Environmental Earth Sciences

The marine labile dissolved organic carbon (DOC) pool is a dynamic reservoir of thousands of molecules that cycles approximately one-quarter of Earth's primary production within days to weeks. After excretion by phytoplankton and other microbes, metabolites are rapidly consumed, resulting in low standing concentrations (picomolar to low nanomolar). Despite the decades-long search for labile DOC sources and molecular identities, marine phytoplankton exometabolomes are not well characterized, largely due to difficulties in measuring small polar molecules in saline water. Here, we profiled the exometabolomes of six axenic phytoplankton species representing key functional groups including a diatom (Thalassiosira pseudonana CCMP1335), a picoeukaryote (Micromonas commoda RCC299), a coccolithophore (Gephyrocapsa huxleyi CCMP371), a diazotrophic cyanobacterium (Crocosphaera watsonii WH8501), and two picocyanobacteria (Prochlorococcus marinus MIT9301 and Synechococcus WH8102). From these cultures, we quantified 56 amine- and alcohol-containing exometabolites representing 11 compound classes which in sum comprised up to 23.4% of phytoplankton-excreted DOC. We estimated that these phytoplankton-derived exometabolites could supply up to 5% of the daily carbon quota of the dominant heterotrophic bacterium SAR11 in the surface ocean. Substantial variations in exometabolite identity and concentration across phytoplankton taxa underscore taxonomic diversity as a key driver in the supply and composition of labile DOC. This taxonomic variation predicts geographic and seasonal differences in the distribution of marine dissolved metabolites that underpin the cycling of labile DOC back to carbon dioxide (CO2). Overall, our work suggests that phytoplankton exometabolites are key chemical currencies that mediate significant carbon fluxes within the ocean's carbon cycle.

Direct evidence for the assimilation of nitrous oxide (N₂O), a potent greenhouse gas, by freshwater cyanobacteria has been lacking. Here, we report a cyanobacterium, isolated from a nitrogen-polluted river, that fixes N2O via dinitrogen (N2) gas by nitrogenase activity. N2O-reducing bacteria were enriched from river samples, under alternating light/dark conditions in the presence of atmospheric N2 and the absence of oxygen (O2), followed by isolation using fluorescence-activated cell sorting. The isolated strain, Nostoc sp. strain MS1 (NIES-4466), consists of moniliform coccoid cells and is phylogenetically affiliated with the genus Nostoc. A high-quality draft genome of strain MS1 revealed the presence of nitrogenase genes encoding the MoFe protein but the absence of N2O reductase genes, i.e., clades I, II, and III nosZ. When incubated in a He (95%)/CO2 (5%) atmosphere with 0.01% 15N-labeled N2O, the cells exhibited elevated 15N content relative to natural abundance (0.36%). The degree of 15N incorporation positively correlated with ethylene production from acetylene, implicating nitrogenase in N2O assimilation by strain MS1. While replacing He with N2 reduced N2O uptake, likely due to substrate competition, N2O consumption activity persisted, suggesting that freshwater cyanobacteria can function as an N2O sink. These findings, supported by genomic and 15N tracer analyses, highlight the previously unrecognized role of cyanobacteria in mitigating N₂O emissions in freshwater environments.

Cover cropping and diversified crop rotations are widely promoted as strategies to enhance soil carbon inputs and contribute to climate change mitigation. However, including additional crop cover could have detrimental impact of water use efficiency (WUE) and has not been sufficiently quantified under continuous field conditions. We evaluated two contrasting crop management systems over two full rotation cycles (2017–2022): a diversified winter wheat–corn–soybean rotation with cover crops and a conventional soybean–corn–soybean rotation without cover crops. Evapotranspiration (ET) was measured at half-hourly resolution using large weighing lysimeters at the Elora Research Station in Ontario, Canada. Analyses focused on the growing season (1 May–1 November), and cumulative seasonal ET was calculated for each lysimeter. Crop yield was measured annually, and WUE was calculated as the ratio of yield to cumulative ET. The experiment included 12 lysimeters across two soil types (silt loam and loamy sand). Across six growing seasons, cumulative ET ranged from 350 to 800 mm, yield from 903 to 13,203 kg ha⁻¹, and WUE from 0.17 to 2.1 kg m⁻³. Despite clear differences in management practices, ET, yield, and WUE did not differ significantly between the diversified and conventional systems in five of six years. In 2017, significant differences were observed between managements for ET in loamy sand soil and for yield and WUE in silt loam soil, largely reflecting differences in crop type between systems. In contrast, soil type consistently exerted a strong influence on ET, yield, and WUE across all years. These results suggest that, in the studied agroecosystem, there are minimal trade-offs between climate change mitigation benefits associated with cover crop use and the water-use efficiency of diversified cropping systems, with soil properties exerting dominant control on crop water use and productivity.

Pressure-temperature (P-T) estimates from subduction-related metamorphic rocks such as eclogites and blueschists are often used to constrain the thermal conditions of fossil subduction zone plate interfaces. However, the metamorphic rock record indicates peak temperatures 100-300°C warmer on average than those predicted by geodynamic models for modern subduction zones.. To shed light on the difference in the fossil and modern subduction zone thermal structures, we compare newly acquired P-T estimates using quartz-in-garnet and zircon-in-garnet elastic thermobarometry combined with Zr-in-rutile thermometry to newly constructed geodynamic models for the thermal evolution of the Rio San Juan Complex, Dominican Republic. The geodynamic models are 2-D coupled kinematic-dynamic models that use the fossil subduction parameters, such as time-dependent convergence velocity and plate age. Global plate reconstruction models provide constraints on these parameters. Data from regional geological and petrological studies constrain the duration of subduction and the thermal history of the subducting plate. New analyses on an eclogite sample reveal a counter-clockwise pressure-temperature (P-T) path, with peak conditions at ~1.7 GPa and ~650ºC. Previous thermobarometric studies on the same sample indicate similar temperatures but significantly higher peak pressures, up to ~2.3 GPa, during the early stage of subduction around 110–104 Ma. In contrast, lower-grade blocks indicate isobaric cooling to ~400ºC and clockwise P-T paths during a later stage (80–62 Ma). Our thermal modeling results indicate that subduction initiation between two relatively young tectonic plates (< 30 Ma) can explain the relatively low-P, high-T data from both new and previous analyses. However, the highest pressures derived by the previous study require a rapid deepening of the maximum depth of slab-mantle decoupling during the early stages of subduction. Such deepening can be explained by a plate velocity increase around 110 Ma. Our modeling results further indicate the subduction rate increase results in significant cooling of the interface at a given depth, consistent with the isobaric cooling observed in the rock record. Migration of the spreading center across the study area may explain the change from counter-clockwise to clockwise paths, and higher temperature gradients of some of the blocks.

A major concern about the 2019-2020 Australian ‘Black Summer’ bushfires, along with other recent wildfire events worldwide, is whether they signal a shift toward a more extreme fire regime characterized by greater frequency, intensity, or burned area. Although fire has shaped Australia’s terrestrial ecosystems over evolutionary timescales, climate variability, and increasingly severe fire weather, perhaps exasperated by human-induced climate change or decisions regarding natural resource management, may be contributing to more extreme wildfires. Charcoal preserved in undisturbed, well-dated sediments holds significant potential for reconstructing long-term fire history. This study employed high-resolution ¹⁴C dating, charcoal accumulation (CHAR), and of a calibration experiment between Raman spectroscopy and Eucalypt species burnt in a calorimeter under controlled energy conditions to simulate a gradient from low-intensity to high-intensity wildfires. Our focus was on examining changes in fire intensity, severity and area burned in the upper Blue Mountains of NSW, in eastern Australia, over the Twentieth Century. We evaluated several previously proposed Raman-derived indicators of thermal maturity, including Raman band separation (RBS or G-D), the ratio of peak maximum intensities in the D- and G-bands (ID/IG), the ratio of the area under these bands (AD/AG), and the ratio of the full width at half maximum for the D- and G-bands (WD/WG). AD/AG produced the best relationship with increasing applied energy, but all these Raman-derived parameters were found to be less capable at higher fire intensities. To address this issue, a chemometric (backward interval partial least squares (PLS) regression) modelling approach was used which provided a more robust model linking Raman spectra and fire intensity. The application of this model across multiple upper Blue Mountains sites does not support the hypothesis that fire is becoming more severe. In contrast, CHAR results suggest that area burned across the region is increasing. We present a consideration of the drivers of these changes across the Twentieth Century, and further work seeks to place these trends in the context of the characteristics of fire regimes over the many thousands of years (represented by the sediments in the mires of the Blue Mountains).

Coal deposits derived from peat play a key role in Earth’s carbon cycle by facilitating the burial and transformation of organic carbon. However, the role of mineral precipitation in peat-forming environments, particularly the microbially mediated formation of siderite and its contribution to carbon cycling, remains poorly understood. Here, we investigate a Jurassic siderite-bearing coal seam to reconstruct mineral paragenesis and constrain diagenetic conditions during peat accumulation. Authigenic, 13C-rich siderite associated with methanogenesis formed under acidic and reducing conditions, consistent with precipitation equilibrium simulations that show enhanced siderite saturation at elevated CO2 pressures. Our results suggest that siderite genetically associated with coal may have acted as an inorganic carbon sink by limiting greenhouse-gas release during peat decomposition, thereby reducing CO2 emission by 2% within the studied paleo-peat deposit and sequestering ∼12.67 Gt of CO2 across three Jurassic coal basins. These findings reveal a previously overlooked pathway for carbon fixation in coal measures, with potential for both deep-time climate and modern greenhouse gas mitigation.

紀伊半島東部，三重県朝柄地域の中央構造線沿いに分布する珪長質火山砕屑岩からなる朝柄層と付随する黒雲母トーナル岩のジルコンU–Pb年代測定を行った．朝柄層の層序を再検討し，下位より，松葉谷流紋岩質凝灰岩部層，上出流紋岩質凝灰質砂岩泥岩互層部層，不動谷流紋岩質凝灰角礫岩部層，中出流紋岩質凝灰岩部層に層序区分した．松葉谷流紋岩質凝灰岩部層からは96.1±0.9 Ma，上部の中出流紋岩質凝灰岩部層からは91.7±1.3 Maの年代を得た．これらの年代から，朝柄層は紀伊半島西部から淡路島に分布する泉南流紋岩類に対比される．一方，付随する黒雲母トーナル岩からは79.9±8.9 Maの年代が得られ，この年代は松葉谷流紋岩質凝灰岩部層の約78 Maの小クラスター年代に類似する．このことは黒雲母トーナル岩が松葉谷流紋岩質凝灰岩部層の近傍で約78 Maに貫入し，熱水変質作用を伴った弱い接触変成作用を起こしたと考えられる．

Foliar selenium (Se) application can mitigate arsenic (As) toxicity and enhance Se accumulation in highland barley; however, the optimal dose, Se type, and application stage remain unclear. Field surveys across the Tibetan Plateau have revealed a widespread Se deficiency in grains. Therefore, pot experiments were conducted using sodium selenite and nanoselenium at three concentrations (12, 24, and 48 mg/L) under three As levels (20, 40, and 60 mg/kg), applied at the booting, filling, and booting + filling stages. The results showed that sodium selenite reduced grain As by 2.1–54.2% and increased grain Se by 197.1–2352.4 times, outperforming nanoselenium. Application of 24 mg/L Se at the filling stage achieved the greatest reduction in the amount of grain As. Tissue Se accumulation increased with the application dosage, particularly at the filling stage. Moreover, foliar Se application inhibited As translocation while promoting Se translocation. These findings support foliar Se application as a practical strategy to reduce As accumulation and alleviate Se deficiency in crops.

Environmental DNA (eDNA) is an emerging tool for wildlife biologists and citizen scientists to build local knowledge about biodiversity and detect invasive species. While single-species eDNA assays are widely used to detect cryptic species, metabarcoding approaches can simultaneously detect a wide range of taxa depending on the chosen genetic target. In a partnership pilot study between Traditional Owners, scientists, and Government, we used a 12S metabarcoding approach to identify the vertebrate prey consumed by predators in the Burragorang Valley on Gundungurra Country (NSW, Australia). Here, we report an incidental but important detection of an invasive fish species, Oriental Weatherloach Misgurnus anguillicaudatus, in multiple Red Fox Vulpes vulpes scats within the high conservation value landscape of the Burragorang Valley (encompassing the WaterNSW Warragamba Special Areas). Our eDNA identification of Oriental Weatherloach was later validated by a physical capture of the species. We reviewed public databases to better understand the distribution of this invasive species in the region and discovered that Oriental Weatherloach had been detected upstream of the Warragamba Special Area on the Wollondilly and Wingecarribee Rivers, expanding its range since the 1990s. However, this was the first detection within the strictly controlled inner conservation zone for Lake Burragorang, Sydney’s drinking water supply. The recent invasion of this exotic fish species was not widely known by Traditional Owners, local community, Government departments or conservation groups and is significant as it is within the Greater Blue Mountains World Heritage Area. Our preliminary study highlights the value of using eDNA metabarcoding on predator scats as a biomonitoring tool for the incidental detection of invasive species and shows that freshwater fish may play a more significant role in the diet of foxes than previously understood.

Abstract Solar superflares of S‐class (>X10 in soft X‐rays) pose extreme space weather hazards, yet their prediction remains a fundamental challenge owing to their rapid and transient natures and the limitations of conventional event‐based forecasts. We introduce for the first time, a probabilistic spatiotemporal framework designed to identify extended epochs and heliographic zones with heightened superflare likelihood, rather than predicting individual events. Our methodology analyzes nearly five decades of Geostationary Operational Environmental Satellites SXR observations (1975–2025) and introduces two novel components: (a) a cross‐Laplacian diagnostic, derived from the butterfly diagrams of sunspot area and SXR flare density, which identifies photosphere‐corona coupling and pinpoints magnetic energy accumulation sites as preferential zones for S‐flare occurrence; and (b) a temporal model based on the coupled phase states of 1.7‐year and 7‐year oscillations, extracted via wavelet analysis and projected forward using a Bayesian Least‐Squares Support Vector Machine method. Integrating these spatial and temporal diagnostics yields a unified forecast, we identify 2025.7–2026.6 and 2027.2–2027.9 years, as the primary high‐probability windows for S‐class flare activity during Solar Cycle 25, corresponding preferentially to latitudinal bands in the southern (S–S) and northern (N–N) hemispheres, respectively. Subsequent windows are projected for Cycle 26. This physics‐informed, probabilistic approach provides a robust strategy for anticipating periods of extreme solar activity by characterizing the inherent patterns of magnetic energy storage and release in the solar atmosphere.