)

We investigated the community structure of freshwater picocyanobacteria (Pcy) in the oligotrophic subalpine Lake Maggiore (Northern Italy) using the least conserved sequences of the Internal Transcribed Spacer (ITS-1) fragment. The aim was to characterize the succession of Pcy Operational Taxonomic Units (OTUs) during a seasonal cycle at two environmental niches with distinctive light, temperature and mixing characteristics. ITS fragment length outlined a succession of OTUs from spring to summer, even in the absence of a pronounced spring peak. The vertical partitioning of Pcy OTUs down the water column, more evident in summer, indicates different community composition at the two depths. Redundancy Data Analysis (RDA) was carried out using temperature, pH and depth of mixing layer (Zm), which passed the Monte Carlo test (p<0.05). A correlation between axis 1 and temperature, pH and Zm of respectively 0.85, 0.64 and 0.61 was found. Our results indicate that lake thermal structure, defined by temperature and Zm, has a significant influence on the succession of Pcy OTUs and on their dynamics in colonizing different ecological niches in the vertical gradient.