Microbial degradation of DOC released by the mixed culture of Crassostrea gigas and Undaria pinnatifida

To determine the potential of shellfish-algae aquaculture on carbon sequestration, we studied the microbial degradation process of dissolved organic carbon (DOC) released in a mixed culture of Crassostrea gigas and Undaria pinnatifida . The results showed that DOC concentration tended to rapidly decline followed by a slow decline period and then a steady phase with 28% concentration of DOC remaining over a period of 120 days after removing the Crassostrea gigas and Undaria pinnatifida . Bacterial community structure was dynamic over time and changed in response to three stages of DOC degradation. In the three stages of DOC degradation, the dominant microbial classes shifted from Alphaproteobacteria (39%) and Gammaproteobacteria (43%) in the rapid decline period to Planctomycetia (28%) and Flavobacteria (19%) in the slow decline period to Alphaproteobacteria (29%) in the stable period. The amino acid metabolism and carbohydrate metabolism were observed to be highly active in the rapid DOC decline and slow DOC decline periods, respectively. The carbon-fixing genes smtA and rbcL were most abundant during the DOC rapid degradation stage. The DOC concentration was significantly negatively correlated with Planctomycetia, Anaerolineae (p < 0.05) and the tricarboxylic acid cycle (TCA cycle) (p < 0.001). These results suggested that the main carbon fixation pathways TCA cycle and its related carbon-fixing genes may be involved in the degradation of DOC released by the mixed culture of shellfish-algae.