The Elusive Fraction of Marine Plankton Diversity: Size, Abundance and Taxonomic Composition of 0.2 μm‐Filterable Prokaryotes Across Three Contrasting Coastal Environments

Abstract

The existence of prokaryotes escaping filtration through 0.2‐μm filters has been known for decades. Such ‘filterable’ prokaryotes (hereafter ‘FP’) might include true ultra‐small groups (i.e., ultramicrobacteria), but also cells that miniaturize temporally as a physiological strategy to persist under unfavourable conditions, representing a ‘seed bank’ that can flourish when conditions change. However, the taxonomy, environmental drivers and ecological relevance of these microorganisms remain unknown due to a scarcity of studies targeting this size‐fraction. We characterized prokaryotic assemblages from the 0.1–0.22 μm and the > 0.22‐μm (‘0.2RP’ for 0.22‐μm‐retained prokaryotes) size‐fractions across different sites in the NE Atlantic (Bay of Biscay), the NW Mediterranean Sea and the Red Sea. 9%–15% of total cells passed through 0.22‐μm filters and their mean cell size was 16%–46% smaller than that of the unfiltered community. 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. Distinct environmental drivers shaped different FP subpopulations, as the contribution of specific taxa within the FP communities varied with organic carbon (OC) concentration and temperature. Notably, taxonomic dissimilarity between both fractions increased with higher OC concentration, suggesting that more normal‐sized taxa may be miniaturized towards conditions of less OC availability. Overall, our results suggest that the widespread use of 0.2‐μm filters likely results in significant losses of the extant diversity, potentially hiding ecologically important roles in marine ecosystems.

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