A novel free-living prochlorophyte abundant in the oceanic euphotic zone

Abstract

The recent discovery of photosynthetic picoplankton has changed our understanding of marine food webs1. Both prokaryotic2,3 and eukaryotic4,5 species occur in most of the world's oceans and account for a significant proportion of global productivity6. Using shipboard flow cytometry, we have identified a new group of picoplankters which are extremely abundant, and barely visible using traditional microscopic techniques. These cells are smaller than the coccoid cyanobacteria and reach concentrations greater than 105 cells ml–1 in the deep euphotic zone. They fluoresce red and contain a divinyl chlorophyll a-like pigment, as well as chlorophyll b, α-carotene, and zeaxanthin. This unusual combination of pigments, and a distinctive prokaryotic ultrastructure, suggests that these picoplankters are free-living relatives of Prochloron7. They differ from previously reported prochlorophytes—the putative ancestors of the chloroplasts of higher plants—in that they contain α-carotene rather than β-carotene and contain a divinyl chlorophyll a-like pigment as the dominant chlorophyll.