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Lehman Caves is an extensively decorated high desert cave that represents one of the main tourist attractions in Great Basin National Park, Nevada. Although traditionally considered a water table cave, recent studies identified abundant speleogenetic features consistent with a hypogenic and, potentially, sulfuric acid origin. Here, we characterized white mineral deposits in the Gypsum Annex (GA) passage to determine whether these secondary deposits represent biogenic minerals formed during sulfuric acid corrosion and explored microbial communities associated with these and other mineral deposits throughout the cave. Powder X‐ray diffraction (pXRD), scanning electron microscopy with electron dispersive spectroscopy (SEM‐EDS), and electron microprobe analyses (EPMA) showed that, while most white mineral deposits from the GA contain gypsum, they also contain abundant calcite, silica, and other phases. Gypsum and carbonate‐associated sulfate isotopic values of these deposits are variable, with δ34SV‐CDT between +9.7‰ and +26.1‰, and do not reflect depleted values typically associated with replacement gypsum formed during sulfuric acid speleogenesis. Petrographic observations show that the sulfates likely co‐precipitated with carbonate and SiO2 phases. Taken together, these data suggest that the deposits resulted from later‐stage meteoric events and not during an initial episode of sulfuric acid speleogenesis. Most sedimentary and mineral deposits in Lehman Caves have very low microbial biomass, with the exception of select areas along the main tour route that have been impacted by tourist traffic. High‐throughput 16S rRNA gene amplicon sequencing showed that microbial communities in GA sediments are distinct from those in other parts of the cave. The microbial communities that inhabit these oligotrophic secondary mineral deposits include OTUs related to known ammonia‐oxidizing Nitrosococcales and Thaumarchaeota, as well as common soil taxa such as Acidobacteriota and Proteobacteria. This study reveals microbial and mineralogical diversity in a previously understudied cave and expands our understanding of the geomicrobiology of desert hypogene cave systems. [ABSTRACT FROM AUTHOR]

The Duluth Complex (DC) contains sulfide‐rich magmatic intrusions that represent one of the largest known economic deposits of copper, nickel, and platinum group elements. Previous work showed that microbial communities associated with experimentally‐weathered DC waste rock and tailings were dominated by uncultivated taxa and organisms not typically associated with mine waste. However, those experiments were designed for kinetic testing and do not necessarily represent the conditions expected for long‐term environmental weathering. We used 16S rRNA gene methods to characterize the microbial communities present on the surfaces of naturally‐weathered and historically disturbed outcrops of DC material. Rock surfaces were dominated by diverse uncultured Ktedonobacteria, Acetobacteria, and Actinobacteria, with abundant algae and other phototrophs. These communities were distinct from microbial assemblages from experimentally‐weathered DC rocks, suggesting different energy and nutrient resources in environmental samples. Sulfide mineral incubations performed with and without algae showed that photosynthetic microorganisms could have an inhibitory effect on autotrophic populations, resulting in slightly lower sulfate release and differences in dominant microorganisms. The microbial assemblages from these weathered outcrops show how communities develop during weathering of sulfide‐rich DC rocks and represent baseline data that could evaluate the effectiveness of future reclamation of waste produced by large‐scale mining operations. [ABSTRACT FROM AUTHOR]

The Duluth Complex in northeastern Minnesota hosts economically significant deposits of copper, nickel, and platinum group elements (PGEs). The primary sulfide mineralogy of these deposits includes the minerals pyrrhotite, chalcopyrite, pentlandite, and cubanite, and weathering experiments show that most sulfidebearing rock from the Duluth Complex generates moderately acidic leachate (pH 4 to 6). Microorganisms are important catalysts for metal sulfide oxidation and could influence the quality of water from mines in the Duluth Complex. Nevertheless, compared with that of extremely acidic environments, much less is known about the microbial ecology of moderately acidic sulfide-bearing mine waste, and so existing information may have little relevance to those microorganisms catalyzing oxidation reactions in the Duluth Complex. Here, we characterized the microbial communities in decade-long weathering experiments (kinetic tests) conducted on crushed rock and tailings from the Duluth Complex. Analyses of 16S rRNA genes and transcripts showed that differences among microbial communities correspond to pH, rock type, and experimental treatment. Moreover, microbial communities from the weathered Duluth Complex rock were dominated by taxa that are not typically associated with acidic mine waste. The most abundant operational taxonomic units (OTUs) were from the genera Meiothermus and Sulfuriferula, as well as from diverse clades of uncultivated Chloroflexi, Acidobacteria, and Betaproteobacteria. Specific taxa, including putative sulfur-oxidizing Sulfuriferula spp., appeared to be primarily associated with Duluth Complex rock, but not pyrite-bearing rocks subjected to the same experimental treatment. We discuss the implications of these results for the microbial ecology of moderately acidic mine waste with low sulfide content, as well as for kinetic testing of mine waste. IMPORTANCE Economic sulfide mineral deposits in the Duluth Complex may represent the largest undeveloped source of copper and nickel on Earth. Microorganisms are important catalysts for sulfide mineral oxidation, and research on extreme acidophiles has improved our ability to manage and remediate mine wastes. We found that the microbial assemblages associated with weathered rock from the Duluth Complex are dominated by organisms not widely associated with mine waste or mining-impacted environments, and we describe geochemical and experimental influences on community composition. This report will be a useful foundation for understanding the microbial biogeochemistry of moderately acidic mine waste from these and similar deposits. [ABSTRACT FROM AUTHOR]

In recent years there has been a surge in the recovery of ancient organic molecules from archaeological contexts. These analyses are yielding unprecedented insights into human evolution and cultural practices, and are providing valuable data for reconstructing paleoenvironments. However, contamination of archaeological sediments by microorganisms can alter ancient biomolecular data. Furthermore, the extent to which microbes can penetrate ancient archaeological sediments once these are exposed by excavation is unknown. We tested this question at Crvena Stijena, a rock shelter in the Dinaric alps in Montenegro that contains archaeological deposits spanning more than 80,000 years. Excavations in the early 1960s exposed these profiles, which have been cleaned several times to permit sampling for archaeological, geological, and biomolecular analyses. The growth of green biofilms on the exposed profiles after cleaning has prompted the question of whether this surface contamination extends into the profile. To test this question, we examined five different geological layers by sampling sediments from the exposed surface and at 1 cm intervals horizontally into the profile. Results from 16S rRNA gene sequencing show that samples from sediment surfaces have distinct microbial communities from most samples collected more than 1 cm deep, and microbial biomass from the deeper samples is very low. Together, this evidence strongly indicates that microbial contamination is limited to the profile surfaces. This lowers the likelihood that ancient biomolecules in these sediments have been altered by recent changes to the in situ microbial community, and that cleaning of the profiles before sampling may not need to exceed 2 cm in depth. These results lend further support to the research utility of limited vertical sampling along archaeological profiles and witness sections, a strategy which conserves rare and limited archaeological deposits while helping to tackle key questions about the past. • Growth of green biofilms on recently excavated surfaces raised concerns about contamination of archaeological sediments. • Microbial communities deeper than 1 cm into profiles were distinct from surface communities, and had very low biomass. • These communities were populated by microorganisms found in deep oligotrophic soils and sediments. • It is unlikely that ancient biomolecules have been altered by recent changes to the in situ microbial community. [ABSTRACT FROM AUTHOR]

Biovermiculations are uniquely patterned organic rich sediment formations found on the walls of caves and other subterranean environments. These distinctive worm-like features are the combined result of physical and biological processes. The diverse microbial communities that inhabit biovermiculations may corrode the host rock, form secondary minerals, and produce biofilms that stabilize the sediment matrix, thus altering cave surfaces and contributing to the formation of these wall deposits. In this study, we incubated basalt, limestone, and monzonite rock billets in biovermiculation mixed natural community enrichments for 468–604 days, and used scanning electron microscopy (SEM) to assess surface textures and biofilms that developed over the course of the experiment. We observed alteration of rock billet surfaces associated with biofilms and microbial filaments, particularly etch pits and other corrosion features in olivine and other silicates, calcite dissolution textures, and the formation of secondary minerals including phosphates, clays, and iron oxides. We identified twelve distinct biofilm morphotypes that varied based on rock type and the drying method used in sample preparation. These corrosion features and microbial structures inform potential biological mechanisms for the alteration of cave walls, and provide insight into possible small-scale macroscopically visible biosignatures that could augment the utility of biovermiculations and similarly patterned deposits for astrobiology and life detection applications. [ABSTRACT FROM AUTHOR]

Photosynthetic cave communities ("lampenflora") proliferate in Carlsbad Cavern and other show caves worldwide due to artificial lighting. These biofilms mar the esthetics and can degrade underlying cave surfaces. The National Park Service recently modernized the lighting in Carlsbad Cavern to a light-emitting diode (LED) system that allows adjustment of the color temperature and intensity. We hypothesized that lowering the color temperature would reduce photopigment development. We therefore assessed lampenflora responses to changes in lighting by monitoring photosynthetic communities over the course of a year. We measured photopigments using reflected-light spectrophotometric observations and analyzed microbial community composition with 16S and 18S rRNA gene amplicon sequencing. Reflected-light spectrophotometry revealed that photosynthetic biofilm development is affected by lighting intensity, color temperature, substrate type, and cleaning of the substrate. Gene sequencing showed that the most abundant phototrophs were Cyanobacteria and members of the algal phyla Chlorophyta and Ochrophyta. At the end of the study, visible growth of lampenflora was seen at all sites. At sites that had no established biofilm at the start of the study period, Cyanobacteria became abundant and outpaced an increase in eukaryotic algae. Microbial diversity also increased over time at these sites, suggesting a possible pattern of early colonization and succession. Bacterial community structure showed significant effects of all variables: color temperature, light intensity, substrate type, site, and previous cleaning of the substrate. These findings provide fundamental information that can inform management practices; they suggest that altering lighting conditions alone may be insufficient to prevent lampenflora growth. IMPORTANCE Artificial lighting in caves visited by tourists ("show caves") can stimulate photosynthetic algae and cyanobacteria, called "lampenflora," which are unsightly and damage speleothems and other cave surfaces. The most common mitigation strategy employs bleach, but altering intensities and wavelengths of light might be effective and less harsh. Carlsbad Cavern in New Mexico, a U.S. National Park and UNESCO World Heritage Site, has visible lampenflora despite adjustment of LED lamps to decrease the energetic blue light. This study characterized the lampenflora communities and tested the effects of color temperature, light intensity, rock or sediment texture, and time on lampenflora development. DNA amplicon sequence data show a variety of algae and cyanobacteria and also heterotrophic bacteria. This study reveals microbial dynamics during colonization of artificially lit surfaces and indicates that while lowering the color temperature may have an effect, management of lampenflora will likely require additional chemical or UV treatment. [ABSTRACT FROM AUTHOR]