High dark carbon fixation in the tropical South China Sea

Dark carbon fixation (DCF) has been usually assumed to be insignificant in the study of microbial production and carbon cycling. In order to evaluate DCF distribution and its contribution to biogenic carbon flux, surface and vertical distribution of DCF, primary production (PP) and bacterial production (BP) were investigated in both offshore and coastal waters in the South China Sea (SCS). Surface DCF was ~ 0.058 μg C L−1 h−1 (~ 10% of PP), within the same order of magnitude as BP of ~ 0.047 μg C L−1 h−1 in the offshore waters. Integrated over the 1500 m water column, DCF was ~ 196 mg C m−2 h−1, corresponding to ~ 384% of PP, and represented a newly produced source of organic matter. This suggested that DCF was an important microbial metabolic pathway in the SCS, which might support ~ 83% carbon demand of heterotrophic prokaryotes. Interestingly, the DCF was higher in the deep water (~ 0.140 μg C L−1 h−1) than the surface water (~ 0.089 μg C L−1 h−1). In addition to the different microbial community, this different vertical distribution of DCF was likely due to the nutrient status, as our nutrient enrichment experiment showed that the addition of glucose, ammonium and phosphate stimulated the DCF rates, especially the addition of glucose plus ammonium.