| 01 Aug 2025
The mirabilite microbiocosm in a Carpathian contact cave
Abstract. This study examines the microbial and geochemical environment surrounding mirabilite (sodium sulfate decahydrate) deposits in Izvorul Tăușoarelor Cave, located in the Romanian Carpathians. Using a metabarcoding approach, we analysed mirabilite, sediments, dipluran insects, drip water, and moonmilk deposits to investigate the microbial communities and elemental profiles linked to mirabilite formation. Elemental analysis revealed a geochemical signature in mirabilite samples that was dominated by sodium, sulfur, and calcium. Microbial profiling revealed a unique pattern: sulfur-reducing bacteria, such as Desulfobacterota, were absent in mirabilite samples, whereas Pseudomonas dominated, suggesting an alternative sulfur cycling pathway that potentially involves sulfide biooxidation. The presence of ammonia-oxidising archaea (Ca. Nitrocosmicus) exclusively in the mirabilite area, and of bacteria (Nitrococcus), indicates a possible influence from a small bat colony, which contributes minimal ammonia that may support the microbial equilibrium required for mirabilite growth. Actinomycetota, abundant in mirabilite, may facilitate mineral crystallisation through mycelium-like structures. We propose the term “microbiocosm” to describe the interconnected network of biotic and abiotic elements surrounding the mirabilite environment, proposing a novel framework for investigating microbial contributions to this mineral formation.
The manuscript presents a metabarcoding and geochemical study of the microbial communities associated with mirabilite deposits in the Izvorul Tăușoarelor Cave (Romania). The authors introduce the term “microbiocosm” to describe the interconnected biotic and abiotic components influencing mirabilite formation. The study combines elemental analyses, microbial diversity profiling, and limited ecological interpretation.
While the topic may be of local speleological and microbial ecological interest, I find that the paper, in its current form, does not meet the standards or scientific scope of Biogeosciences. The study is primarily descriptive, lacks robust quantitative data, and provides limited mechanistic insight into biogeochemical processes. Moreover, there are numerous speculative statements throughout the manuscript that are not sufficiently supported by data. For instance, the absence of sulfate-reducing bacteria in mirabilite samples is over-interpreted as mechanistically significant, yet no supporting metabolic or environmental evidence is provided.
In my opinion, the main limitation is the low number of mirabilite replicates, which should be the focal point of the study. Two samples are insufficient to provide a statistically or biologically meaningful description of the microbial communities involved. The authors should substantially increase the number of replicates if they wish to maintain this focus. In addition, too many functional assumptions about bacterial roles are made based solely on 16S rRNA gene data. This approach cannot robustly infer metabolic functions or biogeochemical activity. To strengthen the manuscript, metagenomic sequencing should be performed, at least on the mirabilite samples. I also recommend quantitative PCR analyses targeting both prokaryotic and eukaryotic (fungal) domains, as fungi are likely to play a significant ecological role in cave environments.
There are also important issues regarding the presentation and interpretation of results. The Results section is difficult to follow and should be completely reorganized. Taxonomic nomenclature and database usage need clarification: please specify the reference database and version used for classification. For example, according to the latest taxonomic updates, Firmicutes are now referred to as Bacillota, and Actinobacteriota as Actinomycetota. I strongly recommend restructuring the Results section hierarchically—starting from higher taxonomic levels (phylum, class) and progressing to the dominant genera in each sample. Consequently, Figure 2 should be redesigned to display phylum- or genus-level distributions, which would improve readability. If the authors wish to retain the ASV-based information, this could be shown as a heatmap or supplementary table. The rationale behind the current Figure 3A–C is unclear, the plots are confusing and provide limited information.
The Discussion is very limited. Only a few microbial groups are addressed, and the manuscript does not sufficiently engage with relevant literature on cave microbiology. The authors should consult and integrate more studies on cave microbial ecology and substantially revise the Discussion in light of the additional analyses recommended above.
The ecological analyses (e.g., beta diversity and PCA) are well presented. However, alpha diversity indices are missing and should be included, as they are essential for describing within-sample richness and diversity.
Overall, the topic has potential, but the manuscript requires an extensive and fundamental revision before it could be reconsidered for publication.