A Single Species, Micromonas pusilla (Prasinophyceae), Dominates the Eukaryotic Picoplankton in the Western English Channel

Abstract

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...

Mixotrophic haptophytes are key bacterial grazers in oligotrophic coastal waters

“…Picoplanktonic chlorophytes cells (o3 mm) were represented almost exclusively by the Prasinophyceae Micromonas pusilla. The great abundance of Micromonas is consistent with previous estimation of picoplankton composition in the Mediterranean Sea including Blanes Bay (Foulon et al, 2008) but also in other oceanic regions (for example, Not et al, 2007;2004). Our results also revealed the existence of marked changes in the abundance of some nano-PFs (3-20 mm) along the year (cryptophytes and chlorophytes dominated at the end of winter, pigmented dinoflagellates increased in summer, and at the beginning of winter all groups increased significantly), whereas haptophytes were present all year-round always at a significant number without major fluctuations (Figure 2).…”

“…Cells were then immediately observed by epifluorescence microscopy under green light excitation. Chlorophytes and the genus Micromonas (Mamiellophyceae, Prasinophyceae and Chlorophyta) were enumerated by tyramide signal amplification-FISH (TSA-FISH), also named catalyzed reporter deposition-FISH, using the probes CHLO02 and MICRO01, respectively (Simon et al, 2000;Not et al, 2004). Probes labeled with a horseradish peroxidase enzyme were also provided by Thermo Fisher Scientific.…”

Significant CO2 fixation by small prymnesiophytes in the subtropical and tropical northeast Atlantic Ocean

“…For Synechococcus, the probes used targeted the following specific lineages: clade I (SYN1006), clade II (SYN1006RS), clade III (SYN262), clade IV (SYN635) or more generally clades I-VII and X (SYN1258). The nuclear 18S rRNA oligonucleotide probes used were EUK1209r, NCHLO01 and CHLO02 to target all eukaryotes and the class-specific probes PRAS01, PRAS02, PRAS05 (targeting prasinophytes), and CRYPTO13, PRYM02 and PELA01 targeting cryptophytes, prymnesiophytes and pelagophytes, respectively (Giovannoni et al, 1988;Simon et al, 1995Simon et al, , 2000Biegala et al, 2003;Not et al, 2002Not et al, , 2004. The chrysophytespecific probe (CHRYSO1037) targeted the chloroplast 16S rRNA gene, as used previously for dot blot hybridization analysis (Fuller et al, 2006b) and validated here for tyramide signal amplification (TSA)-FISH analysis (see below).…”

“…Moreover, studies based on size-fractionated pigment analysis also suggest a wide distribution of these small photosynthetic eukaryotes (Latasa and Bidigare, 1998;Wright et al, 2009). Recent progress using molecular approaches has begun to reveal their diversity (Moon-van der Staay et al, 2000;Díez et al, 2001;Vaulot et al, 2008;Worden and Not, 2008) showing a dominance of prymnesiophytes, pelagophytes and prasinophytes in marine environments (for example, see Moon-van der Staay et al, 2001;Romari and Vaulot, 2004;Not et al, 2004;Fuller et al, 2006a, b;Viprey et al, 2008;Liu et al, 2009) and lately of chrysophytes (Fuller et al, 2006b;McDonald et al, 2007;Lepère et al, 2009). However, we still have a poor understanding of the phylogenetic affiliation of the smallest cells (for example, see Liu et al, 2010), and particularly in directly linking taxonomic identity with 14 C primary production measurements.…”

Ecology and Diversity of Picoeukaryotes

“…Eustigmatophyceae Nannochloropsis granulata Zapata et al (2001) (Chrysochromulina), Guillou et al (1999b), and F. Rodriguez (personal communication) (Bolidophyceae), Latasa et al (2004) (Prasinophyceae), Kawachi et al (2002a, b) (Pinguiophyceae), Henley et al (2004) (Trebouxiophyceae), and Karlson et al (1996) (Eustigmatophyceae). …”