Review Reports - Floating Rafts from Coastal Hypersaline Environments in Brazil

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The article entitled “Floating rafts from hypersaline environments” is an interesting study. However, a few issues should be addressed before the manuscript can be considered for publication. Therefore, major revision is recommended.

Please elaborate on the characterization techniques used and ensure they are clearly summarized in the abstract.

Throughout the manuscript, chemical formulas (e.g., CaMg(CO₃)₂) should be written in the correct and consistent format.

How was aragonite conclusively distinguished from calcite and vaterite using electron diffraction alone? I strongly suggest the authors that, if samples are available, take xrd and or ftir and confirm the polymorphs and present in the revised manuscript.

The authors must address the evidences that supports the role of organic matter in controlling raft morphology and nanocrystal organization in natural evaporitic environments.

What are the analytical limitations of SEM–EDS in detecting minor phases such as gypsum or halite?

Why are crystal sizes smaller in organic-rich environments compared to those in cave systems?

The overall English language and grammar can be improved throughout the manuscript for better clarity and readability

Author Response

Comment 1: The article entitled “Floating rafts from hypersaline environments” is an interesting study. However, a few issues should be addressed before the manuscript can be considered for publication. Therefore, major revision is recommended.

Please elaborate on the characterization techniques used and ensure they are clearly summarized in the abstract.

Response 1: The techniques “scanning electron microscopy (SEM)”, “energy-dispersive spectroscopy (EDS)” and “electron energy-loss spectroscopy (EELS)” were added to the abstract as suggested, as follows:

“Scanning electron microscopy (SEM) showed that the top surfaces were flat, whereas in the bottom surfaces the grains protrude into the water. … Electron diffraction of two of the rafts indicates that they consist of aragonite. Accordingly, electron energy-loss spectroscopy (EELS) shows the C K-edges characteristic of carbonates, along with O and Ca edges. Energy-dispersive spectroscopy (EDS) in the SEM also revealed a few Ca sulfate crystals on the bottom surface.”

Comment 2: Throughout the manuscript, chemical formulas (e.g., CaMg(CO₃)₂) should be written in the correct and consistent format.

Response 2: We thank the reviewer for this comment. We corrected the chemical formulae throughout the text.

Comment 3: How was aragonite conclusively distinguished from calcite and vaterite using electron diffraction alone? I strongly suggest the authors that, if samples are available, take xrd and or ftir and confirm the polymorphs and present in the revised manuscript.

Response 3: We do not have sufficient amounts of the samples for XRD. We disagree with the reviewer in that SAED alone is not sufficient, since we analyzed several diffraction patterns and several spots in each of them. We identified several spots for aragonite by selected area electron diffraction, including the 100% (which is the {111}), but not for calcite or vaterite (Figure 10). If there were calcite and/or vaterite in the samples, they would be minor phases. In addition, the identification of aragonite was not made with SAED alone. EDS and EELS spectra indicate that Ca carbonate(s) predominate in the samples (figures 2c, 3c, 6c, 11). The presence of Sr peaks in the EDS spectra of the rafts in Figures 3 and 6 also indicate the predominance of aragonite, since Sr fits much better in the aragonite lattice than in calcite due to its larger atomic radius (Borchardt-Ott, W. Crystallography – An introduction. Springer-Verlag Berlin Heidelberg 2011. Pp. 267-268). Accordingly, Sr is commonly found in natural aragonites (Doubleday, Z.A.; Harris, H.H.; Izzo, C.; Gillanders, B.M. Strontium randomly substituting for calcium in fish otolith aragonite. Anal. Chem. 2014 86, 865−869. dx.doi.org/10.1021/ac4034278). In addition to the electron diffraction, EELS and EDS data, morphologies are consistent with aragonite, for example the helical twins of nanocrystals in Figure 3g, the pseudo-hexagonal twins in Figure 7c, and the long prisms in Figures 6d-e and 7a-c. Thus, the set of techniques used were sufficient to identify aragonite as the main mineral in the rafts, and there is no need for the use of XRD and FTIR.

Comment 4: The authors must address the evidences that supports the role of organic matter in controlling raft morphology and nanocrystal organization in natural evaporitic environments.

Response 4: We have no evidence; we proposed a hypothesis to explain the main similarities with rafts from open environments, as well as the differences from cave rafts where most work on carbonate rafts have been done. Caves are notoriously poor in organic matter because of the absence of primary production. On the other hand, the environments where our samples were collected are rich in organic matter because they are eutrophicated (Laut et al., 2017, https://doi.org/10.1371/journal.pone.0184819; Cotovicz Jr. et al., 2021, https://doi.org/10.1007/s10533-021-00842-3). Indeed, there are remains of microbial cells and/or EPS (extracellular polymeric substance) in several SEM images (e.g., Figures 2-3). We improved the discussion on the possible roles of organic molecules in Ca carbonate mineralization, as follows:

“Natural environments contain a wide variety of organic molecules that can interact with inorganic ions and minerals. In some cases, these molecules promote nucleation, whereas in others they act as poisons to mineral growth [54]. Indeed, soluble organic molecules have been shown to regulate the polymorph precipitated, as well as the elemental composition, morphology and size of Ca carbonates [55-58]. Furthermore, Cölfen and Antonietti [52] highlighted the role of organic molecules in nucleation, crystal growth, and their aggregation into mesocrystals, thereby controlling the shape, size and properties of the material.”

Comment 5: What are the analytical limitations of SEM–EDS in detecting minor phases such as gypsum or halite?

Response 5: SEM-EDS show morphologies and elemental composition, but none about the atomic structure. Lagoa Vermelha and the saltwork where samples were collected contains the Ca sulfates gypsum and anhydrite, as well as the Na chloride halite (Moreira et al., 1987; Guedes et al., 2022; Shiraishi et al., 2023). This is why we did name the cubic NaCl as halite, but not the layered Ca sulfates. The text was changed as follows:

“Given the high salinity of the water (110‰) and the occurrence of halite in microbial mats from the saltwork (38), it is likely that halite precipitated shortly after the onset of CaCO3 precipitation. Thus, the saltwork rafts are composed predominantly of CaCO3, with minor amounts of halite and Ca sulfate, although the halite was dissolved during washing with distilled water in the sample preparation process.”

Comment 6: Why are crystal sizes smaller in organic-rich environments compared to those in cave systems?

Response 6: Most of the time, organic matter acts as poisons to mineral nucleation and/or growth. This would explain why seawater is supersaturated with respect to calcite, aragonite, vaterite and dolomite but there is no widespread inorganic Ca-Mg carbonate precipitation – biomineralization is responsible for most CaCO3 production in marine environments (Morse et al., 2007). Some text was added to the text to clarify this point:

Comment 7: The overall English language and grammar can be improved throughout the manuscript for better clarity and readability

Response 7: We used the chatGPT for language improvement.

Reviewer 2 Report

Comments and Suggestions for Authors

The article was prepared based on several selected samples from the two lagoons in Brazil (Lagoa de Araruama and Lagoa Vermelha). The title “Floating rafts from hypersaline environments” suggests that it concerns a broader, synthetic, general study of hypersaline environments worldwide. Hypersaline environments across the world are highly diverse, and therefore, in the reviewer’s opinion, the title should include more specific information about which lagoons are the subject of this study.

The article presents the results of studies, including microscopic (transmitted light microscope, SEM-EDS, and TEM) on selected individual rafts that precipitated on the surface of the brine due of evaporation. These results are well described using appropriate analytical methods.

However, a weakness of the study lies in the discussion of the results, which lacks an explanation of the mechanisms behind the precipitation, particularly of carbonates, in the absence of sulfates and halite. According to the reviewer, supplementing the discussion with an explanation of these processes, including the role of biogeochemical mechanisms and carbonate deposition in saline waters, based on previous studies (e.g., Dupraz et al., 2004, 2009; Cotovicz et al., 2021; Babel & Schreiber, 2014; Shiraishi et al., 2023 b), would significantly enhance the scientific value of the article. Of course, where possible, it would add value to include the chemical composition (major ions) of the brine from which the studied rafts originated. This would allow for the conclusion of their genesis.

The article also requires editorial revisions: the citation style of the article is inconsistent with MDPI's style. Below are several detailed comments referring to selected sections:

Line 51: It would be worth adding trona rafts to this list (see Warren, 2016).

Line 78: The authors state that halite rafts were absent in the described hypersaline coastal environments, but no explanation is provided.

Line 91: “Araruama Lagoon is described as the world's largest hypersaline lagoon”. According to many sources, the largest lagoon is Laguna Madre in Mexico and the US, with a length of about 209 km and an average depth of 1–1.5 m; additionally, Laguna Ojo de Liebre on the Pacific coast of Baja California, Mexico, is larger than Araruama.

Line 119: “Leading to precipitation of halite (NaCl) and other minerals” – which other minerals? This should be specified.

Lines 183–184: “To our knowledge, this is the first time that a Ca carbonate raft is described in hypersaline environments.” The study of the rafts themselves may be novel, but the mechanism of Ca carbonate precipitation in Araruama Lagoon was already presented by Cotovich et al., 2021; for other hypersaline environments, see also:

Dupraz, C., Visscher, P.T., Baumgartner, L.K., and Reid, R.P. (2004), Microbe–mineral interactions: early carbonate precipitation in a hypersaline lake (Eleuthera Island, Bahamas). Sedimentology, 51: 745–765. https://doi.org/10.1111/j.1365-3091.2004.00649.x

Dupraz, C., Visscher, P.T., et al., 2009. Processes of carbonate precipitation in modern microbial mats. Earth-Science Reviews, 96, 141–162.

Lines 191–192: “Strontium and S appear as minor elements and may represent impurities and/or minor phases.” This statement is too general and needs clarification. Strontium is present in calcium sulfates and carbonates, generally substituting for calcium in the crystal lattice due to its similar ionic radius.

Literature: The reference list is extensive but lacks classical studies directly relevant to the topics discussed. I encourage the authors to consult works describing raft formation mechanisms and numerous examples, such as:

Arthurton, R.S., 1973. Experimentally produced halite compared to Triassic layered halite: rocks from Cheshire, England. Sedimentology, 20, 145–160.

Warren, J.K., 2016. Evaporites: A Compendium. Berlin: Springer, 1854 pp.

Author Response

Comment 1: The article was prepared based on several selected samples from the two lagoons in Brazil (Lagoa de Araruama and Lagoa Vermelha). The title “Floating rafts from hypersaline environments” suggests that it concerns a broader, synthetic, general study of hypersaline environments worldwide. Hypersaline environments across the world are highly diverse, and therefore, in the reviewer’s opinion, the title should include more specific information about which lagoons are the subject of this study.

Response 1: We thank the reviewer for this comment. We changed the title to “Floating rafts from coastal hypersaline environments in Brazil”

Comment 2: The article presents the results of studies, including microscopic (transmitted light microscope, SEM-EDS, and TEM) on selected individual rafts that precipitated on the surface of the brine due of evaporation. These results are well described using appropriate analytical methods.

Response 2: Ca-Mg carbonates are the first minerals to precipitate in evaporites. With increasing water evaporation and salt concentration, precipitation of Ca sulfates and halite takes place (Warren, 1996). Although they were minor minerals, sulfates and halite were present in the rafts from the saltwork (Figure 5). It is not possible to measure que major ions in the brines because we did not maintain the samples, and the ion concentrations in lagoons and the saltwork change with the weather. About the mechanisms behind raft precipitation, we have already pointed to the role of the air-water interface in nucleation, to the mechanism of crystal growth through aggregation of nanocrystals to form mesocrystals, and discussed CaCO₃ precipitation in the context of the environments studied. We improved the discussion on the mesocrystals as a major mechanism behind raft precipitation and the role of organic molecules, as follows:

“The polycrystalline grains are composed of micrometric euhedral aragonite crystals, similar to those reported in the aragonite rafts from brackish waters [11] and/or apparently isometric nanoparticles comparable to those described in early stages of aragonite precipitation in microbial mats [50], corals and sponges [51]. These nano-crystals are aligned parallel to each other (Figures 3d-e, g; 5d; 6d-e), resulting in a birefringence similar to that of single crystals (Figures 3a, 6a). Even in fan-shaped structures, the organization of the nanocrystals is maintained over several micrometers (Figure 4b). The alignment of nanocrystals to form larger composite unities with different levels of crystallinity and purity is widespread among biominerals and synthetic minerals. This process results in faster growth, complex morphologies, and specific properties [52]. In this context, the similarity in shape and size of the grains within each sample, as well as among the four samples studied here, is remarkable. This suggests the presence of some kind of self-organization process in this system, possibly related to growth by aggregation of nanocrystals into mesocrystals, as proposed by Cölfen and Antonietti [52] and observed in synthetic aragonite [53].”

Comment 3: The article also requires editorial revisions: the citation style of the article is inconsistent with MDPI's style. Below are several detailed comments referring to selected sections:

Response 3: The citation style was changed to the MDPI style.

Comment 4, Line 51: It would be worth adding trona rafts to this list (see Warren, 2016).

Response 4: We added trona and carnallite to the list.

Comment 5, Line 78: The authors state that halite rafts were absent in the described hypersaline coastal environments, but no explanation is provided.

Response 5: We state that Ca carbonate rafts were present in the environments studied, and cite halite rafts described in other environments. We did not observe halite rafts, but it does not mean that they are not there. Our work in Araruama Lagoon focuses on magnetotactic microorganisms; in Vermelha Lagoon, the focus is on Ca-Mg carbonates. During evaporation of seawater in coastal lagoons and saltworks, mineral precipitation usually begins with Ca-Mg carbonates, then Ca sulfates, and then halite. Probably NaCl in most of the brines studied were not concentrated sufficiently to enable substantial amounts of halite precipitation. If we went further in the halite concentration ponds, possibly we would find halite rafts. But we didn’t.

Comment 6, Line 91: “Araruama Lagoon is described as the world's largest hypersaline lagoon”. According to many sources, the largest lagoon is Laguna Madre in Mexico and the US, with a length of about 209 km and an average depth of 1–1.5 m; additionally, Laguna Ojo de Liebre on the Pacific coast of Baja California, Mexico, is larger than Araruama.

Response 6: Araruama Lagoon is the largest hypersaline lagoon in Brazil. We thank the reviewer and corrected the text.

Comment 7, Line 119: “Leading to precipitation of halite (NaCl) and other minerals” – which other minerals? This should be specified.

Response 7: We changed the text as follows: “…leading to precipitation of halite [29]” The other minerals (Mg-calcite, aragonite and gypsum) are already listed at the end of the paragraph.

Comment 8, Lines 183–184: “To our knowledge, this is the first time that a Ca carbonate raft is described in hypersaline environments.” The study of the rafts themselves may be novel, but the mechanism of Ca carbonate precipitation in Araruama Lagoon was already presented by Cotovich et al., 2021; for other hypersaline environments, see also:

Dupraz, C., Visscher, P.T., et al., 2009. Processes of carbonate precipitation in modern microbial mats. Earth-Science Reviews, 96, 141–162.

Response 8: We could not find any descriptions in the literature of Ca carbonate rafts in hypersaline environments. Nevertheless, we deleted this phrase because there is the possibility that some work was missed. About the work suggested, they are about a hypothesis of biomineralization/organominerazation influenced by microorganisms and their biomolecules, which we think is a bit outdated and a bit out of context. Although there is probably influence of organic molecules in the polymorph precipitated and also in crystal size and shape, these rafts arise primarily due to supersaturation. This is certainly not biomineralization (no living beings involved), perhaps has a bit of organomineralization – but we believe they are beyond organomineralization, somewhere between organomineralization and completely inorganic precipitation. We added a deeper discussion on the possible roles of organic molecules and also mesocrystal self-organization to the text, cited above.

Comment 9, Lines 191–192: “Strontium and S appear as minor elements and may represent impurities and/or minor phases.” This statement is too general and needs clarification. Strontium is present in calcium sulfates and carbonates, generally substituting for calcium in the crystal lattice due to its similar ionic radius.

Response 9: we thank the reviewer for this comment. We changed the text as follows:

“Strontium readily substitutes into the aragonite lattice due to its larger atomic radius [47] and is commonly found in natural aragonite [e.g., 48-49]. In contrast, S may be present in minor phases such as Ca sulfates (see below).”

Comment 10, Literature: The reference list is extensive but lacks classical studies directly relevant to the topics discussed. I encourage the authors to consult works describing raft formation mechanisms and numerous examples, such as:

Arthurton, R.S., 1973. Experimentally produced halite compared to Triassic layered halite: rocks from Cheshire, England. Sedimentology, 20, 145–160.

Warren, J.K., 2016. Evaporites: A Compendium. Berlin: Springer, 1854 pp.

Response 10: We thank the reviewer for this comment. We found some classic work on rafts from evaporites consisting of gypsum, trona and carnallite, and added the information to the “Introduction” section and the references to the “References” section. The work of Arthurton was not chosen because it contains synthetic and fossil rafts but not contemporary natural rafts, which are the object of the present work. We have no access to the book by Warren, but we explored further the content of the review published by Warren in 1996.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors have made necessary modifications and the paper may be accepted

Reviewer 2 Report

Comments and Suggestions for Authors

The revisions were implemented in accordance with the reviewer’s suggestions, and the explanations are sufficient.