24 Apr 2023
 | 24 Apr 2023
Status: this preprint is currently under review for the journal BG.

Bacterioplankton dark CO2 fixation in oligotrophic waters

Afrah Alothman, Daffne López-Sandoval, Carlos M. Duarte, and Susana Agustí

Abstract. 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 (TBDIC 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 (TBDIC %) relative to the total DIC fixation (TotalDIC fixation). Our study demonstrated that TBDIC fixation increased the TotalDIC fixation from 2.03 to 60.45 µg 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 TotalDIC fixation. The highest TBDIC fixation values were measured at the surface and deep (400 m) water with an average value of 5.23 ± 0.45 µg µg C L−1 d−1, and 4.95 ± 1.33 µg C L−1 d−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 TBDIC contribution to TotalDIC 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 as a crucial component of the carbon dioxide flux in addition to photosynthesis.

Afrah Alothman et al.

Status: open (until 05 Jun 2023)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on bg-2023-65', Federico Baltar, 26 Apr 2023 reply
    • AC1: 'Reply on RC1', Afrah Alothman, 22 May 2023 reply
      • RC3: 'Reply on AC1', Federico Baltar, 22 May 2023 reply
  • CC1: 'Review on bg-2023-65', Marc Llirós Dupré, 27 Apr 2023 reply
    • AC3: 'Reply on CC1', Afrah Alothman, 22 May 2023 reply
  • RC2: 'RC- Comment on bg-2023-65', Marc Llirós Dupré, 28 Apr 2023 reply
    • AC2: 'Reply on RC2', Afrah Alothman, 22 May 2023 reply
      • RC4: 'Reply on AC2', Marc Llirós Dupré, 23 May 2023 reply

Afrah Alothman et al.

Afrah Alothman et al.


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Short summary
The study validated the significant involvement of bacteria in CO2 fixation throughout the entire oxygenated water column of the oligotrophic Red Sea. Our study showed that the importance of dark CO2 fixation influencing the carbon flux when contributing to the large fraction of total CO2 fixation in case of low or absent primary production. Therefore, dark anaplerotic DIC fixation and chemo-autotrophic processes must be accounted as a crucial component of the CO2 flux in the oceans.