the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 12 Nov 2025
Influences of CO2 and Fungus-Assisted Bioweathering on Fluoridated Apatite
Abstract. Fluoridated apatite (FAP) is the dominant P source for ecosystems. However, the bioweathering of FAP is still not fully elucidated. In this study, phosphate-solubilizing fungus (Aspergillus niger) was firstly incubated in soil to examine the weathering of FAP, on both cross and longitudinal sections. It showed that the fungus induced more pronounced erosion channels on the cross sections in a P-deficient soil. We therefore aimed to disentangle the relative contributions of biological (phosphate-solubilizing fungus) and abiotic factors (CO2 and crystal face orientation) to the observed weathering contrasts. To further investigate the weathering contrasts on different sections of FAP, incubation was conducted in a culture medium. Fungal colonization on the cross section of FAP resulted in deeper P depletion zones and enriched secondary minerals (primarily calcium oxalate) than those on the longitudinal section. Additionally, elevated CO2 (10 %) significantly accelerated the weathering of FAP on the cross section, which was confirmed by its enhanced surface roughness, further promoted fungal colonization and subsequent bioweathering for FAP. Synergistic interactions between fungi and elevated CO2 accelerate phosphate mineral weathering, providing a new insight on P cycling in soil microenvironments such as the rhizosphere.
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Status: open (until 09 Jan 2026)
- RC1: 'Comments on egusphere-2025-5242', Anonymous Referee #1, 08 Dec 2025 reply
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- 1
General Comments
Introduction is well structured. More detail should be added however on the selection of the Aspergillus strain. For example, environments found in and function, use in other rock weathering studies, use in industry (strains used for citrate production). This is important justification which grounds the theory present in the introduction to the specifics of this study and the use of Aspergillus.
There are some methodological issues (for example, lack of control treatments and number of replicates) that minimise the power of the analyses and conclusions.
The discussion requires significant work to bolster conclusions by referencing to relevant literature, appropriately calling out results and adjusting the structure for readability. Numerous unsupported claims are made that require support from the literature.
Specific Comments
Lines 80 -84: two sentences discuss results presented in supplementary information in introduction. Understandable as to why, however results/context from other work should be presented here, discussion of results should be saved for discussion.
Two replicates per treatment were used – the statistical power of this should be noted in the statistics section. More replicates would be preferred or the adoption of a gradient method (e.g. a range of CO2 concentrations).
Section 2.3 details the soil experiments. This is difficult to follow, and would benefit from stating the number of bottles used, consistent use of ‘samples’ or ‘glass sheets’, and explanation of why all FAP slides were not removed after 30 days (only 2 what about the others?).
A potentially significant issue is the lack of controls, it is not clear that incubations were performed without Aspergillus inoculation in the soil experiments. For example, in the soil incubation the role of abiotic soil factors on weathering was not accounted for (or appear not to be). The soil was acidic, which could result in weathering.
How were cell concentrations determined? Not presently stated.
Soil sterilisation requires detail. Sterilisation at 121 C in a dry sterilisation mode? Using what instrument (autoclave, oven…)? Heat treatment can alter organics structure; did it affect organic P mobilisation from the soil? Gamma sterilisation would be preferable, why was it not used?
The addition of a table of treatments in the methods would be a useful look up tool when reading the discussion.
Line 176: How was roughness gauged. Not enough to say they were compared. How were they compared? Visual assessment is not sufficient, it is a qualitative method. Number and size of pits/etches/deposits is an example of data that could be used. Covered later, to some extent in section 3.3, but more explicit description on how roughness was quantified in methods is required.
The discussion begins with reference to supplementary figures - given importance to the discussion they should be included in the main body of the manuscript.
Lines 234-242: cause of reductions in structural stability of mineral crystals, with regards to ‘screw dislocation’, not discussed with reference to LFAP and CFAP.
Lines 238-239 require referencing of the relationship of screw dislocation to reduced stability. Further discussion on how screw dislocations and how they result in reduced stability also needed.
Paragraph 2 discussion should be split into two at “fungal hyphae can accelerate...” because it deals with different weathering concepts/causes than start of paragraph.
Line 257: Aspergillus niger’s ability to accelerate bioweathering through fungal growth requires referencing, specifically with reference to Aspergillus niger.
Line 260: the link between screw dislocations and increased susceptibility to bio weathering is inferred not explicitly tested. It should be stated that it ‘may’ have resulted in increased susceptibility.
Line 262: Potential for fluorine toxicity introduced, but not relevant to the rest of paragraph and not fully discussed (how was it determined, why is it relevant…)
Line 264: reference to literature. How does carbonic acid increase weathering?
Lines 269-271: conclusion that “PSF would survive and perform its solubilizing ability under elevated CO2” does not follow from results. It can or may survive. Further, that “that elevated CO₂ not only accelerates chemical dissolution but also sustains fungal colonization and activity on mineral surfaces” also doesn’t follow from the results. The results do not show fungal colonisation was sustained by elevated CO2 rather were not inhibited. Increases in weathering markers between CO2 and fungi+CO2 treatments necessary to support these conclusions. If they are present, explicitly detail them.
Line 272: statement requires referencing.
Line 273: statement requires referencing.
Line 276-278: statement requires referencing.
Line 280-281: statement requires referencing.
Line 283-285: traditional view statement requires referencing.
Technical Corrections
Lines 69-71: sentence requires reference(s).
Line 185: forming should be formed.