Freshwater picocyanobacteria: single cells, microcolonies and colonial forms

This chapter deals with some taxonomic and ecological aspects of picocyanobacteria (Pcy) single-cells, microcolonies and other colonial (CPcy), that are common in lakes throughout the world, and abundant across a wide spectrum of trophic conditions. We discuss phenotypic diversity of Pcy in conjunction with a genotypic approach in order to resolve whether a similar morphology also re fl ects a phylogenetic relationship. Microcolonies of different size (from 5 to 50 cells) constitute a gradient without a net separation from single-celled types and should be consi dered Pcy, as transition forms from single-cell to colonial morphotypes. The single-celled Pcy populations tend to be predominant in large, deep oligo-mesotrophic lakes, while the CPcy fi nd optimal conditions in warmer, shallower and more nutrient rich lakes. The knowledge of Pcy diversity in pelagic and littoral zone habitats is a key to understand the dominance of certain genotypes in the water column and of their ubiquity. An anlysis is included of the factors (biotic and abiotic) in fl uencing the dynamics of the different Pcy forms.