The Bioavailability Of Typical Marine Dissolved Organic Matter D-amino Acids

The ocean contains a huge dissolved organic carbon(DOC)pool,which can compare the amount of carbon in the atmosphere CO2.However,more than 95%of ocean DOC is refractory DOC(RDOC)that resists microbial degradation and is stored in the ocean for a long time.The average age of RDOC can reach 4000-5000 years.So far,the origins and mechanisms of RDOC are not clear.According to the the theoretical framework of the Microbial Carbon Pump(MCP)proposed by Professor Nianzhi Jiao,microorganisms are the main contributors to the marine RDOC.However,the composiltion of the RDOC and the reasons for its refractory still need to be further studied.All amino acids,except for glycine,Amino acids are chiral compounds that can be divided into D-amino acid(D-AA)and L-amino acids(L-AA)according to their optical characteristics.Biochemical studies have shown that L-AA is an important basic component of protein.Although not involved in protein synthesis,D-AA has many essential functions in microbial physiology regulation and microbial ecology.D-AA can be classified into canonical D-AA and non-canonical D-AA depending on whether or not it participates in the synthetic of peptidoglycan.D-AA is a major constituent of ocean DOC pool.Many marine microorganisms can produce D-AA.According to the distribution and content of D-AA in the ocean,it is speculated that about a quarter of DOC comes from microbial meta'bolic activities.Nevertheless,there are few reports on the bioavailability of D-AA by marine microorganisms.How marine environmental microorganisms respond to the addition of exogenous D-AA and how microbial communities metabolize different types of D-AA is not clear.The study on bioavailability of D-AA is essential for understanding the metabolism and the fate of D-AA in the ocean.Based on the above,peptidoglycan component D-Alanine(Ala),D-Glutamic acid(Glu)and non-canonical D-amino acids D-Methionine(Met),D-Valine(Val)as well as their L-types are added separately to the natural seawater from three different environmental(A9:5 meters of estuary;SEATS surface:5 meters of oligotrophic open sea;SEATS deep:2000 meters of oligotrophic open sea)in the South China Sea for long periods(745 days)of culture.Combined with microbial abundance,amino acid concentration and microbial community structure,the bioavailability of D-AA by marine microbes and the major microorganisms that responses to D-AA were studied.The main experimental results are as follows:The utilization of amino acids by microorganisms im each environment was difference.Compared to SEATS station(oligotrophic open sea),the A9 station(estuary)microorganisms show a relatively rapid utilization capability for various amino acids.In A9 and SEATS surface station,microorganisms showed a relatively rapid response within 7 days.Microbial abundance in A9 station increased 4 to 5 times,and in SEATS surface station,it increased 2 times.However,in SEATS deep station,experimental groups,besides L-Glu group,did not respond until 15 days later.Among the four types of D-AA,D-Ala is the most easily utilized,while D-Met is difficult to use.There was no obvious difference in the utilization of Ala and Glu by microorganisms in their two configurations,but the utilization of D-Met and D-Val was significantly lower than their corresponding L-form,especially in SEATS station.After 745 days culture,all the amino acids at A9 Station were fully utilized,while the relative consumption ratio of D-Met and D-Val in SEATS station was lower than their corresponding L-form,indicating that D-Met and D-Val were relatively recalcitrant than the L-form.The succession of microbial community during culture showed that the microorganisms of A9 station had a rapid response to the addition of different amino acids.In SEATS surface station,the microbial community composition changes rapidly in the D-Glu.D-Val and L-Met-added groups,and there is significant difference in the response between the D-type and L-type of Met and Val.In the SEATS deep Station,the composition of the microbial community changes rapidly within the first 4 days after the addition of D-AA,while the response to L-AA is generally after the 8th day.Through comparison,potential groups that can utilize D-AA were found,which mainly distributed in 8 families.A9 Station includes Oceanospirillaceae,Hyphomonadaceae,Hyphomicrobiaceae,Unclassified Gammaproteobacteria;At SEATS surface station includes Rhodobacteraceae,Oceanospirillaceae,Phyllobacteriaceae;SEATS deep station includes Oceanospirillac eae,Pseudomonadaceae,Pseudoalteromonadaceae.In this study,we compared and analyzed the responses and community structure succession after the addition of D-AA by different environmental microorganisms,which provided clues and evidence for explaining the transformation and metabolism of D-AA by in situ microorganisms.