Marine organic matter fuels the growth of microbial communities, shaping bacterial compositions that specialize in its degradation. Here, we investigated the composition of size-fractionated bacterial communities, functional genes, as well as dissolved (DOM) and particulate organic matters (POM) in coastal waters over a 22-day period that includes a diatom bloom. Co-occurrence analysis showed that the free-living bacterial communities were significantly less stable than particle-associated communities. The relative proportion of amino acids and peptides in DOM were significantly increased during bloom-affected period, while the major classes of POM molecules exhibited relative stability across all samples. We revealed a strong correlation between bacterial communities and the chemical composition of organic matter. Notably, the composition of organic matter, including both DOM and POM, was more strongly related to the composition of free-living bacterial communities than to particle-associated communities. This suggests that free-living bacteria exhibit greater variations in temporal dynamics and higher sensitivity to the specific structure of organic matter molecules. Additionally, functional analysis revealed the divergence of extracellular enzymes and transporters, enabling bacteria to occupy separate ecological niches throughout the water column despite the heterogeneous composition of organic matter supply. Specifically, we found that TonB-dependent transporters gene abundances were more positively correlated with extracellular enzymes at the community and metagenome-assembled genomes level compared to ATP-binding cassette transporters. The correlations suggesting a potential co-regulation or functional linkage between extracellular enzymes and TonB-dependent transporters in the metabolism of marine heterotrophic prokaryotes. Our findings provide new insights into the metabolic strategies employed by these microbes in the dynamic marine environment.
 

bacteria, metagenome, LC-MS/MS, coastal zone, time-series sequences