Reply on AC2

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> Dark CO₂ 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 CO₂ 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 (¹³C-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 &micro;g C L^&minus;1 d^&minus;1 within the photic zone, contributing 13.18 % to 71.68 % with an average value of 33.95 &plusmn; 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 &plusmn; 0.45 &micro;g &micro;g C L^&minus;1 d^&minus;1, and 4.95 &plusmn; 1.33 &micro;g C L^&minus;1 d^&minus;1, respectively. These findings suggest that the non-photosynthetic processes such as anaplerotic DIC reactions and chemo-autotrophic CO₂ fixation extended to the entire oxygenated water column. On the other hand, the % of TB_DIC contribution to Total_DIC fixation increased as primary production decreased (R² = 0.45, p &lt;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 as a crucial component of the carbon dioxide flux in addition to photosynthesis.