The deep biosphere harbors diverse microbial communities amounting to a substantial fraction of Earth's biomass for much of its history. Analyses of fracture-hosted minerals (e.g., pyrite, calcite) and their C-O-S isotope signatures are beginning to establish a fossil record of subsurface microbial communities and their relationship with major geological events. Fracture-coating pyrite from deep within the Fennoscandian Shield records the largest 34S-enrichment observed on Earth to date (δ34S = +147 ‰) – likely the result of late-stage Rayleigh distillation during closed-system microbial sulfate reduction (MSR). This implies even heavier S isotope values for the complementary sulfate reservoir during pyrite formation, possibly recorded in coeval sulfate minerals such as barite [BaSO4]. However, barite has been poorly explored as an archive of ancient deep subsurface biosignatures, particularly in samples with extreme 34S-enrichment in pyrite. Here, we compiled published microscale δ13Ccalcite, δ18Ocalcite, and δ34Spyrite data with new secondary ion mass spectrometry (SIMS) δ18Obarite and δ34Sbarite analyses from two localities on the Fennoscandian Shield (Forsmark and Laxemar, Southeastern Sweden), aiming to constrain the record of microbial processes in barite. We identified two groups of barites distinguished by their distinct δ18O/δ34S trends. Comparison of these trends with δ13C values of cogenetic calcite reveals variable associations with sulfate-dependent anaerobic oxidation of methane (AOM) and organotrophic sulfate reduction (OSR). Strongly 13C-depleted calcite (δ13Ccalcite < −40 ‰), indicating a relationship with AOM, almost exclusively occurs in the steeper-trending Group 1. Evidence for barite affected by OSR (δ13Ccalcite > −30 ‰) occurs in both groups and dominates the shallower-trending Group 2. The different trends likely resulted from an interplay of MSR pathways (AOM vs. OSR) and variations in sulfate reduction rates (SRR), which we attribute to the relative abundances of sulfate and electron donors in the parent fluid of barite. Lower sulfate and electron donor abundances likely permitted methanogenesis and AOM at lower SRR, resulting in a steeper δ18O/δ34S trend in Group 1 barite. Conversely, higher sulfate and electron donor abundances inhibited methanogenesis and AOM, favoring OSR at a higher SRR, which is reflected by the shallower δ18O/δ34S trend in Group 2 barite. Comparison between the δ34Spyrite range (−53.9 ‰ to +131.7 ‰) and the δ34Sbarite range (+7.6 ‰ to +52.0 ‰) across both localities demonstrates that the sulfate reservoir corresponding to extremely 34S-enriched pyrite is not recorded in barite. The discrepancy between barite and pyrite could reflect their formation in separate microenvironments or barite undersaturation at high degrees of Rayleigh distillation. Our study highlights the need for coupled microscale C-O-S analyses in fracture-hosted mineral assemblages to understand metabolic processes in the ancient deep biosphere, while stressing that these records are affected by local hydrogeochemical conditions.
In this study, we combined high resolution Transmission Electron Microscopy (TEM) bright field and High-Angle Annular Dark-Field Scanning TEM imaging with nanoscale Secondary Ion Mass Spectrometry (NanoSIMS) analyses of Rosalina sp. foraminifer cells cultured in 33S-labeled seawater to understand the origin of sulfur in the test and the cytoplasm, its interaction with the ultrastructure of the test and the co-distribution of sulfur with phosphorus, calcium and magnesium. Cells that lived in 33SO4-labeled seawater revealed that at least 1/4 of the sulfur observed in the cytoplasm comes from seawater sulfate incorporated during the duration of the growth experiment. Our observations reveal a global co-occurrence of S and Mg within the last two chambers of the test, that all appear more concentrated in two areas, mostly near the Primary Organic Sheet (POS) and in the Outer Calcitic Layer (OCL) that corresponds to the calcite added during the growth of the last chamber. Less crystalline grains can be found in the S and Mg-richest part of the POS and within OCLs. We interpret these grains as a non-crystalline phase likely containing Amorphous Calcium Carbonate (ACC) and organic matter, formed or assembled at the sites of calcification, revealing a complex interplay between S, Mg and Ca. Labeled sulfur is also found within smaller vesicles in the cytoplasm and represents overall 10 % to 25 % of the sulfur we observe. Though we cannot confirm the occurrence of an assimilatory pathway for sulfur in foraminifera, the clear presence of seawater sulfate in the cell demonstrates that sulfate is involved in many aspects of foraminifer biological activity, in close association with Ca-transportation, Mg distribution, and likely with ACC formation and its evolution to calcite.
Groundwater ecosystems fulfil functions that humankind relies upon, e.g. for sustainable drinking water production. Quantification of these services is lacking so far. Thus, it is not possible to predict scenarios (e.g. future climates). Based on data from a comprehensive groundwater ecosystem study comprising four zones of varying land use and groundwater/surface water exchange, we parameterized a quantitative dynamic food web model (recharged organic carbon, microorganisms using the biodegradable fraction of this carbon, and fauna grazing on the microorganisms), in a proof-of-principle approach. With the model satisfactorily reflecting the field data, we calculated five further scenarios, three of which without fauna (mortality e.g. due to contamination, sudden peaks of temperature etc.). Two of the “fauna” and two of the “no fauna” scenarios were run with temperature elevated by 1.5 and 3 °C, respectively. The ecosystem service of carbon degradation was expressed as the difference in carbon concentration between the beginning of the simulation and the end of the simulation. In most scenarios, remaining carbon increased over time. The remaining carbon in some scenarios was up to 6.6 times as high in the “no fauna” scenarios compared to the reference case. Fauna was thus shown to fulfil a service by promoting microbial carbon degradation that may be substantial. This model set up can serve to test other cases of varying physical and chemical variations and disturbances.
Boreal forests play an important role in the global terrestrial carbon cycle, yet the effects of climate-driven permafrost degradation on ecosystem carbon dynamics remain poorly represented in current models. Earth system models often simplify subsurface thermal and hydrological processes, limiting their ability to capture ecosystem responses in permafrost-affected regions. Here, we improve the soil module of the LPJ-GUESS model by implementing a deep soil profile (3 m) with refined vertical discretization (30 layers) and explicit ice-impedance effects. The improved LPJ-GUESS configuration is then applied to simulate carbon and hydrothermal dynamics in the climate-sensitive boreal forests of Northeast China. Model evaluation indicates that the improved configuration generally enhances the simulation of soil hydrothermal dynamics relative to the default LPJ-GUESS configuration. The correlation coefficient for deep soil temperature (1.0 m) increases from 0.818 to 0.894, while the systematic overestimation of soil water content is substantially reduced, with the bias at 1.0 m reduced from 0.321 to −0.006. These improvements are associated with better agreement between simulated and observed vegetation composition. The simulated dominance of needleleaf forests decreases from near-complete dominance to 24.1 %, resulting in vegetation patterns that are more consistent with regional observations. Consequently, biases in carbon estimates are reduced, with simulated net primary productivity decreasing from 583.5 to 560.1 g C m−2 yr−1 and aboveground biomass from 112.9 to 99.0 t ha−1, resulting in better agreement with estimates from satellite-based observations. Model sensitivity analysis indicates that vegetation composition is highly sensitive to changes in soil hydrothermal conditions compared with the control simulation. Pronounced shifts in vegetation composition occur when soil temperature increases exceed 0.6 °C and soil water deficits exceed 0.6 m3 m−3 compared to the default model configuration. Overall, this study highlights the importance of improving soil hydrothermal representation for simulating vegetation dynamics and carbon cycling in permafrost-affected boreal ecosystems.
Forests are major sinks for atmospheric mercury (Hg) due to the efficiency of stomatal uptake and litter deposition. Tropical forests, highly productive ecosystems, remain understudied despite their pronounced climatic and phenological variability. We investigated whether seasonal rainfall regimes and associated tree adaptations influence Hg dynamics in secondary tropical moist broadleaf (TMBF) and tropical dry broadleaf (TDBF) forests in Costa Rica by examining foliar Hg concentrations, litter Hg standing stocks, and soil Hg concentrations. Seasonal differences in rainfall and associated environmental conditions strongly influenced foliar Hg concentration in the TDBF. Deciduous trees showed 4.7 times higher foliar Hg concentration in the wet season, when leaves were mature, compared to the dry season, when newly flushed leaves emerged after leaf shedding. Evergreen trees in TDBF demonstrated 2.3 times lower foliar Hg concentrations in the dry season than in the wet season (23 vs. 53 µg kg−1), likely due to various associated physiological processes (e.g., leaf flushing). However, the primary mechanism remains unclear given the complex and unexplored Hg dynamics in TDBF. TMBF showed no clear seasonal variation in foliar Hg in either deciduous (dry: 48; wet: 54 µg kg−1) or evergreen trees (dry: 57; wet: 53 µg kg−1), likely due to longer leaf lifespan sustaining year-round transpiration and stomatal Hg uptake under high humidity. Atmospheric Hg concentrations in TDBF were two times higher than in TMBF (1.2 vs. 0.7 ng m−3) across both seasons, likely reflecting greater Hg capture per unit area in TMBF due to denser vegetation and enhanced wet deposition via rainfall. Foliar Hg was not correlated with stomatal density or specific leaf area in either forest type. Soil Hg concentrations, however, were correlated with litter-derived inputs, supporting litter as the dominant Hg transfer pathway. Higher seasonally averaged litter Hg standing stocks (34 µg m−2) in TMBF than in the TDBF (19 µg m−2) resulted in 3.4 times higher soil Hg concentrations (0–30 cm) in TMBF (115 µg kg−1) than in TDBF (34 µg kg−1). Soil Hg concentrations were 2.6 times lower at a previously deforested TMBF site, indicating persistent disturbance effects despite almost three decades of reforestation. Overall, foliar Hg concentrations in the TDBF varied markedly between seasons, while both TDBF and TMBF appeared to be important Hg0 reservoirs, with contrasting dynamics that are potentially sensitive to climate change.
