Abstract
Significance The oceans are populated by an astronomical number of predominantly uncultured microbes, which altogether guarantee ecosystem function. Unicellular eukaryotic predators represent basal links in marine food webs and have so far been predominantly characterized as a functional group, despite having different ecologies and evolutionary histories. In order to better understand the ecoevolution of the ocean’s smallest predators, we have investigated four species belonging to an uncultured cosmopolitan family: marine stramenopiles (MAST)-4. Using state-of-the-art single-cell genomics and metaomics approaches, we found that members of this predatory family have different distributions in the surface ocean and different genes to degrade food, which likely represent niche adaptations. Our work highlights the importance of understanding the species-level ecology and genomics of tiny ocean predators. , Unicellular eukaryotic predators play a crucial role in the functioning of the ocean ecosystem by recycling nutrients and energy that are channeled to upper trophic levels. Traditionally, these evolutionarily diverse organisms have been combined into a single functional group (heterotrophic flagellates), overlooking their organismal differences. Here, we investigated four evolutionarily related species belonging to one cosmopolitan group of uncultured marine picoeukaryotic predators: marine stramenopiles (MAST)-4 (species A, B, C, and E). Co-occurrence and distribution analyses in the global surface ocean indicated contrasting patterns in MAST-4A and C, suggesting adaptation to different temperatures. We then investigated whether these spatial distribution patterns were mirrored by MAST-4 genomic content using single-cell genomics. Analyses of 69 single cells recovered 66 to 83% of the MAST-4A/B/C/E genomes, which displayed substantial interspecies divergence. MAST-4 genomes were similar in terms of broad gene functional categories, but they differed in enzymes of ecological relevance, such as glycoside hydrolases (GHs), which are part of the food degradation machinery in MAST-4. Interestingly, MAST-4 species featuring a similar GH composition (A and C) coexcluded each other in the surface global ocean, while species with a different set of GHs (B and C) appeared to be able to coexist, suggesting further niche diversification associated with prey digestion. We propose that differential niche adaptation to temperature and prey type has promoted adaptive evolutionary diversification in MAST-4. We show that minute ocean predators from the same phylogenetic group may have different biogeography and genomic content, which needs to be accounted for to better comprehend marine food webs.
Francisco Latorre
Technician
Aleix Obiol
Postdoctoral scientist
Ramon Massana
Staff scientist
I am microbial ecologists with a deep interest in protist ecology and evolution
