Microbial alteration in marine sediments: Insights from compound-specific isotopic compositions of amino acids in subseafloor environments

Abstract

The proportion of amino acids (AAs) in sediment can be used as an indicator of microbial degradation, which is primarily the product of benthic prokaryote activity. The microbial activity would be reflected with the stable isotope ratio of nitrogen (delta N-15) at the time of mineralization and resynthesis of AAs. In this study, the compound-specific isotope analysis of individual AAs was used to investigate delta N-15 variation associated with microbial processes in marine sediment samples. Our results showed a decrease in AA concentrations in core-top sediment was accompanied by an increase in delta N-15 values, suggesting large N-15 enrichment in buried AAs. Phenylalanine displayed an increase in delta N-15 from the surface to depths greater than 2 cm, whereas relatively constant delta N-15 values at depths below 2 cm, suggesting that microbial utilization of phenylalanine varies with depth. Glycine showed the highest relative molar contribution (from 12.1 to 36.4%), with the largest delta N-15 increase (from 0.8 to 8.7) in deep sediment, implying that such information can serve as a measure of AA diagenesis in sedimentary environments. Our results also indicated that the delta N-15 values of individual AAs in sediment reflected the microbial alteration of organic matter at water-sediment interfaces and in sub-surface environments. These findings form an important basis for interpreting the delta N-15 values of AAs in sediment.