Looking for the dominant marine bacterivores

Abstract

Heterotrophic flagellates are central in marine food webs, as was already recognized in the seminal papers conceptualizing the microbial loop. Indeed, if planktonic bacteria were abundant, growing actively and with balanced abundances, an equally significant bacterial loss mechanism was required. We know now that grazing by small heterotrophic (and mixotrophic) flagellates and viral lysis are the main mortality factors for marine bacteria. Despite this recognized ecological role, and contrasting with the significant advances achieved with marine bacteria, little is known on the diversity of marine heterotrophic flagellates. This heterogeneous group of cells is poorly captured by microscopic and culturing approaches, and has been recently targeted by environmental molecular surveys. These have illuminated several uncultured clades, collectively named Marine Stramenopiles (MAST), which appear as abundant and widespread marine bacterivores. One of these clades, MAST-4, is formed by tiny cells (2-3 μm) that account on average for ~10% of marine heterotrophic flagellates. MAST-4 displays low genetic variability indicating this widely successful group has suffered low evolutionary diversification. Future research based on novel isolation attempts, single cells analyses and in situ experiments combined with FISH are promising to reveal the true extent of their ecological and evolutionary significance