Delineating ecotypes of marine photosynthetic picoeukaryotes in the wild

Extremely small eukaryotic green algae are abundant primary producers found in diverse marine habitats. Over the last decade several studies have revealed extensive diversity within the "pico-prasinophytes" (≤2 µm diameter) that was previously unrecognized due to a lack of distinguishing morphological features. Using whole genome and marker gene analyses, distinct species have since been recognized within the Micromonas and Ostreococcus genera. Relatively little is known about environmental factors driving distributions of these species, but for Ostreococcus, laboratory studies suggested that differentiation reflects high- and low-light adapted ecotypes. Subsequent field studies indicated that Ostreococcus Clade OI and Clade OII rarely co-occur but partition according to distinct habitats - representing 'mesotrophic' and 'oligotrophic' ecotypes, respectively. Unlike Micromonas and Ostreococcus, Bathycoccus was presumed to be a single cosmopolitan species because identical 18S rRNA gene sequences are observed in cultured isolates and in environmental surveys. However, analysis of a targeted metagenome from a Bathycoccus population in the tropical Atlantic led to the hypothesis that Bathycoccus also harbors distinct ecotypes. Here, we have developed qPCR assays to enumerate the two Bathycoccus types which can be discriminated based on the internal transcribed spacer (ITS). Statistical analysis of qPCR and environmental data from >200 North Pacific Ocean samples shows that the two Bathycoccus clades are only somewhat analogous to oligotrophic and mesotrophic Ostreococcus clades. The two Bathycoccus clades co-occurred more than twice as often as the Ostreococcus clades. Additionally, while Bathycoccus BII and oligotrophic Ostreococcus OII were found at warm temperatures up to 26°C, BII extended into colder waters than OII. Similarly, Bathycoccus BI extended into warmer waters than mesotrophic Ostreococcus OI. Currently, we are analyzing metatranscriptomes to understand niche partitioning by these ecotypes. The patterns observed thus far suggest that warming trends, or co-associated changes would differentially impact geographic distributions of these clades with possible consequences for primary production and community structure.