Journal of Geophysical Research: Biogeosciences

Permafrost thaw exposes ancient organic matter to microbial degradation, which is predicted to release globally significant quantities of greenhouse gases into the atmosphere. Though microorganisms drive these processes, the relative importance of biotic (taxonomic and functional community composition) versus environmental (e.g., soil physicochemistry) drivers and their interactions are unknown. Using a novel in situ thaw experiment conducted at the Cold Regions Research and Engineering Laboratorys Permafrost Tunnel near Fairbanks, Alaska, we experimentally separated the effects of soil physicochemistry and microbial communities under "real-world" thaw conditions. To simulate thaw, active layer soil, Holocene permafrost (2 kya), and Pleistocene permafrost (40 kya) were sterilized, inoculated with microbial communities from the different soils, enclosed in 0.22 {micro}m membrane bags to prevent immigration, and buried in the active layer. We retrieved the bags after two weeks and two months of thaw and characterized microbial community structure (16S rRNA and ITS2 amplicon sequencing), functional potential (metagenome sequencing), and soil organic matter (OM) composition at the molecular level (FT-ICR MS). Soil had a stronger effect on bacterial community and gene assemblages than inoculum, and the effects of inoculum were stronger and longer-lasting on community structure than functional potential. Pleistocene permafrost initially contained approximately eleven times more dissolved organic carbon than the other soils, and was enriched in OM derived from microbial necromass and low molecular weight organic acids. This carbon was rapidly depleted during thaw and OM compositional characteristics became increasingly similar to active layer and Holocene permafrost, paralleling shifts in Pleistocene permafrost functional gene profiles and bacterial community structure towards those of other soils. Overall, this work provides new insights into the susceptibility of OM to microbial degradation in compositionally distinct permafrost soils, and ways in which Pleistocene Yedoma permafrost carbon is likely to be particularly vulnerable to permafrost thaw.

Mars relatively moderate surface conditions, availability of solar energy, and in situ resources like water ice, carbon dioxide, and mineral-rich regolith make it a compelling target for supporting life beyond Earth. However, existing experiments testing habitability in Mars conditions generally rely on leachates of physical regolith simulants, which vary in composition across simulant types, leaching conditions, and production batches. We introduce a defined Mars media (DMM) that accurately simulates the biologically relevant nutrients (nitrogen, phosphorus, and sulfur) and stressors (perchlorates, heavy metals) in Martian regolith when it is leached in water at neutral pH. We formulated DMM by combining direct rover and lander measurements from Mars with laboratory measurements of regolith simulant leachates. We validate DMM from a lx to 20x concentrate, equivalent to 40 g/L to 800 g/L of leached regolith. Using DMM with acetate as a Mars atmosphere-derived carbon source, we grew eight heterotrophic bacteria, confirming that organisms can source all essential nutrients from Martian resources. We also show that microbial growth in DMM is robust to uncertainties in Martian regolith composition: sensitivity experiments can identify limiting trace element nutrients and toxins in DMM, and demonstrate that bacterial growth is maintained across at least an order of magnitude variation in their concentrations. This is the first defined Mars regolith media recipe containing both macro- and micro- nutrients, and designed specifically for biological experimentation. By shifting from variable leachate-based approaches to a defined aqueous analog, we enable controlled hypothesis testing of microbial survival, growth, and function. DMM will enable further research on astrobiology, biological in situ resource utilization, large-scale soil remediation, and terraforming. O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=121 SRC="FIGDIR/small/719001v1_ufig1.gif" ALT="Figure 1"> View larger version (28K): org.highwire.dtl.DTLVardef@1314b20org.highwire.dtl.DTLVardef@13b57d4org.highwire.dtl.DTLVardef@103315eorg.highwire.dtl.DTLVardef@9e18fe_HPS_FORMAT_FIGEXP M_FIG C_FIG

Warming of the Arctic enhances microbial activity and the decomposition of large stocks of organic matter retained in permafrost soil. Resulting changes in the availability of sulfur may lead to increased emissions of volatile sulfur compounds (VSCs), which impact atmospheric particle and cloud formation, terrestrial and aquatic acidification, and malodor. Marine microbial production of dimethyl sulfide (DMS) has been studied for decades but other VSCs have been largely ignored, while VSC emissions from terrestrial ecosystems are even less studied. Currently, we lack fundamental understanding of the metabolic processes behind VSC production in permafrost soil bacteria, essential for estimating how emissions may change due to thawing. To fill this knowledge gap, we measured VSC emissions from thawing permafrost and three bacterial strains isolated from Greenlandic permafrost and biological soil crust. We show that the bacterial strains produced high levels of VSCs in vitro - including hydrogen sulfide, methanethiol, DMS, dimethyl disulfide, and dimethyl trisulfide. We further show that the same VSCs were also emitted from permafrost upon thaw. Metabolic pathway mapping of the bacterial strains revealed both inorganic sulfate reduction pathways and amino acid metabolism behind bacterial VSC production. High production of VSCs in the late-active and stationary phase suggests connection to secondary metabolism, except for DMS which was linked to early growth, and possibly, primary energy metabolism. Our findings suggest that thawing increases VSC emissions from permafrost soil, possibly leading to higher input of sulfur into the atmosphere from the warming Arctic in the future.

Snow algae darken snowpacks and accelerate melt worldwide. Although elevation strongly structures the physical conditions of mountain snowfields, its influence on snow algal traits and their effects on snowpack reflectance remains unclear. Here, we investigated snow algal composition, cellular traits, and optical properties in summer blooms across an elevational range of 1,059-3,423 m a.s.l. in the western United States, spanning two elevational gradients in the Cascade Range (CA, OR, WA) and the Rocky Mountains (UT, WY, MT). Across all samples (n = 294), snow albedo declined strongly with increasing algal cell density, indicating that total biomass, rather than pigment composition, is the dominant driver of albedo reduction. However, within Sanguina-dominated blooms (117 of 206 samples bloom samples identified across the dataset), neither relative abundance nor algal cell density varied systematically with elevation. Instead, mean cell size increased with elevation, while per-cell pigment concentrations declined, leading to higher astaxanthin:chlorophyll-a ratios driven primarily by reductions in chlorophyll-a per cell. These elevation-dependent shifts in cell size and pigment balance were consistent across both mountain ranges, indicating phenotypic acclimation to increasing environmental stress with elevation. Together, these findings link cellular-scale acclimation of a widespread snow alga to radiative processes shaping mountain snowpacks.

Northern peatlands store large carbon stocks but are sensitive to disturbance. Hydrology, vegetation, herbivory and snow conditions may affect the soil microorganisms driving methane (CH) and nitrous oxide (N2O) cycling. We investigated how reindeer exclusion and snow depth (increased and reduced relative to ambient) manipulations (ongoing for three seasons) influenced archaeal and bacterial communities in a boreal rich fen. Metagenomic (MG) and metatranscriptomic (MT) sequencing were combined with pore-water chemistry and CH flux measurements to link the microbiome to ecosystem processes. Microbial communities differed between outside and inside the exclosure. However, these patterns primarily reflected underlying hydrological variation. Slightly wetter inside plots showed higher expression of denitrification genes (norB, nosZ) and lower (nirS+nirK)/nosZ ratios, indicating greater potential for complete denitrification to N2 instead of N2O. Methane dynamics were mainly associated with vegetation: plots associated with Carex rostrata exhibited lower pmoA/mcrA ratios and elevated CH fluxes. Snow manipulations had subtle effects: reduced snow depth decreased the expression of taxa dependent on microbial interactions, while the effect to the investigated metabolic marker genes was small. Overall hydrology, leading to variations in redox conditions and nutrient availability, together with vegetation appeared as the primary drivers on microbial greenhouse gas processes in this peatland.

Quantifying the lipid biosynthesis rate of archaea in hot spring sediments is necessary to interpret the abundance, isotopic patterns, and environmental significance of archaeal lipid biosignatures, with implications for modern biogeochemical cycling and astrobiology. Here, we performed lipid hydrogen stable isotope probing (LH-SIP) experiments on whole sediments collected from two high-temperature, suboxic, circumneutral hot springs in Yellowstone National Park (USA) and El Tatio Geyserfield (Chile). We determined the incorporation of 2H2O into intact polar lipids (IPLs) which provides a taxon- and metabolism-agnostic quantification of biosynthesis under near-natural conditions. We targeted isoprenoid glycerol dialkyl glycerol tetraether lipids (IPL iGDGTs) and recovered structures with 0 to 7 cyclopentyl rings from both springs. We observed minor 2H-uptake into archaeal IPLs in spring sediments in Yellowstone, corresponding to decadal-scale apparent generation times (16 {+/-} 7 years), and no uptake in El Tatio sediments (consistent with minimum generation times of 35 {+/-} 5 years). We infer that net production of sedimentary IPL-iGDGTs is very slow, consistent with a combination of slow archaeal growth, persistence of older IPLs, lipid recycling, and/or contributions from recently sedimented planktonic biomass. These are the first direct, ex situ estimates of archaeal lipid production rates in terrestrial hydrothermal systems using LH-SIP incubations and provide critical constraints for interpreting archaeal lipids in ancient hot spring deposits. This research establishes a framework for assessing activity by slow-growing extremophilic archaea in hydrothermal environments and provides support for targeting hydrothermal deposits on Mars for biosignature detection efforts. Plain Language SummaryHot springs on Earth are important natural laboratories for understanding how signs of life might form and be preserved in hydrothermal environments on early Earth or Mars. In this study, we examine the rate of archaeal lipid biosignature production in sediments from two hot springs in Yellowstone National Park and the El Tatio Geyserfield in Chile. We used a method that measures new microbial production by tracing heavy hydrogen from labeled water as microbes incorporate that hydrogen into newly made lipids in their cell membranes. We found that archaeal lipids in hot spring sediments are produced very slowly, on timescales of decades. This result, along with the chemical stability of lipids and the rapid mineralization rate in hot springs, may allow these molecular biosignatures to be entombed and preserved in hot spring mineral deposits. These results help us better interpret ancient hydrothermal deposits on Earth and support the idea that slowly growing microbial communities could still leave detectable molecular traces in similar environments on Mars and other rocky planets. Key PointsO_LILipid hydrogen stable isotope probing is applied to high temperature hot spring sediments for the first time C_LIO_LIIn hot spring sediments, archaeal lipid production occurs on decadal timescales comparable to some marine sediments, but are much faster than the century- to millennia-scale rates observed in the deep subsurface C_LIO_LIConfirmation of archaeal lipid synthesis in hot spring sediments adds additional support for targeting Martian hydrothermal deposits for biosignature detection efforts C_LI

The Arctic is warming rapidly, causing permafrost thaw and accelerating the release of greenhouse gases. Rapid thaw features such as retrogressive thaw slumps are increasing in frequency and severity across the Arctic; however, their associated greenhouse gas emissions are poorly constrained. Current estimates of emissions from retrogressive thaw slumps rely largely on laboratory incubations and carbon stock estimates rather than in-situ field measurements. Here we directly quantify methane and carbon dioxide fluxes from the exposed headwall of an active retrogressive thaw slump. We show that thaw immediately releases biogenic methane and carbon dioxide, originating from gases trapped within the frozen soil matrix. Microbial transcription of methyl-coenzyme M reductase suggests archaea carrying out methanogenesis at subzero temperatures are the source of trapped methane. Carbon emissions varied by an order of magnitude among cryostratigraphic units, reflecting differences in geomorphologic history, organic carbon and nitrogen content, and microbial community composition. Carbon emissions were highest from organic-rich paleo cryosols from the Late Holocene that contained abundant methanogenic archaea. We estimate that [~]300 kg C (CO2 equivalents) is emitted annually from the headwall of this small thaw slump (surface area of [~]1200 m2). Considering the thousands of active slumps and extensive coastal permafrost erosion across the northern continuous permafrost zone, such features may represent a growing natural source of GHG emissions. These findings indicate that current permafrost carbon feedback models underestimate GHG release by omitting the direct release of trapped gases stored in permafrost.

Evidence from the Cassini mission confirmed that Saturn's moon Enceladus hosts a subsurface alkaline ocean where rock-water reactions may generate redox disequilibria capable of supporting microbial metabolisms. To investigate potential microbial survival under simulated Enceladus ocean conditions, we used thermodynamic modeling to develop a salt formulation consistent with one possible Enceladus ocean composition and supplemented it with putative microbial energy sources to create a growth medium. The medium was inoculated with samples from diverse ocean world analog environments on Earth to determine which microorganisms could persist under Enceladus-like conditions. The microorganisms persisting in this geochemically bounded medium were heterotrophic, metabolically versatile bacteria with low carbon requirements. Genomic and physiological analyses further showed the presence of multiple stress-response pathways, sodium- based bioenergetic systems, osmoregulation strategies, and other adaptations consistent with survival in alkaline, low-nutrient settings. These results suggest that some stress-tolerant heterotrophic bacteria may serve as useful model organisms for life in Enceladus' subsurface ocean. These findings demonstrate the value of geochemically modeled media as a framework for constraining habitability, identifying relevant biosignatures, and probing potential microbial survival strategies beyond Earth.

Soil protists shape microbial food webs and nutrient cycling, yet methods for measuring their population growth in soil have lagged behind the taxonomic resolution available from 18S rRNA gene sequencing. Microscopy-based approaches can estimate abundance and growth, but with limited taxonomic resolution. Here, we tested whether quantitative stable isotope probing (qSIP) can provide reproducible, sequencing-resolved growth measurements for soil protists. We incubated soil with natural-abundance or {superscript 1} O-labeled water and measured taxon-specific 18O enrichment in DNA using three common 18S rRNA gene primer sets. 18O enrichment values were positively correlated across datasets, with relationships closest to 1:1 after poorly resolved taxonomic assignments were excluded, indicating that qSIP provides reproducible population-level growth signals across marker choices. We then compared 18S amplicon profiles from unfractionated DNA with qSIP-derived growth measurements across a soil moisture gradient. Amplicon profiles showed small shifts in relative abundances of major protist groups, whereas qSIP revealed a large moisture response in the growing community: 7 ASVs were growing at 20% field capacity compared with 143 at 80% field capacity, representing 1.6% and 63.3% of total protist relative abundance, respectively. Average growth was <1% day{square}1 in the two driest treatments, increasing to 2.1% day{square}1 at 60% and 5.6% day{square}1 at 80% field capacity; among growing taxa, rates averaged 8.7% and 8.3% day{square}1 in the two wetter treatments. By pairing taxonomic resolution with isotope-based growth estimates, qSIP with 18O-H2O moves soil protist ecology toward quantitative population dynamics: identifying which taxa grow, how fast, and how growth responds to the environment.

Mangrove forests are major blue carbon ecosystems but are often characterized by low surface methane (CH4) emissions. Such low emissions, however, do not necessarily indicate weak methanogenesis, because CH4 production may be offset by internal CH4 consumption before reaching the atmosphere. Although previous community, genomic, and transcriptomic studies have implicated methylotrophic methanogenesis in mangrove sediments, direct taxon-resolved evidence linking methylated carbon assimilation to potentially active methanogens remains limited. Here, we combined methanogenic activity assays, DNA stable isotope probing (DNA-SIP), mcrA and 16S rRNA gene analyses, and phylogenetic comparisons to identify potentially active methanogens across saline-influenced mangrove soils. The results showed that CH4 production potentials were consistently dominated by methylotrophic pathways (1.86-2.78 g CH4 g-1 soil hr-1) across all sites. DNA-SIP, together with consistent community patterns in fresh soils, indicated the potential activity of methylotrophic and mixotrophic methanogens under saline conditions. Methanolobus-affiliated methanogens were associated with salinity, Na+, Cl-, and NH4+, whereas Methanosarcina and unclassified Methanosarcinaceae were linked to soil soluble organic carbon availability and water content, indicating niche differentiation among active methanogenic groups. Phylogenetic analyses incorporating reference sequences from diverse environments further showed that potentially active mangrove methanogens were dominated by saline-associated lineages. Together with our previous methanotrophic evidence from the same sites, these findings suggest that low CH4 emissions from mangrove blue carbon ecosystems can mask substantial internal CH4 cycling sustained by active methanogenesis and CH4 consumption.

A substantial fraction of marine sediments experience elevated temperatures due to burial or hydrothermal activity. These conditions can fundamentally reshape both microbial activity and the chemical nature of sedimentary organic matter (OM). Laboratory incubations have demonstrated that moderate heating of marine sediments can lead to the production of labile organic compounds such as acetate, however, it remains unclear whether heating alters the bioavailability of the remaining OM pool. In this study, we experimentally tested the effect of temperature on the bioavailability of OM through a series of bioreactor experiments using deeply buried sediment collected from Guaymas Basin (Gulf of California). We measured acetate concentrations in sterilized Guaymas Basin sediments before and after artificial heating (70{degrees}C for 7 days) to quantify abiotic acetate generation. We then conducted incubations of a model marine bacterium with sterilized, artificially heated sediment and tracked respired CO2 production and its associated 13C and 14C signatures. Our study revealed that sediment depth and hydrothermal history strongly control abiotic acetate production, with higher acetate yields from shallower, cooler sediments. Respiration rates in control and heated sediment incubations were nearly identical, indicating that heating does not measurably alter the bioavailability of bulk sedimentary OM. Moreover, the {delta}13C values of respired CO2 were indistinguishable between control and heated sediment incubations while the {Delta}14C values were more depleted in the first 24 hours in incubations with heated sediment. This transient offset suggests that low-temperature heating mobilizes a small pool of older material due to desorption of mineral-bound OM without altering overall bioavailability. Our findings shed light on the role of thermal alteration in shaping carbon cycling in marine sediments by influencing how OM is made available to sedimentary microorganisms.

Rock surface weathering is a critical element in the process of early soil formation, in which lichens are thought to play a significant role. Crustose lichens, with a large area of rock-surface contact, are generally considered more influential in rock weathering, while foliose and fruticose growth forms, with more developed three-dimensional structure and less rock-surface contact, are rarely considered in this context. Here, we test the extent to which all three growth forms contribute to granitic rock surface weathering in Maritime Antarctic ecosystems, by quantifying rock hardness beneath foliose (n = 2 species), fruticose (n = 2) and crustose lichens (n= 5). Our data confirm that foliose lichens reduced rock surface hardness by 9%, to a lesser extent than crustose and foliose lichens (40% and 31% reduction, respectively). To disentangle whether these effects result from lichen-induced weathering or lichen preference for pre-weathered rock, we also analyzed a dated deglaciation sequence on granitic rocks from the Morteratsch Glacier forefield in the Swiss Alps. At this location, the impact of crustose lichens on rock substrate hardness generally increased with time since exposure from glacial retreat and with lichen thallus size. We conclude that lichen presence on rock surfaces significantly reduces rock hardness, with crustose lichens having a greater impact than foliose and fruticose forms, highlighting the potential role of lichens of all three growth forms in driving substrate breakdown and shaping early-stage ecosystem processes in polar and alpine regions.

In the next years, several space missions will search for evidence of life on exoplanets, focusing on robust biosignatures associated with oxygenic photosynthesis, including atmospheric oxygen accumulation and the Vegetation Red-Edge in surface reflectance spectra. Many potentially habitable rocky exoplanets orbit M-dwarf stars, whose spectral energy distribution may challenge oxygenic photosynthesis. Differently from the Sun, M-dwarf stars emit predominantly far-red (700- 750 nm) and infrared (750-1000 nm) light, and relatively little visible (400-700 nm) radiation, which constitutes photosynthetically active radiation. Some organisms have been found to photosynthesize under such spectrum but less efficiently than under solar light, as their photosynthetic apparatus evolved to harvest visible light emitted by the Sun. Around M-dwarfs, such different irradiation might have selected adaptations optimized for harvesting far-red / infra-red light. On Earth, similar selection can be found in Acaryochloris marina strains, constitutively presenting high chlorophyll d content in photosystem II & I, with in vivo absorption peaks beyond 700 nm. Here we tested the Moss Beach strain under a simulated M-dwarf spectrum and a simulated primeval atmosphere - anoxic and enriched in carbon dioxide. Results underline how this permanently red-shifted photosynthetic apparatus does not require acclimation to the stellar spectrum and enables for a strong growth and oxygen production, higher than under simulated solar light. Moreover, cells reflectance spectrum highlights a shift of the canonical red-edge toward longer wavelengths, resulting in a Chl d-near-infrared edge, suggesting a similar metabolism on exoplanets orbiting M-dwarfs could successfully produce both a gaseous biosignature and a characteristic surface biosignature. Graphical abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=144 SRC="FIGDIR/small/719884v1_ufig1.gif" ALT="Figure 1"> View larger version (39K): org.highwire.dtl.DTLVardef@7f91bdorg.highwire.dtl.DTLVardef@1391bdborg.highwire.dtl.DTLVardef@53f7b4org.highwire.dtl.DTLVardef@ab59fa_HPS_FORMAT_FIGEXP M_FIG C_FIG Created in BioRender. Liistro, E. (2026) https://BioRender.com/j2de4ay

Biochar amendments are increasingly applied to improve soil physical functioning and support carbon dioxide removal, but their effects on intrinsic soil thermal properties remain poorly characterised. We conducted the first global systematic meta-analysis of 19 independent studies, 231 control-biochar comparisons, and 529 property-specific effect sizes to test how biochar changes soil heat transfer and storage. Biochar reduced thermal conductivity by 17.6% (95% CI, -22.7 to -12.2), thermal diffusivity by 11.0% (-14.5 to -7.3), and volumetric heat capacity by 8.3% (-12.3 to -4.1). Gravimetric heat capacity showed no significant overall response (+3.3%; -7.6 to 15.4) but was supported by fewer studies. Negative responses were directionally consistent for thermal conductivity, diffusivity, and volumetric heat capacity. Moderator analyses showed that responses were most consistently associated with post-application bulk density and changes in bulk density, while application rate modulated response magnitude and soil texture constrained context dependence. Co-variation among thermal conductivity, thermal diffusivity, and volumetric heat capacity matched expected physical dependencies, indicating coordinated structural reorganisation rather than independent shifts in isolated parameters. These estimates describe intrinsic conductive and storage properties; field-scale soil temperature responses may also be modified by albedo, evaporation, vegetation, and surface energy balance. Improved integration of soil thermal measurements with moisture dynamics, structural changes, and carbon cycling is essential to accurately represent biochar effects in soil and land-surface models.

Microbial communities regulate carbon and nitrogen (N) cycling, yet their long-term responses to chronic global changes remain unclear. Using 12 years of grassland litter samples from the Loma Ridge Global Change Experiment in Irvine, California, we tested whether interactions between experimental drought and N deposition, and previously observed temporal variability are driven by background climatic conditions, including precipitation and temperature. Consistent with short-term studies, drought and N addition had relatively small effects on bacterial community composition compared to pronounced seasonal and interannual variability, with drought-by-year interactions explaining more variation than drought alone. Seasonal shifts were largely driven by short-term fluctuations in rainfall and temperature, whereas the substantial interannual variability in community composition was not captured by site-level climate metrics. Contrary to expectations, drought effects were influenced more by background temperature than precipitation, with the strongest effects observed in cooler years. Lastly, a bacterial taxons sensitivity to climate variability under ambient conditions did not predict its response to chronic drought. Together, our findings show that bacterial responses to drought are temporally dynamic and influenced by background temperature, underscoring the need for long-term longitudinal studies of soil microbial communities to better predict microbial responses under future global change. ImportanceMicrobial responses to global change, particularly drought and nitrogen addition, are often inferred from short-term studies (< 2 years), yet natural temporal variability may overshadow experimental effects. Using a 12-year dataset of grassland leaf litter communities, we show that temporal variability, both seasonal and interannual, exert a stronger influence on bacterial community composition than chronic drought or nitrogen deposition. These findings challenge assumptions about the magnitude of drought effects, particularly in naturally drought-affected ecosystem such as California grasslands and highlight the importance of long-term datasets for predicting microbial responses to climate change. By demonstrating that bacterial communities are strongly shaped by background climatic variability (baseline precipitation and temperature independent of imposed chronic treatments) and may be buffered to sustained drought, this work improves forecasts of ecosystem responses and informs the design of global change experiments and restoration strategies in future research studies.

Although wetlands are increasingly recognized as important contributors to the global methane budget, the microorganisms and processes involved methane cycling are poorly characterized, particularly in coastal brackish and saline systems. Here, we investigated microbial and geochemical factors contributing to methane dynamics in three coastal wetlands with different salinities, dominant vegetation types, and soil chemical characteristics. These included a freshwater flotant marsh, a cypress swamp, and a mesohaline salt marsh. Specifically, we paired methane porewater concentrations, surface fluxes, geochemistry, and 16S rRNA gene sequencing to address how microbial community composition links to porewater concentrations and its potential effects on emissions. We found that porewater methane concentrations across sites were the highest in the swamp, followed by the salt marsh and the flotant marsh, and were explained by methanogen richness and abundance. While methane-cycling microbial communities were significantly structured by salinity, two microbial taxa (Methanosaeta and Methanomicrobiaceae) were present across all sites. Hydrogenotrophs were the most abundant methanogen functional group, with Methanomicrobiaceae and Methanobacterium discriminant among wetlands. In contrast, methanotroph functional types varied among wetlands. Type I dominated the freshwater flotant marsh, while the anaerobic methanotrophic archaea the saltwater marsh. These findings contribute to an enhanced understanding of the microbiological contributions to methane emissions from coastal wetlands. Scientific Significance Statement TopicThis study provides critical insights into the microbial and geochemical controls on methane emissions across coastal wetlands along a salinity gradient. Challenging the prevailing paradigm, methane porewater concentrations did not inversely correlate with salinity, as the swamp site with intermediate salinity exhibited the highest concentrations. Methanogen richness and abundance emerged as strong predictors of methane concentrations, while methanotroph richness had no predictive value. Two core methanogens, Methanosaeta and Methanomicrobiaceae, were consistently present across all wetland types. The findings highlight potential role of the water column as a biological methane filter, especially in saline environments. This study significantly advances the understanding of methane cycling in coastal wetlands by decoupling methane emissions from salinity gradients and emphasizing the role of microbial communities and local environmental factors. These insights are essential for refining biogeochemical models to forecast greenhouse gas emissions under sea-level rise and saltwater intrusion scenarios. Scientific Significance Statement OutletThis work integrates microbial ecology, geochemistry, and ecosystem structure to address interdisciplinary questions relevant to the limnological community. Here, we reveal how microbial community composition, rather than salinity alone, predicts methane emissions, offering a fresh perspective into carbon cycling in estuarine and coastal environments. The findings also provide critical insights for improving greenhouse gas emission models to predict climate change feedback from coastal areas.

The assimilation of carbon dioxide by plants can be predicted by the Farquhar, von Caemmerer and Berry model of photosynthesis. This largely mechanistic model is central to understanding how plants influence Earths climate. However, it represents the use of light by photosynthesis using an empirical formulation. Johnson and Berry proposed an alternative mechanistic formulation based on the functioning of the cytochrome b6f complex that includes key steps in light harvesting and electron transport. We compared both formulations using photosynthetic light response measurements from 146 C3 species spanning arctic to tropical biomes and implemented them in the terrestrial biosphere model ELM-FATES to simulate global photosynthesis. The Johnson and Berry formulation better fitted the measured response of leaf-level photosynthesis to light, and predicted lower photosynthetic rates at intermediate light levels, which decreased global estimations of terrestrial photosynthesis by 8%. Our findings support adopting the Johnson and Berry formulation to improve model representation of global carbon cycle modeling.

The slightly-alkaline (pH [~]8.5), boiling ([~]90{degrees}C) vent-water of a Trans-Himalayan geothermal spring, moderately-rich in dissolved solids ([~]1500 ppm), was explored six times over a year. 11 archaeal and 46 bacterial species were detected consistently, while nine bacteria occurred intermittently, in the vent-epicenter featuring a largely-stable physicochemical milieu. All 11 archaea were detected as metagenome-assembled genomes ascribable to Thermoproteota. Of the total 55 bacteria detected, 32 were retrieved as MAGs, 20 as isolates, and three in both forms. Four bacteria could not be classified below the domain-level; three and four belonged to hyperthermophilic (Aquificia) and thermophilic (Thermaceae and Thermoflexaceae) taxa respectively; 27 belonged to taxa having some moderately-thermophilic members; 17 belonged to mesophilic taxa. According to metagenomics, an Aquificia, followed by two Thermoprotei and one Thermoproteales, dominated the microbiome overwhelmingly. Metatranscriptomically, however, the Thermoproteales was most active. Metatranscriptomic signatures envisaged the in situ metabolic status of the 66 species discovered as follows. Among the 18 putative hyperthermophiles and thermophiles identified, 17 rendered wide-ranging activities including growth; one Thermoproteota species had considerable activities sans growth. One new-phylum-level bacterium rendered wide-ranging activities including growth, while three such entities had considerable/minimal activities sans growth. Among the 27 potential moderate-thermophiles discovered, two Armatimonadota and one Thermosynechococcus species rendered wide-ranging activities including growth, 20 had considerable/minimal activities sans growth, whereas four had zero activities. Among the 17 mesophiles identified, 16 rendered considerable/minimal activities sans growth, whereas one had zero activity. Molecular drivers were envisaged from the metatranscriptomic data to explain the trends of inequitable population ecology.

Primary production in the Southern Ocean (SO) plays a critical role in regulating the global carbon cycle, yet the physiological mechanisms governing phytoplankton responses to iron (Fe) limitation and variable light remain poorly constrained. Using a custom made incubation system that simulated natural diel solar variability, we examined the interactive effects of Fe availability, light intensity, and photoperiod (continuous vs. variable) on three ecologically important SO phytoplankton: Fragilariopsis cylindrus, Phaeocystis antarctica, and Thalassiosira antarctica. Physiological, photophysiological, and proteomic measurements revealed that Fe availability was the dominant factor regulating growth, carbon production, photosynthetic performance and protein expression across all species. Distinct acclimation strategies emerged: F. cylindrus exhibited marked trade-offs between productivity and photoprotection under Fe stress, consistent with adaptation to stable, low-light, Fe-poor environments; P. antarctica maintained growth by flexibly modulating photoprotective and photosynthetic capacity, reflecting high plasticity suited to dynamic, open-ocean conditions; and T. antarctica expressed a balanced strategy, sustaining productivity and photoprotection simultaneously, characteristic of coastal bloom formers with higher Fe demand. Dynamic light regimes produced smaller, species-specific effects, influencing chlorophyll content and carbon storage primarily in T. antarctica. Correlation and z-score analyses demonstrated that Fe-rich photosynthetic proteins co-varied with biomass production, whereas photoprotective traits clustered independently, underscoring divergent energy-allocation strategies. Together, these results reveal how SO phytoplankton partition resources between productivity and photoprotection under shifting Fe-light regimes, providing mechanistic insight into their ecological niches.

Subalpine forests in the Alps are fragile ecosystems that play a crucial role in regional water resources and the local climate. These ecosystems are ecologically significant due to their unique biodiversity and vulnerability to climate change. While several components of the hydrological balance have been studied, the interplay between catchment-scale processes and plot-scale drivers such as fog presence and forest age remains insufficiently understood. To address this, we investigated the hydrological balance of a subalpine coniferous forest catchment at the Renon site in the Italian Alps, integrating observations across spatial scales. The study area includes a mosaic of mature and younger regrowth forest, where both interannual and seasonal variability in precipitation and fog presence are pronounced. At the catchment scale, we quantified above-canopy precipitation, evapotranspiration (ET, measured via eddy covariance at the ICOS tower), stream discharge, and soil moisture dynamics. Within the catchment, we characterised water partitioning using sap flow sensors for tree transpiration, throughfall and stemflow collectors with rain gauges above and below the canopy and epiphyte sampling. Mixed fog-rain events frequently coincided with higher throughfall. However, these changes had a minor effect on soil water storage and catchment discharge in the annual water balance, which was nearly closed. At the plot scale, our results show that tree transpiration was higher in the younger forest structure, while canopy interception is a dominant process in water partitioning in the older forest structure, where lichen abundance likely enhances interception. This study highlights the importance of multi-scale monitoring in temperate mountain forests, where forest age influences water partitioning, and fog presence, though not directly quantified, can still contribute to reducing evaporative processes. Such contributions may gain importance under changing climate conditions, albeit less prominently than in tropical or subtropical cloud forests.