Bacterioplankton dark CO2 fixation in oligotrophic waters.

Dark CO 2 fixation by bacteria is believed to be particularly important in oligotrophic ecosystems. However, only a few studies have characterized the role of bacterial dissolved inorganic carbon (DIC) fixation in global carbon dynamics. Therefore, this study quantified the primary production (PP), total bacteria dark CO 2 fixation (TB DIC fixation), and heterotrophic bacterial production (HBP) in the warm and oligotrophic Red Sea using stable-isotope labeling and cavity ring-down spectroscopy (13 C–CRDS). Additionally, we assessed the contribution of bacterial DIC fixation (TB DIC %) relative to the total DIC fixation (total DIC fixation). Our study demonstrated that TB DIC fixation increased the total DIC fixation from 2.03 to 60.45 µ g C L -1 d -1 within the photic zone, contributing 13.18 % to 71.68 % with an average value of 33.95 ± 0.02 % of the photic layer total DIC fixation. The highest TB DIC fixation values were measured at the surface and deep (400 m) water with an average value of 5.23 ± 0.45 and 4.95 ± 1.33 µ g C L -1 d -1 , respectively. These findings suggest that the non-photosynthetic processes such as anaplerotic DIC reactions and chemoautotrophic CO 2 fixation extended to the entire oxygenated water column. On the other hand, the percent of TB DIC contribution to total DIC fixation increased as primary production decreased (R2=0.45 , p<0.0001), suggesting the relevance of increased dark DIC fixation when photosynthetic production was low or absent, as observed in other systems. Therefore, when estimating the total carbon dioxide production in the ocean, dark DIC fixation must also be accounted for as a crucial component of the carbon dioxide flux in addition to photosynthesis. [ABSTRACT FROM AUTHOR]