MARUMBA. The overlooked marine diversity: cell size, abundance and composition of 0.2 µm-filterable prokaryotes across different coastal sites

The existence of prokaryotes able to escape the 0.2-μm pore-size filter has been known for decades. Such “filterable” prokaryotes might include both true ultra-small groups (i.e., ultramicrobacteria), but also cells that miniaturize temporally as a physiological strategy to persist under unfavorable conditions, representing a “seed bank” that can flourish when conditions change. However, the taxonomy, spatial patterns, environmental drivers, and ecological relevance of these microorganisms remain unknown due to a scarcity of studies targeting this size-fraction. Here, we characterized the prokaryotic assemblages from the 0.1-0.22 μm size-fraction (hereafter “FP” for 0.22 μm filterable prokaryotes) and compared them with the >0.22-μm prokaryotic fraction (hereafter “0.2RP” for 0.22 μm retained prokaryotes) across different sites in the NE Atlantic (Bay of Biscay), the NW Mediterranean Sea, and the Red Sea. 8.5-15.1% of total cells passed through 0.22-μm pore-size filters, and their mean cell size was 16%-46% smaller than that of total prokaryotes. FP consistently showed lower taxonomic richness than the 0.2RP fraction. Known small-sized groups like Pelagibacterales, Actinobacteriota and Thermoplasmatota were significantly enriched in FP assemblages, but typical normal-sized bacteria such as Flavobacteriales were also detected, suggesting the presence of miniaturized cells of widespread marine prokaryotes. Notably, community dissimilarity between both fractions was higher in sites with high organic carbon (TOC) concentration, as was the proportion of taxa with a preference for the 0.2RP fraction. This might indicate higher proportion of starvation forms towards conditions of less OC availability. Overall, our results suggest that, using 0.22 μm filters we may be missing a significant fraction of the extant diversity, which may harbor ecologically important roles in marine ecosystems.