Due to the high productivity of cold seep ecosystems, sediments often exhibit significant enrichment of organic compounds, particularly biomacromolecules. Notably, refractory biomacromolecules like chitin can serve as a valuable resource for the local microbial community within cold seep sediments. However, our understanding of the roles of these biomacromolecules and the associated organotrophs in geochemical cycling within cold seeps remains limited. In this study, we aimed to investigate the metabolism of biomacromolecules by key participants involved in geochemical cycles. To achieve this, we analyzed 225 nonredundant metagenome assembled genomes (MAGs) obtained from Haima cold seep sediments. Our findings revealed distinct niche preferences among taxa involved in sulfur and nitrogen cycling, with a preference for active or extinct seep habitats. Additionally, many of these taxa were found to possess genes enabling the utilization of hydrocarbons and various biomacromolecules, such as proteins, nucleic acids, and polysaccharides. These observations strongly suggest the potential contribution of organotrophs to the geochemical cycling processes within cold seeps. Of particular interest, we identified a prevalent MAG in both active and extinct seep habitats that exhibited genes associated with chitin degradation, dissimilatory nitrate reduction to ammonia (DNRA), and nitrogen fixation. This suggests its potential role in coupling carbon and nitrogen cycling through the utilization of chitin. Further enzymatic analysis indicated a slightly increased activity of the encoded chitinase under high pressure (15 MPa) and low temperature (4 ℃) conditions, indicating the adaptation of chitinase to the deep-sea environment. Collectively, our work shed light on the contribution of biomacromolecules in geochemical cycling of carbon, nitrogen, and sulfur in deep-sea environment, proposing chitin as a long-term carbon and ammonia resource in cold seeps.
 

Biomacromolecule,Geochemical cycling,Chitin,Chitinase