The class Prasinophyceae (Chlorophyta) contains several photosynthetic picoeukaryotic species described from cultured isolates. The ecology of these organisms and their contributions to the picoeukaryotic community in aquatic ecosystems have received little consideration. We have designed and tested eight new 18S ribosomal DNA oligonucleotide probes specific for different Prasinophyceae clades, genera, and species. Using fluorescent in situ hybridization associated with tyramide signal amplification, these probes, along with more general probes, have been applied to samples from a marine coastal site off Roscoff (France) collected every 2 weeks between July 2000 and September 2001. The abundance of eukaryotic picoplankton remained high (>10 3 cells ml ؊1 ) during the sampling period, with maxima in summer (up to 2 ؋ 10 4 cells ml ؊1 ), and a single green algal species, Micromonas pusilla (Prasinophyceae), dominated the community all year round. Members of the order Prasinococcales and the species Bathycoccus prasinos (Mamiellales) displayed sporadic occurrences, while the abundances of all other Prasinophyceae groups targeted remained negligible.Several studies have demonstrated the importance of eukaryotic picoplankton (cell size, 0.2-to 3-m) in terms of biomass and productivity in the euphotic zone of oceanic oligotrophic waters (15), as well as in coastal waters (10). To date, only ϳ40 species belonging to nine algal classes (Chlorophyceae, Prasinophyceae, Trebouxiophyceae, Prymnesiophyceae, Bolidophyceae, Eustigmatophyceae, Pinguiophyceae, Bacillariophyceae, and Pelagophyceae) of photosynthetic picoplanktonic eukaryotes have been formerly described (41). However, phylogenetic analyses of sequences retrieved from natural samples in different oceanic regions have demonstrated much higher diversity, since many of these sequences do not correspond to any described taxa (19). The contributions of the different taxonomic groups to the picoplanktonic biomass, diversity, and ecology are poorly known because simple and reliable methods to detect and quantify such organisms in natural samples are lacking. Pigment signatures, scanning electron microscopy, and serial dilution cultures suggest that the classes Prasinophyceae (division Chlorophyta), Pelagophyceae (division Heterokontophyta), and Prymnesiophyceae are major components of the picoplankton biomass in different marine systems (20,35).Among these, the class Prasinophyceae contains several photosynthetic picoeukaryote species. This class is considered to be the most primitive in the green lineage and to have given rise to all other green algal classes, as well as to the land plants (34). Members are known to be common in temperate and cold regions and can occur as prominent constituents of marine picoplankton (38). Within these organisms, genera such as Ostreococcus, Bathycoccus, and Micromonas have been described in coastal waters (4b, 6). Micromonas pusilla (the only described species in the genus Micromonas) has been identified as a major component of the pic...

Ocean Sampling Day was initiated by the EU-funded Micro B3 (Marine Microbial Biodiversity, Bioinformatics, Biotechnology) project to obtain a snapshot of the marine microbial biodiversity and function of the world’s oceans. It is a simultaneous global mega-sequencing campaign aiming to generate the largest standardized microbial data set in a single day. This will be achievable only through the coordinated efforts of an Ocean Sampling Day Consortium, supportive partnerships and networks between sites. This commentary outlines the establishment, function and aims of the Consortium and describes our vision for a sustainable study of marine microbial communities and their embedded functional traits.