Amino acids and amino sugars as indicators of the source and degradation state of sedimentary organic matter

Abstract

Amino acids (AAs) and amino sugars (ASs), compounds ubiquitous to living organisms, can provide information regarding degradation states and bacterial contributions to sedimentary organic matter (SOM). However, very few studies have utilized the combination of AAs and ASs as a proxy to assess the influence of bacterial degradation on SOM in the Yellow Sea (YS). In this study, surface sediments from the YS were analyzed for AAs, ASs, total organic carbon (TOC), total nitrogen (TN), stable carbon isotope (δ13C), and grain size. The concentrations of AAs and ASs ranged from 3.29 to 26.65 μmol g−1 and from 0.50 to 9.11 μmol g−1, respectively, and together contributed 13.1% to TOC and 27.5% to TN. The concentrations of AAs and ASs were both positively correlated with the relative abundance of clay (<4 μm) (R2 = 0.73 and 0.79, respectively, p < 0.01), suggesting that hydrodynamic sorting influenced the distribution of SOM in the study area. Comparisons of the degradation indices (TOC normalized yields of AAs and ratio of AAs to ASs) indicated that the SOM in the South Yellow Sea (SYS) was more degraded than that in the North Yellow Sea (NYS). Variations in the glycine/serine and glucosamine/galactosamine ratios reflected a transition from fresh phytoplankton sourced organic matter (OM) to bacterial OM. Based on the yields of muramic acid (Mur), the bacterial OM contributed 17 ± 13% and 26 ± 19% to TOC and TN, respectively. High ratios of glucosamine to Mur (GlcN/Mur) indicated that the fraction of bacterial debris in the SYS was larger than that in the NYS. This study demonstrated the importance of bacteria in regulating the source and fate of SOM in the YS.