Mónica Sánchez-Román

Early diagenetic dolomite forms at low temperatures (13 C = −10.0–3.4‰; δ ¹⁸ O = −1.6–7.3‰) reflect variations in palaeoproductivity, temperature and hydrological regimes, and ⁸⁷ Sr/ ⁸⁶ Sr ratios (0.708213–0.709243) indicate fluid sources. Sr (up to 5828 ppm), Fe (up to 5187 ppm) and Mn (up to 1035 ppm) reach concentrations above those expected for equilibrium precipitation. Sr enrichment does not correlate with Mg/Ca, which we attribute to kinetically controlled uptake during rapid, microbially mediated growth rather than to equilibrium partitioning into the Ca lattice site. Mn and Fe concentrations vary independently, suggesting that they are not controlled by the same redox conditions. Instead, their distribution mainly reflects environmental conditions rather than direct microbial control. In addition, δ ⁵⁶ Fe values vary between sites (−0.98–0.09‰), providing additional insights into redox conditions during carbonate formation. Together, the data indicate that early diagenetic dolomite formation is governed by site‐specific combinations of sulphate reduction, aerobic heterotrophy, EPS‐mediated divalent cation binding and fluid pathways, rather than a single mechanism. This study provides a geochemical framework for low‐temperature, microbially mediated dolomite formation and identifies which proxies are, and are not, diagnostic of microbial influence in ancient sedimentary records.

IntroductionThe question if life exists outside of Earth has been an important driver of Solar system exploration and exoplanet research in the last few decades [1-3]. Although terrestrial analogue research provides insight into this question [4-5], no definitive evidence of life outside Earth has been found at this moment [6]. This raises the question whether the current methodology for analogue research is sufficient, or needs changes.To answer this, we use AI to systematically analyse 2500 papers that use terrestrial analogues or analogue research methods to study life outside Earth. For each, we determined the used methods, scales at which they are applied, and investigated research questions. This gives new insights in which methods are used and underused, and which methods should be used in combination to improve the methodology towards understanding and detection of life outside Earth MethodsA systematic literature search was performed for analogue research related to life outside Earth. The resulting initial set of articles were analysed manually to determine the used methods, scales, and research questions. Then, articles referred to in review papers were collected to complete the literature compilation.This literature compilation was then analysed by using the Ollama framework [7] in combination with Langchain [8] to run the qwen3:0.6b LLM [9]. Analysis uses the following pipeline: 1) conversion to text-containing pdfs, 2) extraction of text sections 3) extraction of method sections from other sections 4) asking a series of binary question on sections relevant for methods, scales, and research questions to check the usage within each single article. ResultsThe initial compilation included ~700 articles, ~100 of which could be classified as reviews. The manual analysis showed the following:Interdisciplinary research is done, but remains limited. This hinders finding connections between biosignatures and other observables.Research on how to span different spatial scales, ranging from molecular to planetary scales and exoplanetary scales, is lacking. Here, especially the field-scale is underutilised, which may provide importance in linking potentially habitats to actual biosignatures.There is no clear separation between inhabited, uninhabited, and previously inhabited but no longer habitable environments. This affects the reliability of biosignature interpretation. Both false negatives and false positives are overlooked, with almost no research focusing on false negatives.The reviews increased the dataset to a total of ~2500 articles. These were all analysed by AI, with the initial set functioning as a control. AI-analysis deviates significantly from manual analysis, overall indicating higher numbers for each used method, scale, and research question. However, AI confirms that there is a small number of papers that utilises the field scale, indicating at least an agreement with the manual analysis that this scale is underutilised.Figure 1 Correlation diagram showing usage of observational scales and methods. Colours represent that certain methods or scales are used in the same paper (blue) or that they tend to be used separately (red). Light colours to white indicate underused methods and scales. Note that the AI tends give overall higher usage of methods and scales throughout papers, but does not find the same relationships as in the manual analysis. Discussion and conclusionThe research gaps resulting from the manual analysis indicate the need for an integrated, multi-scale framework to advance life-detection strategies. This is partially supported by AI, although improvement of the AI-analysis is needed before final conclusions can be drawn. As examples of suggestions for future research, biologists or biochemists should collaborate with geomorphologists, geophysicists, and imaging spectroscopists, in order to determine the relations between biosignatures on laboratory to field and remote-sensing scales. Futhermore, geologists and biologists should work with exoplanet researchers, to give them insights into the atmospheric processes and exclude false positives and negatives in exoplanetary signs of life. Such collaborations will prove essential to improving the methodology for the search for extraterrestrial life. References[1] C. S. Cockell et al. Astrobiology, 9, 1. (2009). [2] J. Matthey, Platin. Met. Rev., 20, 3. (1976). [3] D. Schulze-Makuch, J. M. Dohm, A. G. Fairén, V. R. Baker, W. Fink, and R. G. Strom Astrobiology, 5, 6. (2005). [4] A. G. Fairén et al. Astrobiology, 10, 8. (2010). [5] F. Biagioli, S. Bay, A. Zerboni, and C. Coleine Int. J. Astrobiol., 24, e29. (2025). [6] J. E. Brandenburg “The Discovery of Microbial Life on Mars.” Int. J. Immunol. Microbiol., 2, 2. (2025). [7] Ollama, Ollama’s documentation. Retrieved from: https://docs.ollama.com (2025). [8] H. Chase LangChain. Retrieved from: https://github.com/langchain-ai/langchain (2022). [9] Qwen Team “Qwen3 Technical Report.” Retrieved from: https://arxiv.org/abs/2505.09388 (2025).

Micritization alters carbonate grain textures through repeated dissolution–reprecipitation cycles. Although microbial activity influences micritization in shallow marine carbonates, few studies integrate sedimentological and microbial analyses in modern settings. Here, we examined how bacterial communities vary along 80‐cm sediment cores collected seasonally over 1 year from Al Kharrar Lagoon (Eastern Red Sea, Saudi Arabia). Micritized grains with microborings and cryptocrystalline aragonite fillings—often associated with extracellular polymeric substances—occurred in ~60% of samples, especially in surface sediments. Bacterial communities were structured primarily by sediment depth rather than season. Key candidates potentially involved in carbonate cycling were inferred from taxonomy and prioritized by abundance, cultured representatives and expected tolerance to lagoon conditions. Surface layers were enriched in aerobic and sulfur‐oxidizing taxa ( Thiogranum , Thioalkalispira , Thiotrichales ), whereas deeper layers contained mainly fermentative lineages ( Aminicenantales , Anaerolineae ). Carbonate precipitation was associated with nitrate reducers ( Fulvivirga ) in surface sediments and sulfate reducers ( Desulfosarcina , Desulfatiglans ) at depth. Versatile taxa such as Cyanobacteria , Tropicimonas , Ruegeria and Woeseia in surface layers and Dehalococcoidia and Anaerolineaceae in deeper layers, may contribute to both dissolution and precipitation processes. These results indicate that, alongside abiotic processes, bacterial communities can significantly influence carbonate cycling and reshape sediment textures in shallow Red Sea environments.

Carbonates are prevalent in the geologic record throughout Earth's history and are also found on Mars and within meteorites. The chemical and isotopic compositions of carbonates have been used to elucidate the composition of ancient oceans and the prevailing conditions during the development of life and its subsequent rapid evolution1-5. Carbonate mineral compositions reflect the environmental conditions under which they were formed4,5 . Dolomite is particularly abundant in ancient rock formations but is scarce in modern sedimentary environments. Many ancient dolomites are suspected of being alteration products of preexisting dolomite phases rather than being originally formed, unaltered dolomites6. The geochemical and neomorphic alteration of dolomite have been studied extensively, with the principal driving forces for neomorphism being the inherent thermodynamic instability of non-stoichiometric dolomites and the surface free energy-driven recrystallization of fine crystalline mosaics to coarser crystalline6. Recent studies have demonstrated that microorganisms can produce dolomite with a geochemical signature distinct from those formed abiotically1-4. Our research integrates field studies, state-of-the-art laboratory experiments, mineralogical, and geochemical analyses to investigate the processes and environmental conditions that control the chemical composition of low-temperature carbonates. The role of inorganic-organic interactions is evaluated in natural field laboratories and carefully controlled laboratory experiments performed under abiotic and biotic conditions. In summary, the research focuses on understanding the processes and environmental conditions that control the chemical composition of low-temperature dolomite. A multidisciplinary approach, integrating field studies, laboratory experiments, microscopic, mineralogical, and geochemical analyses, is employed to investigate the role of inorganic-organic interactions in the formation of these carbonates. This work has the potential to provide insights into the development of life on Earth and the evolution of terrestrial and Martian carbonates. References:[1] Sánchez-Román M., et al. (2011) Chemical Geology 281, 143 - 150.[2] Sánchez-Román M., et al. (2014) Scientific Reports 4, 4767.[3] Sanchez-Roman M., et al (2023) Geochimica et Cosmochimica Acta 356, 66-82.[4] Sánchez-Román M., et al. (2011) Geochimica et Cosmochimica Acta 75, 887-904.[5] Li M., et al. (2021) Geology 49, 698–702.[6] Mazzullo, S.J., et al. (1991) Carbonates Evaporites 7, 21–37.

Palygorskite is assumed to be the predominant clay mineral in playa-lakes, where it may be detrital or authigenic in origin. Discriminating between detrital and authigenic clays is crucial to elucidate paleoenvironmental conditions in lacustrine deposits. This study provides insight into the sedimentary evolution of clay minerals from source, lacustrine Miocene marlstones and mudstones, to sink, represented by three recent hyperalkaline playa-lakes in Central Spain. XRD, TEM, and AEM analyses show concomitant detrital and authigenic palygorskites in the three playa-lakes. The inherited palygorskites exhibit degradation features, larger widths, and common and ideal compositions, in contrast to neoformed particles. The latter are narrower. Depending on the hydrochemical composition of each playa-lake, neoformed palygorskites are enriched in a different octahedral cation (Al₂O₃, MgO, and Fe₂O₃). Iron-rich palygorskites are only formed in association with authigenic saponites in one of the playa-lakes. The same effect of magnesium competition between smectite and palygorskite is observed in Miocene mudstones, where palygorskite is relatively enriched in iron. In hyperalkaline, seasonal playa-lakes lying in the vicinity, slight physicochemical differences play a crucial role in the crystallochemical composition of authigenic palygorskites, highlighting the use of this mineral as a geochemical proxy.

Microbial impacts on early carbonate diagenesis, particularly the formation of Mg-carbonates at low temperatures, have long eluded scientists. Our breakthrough laboratory experiments with two species of halophilic aerobic bacteria and marine carbonate grains reveal that these bacteria created a distinctive protodolomite (disordered dolomite) rim around the grains. Scanning Electron Microscopy (SEM) and X-ray Diffraction (XRD) confirmed the protodolomite formation, while solid-state nuclear magnetic resonance (NMR) revealed bacterial interactions with carboxylated organic matter, such as extracellular polymeric substances (EPS). We observed a significant carbon isotope fractionation (average δ¹³C = 11.3‰) and notable changes in Mg/Ca ratios throughout the experiments. Initial medium δ¹³C was − 18‰, sterile sediments were at 2‰ (n = 12), bacterial-altered sediments were − 6.8‰ (n = 12), and final medium δ¹³C was − 4.7‰. These results highlight the role of bacteria in driving organic carbon sequestration into Mg-rich carbonates and demonstrate the utility of NMR as a tool for detecting microbial biosignatures. This has significant implications for understanding carbonate diagenesis (dissolution and reprecipitation), climate science, and extraterrestrial research.

The Laguna Negra, a high-altitude hypersaline close-basin lake in the Andes of Northwest Argentina, presents a unique environment for studying the formation of microbialites (Gomez et al., 2014; Buongiorno et al., 2019). The extreme conditions like high UV radiation and salinity restrict multicellular life, allowing microbial activity as one of the drivers of carbonate precipitation. Oncolites samples, interpreted as a result from fluid mizing and biologically mediated nucleation (Buongiorno et al., 2019) and collected under Europlanet project have been analyzed for the search of chemical and mineralogical biosignatures. Synchrotron µ-X-Ray Fluorescence provided major-to-trace elemental distributions, revealing elements concentrated in stromatolitic laminae (e.g., Sr, Co, Rb, Mo, Pb), concentrated in not strictly stromatolitic laminations (eg., Fe, Mn, Mo, Ni, As, Pb, Zn), and contrasting with the elements widespread in the matrix (e.g., Ca, Cl, Br, K T, V). UV photoluminescence macroscopy highlighted laminations invisible in naked eye, providing also complementary information of chemical differences. Synchrotron scanning SAXS/WAXS maps provided additional information on mineral composition and crystallites shape and orientation. These combined results provide complementary chemical and mineralogical information for the characterization of the biotic and abiotic layers involved on the oncolites accretion, allowing us to define combination of characters specific to microbially-mediated precipitations. This work also opens the way for the multiscale assessment of mineral biosignatures of microbial precipitation in laminated rocks, from Earth or elsewhere.Figure 1. a) UV photoluminescence macroscopy of the Laguna Negra cross-cut oncolite excited at 385 nm. Emission collected at 571 nm (red), 708 nm (green) and 835 nm (blue). B) Synchrotron XRF map of Sr (red), Zn (green) and Ca (blue). Stromatolitic layers are rich in Sr, while Zn is restricted to outer, likely evaporitic layers. References:Buongiorno, J. et al. (2019) ‘Mineralized microbialites as archives of environmental evolution, Laguna Negra, Catamarca Province, Argentina’, Geobiology, 17(2), pp. 199–222. Available at: https://doi.org/10.1111/gbi.12327.Gomez, F.J. et al. (2014) ‘Microbialites in a high-altitude Andean lake: Multiple controls on carbonate precipitation and lamina accretion’, Palaios, 29(6), pp. 233–249. Available at: https://doi.org/10.2110/palo.2013.049.

Secondary minerals in lava tubes on Earth provide valuable insight into subsurface processes and the preservation of biosignatures on Mars. Inside lava tubes near the Hawaii-Space Exploration and Analog Simulation (HI-SEAS) habitat on the northeast flank of Mauna Loa, Hawaii, a variety of secondary deposits with distinct morphologies were observed consisting of mainly sodium sulphate powders, gypsum crystalline crusts, and small coralloid speleothems that comprise opal and calcite layers. These secondary deposits formed as a result of hydrological processes shortly after the formation and cooling of the lava tubes and are preserved over long periods of time in relatively dry conditions. The coralloid speleothem layers are likely related to wet and dry periods in which opal and calcite precipitates in cycles. Potential biosignatures seem to have been preserved in the form of porous stromatolite-like layers within the coralloid speleothems. Similar secondary deposits and lava tubes have been observed abundantly on the Martian surface suggesting similar formation mechanisms compared to this study. The origin of secondary minerals from tholeiitic basalts together with potential evidence for microbial processes make the lava tubes near HI-SEAS a relevant analog for Martian surface and subsurface environments.