Biology subjects

Wankel, S. D.

Publications and source records attributed to Wankel, S. D..

3 recordsLinked to original sources

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

Nitrogen and oxygen isotope effects during enzymatic nitrate reduction in vitro and by natural lake water consortia

Nitrate (NO3-) isotope ratios are useful indicators for nitrogen (N)-transformation processes if the associated isotope effects and their environmental controls are well-constrained. The NO3- isotope effects in natural environments may depend on the type of dissimilatory nitrate reductases involved and the degree of isotopic overprinting via NO3- regeneration. We measured the coupled N and oxygen (O) isotope effects of NO3- reduction in anoxic incubation experiments with laboratory cultures (Pseudomonas sp. and Escherichia coli) harboring different nitrate reductase enzymes (Nar and/or Nap) as well as with natural freshwater consortia from Lake Lugano (Switzerland) and Lake La Cruz (Spain). For comparison, isotope effects were also evaluated through coupled N and O isotope measurements in the redox transition zone of Lake Lugano North Basin. Incubation-based Rayleigh model N isotope effects ({varepsilon}N) were variable, ranging from 9 to 30{per thousand}. In comparison, in situ {varepsilon}N values (5 to 14{per thousand}) estimated by the closed system model were lower, likely due to substrate limitation in the water column. Experiments with Pseudomonas sp. and E. coli cultures possessing Nar yielded N isotope effects of similar magnitudes and, consistent with previous data, robust {Delta}{delta}18O:{Delta}{delta}15N enrichment ratios of [~]0.9 - 1.0. Nitrate reduction by cultures possessing solely Nap led to lower {Delta}{delta}18O:{Delta}{delta}15N of [~]0.7. In anoxic incubations of lake water, where the effect of nitrification could be excluded, {Delta}{delta}18O:{Delta}{delta}15N values between 0.6 and 1.0 suggest "community activity" of both Nar and Nap. Interestingly, stimulation of lithotrophic nitrate reduction in incubations with amended NO3- + sulfide resulted in a {Delta}{delta}18O:{Delta}{delta}15N slope of 0.90 {+/-} 0.03 (standard error, SE), indicating Nar as the more dominant nitrate-reducing enzyme. On the contrary, stimulation of organotrophic nitrate reduction in NO3- + acetate amended incubations resulted in a significantly lower slope of 0.72 {+/-} 0.03 SE, suggesting a greater contribution by Nap. In contrast to the nitrate-reduction incubation experiments, the in situ {Delta}{delta}18O:{Delta}{delta}15N value of 1.36 {+/-} 0.14 SE observed in the Lake Lugano water column was unusually high for a freshwater environment, likely reflecting the superimposed effect of NO3- production by nitrification. Our study thus underscores that both variations in activity of Nar versus Nap during nitrate reduction, as well as isotopic overprinting by nitrate regeneration may impact ecosystem NO3- isotope dynamics in natural denitrifying environments.

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↗