Microbial processing of organic matter in cold seep sediments revealed by amino compound distributions and...

Cold seeps are oases of life on the seafloor and play a crucial role in carbon and nitrogen cycling. This highlights the need to understand the level, composition, and degradation of organic matter (OM) in these environments. In this study, amino acids (AAs) and amino sugars (ASs) were employed as biomarkers to investigate the OM characteristics in three sediment cores from the Haima cold seep in the South China Sea. The results showed that AA concentrations (1.31 ± 0.64 μmol/g) were significantly higher than those of ASs (0.27 ± 0.09 μmol/g). AAs were dominated by leucine, phenylalanine (Phe), and isoleucine, while ASs were mainly composed of glucosamine (GlcN) and galactosamine. The AAs-based degradation index averaged approximately -0.3, indicating that OM in cold seep sediments was highly degraded. Compared to methane-poor sediments, methane-rich sediments exhibited lower AA concentrations, suggesting that microbial processes, such as anaerobic oxidation of methane (AOM), may enhance OM degradation. This inference is further supported by the reduced Phe proportion, decreased muramic acid (MurA) concentrations, lower bacterial-derived carbon and nitrogen contributions (BDC-C%, BDN-N%), and elevated GlcN/MurA ratios. Moreover, the pronounced depletion of AAs in the methanogenic zone likely reflects their role as key fermentable substrates. Notably, fungal-derived carbon (FDC) contributions to TOC in seepage area exceeded those of BDC-C%, underscoring the underestimated role of fungi in microbial carbon pools at cold seeps. Overall, this study provides molecular-level insights into the fate of organic carbon and nitrogen in methane-rich environments and highlights the microbial mediation of deep-sea carbon reservoir stability.