Bacterioplankton dark CO2 fixation in oligotrophic waters
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)
RC1: 'Comment on bg-2023-65', Federico Baltar, 26 Apr 2023
AC1: 'Reply on RC1', Afrah Alothman, 22 May 2023
- RC3: 'Reply on AC1', Federico Baltar, 22 May 2023 reply
- AC1: 'Reply on RC1', Afrah Alothman, 22 May 2023 reply
CC1: 'Review on bg-2023-65', Marc Llirós Dupré, 27 Apr 2023
- AC3: 'Reply on CC1', Afrah Alothman, 22 May 2023 reply
RC2: 'RC- Comment on bg-2023-65', Marc Llirós Dupré, 28 Apr 2023
AC2: 'Reply on RC2', Afrah Alothman, 22 May 2023
- RC4: 'Reply on AC2', Marc Llirós Dupré, 23 May 2023 reply
- AC2: 'Reply on RC2', Afrah Alothman, 22 May 2023 reply
Afrah Alothman et al.
Afrah Alothman et al.
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Revision of “Bacterioplankton dark CO2 fixation in oligotrophic waters” by Alothman et al.
This is an exciting paper dealing with a very topical issue, which is the contribution of dark CO2 fixation to primary production in the ocean, in particular in oligotrophic waters. Overall the manuscript is well structured and the research strategy is sound, so it should be a valuable contribution to the readerships of Biogeosciences. I have included some detailed comments below that I would like the authors to address — some are trivial, some probably arise from my own misunderstandings, and some should be addressed before publication.
Specific comments (in order of appearance)
l.62-65. I would mention briefly in that sentence what is the range of contribution suggested in that cited study.
l.85-95. I would already cite here Table 1.
Table 1. In the caption it is mentione that N/A is used for not available data, but there are still empty cells with nothing written in the Table.
Table 2. I could not follow the depth description. The 4th column shows the “Depth (m)”, and then the 9th column the “Mean Depth (m)”, but they do not seem to match. E.g. the Deep Cruise had a Depth of 5m but a Mean Depth of 12 m. Please revise the depths of all the cruises in this table. Also, the last row seems to be misplaced. And no need to add decimals in the Mean Depth numbers.
Figure 3. Also no need of 2 decimals in the depth numbers (y-axis).
Figure 4. Consider adding units to the X- and Y-axis.
Figure 5. In the y-axis there is the TBDIC (TBDIC %) which should have % as units, but there are other units (of concentration) included as well. Please revise.
Figure 4 and 5. I am not an expert on the calculations of isotopic signatures, but please double check that there is not autocorrelation in Figure 5 and in some of the relations (the ones derived from isotopic signatures) in Figure 4. What I mean is to check for example that the relation described in Fig.5A is not just an artifact because the parameter in X-axis or in the Y-axis were calculated including some common variable/s. I checked in the methods section how the TBDIC % and the PP were calculated, and it looks like there might be some common variables, but cannot say for certain. I would double-check all the correlations for autocorrelation, and if there is not autocorrelation I would include a sentence somewhere in the text saying/explaining this point.
l.326-328. I would rephrase this sentence” quantitative assessments of dark heterotrophic bacteria CO2 fixation remain few”. I might be mistaken with saying that the contribution of CO2 fixation is low, when I think what you refer to is that what is low is the number of studies on this topic.
l.427. Instead of Prof. Carlos better write also de surname.
I would like to stress that this is a very good paper, and will be better once the wrinkles are ironed out.