Microbes dominate the biomass of oceans, having vital roles in the functioning of ecosystems and the biogeochemical cycling. Yet, little is known about the total extent and structuring patterns of their genetic diversity, especially at the species and population levels. Pointing into this research direction, this proposal has two general aims: (a) to go deeper into assessing the extent of picoeukaryotic (2-5 μm cell size) diversity in marine environments and (b) to investigate genetic patterns at the species and population levels. The chosen lineage for this study is the MArine STramenopiles -4 (MAST-4), a recently discovered group of heterotrophic picoeukaryotes that can be relative abundant (and therefore important) in marine environments. Two novel molecular techniques are considered for this proposal: (1) Pyrosequencing and (2) Single-Cell-Whole-Genome-Amplification. The use of these new techniques and several standard ones, together with the availability of a unique marine environmental DNA set of samples from a variety of geographic locations around the world, will allow a strong assessment of the extent of the MAST-4 diversity, with the potential to unveil new lineages as well as genetic structuring patterns at the species and population levels. The genetic patterns found at different evolutionary hierarchies will be correlated with environmental data in order get insight into ecological adaptation and evolutionary diversification.