A Low-Diversity Microbiota Inhabits Extreme Terrestrial Basaltic Terrains and Their Fumaroles: Implications for the Exploration of Mars

Metabolic versatility of Caldarchaeales from geothermal features of Hawai'i and Chile as revealed by five metagenome-assembled genomes

Members of the archaeal order Caldarchaeales (previously the phylum Aigarchaeota) are poorly sampled and are represented in public databases by relatively few genomes. Additional representative genomes will help resolve their placement among all known members of Archaea and provide insights into their roles in the environment. In this study, we analyzed 16S rRNA gene amplicons belonging to the Caldarchaeales that are available in public databases, which demonstrated that archaea of the order Caldarchaeales are diverse, widespread, and most abundant in geothermal habitats. We also constructed five metagenome-assembled genomes (MAGs) of Caldarchaeales from two geothermal features to investigate their metabolic potential and phylogenomic position in the domain Archaea. Two of the MAGs were assembled from microbial community DNA extracted from fumarolic lava rocks from Mauna Ulu, Hawai'i, and three were assembled from DNA obtained from hot spring sinters from the El Tatio geothermal field in Chile. MAGs from Hawai'i are high quality bins with completeness >95% and contamination <1%, and one likely belongs to a novel species in a new genus recently discovered at a submarine volcano off New Zealand. MAGs from Chile have lower completeness levels ranging from 27 to 70%. Gene content of the MAGs revealed that these members of Caldarchaeales are likely metabolically versatile and exhibit the potential for both chemoorganotrophic and chemolithotrophic lifestyles. The wide array of metabolic capabilities exhibited by these members of Caldarchaeales might help them thrive under diverse harsh environmental conditions. All the MAGs except one from Chile harbor putative prophage regions encoding several auxiliary metabolic genes (AMGs) that may confer a fitness advantage on their Caldarchaeales hosts by increasing their metabolic potential and make them better adapted to new environmental conditions. Phylogenomic analysis of the five MAGs and over 3,000 representative archaeal genomes showed the order Caldarchaeales forms a monophyletic group that is sister to the clade comprising the orders Geothermarchaeales (previously Candidatus Geothermarchaeota), Conexivisphaerales and Nitrososphaerales (formerly known as Thaumarchaeota), supporting the status of Caldarchaeales members as a clade distinct from the Thaumarchaeota.

Unique Geothermal Chemistry Shapes Microbial Communities on Mt. Erebus, Antarctica

Mt. Erebus, Antarctica, is the world's southernmost active volcano and is unique in its isolation from other major active volcanic systems and its distinctive geothermal systems. Using 16S rRNA gene amplicon sequencing and physicochemical analyses, we compared samples collected at two contrasting high-temperature (50°C-65°C) sites on Mt. Erebus: Tramway Ridge, a weather-protected high biomass site, and Western Crater, an extremely exposed low biomass site. Samples were collected along three thermal gradients, one from Western Crater and two within Tramway Ridge, which allowed an examination of the heterogeneity present at Tramway Ridge. We found distinct soil compositions between the two sites, and to a lesser extent within Tramway Ridge, correlated with disparate microbial communities. Notably, pH, not temperature, showed the strongest correlation with these differences. The abundance profiles of several microbial groups were different between the two sites; class Nitrososphaeria amplicon sequence variants (ASVs) dominated the community profiles at Tramway Ridge, whereas Acidobacteriotal ASVs were only found at Western Crater. A co-occurrence network, paired with physicochemical analyses, allowed for finer scale analysis of parameters correlated with differential abundance profiles, with various parameters (total carbon, total nitrogen, soil moisture, soil conductivity, sulfur, phosphorous, and iron) showing significant correlations. ASVs assigned to Chloroflexi classes Ktedonobacteria and Chloroflexia were detected at both sites. Based on the known metabolic capabilities of previously studied members of these groups, we predict that chemolithotrophy is a common strategy in this system. These analyses highlight the importance of conducting broader-scale metagenomics and cultivation efforts at Mt. Erebus to better understand this unique environment.

Small investments with big returns: environmental genomic bioprospecting of microbial life

Microorganisms and their natural products are major drivers of ecological processes and industrial applications. Microbial bioprospecting has been critical for the advancement in various fields such as pharmaceuticals, sustainable industries, food security and bioremediation. Next generation sequencing has been paramount in the exploration of diverse environmental microbiomes. It presents a culture-independent approach to investigating hitherto uncultured taxa, resulting in the creation of massive sequence databases, which are available in the public domain. Genome mining searches available (meta)genomic data for target biosynthetic genes, and combined with the large-scale public data, this in-silico bioprospecting method presents an efficient and extensive way to uncover microbial bioproducts. Bioinformatic tools have progressed to a stage where we can recover genomes from the environment; these metagenome-assembled genomes present a way to understand the metabolic capacity of microorganisms in a physiological and ecological context. Environmental sampling been extensive across various ecological settings, including microbiomes with unique physicochemical properties that could influence the discovery of novel functions and metabolic pathways. Although in-silico methods cannot completely substitute in-vitro studies, the contextual information it provides is invaluable for understanding the ecological and taxonomic distribution of microbial genotypes and to form effective strategies for future microbial bioprospecting efforts.

Habitability Is Binary, But It Is Used by Astrobiologists to Encompass Continuous Ecological Questions

The term "habitability" is pervasive throughout the space sciences and astrobiology literature and is broadly used to describe an environment's ability to support life. Here, we argue that, while it is fundamentally a binary matter whether an organism can persist in an environment or not, these binary assessments lead to continuous ecological measurements that are often collected under the umbrella term "habitability" by astrobiologists. Although the use of habitability in this way has provided a framework for those studying the potential of environments to support life, including comparative analyses between terrestrial and extraterrestrial environments, it can also generate confusion and limit interdisciplinary understanding. Namely, differing ecological metrics used as proxies for habitability can yield differing conclusions depending upon the metrics chosen. Therefore, we suggest that in this continuous sense, the terms habitable and habitability lose meaning unless the specific scientific question and biological metric chosen to address it are defined. As a corollary, the search for universal single metrics to make habitability assessments is not to be encouraged, and as we argue, attempting to do so would oversimply analyses of the ability of environments to support life.

Spectroscopic characterization of samples from different environments in a Volcano-Glacial region in Iceland: Implications for in situ planetary exploration

Raman spectroscopy and laser induced breakdown spectroscopy (LIBS) are complementary techniques that together can provide a comprehensive characterization of geologic environments. For landed missions with constrained access to target materials on other planetary bodies, discerning signatures of life and habitability can be daunting, particularly where the preservation of organic compounds that contain the building blocks of life is limited. The main challenge facing any spectroscopy measurements of natural samples is the complicated spectra that often contain signatures for multiple components, particularly in rocks that are composed of several minerals with surfaces colonized by microbes. The goal of this study was to use the combination of Raman spectroscopy and LIBS to discern different environmental regimes based on the identification of minerals and biomolecules in rocks and sediments. Iceland is a terrestrial volcano-glacial location that offers a range of planetary analog environments, including volcanically active regions, extensive lava fields, geothermal springs, and large swaths of ice-covered terrain that are relevant to both rocky and icy planetary bodies. We combined portable VIS (532 nm) and NIR (785 nm) Raman spectroscopy, VIS micro-Raman spectroscopic mapping, and UV/VIS/NIR (200 - 1000 nm) and Mid-IR (5.6 - 10 μm, 1785 - 1000 cm^-1) laser induced breakdown spectroscopy (LIBS) to characterize the mineral assemblages, hydrated components, and biomolecules in rock and sediment samples collected from three main sites in the volcanically active Kverkfjöll-Vatnajökull region of Iceland: basalt and basalt-hosted carbonate rind from Hveragil geothermal stream, volcanic sediments from the base of Vatnajökull glacier at Kverkfjöll, and lava from the nearby Holuhraun lava field. With our combination of techniques, we were able to identify major mineral polytypes typical for each sample set, as well as a large diversity of biomolecules typical for lichen communities across all samples. The anatase we observed using micro-Raman spectroscopic mapping of the lava compared with the volcanic sediment suggested different formation pathways: lava anatase formed authigenically, sediment anatase could have formed in association with microbial weathering. Mn-oxide, only detected in the carbonate samples, seems to have two possible formation pathways, either by fluvial or microbial weathering or both. Even with our ability to detect a wide diversity of biomolecules and minerals in all of the samples, there was not enough variation between each set to distinguish different environments based on the limited measurements done for this study.

Organic Material Distribution in Mars-Analog Volcanic Rocks, as Determined with Ultraviolet Laser-Induced Fluorescence Spectroscopy

Understanding the distribution of trace organic material in a rocky environment is a key to constraining the material requirements for sustaining microbial life. We used an ultraviolet laser-induced fluorescence (LIF) spectroscopy instrument to characterize the distribution of organic biosignatures in basalts collected from two Mars-analog environments. We correlated the fluorescence results with alteration-related sample properties. These samples exhibit a range of alteration conditions found in the volcanic environments of Hawai'i Volcanoes National Park, Hawai'i (HI), and Craters of the Moon National Monument, Idaho (ID), including fumarolic systems. LIF mapping of the sample surfaces and interiors showed a heterogeneous distribution of areas of highly fluorescent material (point[s]-of-interest [POIs])-with fluorescence characteristics indicative of organic material. Results suggest that POIs are associated with secondary alteration mineral deposits in the rock's vesicles, including zeolites and calcite. Scanning electron microscopy with electron-dispersive X-ray spectroscopy was used to characterize the mineralogy present at POIs and support the evidence of carbon-bearing material. Overall, samples collected proximate to active or relict meteoric fumaroles from Hawai'i were shown to contain evidence for organic deposits. This suggests that these minerals are measurable spectroscopic targets that may be used to inform sample-site selection for astrobiology research.

A rather dry subject; investigating the study of arid-associated microbial communities

Almost one third of Earth's land surface is arid, with deserts alone covering more than 46 million square kilometres. Nearly 2.1 billion people inhabit deserts or drylands and these regions are also home to a great diversity of plant and animal species including many that are unique to them. Aridity is a multifaceted environmental stress combining a lack of water with limited food availability and typically extremes of temperature, impacting animal species across the planet from polar cold valleys, to Andean deserts and the Sahara. These harsh environments are also home to diverse microbial communities, demonstrating the ability of bacteria, fungi and archaea to settle and live in some of the toughest locations known. We now understand that these microbial ecosystems i.e. microbiotas, the sum total of microbial life across and within an environment, interact across both the environment, and the macroscopic organisms residing in these arid environments. Although multiple studies have explored these microbial communities in different arid environments, few studies have examined the microbiota of animals which are themselves arid-adapted. Here we aim to review the interactions between arid environments and the microbial communities which inhabit them, covering hot and cold deserts, the challenges these environments pose and some issues arising from limitations in the field. We also consider the work carried out on arid-adapted animal microbiotas, to investigate if any shared patterns or trends exist, whether between organisms or between the animals and the wider arid environment microbial communities. We determine if there are any patterns across studies potentially demonstrating a general impact of aridity on animal-associated microbiomes or benefits from aridity-adapted microbiomes for animals. In the context of increasing desertification and climate change it is important to understand the connections between the three pillars of microbiome, host genome and environment.

Analysis of bacterial and archaeal communities associated with Fogo volcanic soils of different ages

Basaltic rocks play a significant role in CO2 sequestration from the atmosphere during their weathering. Moreover, the primary microorganisms that colonize them, by providing mineral elements and nutrients, are shown to promote growth of diverse heterotrophic communities and plants, therefore positively impacting Earth's long-term climate balance. However, the first steps of microbial colonization and subsequent rock weathering remain poorly understood, especially regarding microbial communities over a chronological sequence. Here, we analyzed the microbial communities inhabiting the soil developed in crevices on lava flows derived from different eruptions on Fogo Island. Investigated soils show typically low carbon and nitrogen content and are relatively similar to one another regarding their phylogenetic composition, and similar to what was recorded in large soil surveys with dominance of Actinobacteria and Proteobacteria. Moreover, our results suggest a stronger effect of the organic carbon than the lava flow age in shaping microbial communities as well as the possibility of exogenous sources of bacteria as important colonizers. Furthermore, archaea reach up to 8.4% of the total microbial community, dominated by the Soil Crenarchaeotic Group, including the ammonium-oxidizer Candidatus Nitrososphaera sp. Therefore, this group might be largely responsible for ammonia oxidation under the environmental conditions found on Fogo.

The Case for Ancient Hot Springs in Gusev Crater, Mars

The origin and age of opaline silica deposits discovered by the Spirit rover adjacent to the Home Plate feature in the Columbia Hills of Gusev crater remains debated, in part because of their proximity to sulfur-rich soils. Processes related to fumarolic activity and to hot springs and/or geysers are the leading candidates. Both processes are known to produce opaline silica on Earth, but with differences in composition, morphology, texture, and stratigraphy. Here, we incorporate new and existing observations of the Home Plate region with observations from field and laboratory work to address the competing hypotheses. The results, which include new evidence for a hot spring vent mound, demonstrate that a volcanic hydrothermal system manifesting both hot spring/geyser and fumarolic activity best explains the opaline silica rocks and proximal S-rich materials, respectively. The opaline silica rocks most likely are sinter deposits derived from hot spring activity. Stratigraphic evidence indicates that their deposition occurred before the emplacement of the volcaniclastic deposits comprising Home Plate and nearby ridges. Because sinter deposits throughout geologic history on Earth preserve evidence for microbial life, they are a key target in the search for ancient life on Mars.

Mapping metabolic activity at single cell resolution in intact volcanic fumarole sediment

Interactions among microorganisms and their mineralogical substrates govern the structure, function and emergent properties of microbial communities. These interactions are predicated on spatial relationships, which dictate metabolite exchange and access to key substrates. To quantitatively assess links between spatial relationships and metabolic activity, this study presents a novel approach to map all organisms, the metabolically active subset and associated mineral grains, all while maintaining spatial integrity of an environmental microbiome. We applied this method at an outgassing fumarole of Vanuatu's Marum Crater, one of the largest point sources of several environmentally relevant gaseous compounds, including H2O, CO2 and SO2. With increasing distance from the sediment-air surface and from mineral grain outer boundaries, organism abundance decreased but the proportion of metabolically active organisms often increased. These protected niches may provide more stable conditions that promote consistent metabolic activity of a streamlined community. Conversely, exterior surfaces accumulate more organisms that may cover a wider range of preferred conditions, implying that only a subset of the community will be active under any particular environmental regime. More broadly, the approach presented here allows investigators to see microbial communities ‘as they really are’ and explore determinants of metabolic activity across a range of microbiomes.

The BASALT Research Program: Designing and Developing Mission Elements in Support of Human Scientific Exploration of Mars

The articles associated with this Special Collection focus on the NASA BASALT (Biologic Analog Science Associated with Lava Terrains) Research Program, which aims at answering the question, "How do we support and enable scientific exploration during human Mars missions?" To answer this the BASALT team conducted scientific field studies under simulated Mars mission conditions to both broaden our understanding of the habitability potential of basalt-rich terrains on Mars and examine the effects of science on current Mars mission concepts of operations. This article provides an overview of the BASALT research project, from the science, to the operational concepts that were tested and developed, to the technical capabilities that supported all elements of the team's research. Further, this article introduces the 12 articles that are included in this Special Collection.

Requirements for Portable Instrument Suites during Human Scientific Exploration of Mars

Human explorers on the surface of Mars will have access to a far wider array of scientific tools than previous crewed planetary exploration missions, but not every tool will be compatible with the restrictions of this exploration. Spectrometers on flyby, orbital, and landed missions are currently used to determine the composition and mineralogy of geological materials of various types and sizes, from small fragments to celestial bodies in the solar system. Handheld spectrometers that are capable of in situ analyses are already used for geological exploration on Earth; however, their usefulness for human exploration missions and how data from multiple handheld instruments could be combined to enhance scientific return must be further evaluated. As part of the Biologic Analog Science Associated with Lava Terrains (BASALT) research project, we incorporated two handheld instruments, a visible-near infrared spectrometer and an X-Ray Fluorescence spectrometer, into simulated Mars exploration missions conducted on basaltic terrains in Idaho and Hawai'i. To understand the data quality provided by these handheld spectrometers, we evaluated their performance under varying conditions of measurement time, distance, angle, atmosphere, and sample matrix, and we compared data quality between handheld instruments and laboratory techniques. Here, we summarize these findings, provide guidelines and requirements on how to effectively incorporate these instruments into human exploration missions to Mars, and posit that future iterations of these instruments will be beneficial for enhancing science returned from human exploration missions.

Basaltic Terrains in Idaho and Hawai'i as Planetary Analogs for Mars Geology and Astrobiology

Field research target regions within two basaltic geologic provinces are described as Earth analogs to Mars. Regions within the eastern Snake River Plain of Idaho and the Big Island of Hawai'i, the United States, provinces that represent analogs of present-day and early Mars, respectively, were evaluated on the basis of geologic settings, rock lithology and geochemistry, rock alteration, and climate. Each of these factors provides rationale for the selection of specific targets for field research in five analog target regions: (1) Big Craters and (2) Highway lava flows at Craters of the Moon National Monument and Preserve, Idaho, and (3) Mauna Ulu low shield, (4) Kīlauea Iki lava lake, and (5) Kīlauea caldera in the Kīlauea Volcano summit region and the East Rift Zone of Hawai'i. Our evaluation of compositional and textural attributes, as well as the effects of syn- and posteruptive rock alteration, shows that basaltic terrains in Idaho and Hawai'i provide a way to characterize the geology and major geologic substrates that host biological activity of relevance to Mars exploration. This work provides the foundation to better understand the scientific questions related to the habitability of basaltic terrains, the rationale behind selecting analog field targets, and their applicability as analogs to Mars.