Abstract: Picophytoplanktons are distributed worldwide and are ubiquitous in all types of oceans and lakes. They are a fundamental component of aquatic ecosystems, especially in the microbial loop. Previous researches were mainly focused on marine picophytoplankton since their discovery in the 1970s, and only recently freshwater picophytoplankton has been received wide attention. Synechococcus and eukaryotic picophytoplankton are the main component of freshwater picophytoplankton discovered to date, while Prochlorococcus which is widespread in marine is rarely found in freshwater habitats. The eukaryotic component of picophytoplankton is of great diversity which comprises clusters from almost all algal phyla. However, Synechococcus found in lakes in China are mainly composed of phycocyaninrich cells. The abiotic and biotic factors affecting their diversity include nutrients, temperature, light and predation, and picophytoplankton in natural waters could be affected by multiple factors at the same time. Traditional methods including microscopy observation and pure culture techniques hampered the progress of picophytoplankton study, due to its tiny cell size, insufficient distinct morphological features for taxonomic discrimination and being unculturable for most picophytoplankton species. Major advances in picophytoplankton ecology and taxonomy benefited from the introduction of new techniques, including epifluorescence microscopy, flow cytometry and molecular biology techniques. In this paper, we presented the species of freshwater picophytoplankton described to date, and discussed how they are affected by abiotic and biotic factors. In the end, we focused on the techniques recently used in picophytoplankton research.

Key words: high-throughput sequencing, bacterial community, nitrogen deposition, Lefse analysis