Biology subjects

Graham, H. V.

Publications and source records attributed to Graham, H. V..

4 recordsLinked to original sources

bioRxiv2026

Compositional Heterogeneity Structures Microbial Microhabitats across Distinct Mineral Substrates

Microbial communities living on and in rocks operate at the microscale, where interactions with minerals fundamentally shape community structure and function. Yet the relationship between micron scale mineralogical configurations and microbial distributions remains poorly understood. We tested the hypothesis that microbial biomass spatially correlates with areas of heightened mineralogical heterogeneity by applying Raman microspectroscopy to rock samples from three geologically distinct substrates: authigenic carbonates from a marine methane seep, volcanic basalt from Iceland, and polymetallic nodules from the abyssal seafloor. Using spectral decomposition and multiple complementary metrics of compositional heterogeneity, we evaluated intra-pixel and inter-pixel heterogeneity patterns in relation to biomass distribution. Our analyses reveal three patterns across all sample types. 1) When spectra are deconstructed into their constituent components, biomass zones are disproportionately dominated by the biomass spectral component compared with primary mineral components in zones of different minerals. 2) Biomass spectra have more homogeneous compositional profiles than mineral spectra. 3) Biomass is surrounded by more heterogeneous microhabitats than mineral pixels. These findings demonstrate that biomass exerts a distinctive and consistent influence on Raman spectral signatures, both within and between pixels, in ways that mineral components do not. Our results establish generalizable principles linking microscale mineralogical properties to microbial biogeography; these properties could be used as a potential biosignature and may provide a standardized workflow applicable to diverse rock systems and astrobiological exploration strategies.

microbiology↗

bioRxiv2025

Volatilized ammonia supports extremophilic cave ecosystems with unusual nitrogen isotopic signatures

The sulfidic Frasassi cave system hosts a robust, subterranean ecosystem based on microbial lithoautotrophy. Curiously, acidic biofilms forming above degassing sulfidic cave streams, and the invertebrates that feed on them, are extremely depleted in nitrogen-15 ({delta}15N values less than -20{per thousand}). In this study, we tested the hypothesis that these low {delta}15N values result from the volatilization, trapping, and uptake of ammonia degassed from the circumneutral streams. We found that dissolved ammonium in the streams had {delta}15N values near +3{per thousand}, whereas NH3(g) in the cave atmosphere above streams exhibited {delta}15N values as low as -27{per thousand}, consistent with fractionation by NH3 volatilization. Extremely acidic condensation droplets on cave walls efficiently trapped airborne NH3, accumulating up to 4 mM NH4+ with {delta}15N values as low as -29{per thousand}, thereby confirming volatilized and trapped ammonia as the primary N source to cave wall biofilms and the extensive subsurface ecosystem they support. Airborne ammonia trapping represents a novel mechanism for biological N acquisition and provides abundant N for growth in an extreme subsurface environment that otherwise receives very limited nutrient input. SignificanceNitrogen is one of the most abundant elements in organic molecules, and is a key limiting nutrient in many ecosystems. We showed that acidic microbial biofilms in sulfidic caves scavenge trace amounts of airborne ammonia, enabling microbial primary production and supporting associated food webs, including animals, in an environment where nitrogen is otherwise extremely scarce. This process represents a novel mechanism of biological nutrient acquisition and results in biomass and organic matter more depleted in the heavy isotope of nitrogen (15N) than almost all other biological materials on Earth. Extreme 15N isotope depletion is therefore a potential signature of acidic underground ecosystems on Earth and other planetary bodies.

microbiology↗

bioRxiv2025

Microbial ecology of acidic, biogenic gypsum: Community structure and distribution of extremophiles on freshly formed and relict sulfate deposits in a hydrogen sulfide-rich cave

Sulfate minerals are abundant on the Martian surface, and many of these evaporite deposits are thought to have precipitated from acidic fluids. On Earth, gypsum (CaSO4*2H2O) and other sulfates sometimes form under acidic conditions, so exploring the extremophilic life that occurs in these mineral environments can help us evaluate the astrobiological potential of acid sulfate depositional settings. Here, we characterized the microbial communities associated with acidic gypsum deposits in a sulfuric acid cave, where sulfate precipitation is driven by sulfide-oxidizing bacteria and archaea. We used 16S rRNA gene sequencing and cell counts to characterize gypsum-associated microorganisms in freshly formed and relict deposits throughout the cave, in order to test hypotheses about how microbial community composition and abundance would vary with distance from the sulfidic water table and with the concentration of H2S(g) and other gases in the cave atmosphere. We found that actively-forming gypsum in the lower cave levels was colonized by low diversity communities of sulfide-oxidizing chemolithotrophs and other acidophiles that have few cells compared to other environments in the cave. The most abundant taxa were Acidithiobacillus, Metallibacterium, Mycobacteria, and three different Thermoplasmatales-group archaea, which occupied distinct niches based on proximity to sulfidic streams and the concentration of gases in the cave air. In contrast, deposits in older cave levels had more diverse communities that are dominated by chemoorganotrophic and methanotrophic taxa. These findings show that acidic sulfate deposits serve as habitats for extremophilic microorganisms, and broaden our knowledge of the life associated with terrestrial sulfates. ImportanceGypsum and other sulfate salts are common on Mars, and many of these deposits are thought to have formed from acidic fluids early in the planets history. Understanding the life that survives and thrives in similar environments on Earth is therefore crucial for evaluating whether these Martian sulfates are or ever were habitable. One such environment where acidic gypsum occurs is in sulfuric acid caves, where extremophilic microorganisms drive the precipitation of sulfate minerals by oxidizing hydrogen sulfide gas from the cave atmosphere. Here, we characterized the communities of microorganisms on freshly formed and ancient gypsum in the Frasassi Caves, and found that the gypsum deposits hosted microbial communities that changed based on chemical energy availability and the age of the gypsum. Our findings underscore the importance of chemical and microbiological interactions in shaping habitable niches, and provide context for searching for past or present life in acidic Martian sulfates.

microbiology↗

bioRxiv2023

The Vacant Niche Revisited: Using Negative Results to Refine the Limits of Habitability

To define the boundaries of habitability, biologists often search for highly specialized organisms in extreme environments. However, negative life detections--when a method is unable to detect microorganisms in a given setting--are just as important to constrain the environmental limits of life on Earth. In turn, these limits inform the selection of targets for life detection on other worlds. We performed a comprehensive, though non-exhaustive, literature search for negative life detections in polyextreme environments. We then catalogued the physicochemical conditions at these sites to further understand the habitability limits for life on Earth and the effects of multiple stressors on habitability. Using multivariate statistical techniques, our study searched for combinations of environmental parameters where extremes support or inhibit life. Our search raised several methodological and analytical considerations relevant to life detection studies in extreme environments. Incomplete documentation of environmental factors and experimental protocol limitations in the extreme environment literature complicated our analyses. This demonstrates the need to report negative results, particularly in life detection experiments, and the potential value for standardized reporting protocols. Exploring the range of results possible from life-detection methodologies is key to constrain the limits of life on Earth and informs our search for life elsewhere.

microbiology↗