Vertical distribution pattern of cbb genes and bacterial CO₂ fixing potential in the South China sea

In this study, the cbb genes which encode the key enzymes in the Calvin cycle were used as functional markers to investigate the distribution pattern of CO2 fixing potential of autotrophic bacteria at different depths of the South China Sea (SCS). The results revealed that cbb gene abundance was similar between the surface and the deep layer of SCS, over two fold higher than the middle layer. The types of cbb genes showed dramatically different distributions with depths. Metagenomic amplicon sequencing revealed that Synechococcus was the main genus containing cbb genes in the surface layer, along with Rubrivivax and Limnohabitans in the middle, and unclassified Epsilonproteobacteria in the deep. Furthermore, the surface layer had the highest CO2 fixing potential due to that Synechococcus had a high CO2 fixation capacity and relatively high autotrophic growth rate. The difference in the distribution of bacterial species and therefore cbb types with depths was potentially affected by the variation in light intensity and concentration of dissolved oxygen, inorganic carbon and reducing substances.