Source, transformation and degradation of particulate organic matter and its connection to microbial processes in Jiaozhou Bay, North China

Microbial processes during the degradation and transformation of marine organic matter (OM) are thought to play an important role in controlling the strength and efficiency of carbon export and preservation in the ocean. However, the detailed relationship between the diagenetic states of OM and microbial activity and community structures remains poorly understood. In this study, particulate amino acids (PAAs), stable carbon and nitrogen isotopic signatures, heterotrophic bacterial abundance (HBA), bacterial community structure and extracellular hydrolytic enzyme activity (EEA) in particulate OM (POM) were investigated in Jiaozhou Bay (JZB), China, with the aim of investigating bacterial degradation and adaption in response to different sources and reactivity of POM in the coastal environment. The results showed that POM from different sources had distinct PAA compositions, diagenetic states, and bacterial features, such as particle attached HBA, EEA and community structure. High PAA concentrations but low contribution of bacterial carbon to particulate organic carbon (POC) and diagenetic alteration of POM were observed in regions with high in situ marine production, where high Chl-a and autochthonous δ13C and δ15N signatures were observed. In addition, high HBA and D-amino acid (D-AA) concentrations in POM were also found in the same regions, which indicated that fresh POM also induced rapid bacterial growth and reproduction. However, the lowest D-AA proportion to PAA (mol% D-AA) in those regions still indicated a more obvious supply of AAs from marine sources, which was consistent with the low bacterial carbon contribution to POC and fresh states of POM. In contrast, POM in low primary production regions was highly degraded by bacteria, as reflected by an enrichment in D-AAs and high bacterial contribution. However, high EEA probably driven by the priming effect occurred in regions where fresh planktonic POM was mixed with terrigenous POM. High relative abundances of Flavobacteriia, Alphaproteobacteria and Gammaproteobacteria were observed in fresh planktonic, terrestrial and planktonic mixture and highly degraded POM, respectively, indicating a possible bacterial adaptation strategy to their available OM. Our study has important implications for bacteria–POM interactions and provides insight into the role of bacteria in the degradation and transformation of POM of different sources in coastal systems.