This study seeks to identify drivers of differences in ocean acidification (OA) rates throughout the upper ocean at Station ALOHA in the North Pacific Subtropical Gyre (NPSG). OA is intensified in the subsurface due to increases in natural and anthropogenic carbon pools, and their interactions. Enhanced subsurface trends are found for all OA indicators. This includes parameters that have previously been identified to exhibit nonlinear interactions between anthropogenic and natural carbon ([H + ], p CO 2 , pH, Revelle Factor), as well as parameters that are not subject to this response (aragonite saturation state (Ω Ar )). Different parameters have trend maxima in each of the water masses in the upper 500 m, driven by different mechanisms. Enhanced acidification is noted in the North Pacific Tropical Water (NPTW) from 2015 until 2020. This is due to the interplay of a circulation slowdown during a prolonged negative phase of the North Pacific Gyre Oscillation (NPGO) with other anomalous atmospheric forcing that altered source water chemistry, including large‐scale freshening. Sustained increased acidification is also associated with freshening and cooling in the Subsurface Salinity Minimum (SSM) over the whole time‐series, with considerable oxygen loss and nutrient increases. In the North Pacific Intermediate Water (NPIW), a well‐documented circulation slowdown has led to enhanced CO 2 ingrowth from remineralization, buffered by increasing carbonate dissolution. Local changes seem to play a smaller role. In the SSM and NPTW, enhanced acidification is associated with cooling and freshening, providing new insights on how OA can accelerate beyond the well‐documented warming and souring of the ocean.

There is a paucity of records on decadal to millennial scales archiving the evolution of the Antarctic subglacial hydrologic system, leaving a gap in our knowledge of the links between meltwater drainage and ice behavior. We employed a suite of sedimentological and geochemical methods to assess the stratigraphic distribution of lithofacies and evaluate sedimentary processes from sediment cores collected from Mercer Subglacial Lake, located beneath the Mercer Ice Stream in West Antarctica. The composite 2.06 m sediment record consisted of massive-to-stratified diamict, massive muds, and laminated muds. Chloride concentrations indicate sediment porewater was primarily derived from glacial melt with sediment deposition in freshwater conditions. Whereas diamicts are associated with basal ice contact during ice stream grounding events (tills), sorted mud beds buried below diamict lack coarse-grained detritus (>2 mm) indicating deposition from suspension settling in slowly flowing or ponded meltwater. Rhythmically laminated sediments capping the sedimentary sequence capture modern subglacial lake conditions and are likely influenced by a continuum of processes controlled by suspended sediment delivered into the lake, water column velocity changes associated with fill-drain cycles, and sediment fallout from basal ice melt. These sedimentary facies characterize a complex subglacial hydrologic system providing evidence that basal conditions alternated from grounded ice to water-filled cavities fed and drained by subglacial meltwater. Our data provide new information on subglacial sediments beneath an Antarctic ice stream that can be used to refine our knowledge of subglacial hydrology, its coupling with ice dynamics, and as an analog for studying ancient glacial deposits.

Nitrification is an important control on the form and distribution of nitrogen in freshwater ecosystems. However, the seasonality of nitrogen pools and the diversity of organisms catalyzing this process have not been well documented in oligotrophic lakes. Here, we show that nitrogen pools and nitrifying organisms in Flathead Lake are temporally and vertically dynamic, with nitrifiers displaying specific preferences depending on the season. While the ammonia-oxidizing bacteria (AOB) Nitrosomonadaceae and nitrite-oxidizing bacteria (NOB) Nitrotoga dominate at depth in the summer, the ammonia-oxidizing archaea (AOA) Nitrososphaerota and NOB Nitrospirota become abundant in the winter. Given clear seasonality in ammonium, with higher concentrations during the summer, we hypothesize that the succession between these two nitrifying groups may be due to nitrogen affinity, with AOB more competitive when ammonia concentrations are higher and AOA when they are lower. Nitrifiers in Flathead Lake share more than 99% average nucleotide identity with those reported in other North American lakes but are distinct from those in Europe and Asia, indicating a role for geographic isolation as a factor controlling speciation among nitrifiers. Our study shows there are seasonal shifts in nitrogen pools and nitrifying populations, highlighting the dynamic spatial and temporal nature of nitrogen cycling in freshwater ecosystems.

During the 2018-2019 Antarctic field season, the Subglacial Antarctic Lakes Scientific Access project team cleanly accessed Mercer Subglacial Lake, West Antarctica, to sample water and sediments beneath 1087 m of overlying ice. A multicorer was successful in sampling the sediment-water interface, with 4 deployments retrieving 10 cores between 0.3 and 0.4 m in length. Gravity coring was also successful, retrieving cores of 0.97 and 1.78 m in glacial diamict. However, sediment cores retrieved by the gravity cores were shorter than the core barrel penetration (as measured by mud streaks on the outside of the coring system), indicating that the system can likely be improved. This manuscript describes the design, implementation, successes, and lessons learned while coring sediments in a subglacial lake.

Ice streams that flow into Ross Ice Shelf are underlain by water-saturated sediments, a dynamic hydrological system, and subglacial lakes that intermittently discharge water downstream across grounding zones of West Antarctic Ice Sheet (WAIS). A 2.06 m composite sediment profile was recently recovered from Mercer Subglacial Lake, a 15 m deep water cavity beneath a 1087 m thick portion of the Mercer Ice Stream. We examined microbial abundances, used 16S rRNA gene amplicon sequencing to assess community structures, and characterized extracellular polymeric substances (EPS) associated with distinct lithologic units in the sediments. Bacterial and archaeal communities in the surficial sediments are more abundant and diverse, with significantly different compositions from those found deeper in the sediment column. The most abundant taxa are related to chemolithoautotrophs capable of oxidizing reduced nitrogen, sulfur, and iron compounds with oxygen, nitrate, or iron. Concentrations of dissolved methane and total organic carbon together with water content in the sediments are the strongest predictors of taxon and community composition. δ¹³C values for EPS (−25 to −30‰) are consistent with the primary source of carbon for biosynthesis originating from legacy marine organic matter. Comparison of communities to those in lake sediments under an adjacent ice stream (Whillans Subglacial Lake) and near its grounding zone provide seminal evidence for a subglacial metacommunity that is biogeochemically and evolutionarily linked through ice sheet dynamics and the transport of microbes, water, and sediments beneath WAIS.

ABSTRACT While methane is typically produced under anoxic conditions, methane supersaturation in the presence of oxygen has been observed in both marine and fresh waters. The biological cleavage of methylphosphonate (MPn), which releases both phosphate and methane, is one pathway that may contribute to this paradox. Here, we explore the genomic and functional potential for oxic methane production (OMP) via MPn in Flathead Lake, a large oligotrophic freshwater lake in northwest Montana. Time series and depth profile measurements show that epilimnetic methane was persistently supersaturated despite high oxygen levels, suggesting a possible in situ oxic source. Metagenomic sequencing indicated that 10% of microorganisms in the lake, many of which are related to the Burkholderiales (Betaproteobacteria) and Actinomycetota, have the genomic capacity to cleave MPn. We experimentally demonstrated that these organisms produce methane stoichiometrically with MPn consumption across multiple years. However, methane was only produced at appreciable rates in the presence of MPn when a labile organic carbon source was added, suggesting that this process may be limited by both MPn and labile carbon supply. Members of the genera Acidovorax , Rhodoferax , and Allorhizobium , organisms which make up less than 1% of Flathead Lake communities, consistently responded to MPn addition. We demonstrate that the genomic and physiological potential for MPn use exists among diverse, resident members of Flathead Lake and could contribute to OMP in freshwater lakes when substrates are available. IMPORTANCE Methane is an important greenhouse gas that is typically produced under anoxic conditions. We show that methane is supersaturated in a large oligotrophic lake despite the presence of oxygen. Metagenomic sequencing indicates that diverse, widespread microorganisms may contribute to the oxic production of methane through the cleavage of methylphosphonate. We experimentally demonstrate that these organisms, especially members of the genus Acidovorax , can produce methane through this process. However, appreciable rates of methane production only occurred when both methylphosphonate and labile sources of carbon were added, indicating that this process may be limited to specific niches and may not be completely responsible for methane concentrations in Flathead Lake. This work adds to our understanding of methane dynamics by describing the organisms and the rates at which they can produce methane through an oxic pathway in a representative oligotrophic lake.

Considerable attention is given to absolute nutrient levels in lakes, rivers, and oceans, but less is paid to their relative concentrations, their nitrogen:phosphorus (N:P) stoichiometry, and the consequences of imbalanced stoichiometry. Here, we report 38 y of nutrient dynamics in Flathead Lake, a large oligotrophic lake in Montana, and its inflows. While nutrient levels were low, the lake had sustained high total N: total P ratios (TN:TP: 60 to 90:1 molar) throughout the observation period. N and P loading to the lake as well as loading N:P ratios varied considerably among years but showed no systematic long-term trend. Surprisingly, TN:TP ratios in river inflows were consistently lower than in the lake, suggesting that forms of P in riverine loading are removed preferentially to N. In-lake processes, such as differential sedimentation of P relative to N or accumulation of fixed N in excess of denitrification, likely also operate to maintain the lake’s high TN:TP ratios. Regardless of causes, the lake’s stoichiometric imbalance is manifested in P limitation of phytoplankton growth during early and midsummer, resulting in high C:P and N:P ratios in suspended particulate matter that propagate P limitation to zooplankton. Finally, the lake’s imbalanced N:P stoichiometry appears to raise the potential for aerobic methane production via metabolism of phosphonate compounds by P-limited microbes. These data highlight the importance of not only absolute N and P levels in aquatic ecosystems, but also their stoichiometric balance, and they call attention to potential management implications of high N:P ratios.

AbstractThe Subglacial Antarctic Lakes Scientific Access (SALSA) Project accessed Mercer Subglacial Lake using environmentally clean hot-water drilling to examine interactions among ice, water, sediment, rock, microbes and carbon reservoirs within the lake water column and underlying sediments. A ~0.4 m diameter borehole was melted through 1087 m of ice and maintained over ~10 days, allowing observation of ice properties and collection of water and sediment with various tools. Over this period, SALSA collected: 60 L of lake water and 10 L of deep borehole water; microbes >0.2 μm in diameter from in situ filtration of ~100 L of lake water; 10 multicores 0.32–0.49 m long; 1.0 and 1.76 m long gravity cores; three conductivity–temperature–depth profiles of borehole and lake water; five discrete depth current meter measurements in the lake and images of ice, the lake water–ice interface and lake sediments. Temperature and conductivity data showed the hydrodynamic character of water mixing between the borehole and lake after entry. Models simulating melting of the ~6 m thick basal accreted ice layer imply that debris fall-out through the ~15 m water column to the lake sediments from borehole melting had little effect on the stratigraphy of surficial sediment cores.

Station ALOHA (A Long-term Oligotrophic Habitat Assessment) was established in the North Pacific Subtropical Gyre (22 degrees 45' N, 158 degrees W) as an oligotrophic ocean benchmark to improve our understanding of processes that govern the fluxes of carbon (C) into and from the surface ocean. At approximately monthly intervals, measurements of the primary production of particulate C (PC) using the C-14 method, and the export of PC and particulate nitrogen (PN) using surface-tethered sediment traps deployed at 150 m have been made along with a host of complementary physical, biological, and biogeochemical measurements. Euphotic zone depth-integrated (0-200 m) primary production ranged from 220.2 (standard deviation, SD, 10.8) mg C m (-2) d (-1) in Feb 2018 to 1136.5 (SD = 17.1) mg C m (-2) d (-1) in Jun 2000, with a 30-yr (1989-2018) mean of 536.8 (SD = 135.0) mg C m(-2) d (-1) (n = 271). Although the monthly primary production climatology was fairly well constrained, we observed substantial sub-decadal variability and a significant 0-125 m depth-integrated increasing trend of 4.0 (p < 0.01; 95% confidence interval, CI, 2.1-5.9) (mg C m(-2) d (-1)) yr 1 since 1989, displaying a large relative increase of 37% (CI = 18-55%) in the lower portion (75-125 m) of the euphotic zone. Chlorophyll (Chl) a and suspended PC and PN concentrations also displayed significant (p < 0.01) increases in the 75-125 m region of the euphotic zone. PC export at 150 m exhibited both short-term (monthly) and longer-scale variability with a 30-yr mean of 27.9 (SD = 9.7, n = 265) mg C m(-2) d (-1). PC and PN export exhibited extended, multi-year periods of significantly lower or higher values compared to the 30-yr mean. These multi-year periods of anomalously low and high particle export, in the absence of contemporaneous variations in primary production, probably reflect periodic changes in remineralization efficiencies. The PC export ratio (e-ratio; PC export at 150 m divided by 0-150 m depth-integrated C-14-based primary production) was low, with a 30-yr mean of 0.054 (SD = 0.021, n = 248), and exhibited a significant (p < 0.01) long-term decreasing trend over the 30-yr observation period. The 30-yr long-term increases in primary production (similar to 37%), Chl a, and suspended PC and PN concentrations (similar to 17%, 8%, and 8%, respectively) in the 75-125 m portion of the water column are hypothesized to result from an enhanced supply of nutrients to the lower portion of the water column over the past three decades.

Life in environments devoid of photosynthesis, such as on early Earth or in contemporary dark subsurface ecosystems, is supported by chemical energy. How, when, and where chemical nutrients released from the geosphere fuel chemosynthetic biospheres is fundamental to understanding the distribution and diversity of life, both today and in the geologic past. Hydrogen (H-2) is a potent reductant that can be generated when water interacts with reactive components of mineral surfaces such as silicate radicals and ferrous iron. Such reactive mineral surfaces are continually generated by physical comminution of bedrock by glaciers. Here, we show that dissolved H-2 concentrations in meltwaters from an iron and silicate mineral-rich basaltic glacial catchment were an order of magnitude higher than those from a carbonate-dominated catchment. Consistent with higher H-2 abundance, sediment microbial communities from the basaltic catchment exhibited significantly shorter lag times and faster rates of net H-2 oxidation and dark carbon dioxide (CO2) fixation than those from the carbonate catchment, indicating adaptation to use H-2 as a reductant in basaltic catchments. An enrichment culture of basaltic sediments provided with H-2, CO2, and ferric iron produced a chemolithoautotrophic population related to Rhodoferax ferrireducens with a metabolism previously thought to be restricted to (hyper) thermophiles and acidophiles. These findings point to the importance of physical and chemical weathering processes in generating nutrients that support chemosynthetic primary production. Furthermore, they show that differences in bedrock mineral composition can influence the supplies of nutrients like H-2 and, in turn, the diversity, abundance, and activity of microbial inhabitants.

Reports of biogenic methane (CH4) synthesis associated with a range of organisms have steadily accumulated in the literature. This has not happened without controversy and in most cases the process is poorly understood at the gene and enzyme levels. In marine and freshwater environments, CH4 supersaturation of oxic surface waters has been termed the "methane paradox" because biological CH4 synthesis is viewed to be a strictly anaerobic process carried out by O2-sensitive methanogens. Interest in this phenomenon has surged within the past decade because of the importance of understanding sources and sinks of this potent greenhouse gas. In our work on Yellowstone Lake in Yellowstone National Park, we demonstrate microbiological conversion of methylamine to CH4 and isolate and characterize an Acidovorax sp. capable of this activity. Furthermore, we identify and clone a gene critical to this process (encodes pyridoxylamine phosphate-dependent aspartate aminotransferase) and demonstrate that this property can be transferred to Escherichia coli with this gene and will occur as a purified enzyme. This previously unrecognized process sheds light on environmental cycling of CH4, suggesting that O2-insensitive, ecologically relevant aerobic CH4 synthesis is likely of widespread distribution in the environment and should be considered in CH4 modeling efforts.

American bison (Bison bison L.) have recovered from the brink of extinction over the past century. Bison reintroduction creates multiple environmental benefits, but impacts on greenhouse gas emissions are poorly understood. Bison are thought to have produced some 2 Tg yr−1 of the estimated 9–15 Tg yr−1 of pre-industrial enteric methane emissions, but few measurements have been made due to their mobile grazing habits and safety issues associated with measuring non-domesticated animals. Here, we measure methane and carbon dioxide fluxes from a bison herd on an enclosed pasture during daytime periods in winter using eddy covariance. Methane emissions from the study area were negligible in the absence of bison (mean ± standard deviation = −0.0009 ± 0.008 µmol m−2 s−1) and were significantly greater than zero, 0.048 ± 0.082 µmol m−2 s−1, with a positively skewed distribution, when bison were present. We coupled bison location estimates from automated camera images with two independent flux footprint models to calculate a mean per-animal methane efflux of 58.5 µmol s−1 per bison, similar to eddy covariance measurements of methane efflux from a cattle feedlot during winter. When we sum the observations over time with conservative uncertainty estimates we arrive at 81 g CH4 per bison d−1 with 95 % confidence intervals between 54 and 109 g CH4 per bison d−1. Uncertainty was dominated by bison location estimates (46 % of the total uncertainty), then the flux footprint model (33 %) and the eddy covariance measurements (21 %), suggesting that making higher-resolution animal location estimates is a logical starting point for decreasing total uncertainty. Annual measurements are ultimately necessary to determine the full greenhouse gas burden of bison grazing systems. Our observations highlight the need to compare greenhouse gas emissions from different ruminant grazing systems and demonstrate the potential for using eddy covariance to measure methane efflux from non-domesticated animals.

Trace elements sustain biological productivity, yet the significance of trace element mobilization and export in subglacial runoff from ice sheets is poorly constrained at present. Here, we present size-fractionated (0.02, 0.22, and 0.45 µm) concentrations of trace elements in subglacial waters from the Greenland Ice Sheet (GrIS) and the Antarctic Ice Sheet (AIS). Concentrations of immobile trace elements (e.g., Al, Fe, Ti) far exceed global riverine and open ocean mean values and highlight the importance of subglacial aluminosilicate mineral weathering and lack of retention of these species in sediments. Concentrations are higher from the AIS than the GrIS, highlighting the geochemical consequences of prolonged water residence times and hydrological isolation that characterize the former. The enrichment of trace elements (e.g., Co, Fe, Mn, and Zn) in subglacial meltwaters compared with seawater and typical riverine systems, together with the likely sensitivity to future ice sheet melting, suggests that their export in glacial runoff is likely to be important for biological productivity. For example, our dissolved Fe concentration (20,900 nM) and associated flux values (1.4 Gmol y-1) from AIS to the Fe-deplete Southern Ocean exceed most previous estimates by an order of magnitude. The ultimate fate of these micronutrients will depend on the reactivity of the dominant colloidal size fraction (likely controlled by nanoparticulate Al and Fe oxyhydroxide minerals) and estuarine processing. We contend that ice sheets create highly geochemically reactive particulates in subglacial environments, which play a key role in trace elemental cycles, with potentially important consequences for global carbon cycling.

Although subglacial aquatic environments are widespread beneath the Antarctic ice sheet, subglacial biogeochemistry is not well understood, and the contribution of subglacial water to coastal ocean carbon and nutrient cycling remains poorly constrained. The Whillans Subglacial Lake (SLW) ecosystem is upstream from West Antarctica's Gould‐Siple Coast ~800 m beneath the surface of the Whillans Ice Stream. SLW hosts an active microbial ecosystem and is part of an active hydrological system that drains into the marine cavity beneath the adjacent Ross Ice Shelf. Here we examine sources and sinks for organic matter in the lake and estimate the freshwater carbon and nutrient delivery from discharges into the coastal embayment. Fluorescence‐based characterization of dissolved organic matter revealed microbially driven differences between sediment pore waters and lake water, with an increasing contribution from relict humic‐like dissolved organic matter with sediment depth. Mass balance calculations indicated that the pool of dissolved organic carbon in the SLW water column could be produced in 4.8 to 11.9 yr, which is a time frame similar to that of the lakes’ fill‐drain cycle. Based on these estimates, subglacial lake water discharged at the Siple Coast could supply an average of 5,400% more than the heterotrophic carbon demand within Siple Coast embayments (6.5% for the entire Ross Ice Shelf cavity). Our results suggest that subglacial discharge represents a heretofore unappreciated source of microbially processed dissolved organic carbon and other nutrients to the Southern Ocean.

Aerobic methane production in aquatic ecosystems impacts the global atmospheric budget of methane, but the extent, mechanism, and taxa responsible for producing this greenhouse gas are not fully understood. Lake Bonney (LB), a perennially ice‐covered Antarctic lake, has cold hypersaline waters underlying an oxygenated freshwater layer. We present temporal methane concentration profiles in LB indicating methane production in the oxygenated (> 200% air saturation) water. Experiments amended with methylphosphonate (MPn) yielded methane generation, suggesting in situ methanogenesis via the carbon‐phosphorus (C‐P) lyase pathway. Enrichment cultures from the lake were used to isolate five bacterial strains capable of generating methane when supplied with MPn as the sole P source. Based on 16S rRNA gene sequencing, the isolates belong to the Proteobacteria (closely related to Marinomonas, Hoeflea, and Marinobacter genera) and Bacteroidetes (Algoriphagus genus). 16S rRNA metagenomic sequencing confirms the presence of these taxa in LB. None of the isolated species were reported to be capable to produce methane. In addition, orthologs of the phosphoenolpyruvate mutase gene (PepM) and methylphosphonate synthase (MPnS), enzymes involved in phosphonate and MPn biosynthesis, were widely spread in the LB shotgun metagenomic libraries; genes related to C‐P lyase pathways (phn gene clusters) were also abundant. 16S rRNA and mcrA genes of anaerobic methanogens were absent in both 16S rRNA and metagenomics libraries. These data reveal that in situ aerobic biological methane production is likely a significant source of methane in LB.