As an important component of marine organic carbon pools and organic nitrogen pools, amino acids can effectively indicate the degradation states and diagenetic conditions of marine organic matter, and thus have important implications for the scientific evaluation of organic matter migration, transformation, and burial in the ocean. Firstly, the content, composition, configuration, and distribution characteristics of amino acids in particulate matters and surface sediments of Jiaozhou Bay were studied. Combined with carbon-nitrogen ratio and carbon stable isotopes, the degradation indicators, such as carbon and nitrogen normalized yield of amino acid, degradation index, reactivity index, the mole percent of D-amino acid, and mole percent of non-protein amino acid, were used to analyzed the source and degradation degree of organic matter in particulates and sediments of Jiaozhou Bay. Meanwhile, bacterial contributions, bacterial abundance, community structure of bacteria, and extracellular enzyme activity of peptidase were used to analyze the bacterial degradation and bacterial contribution to marine organic matters. The main results are as follows:
1. The degradation indicators based on amino acid content and composition all congruously indicated the degradation statues of organic matters in Jiaozhou Bay. In autumn, the degradation degree of particulate organic matter in Jiaozhou Bay showed that the degradation states in the outer bay were higher than those in the inner bay and the degradation states in the eastern inner bay were higher than those in the western inner bay. While in spring, it was characterized by higher degree of organic matter degradation in the western inner bay and the eastern outer bay. The sources of particulate organic matters were the key factors controlling the organic matter degradation states. The contribution of marine organic matter caused fresher particulate organic matter, at the same time, it promoted the proliferation of bacteria and results in a higher bacterial contribution to organic matter. While the higher contribution of terrestrial organic matter led to a higher degradation degree of organic matters, and conduce to a higher activity of extracellular peptidases to decompose organic matter producing by bacteria. In addition, the changes in bacterial community structure in the particles also change the content and composition of organic matter, Gammaproteobacteria played an important role in the degradation of organic matter.
The content of particulate amino acids in seawater of bacteria in autumn was 1.15 ± 0.87μmol/L. Due to the combined effect of the higher phytoplankton abundance in the inner bay and the higher input of terrigenous organic matter in the eastern inner bay, the concentrations of particulate amino acids showed the tendency of higher values in the surface water of inner bay of Jiaozhou Bay, and the contents in the western area was higher than the western inner bay. The indicators of organic matter degradation, such as carbon and nitrogen normalized yield, degradation index (DI), reactivity index (RI), and mol% D-amino acid all had the similar trend, indicating that the degradation statue in the outer bay was higher than the inner bay, and the particulate organic matters central and northeastern area had the higher degradation states because of higher terrigenous organic matter input. The distributions of concentration of D-AA, bacterial abundance, and bacterial contribution to organic carbon and organic nitrogen all reflected that bacteria were closely related to the organic matter degradation, and bacterial played an important role in the particulate organic matter of Jiaozhou Bay. Fresh organic matter can promote the growth and reproduction of bacteria, thereby promoting the degradation and transformation of organic matter by bacteria. In addition, the biological activity of organic matter can affect the activity of extracellular peptidase from bacteria, and old organic matter can induce the release of extracellular peptidase having high activity. The dominant taxa in Jiaozhou Bay were Alphaproteobacteria, Gammaproteobacteria, Cyanobacteria, and Bacteroidetes. Cyanobacteria are photosynthetic and autotrophic bacteria, and their higher relative proportion can lead to a relative decrease in D-AA. Furthermore, the Gammaproteobacteria had a close relationship with organic matter, and their influence mechanism needs further study.
The particulate amino acids contents of Jiaozhou Bay in spring was 0.98 ± 0.32 μmol/L, which was lower than in autumn. In the surface water, the particulate amino acids had higher contents in the eastern inner bay where organic matter was relatively fresher. The western inner bay had lower concentrations of amino acids, but the degradation states of particulate organic matter were higher, and the contents of particulate amino acids in the western outer bay were lower but fresher, and the amino acids had lower contents and were older in the eastern outer bay. The bacterial contribution to the particulate organic carbon in Jiaozhou Bay in spring was 17.51% ± 8.09%, and 10.25% ± 5.96% to organic nitrogen. The tendency showed the higher value in the outer bay than the inner bay, in which eastern inner bay had higher values. Like autumn, the dominant taxa of bacterial in spring were Proteobacteria, Cyanobacteria, and Bacteroidetes, but the proportion of Cyanobacteria decreased significantly in spring, only accounting for about half of the proportion in autumn. Through the comparison of community structure and concentration of organic matter, it was found that Gammaproteobacteria played an important role in the degradation of particulate organic matter, the high dominance of that taxon was not conducive to the preservation of particulate organic matter. Because photosynthetic autotrophic bacteria organic matter does not contain D-Asx and D-Ser, the proportion of Cyanobacteria has a certain influence on the proportion of D-amino acids.
2. The degradation states of organic matter in the surface sediments were consistent with those of particulate matter of Jiaozhou Bay in the autumn, it also showed that the degradation states of organic matter in the outer bay were higher than those in the inner bay, and the degradation states in the eastern inner bay were higher than those in the western inner bay. The sources of organic matter, microbial activity and depth of overlying water affected the degradation of organic matter. The input of fresh organic matter in the western inner bay was more conducive to the synthesis of bacterial organic matter, which results in higher bacterial contributions to organic matter. The contribution of higher terrestrial organic matter in the eastern inner bay determines the higher degradation states of organic matter and extracellular enzyme activity of peptidase. The deeper water depth in the outer bay caused deeper degradation of the particulate matters before it settled into the sediments, resulting in relatively higher degradation statues of organic matters in the sediments than particulate matters.
The contents of total hydrolyzable amino acids in the surface sediments of Jiaozhou Bay was 7.60 ± 3.64 μmol/g. The horizontal distribution showed that the concentrations were higher in the outer bay than the inner bay, and the concentrations the eastern inner bay were higher than the western inner bay. The input of fresh organic matter, high microbial activity, and high contribution of terrestrial organic matter in the eastern inner bay, and deep depth of water in the outer bay were all important factors resulting in the diversity of degradation states in sediments of Jiaozhou Bay. The indicators of degradation states of organic matter reflected the degradation degrees showed that the organic matters were older in the outer bay, and in the western inner bay organic matters were fresher than the eastern inner bay. Besides, the degradation degrees in sediments were higher than particulate matters, indicating that the organic matter in the sediments had deeper degradation by microorganisms.
The bacterial contributions in the sediments of Jiaozhou Bay presented that the contributions were higher in the western inner bay than the eastern inner bay and outer bay. In addition, the activity of extracellular peptidase was closely related to the degradation degree of organic matter in sediments. Different sources of organic matter have different reactivity, resulting in different contributions of bacterial organic matter and extracellular peptidase activity, the fresher organic matter in the sediments had the higher bacterial contribution but inhibited the expression of extracellular peptidase. Bacteria were involved in the biodegradation process of organic matter in sediments, the D-AA produced by bacteria played an important role in determining the statue of degradation, and the bacterial organic matter had important contributions to organic matter. Therefore, bacteria are the significant regulatory factors in the migration and transformation of organic matter in sediments.