Prokaryotic niche partitioning between suspended and sinking marine particles

Suspended particlesare major organic carbon substrates for heterotrophic microorganisms in the mesopelagic ocean (100–1000 m). Nonetheless, communities associated with these particleshave been overlooked compared with sinking particles, the latter generally considered as main carbon transporters to the deep ocean. This study is the first to differentiate prokaryoticcommunities associated with suspended and sinking particles, collected with a marine snowcatcher at four environmentally distinct stations in the ScotiaSea. Amplicon sequencing of 16S rRNA gene revealed distinct prokaryoticcommunities associated with the two particle‐types in the mixed‐layer (0–100 m) and upper‐ mesopelagic zone(mean dissimilarity 42.5% ± 15.2%). Although common remineralisingtaxa were present within both particle‐types, gammaproteobacterial Pseudomonadalesand Vibrionales, and alphaproteobacterial Rhodobacteraleswere found enriched in sinking particlesup to 32‐fold, while Flavobacteriales( Bacteroidetes) favoured suspended particles. We propose that this niche‐partitioning may be driven by organic matter properties found within both particle‐types: K‐ strategists, specialised in the degradation of complex organic compounds, thrived on semi‐labile suspended particles, while generalists r‐ strategistswere adapted to the transient labile organic contents of sinking particles. Differences between the two particle‐associated communities were more pronounced in the mesopelagic than in the surface ocean, likely resulting from exchanges between particle‐pools enabled by the stronger turbulence.