Biogenic calcium carbonate as evidence for life
Abstract. The history of Earth is a story of co-evolution of minerals and microbes: not only numerous rocks arisen from life, but the life itself may have formed from rocks. To understand the strong association between microbes and inorganic substrates, we investigated the moonmilk, a speleothem of calcium carbonate of microbial origin, present in the Iron Age Etruscan Necropolis of Tarquinia, in Italy. These tombs present a unique environment where the hypogeal walls of the tombs are covered by this speleothem. To study moonmilk formation, we investigated the bacterial community in the rock in which the tombs are carved: calcarenite and hybrid sandstone. We present the first evidence that moonmilk precipitation is driven by microbes within the rocks and not only on the rock surfaces. We also describe how the moonmilk produced within the rocks contributes to rock formation and evolution. The microbial communities of the calcarenite and hybrid sandstone displayed, at phylum level, the same microbial pattern of the moonmilk sampled from the walls of the hypogeal tombs, pointing out that the moonmilk originates from the metabolism of endolytic bacterial community. The calcite speleothem moonmilk is the only known carbonate speleothem on Earth with undoubted biogenic origin, thus representing a robust and credible biosignature of life. Its presence in the inner parts of rocks adds to its characteristics as a biosignature.
Sara Ronca et al.
Status: final response (author comments only)
RC1: 'Comment on egusphere-2022-1457', Anonymous Referee #1, 12 Feb 2023
- AC1: 'Reply on RC1', Teresa Rinaldi, 09 Apr 2023
RC2: 'Comment on egusphere-2022-1457', Anonymous Referee #2, 20 Mar 2023
- AC2: 'Reply on RC2', Teresa Rinaldi, 09 Apr 2023
Sara Ronca et al.
Sara Ronca et al.
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This manuscript has an admirable goal- to determine whether or not certain forms of calcium carbonate constitute a biosignature. This is certainly of interest to the broad geobiologic and astrobiologic community (as outlined in the introduction), and I was intrigued by the sampling site and methods used. However there are some things that should be addressed before publication can proceed. Overall, I am very interested in the results of the paper, but framed such as they are currently, I find them inconclusive. Comments are as follows:
Lines 49-51: The Nutman et al. structures are highly controversial, the subject of great debate, and not at all accepted currently as definitive biogenic ‘stromatolites’. They should not be presented so certainly.
Lines 56-57: are the terms ‘microbialite’ and ‘stromatolite’ used interchangeably? Because the decline in stromatolite form diversity started in the Mesoproterozoic, with a large crash happening prior to the Neoproterozoic-Cambrian boundary, not in the Cambrian period (see papers by Frantz et al 2015; Awramik and Sprinkle, 1999). Also, many of the Archean forms are dubious in their biogenicity (Grotzinger and Knoll, 1999; Mclauglin et al, 2007). This seems to be my main general problem with the manuscript- the assumption is that all carbonates are biogenic instead of acknowledging that these forms are ultimately ambiguous. Certainly, if we are saying that carbonates on Mars mean that there was once life on Mars, the drive is on us to prove beyond a shadow of a doubt that the structures here on Earth are/were all biogenetic. It cannot just be assumed that they are.
Lines 77-78: same problem as above- has it been conclusively proven that most carbonates have biogenic influence? The null hypotheses should be that these structures are abiogenic. Without that, the logic ends up being circular, and it is argued that these structures must be biogenic because carbonate structures are biogenic.
Line 86: The authors rightly acknowledge that direct evidence for biogenicity is lacking. This should be the main point. Many researches assume biogenicity, without direct evidence. The utility of this paper is an attempt to provide a direct link. Reframing the subject around this point would be more impactful.
Lines 93- 95: I am not familiar with the previous work on the subject, but why does containing an active microbial population mean the forms are conclusively biogenic? Microbes may just be living in the structure. Just because I live in a building does not mean I built it (see arguments in Grotzinger and Knoll, 1999, Petryshyn et al. 2021). It is nearly impossible to find a surface on Earth that does not contain an active microbial community.
Line 94: is there any mechanism known by which abiogenic calcite nanofibers are precipitated?
Methods are good, sound, and well-described. I don’t see a 16S plot of results in the figures or supplemental files. This would be interesting to look at.Lines 182-195: It is very interesting that the moonmilk is found within the walls as well. Is it possible that it was deposited there previously, and then covered by a new layer? Or is it possible that endolithic bacteria have burrowed into the wall, causing retrograde neomorphism and creating void space that could later be filled? A scan of the 16S data might reveal the presence of endoliths, and would be helpful in interpretation of the SEM and petrographic analysis.
Line 208-214: This point, and SI Figure 9, are incredibly intriguing. However, with the evidence given, it may just be that the urea in solution fostered the precipitation of the calcium carbonate, as it is known to do, and that this nucleation centered around the colony because the charged surface is a good template for precipitation. Ideally, in order for this to be conclusive, I would like the authors to consider a labeled bicarbonate uptake experiment. If the labeled bicarbonate is incorporated into the calcite crystals, it would conclusively show that microbial metabolism is driving precipitation. Otherwise, this remains intriguing but inconclusive.
Line 226: This should be noted as 16S SSU rRNA analysis, not 16S RNA.
Line 230-235: I agree with the authors, no habitat should be considered ‘extreme’ or a ‘refuge’- microbes like it there!