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As global warming progresses, changes in high-latitude precipitation are expected to impart long-lasting impacts on Earth systems, including glacier mass balance and ecosystem structures. Reconstructing past changes in high-latitude precipitation and hydroclimate from networks of continuous lake records offers one way to improve forecasts of precipitation and precipitation–evaporation balances, but these reconstructions are currently hindered by the incomplete understanding of controls on lake and soil water isotopes. Here, we study the distribution of modern water isotopes in Icelandic lakes, streams, and surface soils collected in 2002, 2003, 2004, 2014, 2019, and 2020 to understand the geographic, geomorphic, and environmental controls on their regional and interannual variability. We find that lake water isotopes in open-basin (through-flowing) lakes reflect local precipitation, with biases toward the cold season, particularly in lakes with sub-annual residence times. Closed-basin lakes have water isotope and deuterium excess values consistent with evaporative enrichment. Interannual and seasonal variabilities of lake water isotopes at repeatedly sampled sites are consistent with instrumental records of winter snowfall; summer relative humidity; and atmospheric circulation patterns, such as the North Atlantic Oscillation. Summer surface soil water isotopes span the entire range of seasonal precipitation values in Iceland and appear to be consistently overprinted by evaporative enrichment, which can occur throughout the year, although the sampling depths were shallower than rooting depths for many plant types. This dataset provides new insight into the functionality of water isotopes in Icelandic environments and offers renewed possibilities for optimized site selection and proxy interpretation in future paleohydrological studies on this North Atlantic outpost. [ABSTRACT FROM AUTHOR]

Many continental paleoclimate archives originate from wetland sedimentary sequences. While several studies have investigated biomarkers derived from peat-generating vegetation typical of temperate/boreal bogs (e.g., Sphagnum), only scant information is available on emergent plants predominant in temperate/subtropical coastal marshlands, peri-lacustrine and fen environments. Here, we address this gap, focusing on two wetlands in the Mediterranean (Nisí fen and Tenaghi Philippon, Greece). We examined the concentration, homologue distribution, and hydrogen stable isotopic composition (δ[sup 2]H) of leaf wax n-alkanes in 13 fen plant species, their surrounding soil, and surface water during the wet growing season (spring) and the declining water table period (summer). Our findings indicate that local graminoid species primarily contribute to the soil n-alkane signal, with a lesser influence from forbs, likely owing to differences in morphology and vegetation structure. The δ[sup 2]H values of surface and soil water align with local average annual precipitation δ[sup 2]H, reflecting winter-spring precipitation. Consistently, the average δ[sup 2]H of local surface, soil, and lower stem water showed negligible evaporative enrichment, confirming minimal [sup 2]H-fractionation during water uptake. We find that δ[sup 2]H values of source water for wax compound synthesis in local fen plants accurately mirror local annual precipitation. Furthermore, despite differences between leaves and lower stems in n-alkane production rates, their δ[sup 2]H values exhibit remarkable similarity, indicating a shared metabolic substrate, likely originating in leaves. Our net [sup 2]H-fractionation values (i.e., precipitation to leaf n-alkanes) align with those in Chinese highlands and other similar environments, suggesting consistency across diverse climatic zones. Notably, our data reveal a seasonal decrease in the carbon preference index (CPI) in plant samples, indicating wax lipid synthesis changes associated with increased aridity. Additionally, we introduce a new parity isotopic difference index (PID) based on the consistent δ[sup 2]H difference between odd and even n-alkane homologues. The PID demonstrates a strong anticorrelation with plant CPI, suggesting a potential avenue to trace long-term aridity shifts through δ[sup 2]H analysis of odd and even n-alkane homologues in sedimentary archives. While further development of the PID is necessary for broad application, these findings highlight the intricate interplay between plant physiology, environmental parameters, and sedimentary n-alkanes in unravelling past climatic conditions. [ABSTRACT FROM AUTHOR]

Shifts in the hydrogen stable isotopic composition (²H/¹H ratio) of lipids relative to water (lipid/water ²H-fractionation) at natural abundances reflect different sources of the central cellular reductant, NADPH, in bacteria. Here, we demonstrate that lipid/water ²H-fractionation (²εfattyacid/water) can also constrain the relative importance of key NADPH pathways in eukaryotes. We used the metabolically flexible yeast Saccharomyces cerevisiae, a microbial model for respiratory and fermentative metabolism in industry and medicine, to investigate ²εfattyacid/water. In chemostats, fatty acids from glycerol-respiring cells were >550‰ ²H-enriched compared to those from cells aerobically fermenting sugars via overflow metabolism, a hallmark feature in cancer. Faster growth decreased ²H/¹H ratios, particularly in glycerol-respiring cells by 200%. Variations in the activities and kinetic isotope effects among NADP+-reducing enzymes indicate cytosolic NADPH supply as the primary control on ²εfattyacid/water. Contributions of cytosolic isocitrate dehydrogenase (cIDH) to NAPDH production drive large ²H-enrichments with substrate metabolism (cIDH is absent during fermentation but contributes up to 20 percent NAPDH during respiration) and slower growth on glycerol (11 percent more NADPH from cIDH). Shifts in NADPH demand associated with cellular lipid abundance explain smaller ²εfattyacid/water variations (<30%) with growth rate during fermentation. Consistent with these results, tests of murine liver cells had ²H-enriched lipids from slower-growing, healthy respiring cells relative to fast-growing, fermenting hepatocellular carcinoma. Our findings point to the broad potential of lipid ²H/¹H ratios as a passive natural tracker of eukaryotic metabolism with applications to distinguish health and disease, complementing studies that rely on complex isotope-tracer addition methods. [ABSTRACT FROM AUTHOR]

The article focuses on the use of hydrogen and carbon isotopes in aquatic plant waxes from Greenland lakes to understand shifts in methane cycling during past Holocene warming, revealing significant changes in methane dynamics in response to elevated temperatures and productivity during the Middle Holocene. This research suggests that ongoing warming may lead to increased methane-derived carbon in Arctic lakes, even in lakes where methane is not a major component of the carbon cycle today.

The rate at which microorganisms grow and reproduce is fundamental to our understanding of microbial physiology and ecology. While soil microbiologists routinely quantify soil microbial biomass levels and the growth rates of individual taxa in culture, there is a limited understanding of how quickly microbes actually grow in soil. For this work, we posed the simple question: what are the growth rates of soil microorganisms? In this study, we measure these rates in three distinct soil environments using hydrogen-stable isotope probing of lipids with ²H-enriched water. This technique provides a taxa-agnostic quantification of in situ microbial growth from the degree of ²H enrichment of intact polar lipid compounds ascribed to bacteria and fungi. We find that growth rates in soil are quite slow and correspond to average generation times of 14 to 45 d but are also highly variable at the compound-specific level (4 to 402 d), suggesting differential growth rates among community subsets. We observe that low-biomass microbial communities exhibit more rapid growth rates than high-biomass communities, highlighting that biomass quantity alone does not predict microbial productivity in soil. Furthermore, within a given soil, the rates at which specific lipids are being synthesized do not relate to their quantity, suggesting a general decoupling of microbial abundance and growth in soil microbiomes. More generally, we demonstrate the utility of lipid-stable isotope probing for measuring microbial growth rates in soil and highlight the importance of measuring growth rates to complement more standard analyses of soil microbial communities. [ABSTRACT FROM AUTHOR]

Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are bacterial membrane lipids that are frequently employed as paleoenvironmental proxies because of the strong empirical correlations between their relative abundances and environmental temperature and pH. Despite the ubiquity of brGDGTs in modern and paleoenvironments, the source organisms of these enigmatic compounds have remained elusive, requiring paleoenvironmental applications to rely solely on observed environmental correlations. Previous laboratory and environmental studies have suggested that the globally abundant bacterial phylum of the Acidobacteria may be an important brGDGT producer in nature. Here, we report on experiments with a cultured Acidobacterium, Solibacter usitatus, that makes a large portion of its cellular membrane (24 ± 9% across all experiments) out of a structurally diverse set of tetraethers including the common brGDGTs Ia, IIa, IIIa, Ib, and IIb. Solibacter usitatus was grown across a range of conditions including temperatures from 15 to 30°C, pH from 5.0 to 6.5, and O2 from 1% to 21%, and demonstrated pronounced shifts in the degree of brGDGT methylation across these growth conditions. The temperature response in culture was in close agreement with trends observed in published environmental datasets, supporting a physiological basis for the empirical relationship between brGDGT methylation number and temperature. However, brGDGT methylation at lower temperatures (15 and 20°C) was modulated by culture pH with higher pH systematically increasing the degree of methylation. In contrast, pH had little effect on brGDGT cyclization, supporting the hypothesis that changes in bacterial community composition may underlie the link between cyclization number and pH observed in environmental samples. Oxygen concentration likewise affected brGDGT methylation highlighting the potential for this environmental parameter to impact paleotemperature reconstruction. Low O2 culture conditions further resulted in the production of uncommon brGDGT isomers that could be indicators of O2 limitation. Finally, the production of brGTGTs (trialkyl tetraethers) in addition to the previously discovered iso‐C15‐based mono‐ and diethers in S. usitatus suggests a potential biosynthetic pathway for brGDGTs that uses homologs of the archaeal tetraether synthase (Tes) enzyme for tetraether synthesis from diethers. [ABSTRACT FROM AUTHOR]

Sediment cores from 13 lakes in a 1500 km transect along the eastern North American Arctic contain up to four superposed stratified interglacial units. All 13 lakes contain one unit with sediment similar in character and mass to Holocene gyttja, with 14C ages >40 ka, luminescence ages 90 to 120 ka, and pollen assemblages that require nearly complete Laurentide deglaciation, supporting a Last Interglacial (LIG; MIS 5e) age. Two lakes preserve an older interglacial, with luminescence ages suggesting an MIS 7 age. Four adjacent lakes record a thin, stratified organic unit between the LIG and Holocene units with 14C ages >50 ka, that is probably from late in MIS 5. Temperature estimates from biotic proxies suggest LIG summer temperatures 4–6°C above mid‐20th century values; pollen, chironomids and DNA document a poleward expansion of woody plants and invertebrate species during the LIG, supporting arguments that positive feedbacks native to the Arctic amplified insolation‐driven summer temperature increases. The stratigraphic succession implies the Laurentide Ice Sheet remained intact with sea level below ‐40 m from ~115 ka to ~11 ka, and places new constraints on the interpretation of cosmogenic radionuclide inventories in erratic boulders older than the Holocene throughout this region. [ABSTRACT FROM AUTHOR]

Reconstructions of Holocene lake water isotopic composition based upon subfossil aquatic organic material offer new insights into Arctic climate. We present quantitative estimates of warmth during the Holocene Thermal Maximum in northwest Greenland, inferred from oxygen isotopes of chironomid head capsules and aquatic moss preserved in lake sediments. δ 18 O values of chironomids from surface sediments of multiple Greenland lakes indicate that these subfossil remains record the δ 18 O values of the lake water in which they grow. Our lake water δ 18 O reconstruction is supported by downcore agreement with δ 18 O values in aquatic moss and chironomid remains. δ 18 O of both organic materials from Secret Lake decrease after 4 ka (ka = thousands of years ago) by 3‰ into the Neoglacial. We argue that lake water at Secret Lake primarily reflects precipitation δ 18 O values, which is strongly correlated with air temperature in NW Greenland, and that this signal is biased towards summer and early autumn conditions. Other factors may have influenced Secret Lake δ 18 O values through the Holocene, including evaporation of lake water and changing seasonality and source of precipitation. The maximum early Holocene summer and early autumn-biased temperature anomaly at Secret Lake is 2.5–4 °C warmer than present from 7.7 (the beginning of our record) to ∼6 ka. The maximum late Holocene cold anomaly (which includes the Little Ice Age) is 1.5–3 °C colder than present. These ranges of possible temperature anomalies reflect uncertainty in the δ 18 O – temperature relationship for precipitation at the study site through the Holocene. [ABSTRACT FROM AUTHOR]

Stable isotope data have made pivotal contributions to nearly every discipline of the physical and natural sciences. As the generation and application of stable isotope data continues to grow exponentially, so does the need for a unifying data repository to improve accessibility and promote collaborative engagement. This paper provides an overview of the design, development, and implementation of IsoBank (www.isobank.org), a community-driven initiative to create an open-access repository for stable isotope data implemented online in 2021. A central goal of IsoBank is to provide a web-accessible database supporting interdisciplinary stable isotope research and educational opportunities. To achieve this goal, we convened a multi-disciplinary group of over 40 analytical experts, stable isotope researchers, database managers, and web developers to collaboratively design the database. This paper outlines the main features of IsoBank and provides a focused description of the core metadata structure. We present plans for future database and tool development and engagement across the scientific community. These efforts will help facilitate interdisciplinary collaboration among the many users of stable isotopic data while also offering useful data resources and standardization of metadata reporting across eco-geoinformatics landscapes., Competing Interests: The authors declare no competing interests., (Copyright: This is an open access article, free of all copyright, and may be freely reproduced, distributed, transmitted, modified, built upon, or otherwise used by anyone for any lawful purpose. The work is made available under the Creative Commons CC0 public domain dedication.)

Shifts in the hydrogen stable isotopic composition ( 2 H/ 1 H ratio) of lipids relative to water (lipid/water 2 H-fractionation) at natural abundances reflect different sources of the central cellular reductant, NADPH, in bacteria. Here, we demonstrate that lipid/water 2 H-fractionation ( 2 ε fattyacid/water ) can also constrain the relative importance of key NADPH pathways in eukaryotes. We used the metabolically flexible yeast Saccharomyces cerevisiae, a microbial model for respiratory and fermentative metabolism in industry and medicine, to investigate 2 ε fattyacid/water . In chemostats, fatty acids from glycerol-respiring cells were >550‰ 2 H-enriched compared to those from cells aerobically fermenting sugars via overflow metabolism, a hallmark feature in cancer. Faster growth decreased 2 H/ 1 H ratios, particularly in glycerol-respiring cells by 200‰. Variations in the activities and kinetic isotope effects among NADP + -reducing enzymes indicate cytosolic NADPH supply as the primary control on 2 ε fattyacid/water . Contributions of cytosolic isocitrate dehydrogenase (cIDH) to NAPDH production drive large 2 H-enrichments with substrate metabolism (cIDH is absent during fermentation but contributes up to 20 percent NAPDH during respiration) and slower growth on glycerol (11 percent more NADPH from cIDH). Shifts in NADPH demand associated with cellular lipid abundance explain smaller 2 ε fattyacid/water variations (<30‰) with growth rate during fermentation. Consistent with these results, tests of murine liver cells had 2 H-enriched lipids from slower-growing, healthy respiring cells relative to fast-growing, fermenting hepatocellular carcinoma. Our findings point to the broad potential of lipid 2 H/ 1 H ratios as a passive natural tracker of eukaryotic metabolism with applications to distinguish health and disease, complementing studies that rely on complex isotope-tracer addition methods., Competing Interests: Competing interests statement:The authors declare no competing interest.

Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are bacterial membrane lipids that are frequently employed as paleoenvironmental proxies because of the strong empirical correlations between their relative abundances and environmental temperature and pH. Despite the ubiquity of brGDGTs in modern and paleoenvironments, the source organisms of these enigmatic compounds have remained elusive, requiring paleoenvironmental applications to rely solely on observed environmental correlations. Previous laboratory and environmental studies have suggested that the globally abundant bacterial phylum of the Acidobacteria may be an important brGDGT producer in nature. Here, we report on experiments with a cultured Acidobacterium, Solibacter usitatus, that makes a large portion of its cellular membrane (24 ± 9% across all experiments) out of a structurally diverse set of tetraethers including the common brGDGTs Ia, IIa, IIIa, Ib, and IIb. Solibacter usitatus was grown across a range of conditions including temperatures from 15 to 30°C, pH from 5.0 to 6.5, and O 2 from 1% to 21%, and demonstrated pronounced shifts in the degree of brGDGT methylation across these growth conditions. The temperature response in culture was in close agreement with trends observed in published environmental datasets, supporting a physiological basis for the empirical relationship between brGDGT methylation number and temperature. However, brGDGT methylation at lower temperatures (15 and 20°C) was modulated by culture pH with higher pH systematically increasing the degree of methylation. In contrast, pH had little effect on brGDGT cyclization, supporting the hypothesis that changes in bacterial community composition may underlie the link between cyclization number and pH observed in environmental samples. Oxygen concentration likewise affected brGDGT methylation highlighting the potential for this environmental parameter to impact paleotemperature reconstruction. Low O 2 culture conditions further resulted in the production of uncommon brGDGT isomers that could be indicators of O 2 limitation. Finally, the production of brGTGTs (trialkyl tetraethers) in addition to the previously discovered iso-C15-based mono- and diethers in S. usitatus suggests a potential biosynthetic pathway for brGDGTs that uses homologs of the archaeal tetraether synthase (Tes) enzyme for tetraether synthesis from diethers., (© 2022 The Authors. Geobiology published by John Wiley & Sons Ltd.)