Microbial community divergence and pH-driven biomineralization in two terrestrial analogs to Mars

Extreme environments on Earth represent powerful natural laboratories for exploring microbe–mineral interactions relevant to Mars. Alkaline systems rich in carbonate and acidic Fe-sulfate-dominated sites mirror key geochemical regimes encountered on the Martian surface. Understanding how microbial communities respond to these contrasting conditions provides insight into the potential formation, alteration, and preservation of biosignatures. In this study, we compare two chemically divergent terrestrial analogs: the alkaline Lake Bagno dell’Acqua, in Pantelleria, Italy and an acidic site in Solforata di Pomezia, Latium, Italy. These environments span a wide pH spectrum and host distinct mineralogical substrates, allowing for an assessment of how microbial metabolisms, including carbonate-forming pathways, Fe-oxidation, and enzyme-mediated biomineralization, drive mineral transformations under Mars-relevant conditions. Water temperature, pH, and electrical conductivity were analyzed on field. Sediment samples were collected from both sites to characterize Microbial communities through 16S rRNA gene sequencing. Genomic data were screened for metabolic pathways associated with biomineralization, including those related to carbonic anhydrase and urease. Sediments collected from the alkaline Lake Bagno dell’Acqua reveal microbial communities that are strongly enriched in taxa typically associated with carbonate-forming microhabitats. This pattern suggests not only the presence of microorganisms capable of surviving under high-pH conditions, but also a pronounced community-driven potential for microbially induced carbonate precipitation (MICP). The taxonomic composition indicates that these communities rely on metabolic strategies wellsuited for alkaline environments, such as the activity of alkalinity-tolerant heterotrophs and microorganisms able to reshape local chemical microgradients. Such microgradients—particularly those involving pH, carbonate saturation, and organic matter turnover—play an essential role in controlling mineral nucleation and subsequent crystal growth. Further support for this interpretation comes from laboratory-cultured isolates obtained from the same sediments. Genomic analyses of these isolates show that they encode key enzymes directly implicated in biomineralization processes, including carbonic anhydrase and urease. These enzymes are known to accelerate carbonate precipitation by locally increasing carbonate alkalinity or inducing pH shifts, thereby reinforcing the notion that the Bagno dell’Acqua system hosts an active and metabolically versatile carbonate-precipitating microbial consortium. In contrast, sediments from the Pomezia solforata exhibit microbial communities with a drastically different ecological and functional character. Here, the dominant taxa belong to acidophilic Proteobacteria specializing in biogeochemical transformations of iron and sulfur. The heavy representation of Fe-oxidizing lineages is striking and aligns with the abundant jarosite—a potassium iron sulfate mineral—found throughout the sediments. Jarosite formation is favored in low-pH, oxidizing, metal-rich environments, and the co-occurrence of these acidophiles with jarosite strongly suggests that biologically mediated iron oxidation contributes to the ongoing development of Fe-sulfate mineral phases at this site. The ecological pressures imposed by the solforata’s acidic and geochemically reactive conditions thus select for microbes with specialized metabolic adaptations distinct from those thriving in the alkaline lake system. A multivariate comparison of community composition using Bray–Curtis principal coordinates analysis (PCoA) underscores the profound differences between the two sites. The ordination displays a pronounced separation, reflecting how extreme pH regimes—highly alkaline at Bagno dell’Acqua and strongly acidic at Pomezia—act as powerful selective forces. These environmental constraints drive the assembly of fundamentally different microbial ecosystems, each capable of mediating distinct biogeochemical processes and shaping characteristic patterns of mineral formation. Taken together, the contrasting microbial signatures and their associated mineralogical imprints highlight two divergent biomineralization pathways operating under opposite physicochemical conditions. These findings have broader implications for the interpretation of potential biosignatures on Mars, where sediments may exhibit comparable chemical heterogeneity. Understanding how microbial processes respond to and exploit extreme pH environments on Earth provides a valuable framework for identifying biologically influenced mineral phases and deciphering the ecological strategies of hypothetical extraterrestrial life in similarly variable settings.