Protistan diversity in environmental molecular surveys

Abstract

Marine protists include a heterogeneous collection of phototrophic and heterotrophic unicells covering a wide cell size range and belonging to virtually all eukaryotic lineages. They have been identified by microscopy, which allows a reasonable level of resolution for the larger specimens but is clearly insufficient for the smallest ones. Moreover, as occurs with their prokaryotic counterparts, a large majority of marine protists are uncultivable. Molecular tools have revolutionized field studies of protists' diversity, allowing exhaustive species inventories especially when combined with High-Throughput sequencing technologies. These surveys have shown that natural assemblages are very diverse, including novel phylogenetic lineages that had remained uncharacterized despite their evident ecological Signifi- Cance. The extent of diversity and novelty is largest within the assemblage of the smallest protists, the picoeukaryotes. The information gathered by sequencing phylogenetic marker genes has been combined with an array of complementary molecular methods such as fingerprinting tools to study diversity changes along spatial and temporal gradients, fluorescence in situ hybridization (FISH) to put a face on the novel lineages and perform specific cell counts, and metagenomics to explore ecological adaptations on the basis of the genetic potential. This chapter presents an overview of the molecular approaches currently applied to gain knowledge on the diversity and function of protists in the environment.