Rare Earth Elements in Shells of Black Sea Molluscs: Anomalies and Biogeochemical Implications

Round 1

Reviewer 1 Report

Dear Authors,

I found this manuscript focused on an interesting topic, very well-written and drafted, complete in almost all its parts and with an adequate amount of analyses and key results.

In my opinion, more attention to the selected species should be paid in the introduction section, explaining the choice of these animal models among several similar species dispersed in the studied area. Particularly, the authors should argue, the choice to select some bivalves and just one gastropod, if there is a biological reason considering the different life traits regarding feeding and development. Please take care of these suggestions during your manuscript revision.

Figure one needs to be improved, in the present form the sampling points of the different sampled species were not reported/highlighted. It is essential to evaluate the results well to know if the specimens come from the same place or if there were differences.

Double-check the reference style and italicize all the scientific names.

Best regards

Author Response

Dear Editors and Reviewer 1,

We cordially thank you for your work aimed to help us improve our manuscript for the publication and for providing us with this opportunity. All your comments and suggestions have been considered, and respective modifications have been included in the manuscript. All essential modifications have been highlighted using the Track Changes function. Please find below our replies to Reviewer 1 specific comments.

Best regards,

Sophia Barinova and coauthors

Response to Reviewer #1

We cordially thank the Reviewer for the attention to the manuscriptandvaluable comments.

Reviewer’s comment 1: “I found this manuscript focused on an interesting topic, very well-written and drafted, complete in almost all its parts and with an adequate amount of analyses and key results.

In my opinion, more attention to the selected species should be paid in the introduction section, explaining the choice of these animal models among several similar species dispersed in the studied area. Particularly, the authors should argue, the choice to select some bivalves and just one gastropod, if there is a biological reason considering the different life traits regarding feeding and development. Please take care of these suggestions during your manuscript revision.”

Answer: We are grateful to the Reviewer for the positive reception of the manuscript. We quite agree that the information on the principle of choice of these molluscswould be useful. The sampling objects are characterized in Section 2.1, and we believe it is more pertinent to add this information to this section rather than to Introduction. This information has been added on p. 3: “These are the common and most abundant mollusc species collected in the same conditions of the mollusc farm environment; three of them (M. galloprovincialis, M. gigas and R. venosa) are popular and commercially valuable seafood [74] and the scallop and the ark clam are potentially important species [94,95] in the Black Sea aquaculture. No any other gastropodspeciesof the similar size, availability, and commercial value as for R. venosahave been detected in this area.” Thus, the only reasons for the choice of these animals were their wide availability on (and relevance to) the mollusc farmin the studyandtheir commercial potential related to mariculture. There have beenno specific biological reasons or feeding- or development-related life traits behind this choice. The shell REE compositionstudied in this work represents a mean imprint of a multitude ofenvironmental and physiological factorson the molluscs during their life. It is likely that the dietary(especially as regards bivalves) and developmentalfeaturesmay change with age and in varying environmental conditions.These issues themselves are extremely underexplored. All the more so is how these factorsaffect REE in mollusc shells, and this is, of course, beyond the scope of the study.

Reviewer’s comment 2: “Figure one needs to be improved, in the present form the sampling points of the different sampled species were not reported/highlighted. It is essential to evaluate the results well to know if the specimens come from the same place or if there were differences.”

Answer: According to the Reviewer’s request, Figure 1 has been modified. The sampling station was essentially single, and it has been represented by a single black dotin the figure. Mussels were sampled from the same collector on which the cages containing the other bivalves were suspended, and spatial uncertainty in sampling the bivalves from these objects was 2-3 m. The linear dispersion in sampling the gastropod under the collector was higher, about 20 m, as these free-living benthic molluscs are far more dispersed. An outline of the above information has been given on p. 3: “The black dot in the sampling site map (Figure 1) approximately corresponds to the seafloor area involved in the sampling of the gastropod, with the spatial dispersion in the sampling of the bivalves being much smaller.”

Reviewer’s comment 3: “Double-check the reference style and italicize all the scientific names.”

Answer: We thank the Reviewer for pointing thisout. According to the suggestion, all species names in References have been italicized, and a uniform reference style consistent with that of MDPI has been applied throughout. In particular, journal article title words are now all in lowercase except the first ones in the main and explanatory clauses (i.e. the first words after full stop, colon or dash). This style, however, is in discord with those accepted by some publishers (e.g. Wiley or Springer–Nature), which capitalize all title words except articles, prepositions and conjunctions, but is in generalagreement with others (e.g. Elsevier). At the same time, an ambiguity persists in relation to book chapter title style. The citation manager EndNote was initially used to compile the reference list, and the corresponding MDPI style file was used.The exported citation files sometimes contained superfluous data, e.g. in the journal title section, that has now been excluded.

Author Response File: Author Response.pdf

Reviewer 2 Report

Manuscript jmse-2971898 entitled “Rare earth elements in shells of Black Sea molluscs: anomalies and biogeochemical implications" is an excellently written MS that is worth studying and falls within the scope of JMSE.

In recent decades, the study and application of rare earths has increased in various fields. In this paper, the authors estimate the rare earth element content in the shells of five molluscs widely distributed in the Black Sea coastal region and sampled at the mariculture site in Sevastopol Bay.

They totaled the REE contents in the shells of the mollusks, which decreased in the following order of species examined: Magallana gigas = Anadara kagoshimensis > Flexopecten glaber ponticus ≥ Rapana venosa > Mytilus galloprovincialis.

This information makes it possible to identify regularities and anomalies of REE accumulation in the studied bivalve shells, especially using local normalisation, and to evaluate the bivalve shells as potential biomonitors and REE source.

In addition, they propose a new universal relationship linking the content of three light and heavy REEs in Black Sea mussel shells.

Comments:

Probably it is not necessary to perform analyses of REE composition for species identification besides the available standard methods (barcoding, nuclear markers, etc.)?

Table 1: Why do you choose the median (min – max) of Al and REE contents?

Author Response

Dear Editors and Reviewer 2,

We cordially thank you for your work aimed to help us improve our manuscript for the publication and for providing us with this opportunity. All your comments and suggestions have been considered, and respective modifications have been included in the manuscript. All essential modifications have been highlighted using the Track Changes function. Please find below our replies to Reviewer 2 specific comments.

Best regards,

Sophia Barinova and coauthors

Response to Reviewer #2

We sincerely thank the Reviewer for the consideration of our manuscript and forthe complimentary review on it. Answers to the specific comments are below.

1. “Probably it is not necessary to perform analyses of REE composition for species identification besides the available standard methods (barcoding, nuclear markers, etc.)?”

Answer: The methods mentioned by the Reviewer are, of course, highly selective and robust. At the same time, they may be rather time-consuming and require quite costly assay kits, reagents, equipment, etc. Secondly, they apply only to undamaged genetic material, which is very labile. The identification based on the trace element composition, although much less conclusive due to severe limitations in the number of elements involved, is very cost-effective and turns out to be applicable not only to molluscs’ fresh soft tissues, but also to their shells. One can even anticipate fossil molluscs’ identification from the trace element inventory of their shells, although such a research has not been conducted to date and this hypothesis may not be confirmed.

2. “Table 1: Why do you choose the median (min – max) of Al and REE contents?”

Answer: This is because the alternative notation of concentrations as mean ± SD (or standard error, or 95% confidence interval) is meaningful only if it is derived from the normal or, at least, symmetric distribution. This case is not very typical in the live nature, and our measurements also show this. If a distribution is not symmetric, a median more adequately represents the expectation value. To not overburden the table with the additional 25th and 75th percentile values, which, to some extent, do reflect the uncertainty in the expectation value, we decided to present only the expected range of the values (as min – max).

Author Response File: Author Response.pdf