Molecular characterization of organic matter transformation mediated by microorganisms under anoxic/hypoxic conditions

Dissolved organic matter (DOM) in the ocean is one of the largest carbon pools on Earth. Microbial metabolism is an important process that shapes the marine DOM pool. Current studies on the interactions between microorganisms and DOM focus mainly on oxic environments. Few studies have addressed the molecular characteristics of DOM in microbial-mediated transformation under anoxic/hypoxic conditions. As a result of deteriorating water quality due to eutrophication and global warming, anoxia occurs frequently in coastal waters. In this study, we performed an experiment to investigate changes in microbial community responses and the molecular characteristics of DOM in microbial-mediated transformation under hypoxic conditions. We compared microbial-mediated DOM transformation at different dissolved oxygen levels (7, 5, and 2 mg L (-1)) and in different media (natural and artificial seawater with and without laminarin). We also investigated differences in DOM composition between groups using spectroscopic analysis and ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry. The results showed decreased microbial metabolic activity and delayed community succession at low oxygen (<= 2 mg L (-1)) in natural seawater supplemented with laminarin. The growth of strictly aerobic bacteria such as Pseudomonadaceae and Sphingomonadaceae was inhibited and the total organic carbon utilization rate was reduced by 36.9-6.7% from 4 to 32 days. Moreover, tyrosine-like and tryptophan-like components were preserved, while DOM humification and modified aromaticity index were significantly reduced under low oxygen conditions. This experiment provides justification for further study of the processes and mechanisms of improved labile DOM preservation in anoxic estuarine and coastal waters.

Automated summary

Dissolved organic matter (DOM) in the ocean is a significant carbon pool, and microbial metabolism plays a crucial role in shaping the marine DOM pool. However, the molecular characteristics of DOM under anoxic/hypoxic conditions are not well-studied. This study investigated the changes in microbial community responses and molecular characteristics of DOM during microbial-mediated transformation under hypoxic conditions. The results showed decreased microbial activity, inhibited growth of aerobic bacteria, and changes in DOM composition under low oxygen conditions.