Marine particle size-fractionation indicates organic matter is processed by differing microbial communities on depth-specific particles

Passive sinking flux of particulate organic matter (POM) in the ocean plays a central role in the biological carbon pump and carbon export to the ocean’s interior. Particle-associated (PA) microbes colonize POM, producing “hotspots” of microbial activity. We evaluated variation in PA microbial communities to 500 m depth across four different particle size fractions (0.2 – 1.2 μm, 1.2 – 5 μm, 5 - 20 μm, >20 μm) collected using in situ pumps at the Bermuda Atlantic Time-series Study (BATS) site. In situ pump collections capture both sinking and suspended particles, complimenting previous studies using sediment or gel traps, which capture only sinking particles. Additionally, diagenetic state of size-fractionated particles was examined using isotopic signatures alongside microbial analysis. Our findings emphasize that different particle sizes contain distinctive microbial communities, and each size category experiences a similar degree of change in communities over depth, contradicting previous findings. The robust patterns observed in this study suggest that particle residence times may be long relative to microbial succession rates, indicating that many of the particles collected in this study may be slow sinking or neutrally buoyant. Alternatively, rapid community succession on sinking particles could explain the change between depths. Complementary isotopic analysis of particles revealed significant differences in composition between particles of different sizes and depths, indicative of organic particle transformation by microbial hydrolysis and metazoan grazing. Our results couple observed patterns in microbial communities with the diagenetic state of associated organic matter, and highlight unique successional patterns in varying particle sizes across depth.