Molybdenum isotopes (δ98Mo) in shales are pivotal for reconstructing the redox history of ancient oceans. To refine this proxy, we investigated Mo concentrations and isotope behavior in Fayetteville Green Lake (FGL), New York— a permanently redox-stratified, euxinic lake. FGL mirrors other euxinic systems in its extreme deep-water Mo depletion and the parity of δ98Mo between sediments and surface-waters. However, we discovered a progressive depletion of δ98Mo with depth across the chemocline in the water column. This inverted fractionation trend is likely driven by FGL’s exceptionally high dissolved organic carbon to Mo ratios (DOC/Mo), which promote the formation of organic colloids that preferentially retain isotopically light Mo. We propose that this isotopically light colloidal Mo is suspended in the water column, while the complementary, isotopically heavy Mo in the truly dissolved fraction converts to thiomolybdates and sequesters into underlying sediments, as is commonly seen in H2S-bearing waters. This hypothesis can be tested in other euxinic water bodies with high DOC/Mo, such as Lake Pavin. Although near-quantitative Mo removal in FGL implies that only a small fraction of Mo is colloidally bound, this mechanism could have been more significant in Precambrian oceans where high DOC and low Mo availability were prevalent. Mass balance modeling indicates that if colloidal Mo was a substantial portion of the dissolved pool, euxinic sediments could have captured heavier-than-seawater δ98Mo, contrary to common expectations and hence leading to erroneous reconstruction of the redox conditions.

Microbial processes regulating carbon cycling in ancient oceans remain poorly understood, yet characterizing these processes is critical for understanding early Earth biogeochemistry. Here, we investigate microbial communities associated with sinking particles regulating carbon cycling in meromictic Fayetteville Green Lake, a mid-Proterozoic marginal ocean analog. The lake's photic zone spans oxic through sulfidic conditions, where prokaryotic photoautotrophs contribute to sinking fluxes and organotrophs mediate remineralization across redox and irradiance gradients. To characterize microbial communities in the sinking flux over time and redox condition, we sequenced 16S rRNA amplicons recovered from sediment traps throughout the lake's water column over the course of an annual photoautotroph bloom. Purple sulfur bacteria dominated deep fluxes, while cyanobacteria and green sulfur bacteria contributed variably across depths but were more abundant in suspended communities. As the bloom waned, chemoautotrophic Epsilonbacteraeota gained dominance in deeper fluxes, possibly due to niche partitioning. The shallow flux was remineralized by microbes exposed to temporally fluctuating biogeochemical conditions. Putative temporal changes in the availability and quality of organic matter and terminal electron acceptors thus promoted a succession of low-diversity communities with few dominant hydrolytic and acidogenic clades. Unchanging conditions at depth promoted higher diversity microbial communities with niches for specialists dominated by sulfur-metabolizing and fermentative clades. These findings improve our understanding of carbon cycling in the ancient ocean and offer insights into future shifts under climate change and meromixis in lakes.

I-Abstract Biological processes in the Proterozoic Ocean are often inferred from modern oxygen-deficient environments (MODEs) or from stable isotopes in preserved sediment. To-date, few MODE studies have simultaneously quantified carbon fixation genes and attendant stable isotopic signatures. Consequently, how carbon isotope patterns reflect these pathways has not been thoroughly vetted. Addressing this, we profiled planktonic productivity and quantified carbon fixation pathway genes and associated carbon isotope values of size-fractionated (0.2 – 2.7 and > 2.7 μm) particulate organic carbon values (8 13 C POC ) from meromictic Fayetteville Green Lake, NY, USA. The high-O 2 Calvin-Benson-Bassham (CBB) gene ( cbbL) was most abundant in the <2.7 μm size fraction in shallow oxic and deep hypoxic waters, corresponding with cyanobacterial populations. The low-O 2 CBB gene ( cbbM) was most abundant near the lower oxycline boundary in the larger size fraction, coincident with purple sulfur bacteria populations. The reverse citric acid cycle gene ( aclB) was equally abundant in both size fractions in the deepest photic zone, coinciding with green sulfur bacteria populations. Methane coenzyme reductase A ( mcrA ), of anaerobic methane cyclers, was most abundant at the lower oxycline boundary in both size fractions, coinciding with Methanoregula populations. 8 13 C POC values overlapped with the high-O 2 CBB fixation range except for two negative excursions near the lower oxycline boundary, likely reflecting assimilation of isotopically-depleted groundwater-derived inorganic carbon by autotrophs and acetate oxidation by sulfate-reducers. Throughout aphotic waters, 8 13 C POC values of the large size fraction became 13C-enriched, likely reflecting abundant purple sulfur bacterial aggregates. Microalgal-like isotopic signatures corresponded with increases in cbbL , cbbM and aclB , and enrichment of exopolymer-rich prokaryotic photoautotrophs aggregates. Results suggest that 8 13 C POC values of preserved sediments from areas of the Proterozoic Ocean with sulfidic photic zones may reflect a mixture of alternate carbon-fixing populations exported from the deep photic zone, challenging the paradigm that sedimentary stable carbon isotope values predominantly reflect oxygenic photosynthesis from surface waters.

In lakes, seasonal phytoplankton blooms and allochthonous plant debris intensify particulate organic carbon fluxes to the lakebed. Microbes associated with these particles likely vary with organic substrate lability and redox conditions. To explore microbial compositional responses to these variables, we analyzed particle-associated and free-living assemblages in the permanently redox-stratified Fayetteville Green Lake using 16 S rRNA amplicon sequencing during the peak and end of cyanobacterial and photoautotrophic sulfur bacterial blooms. Assemblage compositions were strongly influenced by redox conditions and particle association. Assemblage compositions varied seasonally above the lower oxycline boundary (summer-generalist heterotrophs; autumn-iron reducers and specialist heterotrophs), but not in the anoxic region below. Particle-associated assemblages were less diverse than free-living assemblages and were dominated by heterotrophs that putatively metabolize complex organic substrates, purple sulfur bacteria, sulfur-cycling Desulfocapsa, and eukaryotic algae. The least diverse particle-associated assemblages occurred near the lower oxycline boundary, where microbial activities and abundances were highest, and anoxygenic photoautotrophs were enriched. The low-diversity particle-associated heterotrophs likely remineralize complex organic substrates, releasing simpler organic substrates to free-living assemblages during transit, thereby influencing surrounding microbial diversity and function. Our results challenge the paradigm that phytoplankton from the shallow photic zone are the primary contributor to the vertical flux. We suggest that photoautotrophic prokaryotes from the deep photic zone contribute significantly to deep-water carbon in this environment, and possibly in other oxygen-deficient waters with sulfidic photic zones. Furthermore, results suggest that seasonally variable terrestrial carbon and metal inputs also influence microbial diversity and function in similar systems.

Microbial assemblages associated with biogenic particles are phylogenetically distinct from free-living counterparts, yet biogeochemically coupled. Compositions may vary with organic carbon and inorganic substrate availability and with redox conditions, which determine reductant and oxidant availability. To explore microbial assemblage compositional responses to steep oxygen and redox gradients and seasonal variability in particle and substrate availability, we analyzed taxonomic compositions of particle-associated (PA) and free-living (FL) bacteria and archaea in permanently redox-stratified Fayetteville Green Lake. PA and FL assemblages (> 2.7 µ m and 0.2 – 2.7 µ m) were surveyed at the peak (July) and end (October) of concurrent cyanobacteria, purple and green sulfur bacteria blooms that result in substantial vertical fluxes of particulate organic carbon. Assemblage compositions varied significantly among redox conditions and size fractions (PA or FL). Temporal differences were only apparent among samples from the mixolimnion and oxycline, coinciding with seasonal hydrographic changes. PA assemblages of the mixolimnion and oxycline shifted from aerobic heterotrophs in July to fermenters, iron-reducers, and denitrifiers in October, likely reflecting seasonal variability in photoautotroph biomass and inorganic nitrogen. Within a light-scattering layer spanning the lower oxycline and upper monimolimnion, photoautotrophs were more abundant in July than in October, when Desulfocapsa , a sulfate-reducing and sulfur-disproportionating bacterium, and Chlorophyte chloroplasts were abundant in PA assemblages. In this layer, microbial activity and cell concentrations were also highest. Below, the most abundant resident taxa were sulfate-reducing bacteria and anaerobic respirers. Results suggest PA and FL assemblage niche partitioning interconnects multiple elemental cycles that involve particulate and dissolved phases.

The fidelity of uranium isotopes (8238U) in marine carbonates as a paleoredox proxy relies on whether carbonates can record and preserve seawater 8238U. Although modern carbonate sediments deposited under oxic conditions have been shown to track seawater 8238U, it remains unknown whether this is true for carbonates deposited under anoxic conditions. This is a crucial question because many ancient carbonates were likely deposited or reworked under anoxic bottom waters. To better understand the behavior of uranium isotopes under this scenario, we investigated U isotope geochemistry in the meromictic Fayetteville Green Lake (FGL; New York, USA), where primary calcite is precipitated from oxic surface waters, sinks past the chemocline, and is deposited under anoxic bottom waters. We observed significant depletions of dissolved U concentration (from 2.7 to 0.9 ppb) and 8238U (from -0.55%0 to -0.96%0) below the chemocline in FGL. Parallel with these depletions, 8238U of sediment traps increased progressively from -0.51%0 to -0.16%0, suggesting that U(VI) reduction was occurring in the anoxic water column. Carbonate sediments deposited under anoxic bottom waters were enriched in U by 6-18x compared to primary calcite. Our data suggest that such significant authigenic U enrichments resulted from U(VI) reduction in the anoxic water column and below the sediment-water interface. The 8238U value in the top 0.25 cm of sediments was -0.29%0 +/- 0.10%0, overprinting original 8238U in primary calcite (-0.51%0 +/- 0.02%0). Future applications of carbonate 8238U as a paleoredox proxy should consider depositional environments (oxic vs. anoxic) of carbonates.

We document magnetic mineral diagenesis with high-resolution magnetic susceptibility, hysteresis, isothermal remanent magnetization, and other rock magnetic measurements through a shallow sulfate-methane transition (SMT) at Perseverance Drift-a high-accumulation rate Holocene biosiliceous Antarctic marine sediment deposit. The structure of the SMT is defined with porewater measurements from the same core, allowing direct comparison. Dissolution of the detrital (titano)magnetite assemblage, with preferential dissolution of stochiometric magnetite, occurs in the upper SMT. Higher coercivity magnetic minerals dissolve more slowly, continuing to dissolve through the entire SMT and could be a source of ferric iron for microbial respiration following exhaustion of porewater sulfate, as suggested by accumulation of porewater ferrous iron below the SMT. Superparamagnetic ferrimagnetic mineral enrichment/depletion occurs in three phases through the SMT and is coupled tightly to the availability of dissolved ferrous iron relative to dissolved sulfide. High concentrations of authigenic remanence-bearing iron sulfides, including greigite and hexagonal 3C pyrrhotite, which can be detected using remanence parameters but not in-field concentration dependent parameters, accumulate in a transient horizon at the base of the SMT during this early diagenesis, where sulfide is present but limited relative to dissolved ferrous iron. Formation of this remanence-bearing iron sulfide horizon is likely facilitated by continued iron reduction through the SMT. Nonsteady state perturbations that shift the porewater profile, such as changes in carbon flux or sedimentation rate, can lead to preservation of these transient horizons, much like well documented preservation of manganese oxide layers in marine sediments following similar shifts to porewater profiles.

The calving of A-68, the 5,800-km(2), 1-trillion-ton iceberg shed from the Larsen C Ice Shelf in July 2017, is one of over 10 significant ice-shelf loss events in the past few decades resulting from rapid warming around the Antarctic Peninsula. The rapid thinning, retreat, and collapse of ice shelves along the Antarctic Peninsula are harbingers of warming effects around the entire continent. Ice shelves cover more than 1.5 million km(2)and fringe 75% of Antarctica's coastline, delineating the primary connections between the Antarctic continent, the continental ice, and the Southern Ocean. Changes in Antarctic ice shelves bring dramatic and large-scale modifications to Southern Ocean ecosystems and continental ice movements, with global-scale implications. The thinning and rate of future ice-shelf demise is notoriously unpredictable, but models suggest increased shelf-melt and calving will become more common. To date, little is known about sub-ice-shelf ecosystems, and our understanding of ecosystem change following collapse and calving is predominantly based on responsive science once collapses have occurred. In this review, we outline what is known about (a) ice-shelf melt, volume loss, retreat, and calving, (b) ice-shelf-associated ecosystems through sub-ice, sediment-core, and pre-collapse and post-collapse studies, and (c) ecological responses in pelagic, sympagic, and benthic ecosystems. We then discuss major knowledge gaps and how science might address these gaps. This article is categorized under: Climate, Ecology, and Conservation > Modeling Species and Community Interactions

To better understand lateral dispersion of buoyant and nonbuoyant pollutants within the surface waters of large lakes, two lateral dispersion experiments were carried out in Lake Michigan during the stratified period: (1) a dye tracking experiment lasting 1 d; and (2) a drifter tracking experiment lasting 24 d. Both the dye patch and drifters were surface-released at the center of Lake Michigan's southern basin. Near-surface shear induced by near-inertial Poincare waves partially explains elevated dye dispersion rates (1.5-4.2 m(2) s(-1)). During the largely windless first 5 d of the drifter release, the drifters exhibited nearly scale-independent dispersion (K similar to L-0.2), with an average dispersion coefficient of 0.14 m(2) s(-1). Scale-dependent drifter dispersion ensued after 5 d, with K similar to L-1.09 and corresponding dispersion coefficients of 0.3-2.0 m(2) s(-1) for length scales L = 1500-8000 m. The largest drifter dispersion rates were found to be associated with lateral shear-induced spreading along a thermal front. Comparisons with other systems show a wide range of spreading rates for large lakes, and larger rates in both the ocean and the Gulf of Mexico, which may be caused by the relative absence of submesoscale processes in offshore Lake Michigan.

Climatic, cryospheric, and biologic changes taking place in the northern Antarctic Peninsula provide examples for how ongoing systemic change may progress through the entire Antarctic system.A large, interdisciplinary research project focused on the Larsen Ice Shelf system, synthesized here, has documented dramatic ice cover, oceanographic, and ecosystem changes in the Antarctic Peninsula during the Holocene and the present period of rapid regional warming.The responsiveness of the region results from its position in the climate and ocean system, in which a narrow continental block extends across zonal atmospheric and ocean flow, creating high snow accumulation, strong gradients and gyres, dynamic oceanography, outlet glaciers feeding into many fjords and bays having steep topography, and a continental shelf that contains many glacially carved troughs separated by areas of glacial sediment accumulation.The microcosm of the northern Antarctic Peninsula has a tendency to change rapidly-rapid relative not just to Antarctica's mainland but compared to the rest of the planet as well-and it is generally warmer than the rest of Antarctica.Both its Holocene and modern glaciological retreats offer a picture of how larger areas of Antarctica farther south might change under future warming.

Carbon cycling in natural ecosystems is a biologically mediated process with global consequences. Recent work has revealed the important role that lakes play in the global carbon cycle, suggesting organic carbon burial in small lakes and reservoirs matching and even surpassing that of the world's oceans. While much is known regarding biogeochemical cycling of carbon in the water column and underlying sediments of freshwater and marine systems, less is known about permanently redox-stratified water bodies. The modern ocean is fully oxygenated, however, the ocean is thought to have been redox-stratified throughout much of Earth's history, and seasonal redox-stratification is an increasing problem in many freshwater systems due to eutrophication driven by human land usage and warming resulting from global climate change. To better understand carbon signals preserved in the rock record from times of ocean redox-stratification as well as the effects of increasing redox-stratification on carbon cycling in modern freshwater systems, we have characterized the concentration and stable isotopic signal of inorganic and organic carbon in permanently redox-stratified Fayetteville Green Lake (FGL), New York. The results of these analyses indicate that: (1) groundwater is the primary source of dissolved inorganic carbon (DIC) at FGL; (2) organic carbon is extensively cycled within the water column and upper sediments resulting in an increasingly isotopically depleted DIC pool; (3) cyanobacteria-driven carbonate precipitation in the oxic zone is the primary source of carbonate in the sediments; (4) methane concentrations increase below the chemocline with extremely negative delta C-13 values (-99.1 parts per thousand to -102.3 parts per thousand).

Manganese and iron are redox-sensitive elements that yield clues about biogeochemistry and redox conditions both in modern environments and in the geologic past. Here, we investigated Mn and Fe-bearing minerals preserved in basin sediments underlying Fayetteville Green Lake, a redox-stratified lake that serves as a geochemical analogue for Paleoproterozoic oceans. Synchrotron-source microprobe techniques (mu XRF, mu XANES, and mu XRD) and bulk geochemical analyses were used to examine the microscale distribution and speciation of Mn, Fe, and S as a function of depth in the top 48 cm of anoxic lake sediments. Manganese was primarily associated with calcite grains as a manganese-rich carbonate that precipitated in the chemocline of the water column and settled through the euxinic basin to collect in lake sediments. Iron was preserved in framboidal iron sulfides that precipitated in euxinic bottom waters and underwent transformation to pyrite and marcasite in the sediments. Previous studies attribute the formation of manganese-rich carbonates to the diagenetic alteration of manganese oxides deposited in basins underlying oxygenated water. Our study challenges this paradigm by providing evidence that Mn-bearing carbonates form in the water column and accumulate in sediments below anoxic waters. Consequently, manganoan carbonates preserved in the rock record do not necessarily denote the presence of oxygenated bottom waters in ocean basins. (C) 2018 Elsevier Ltd. All rights reserved.

The calving of A-68, the 5,800 km2 iceberg that separated from the Larsen C Ice Shelf in July 2017, presents a unique and time-sensitive research opportunity. This event and other ice-shelf losses (e.g., from Larsen A and B, Wilkins, Wordie) are harbingers of warming effects along the Antarctic Peninsula in particular, and ultimately around all of Antarctica. The scientific momentum and public interest created by this most recent event led to an NSF funded workshop in November 2017. A consortium of US and international researchers with a diversity of expertise in Antarctic glaciological, geological, biological, and ecological sciences shared and reviewed the latest research, identified important research priorities and knowlegde gaps, and outlined strategic plans for the research community to advance understanding of the continent-wide changes that Antarctic ice shelves and surrounding ecosystems will experience in response to warming. The workshop has leveraged an opportunity to synergize efforts in investigating Antarctic ecosystems under the direct and indirect effects of ice-shelf collapse, and climatic warming in general. Here we present a review on the known and unknown ecosystem processes that will occur in the wake of massive, abrupt ice-shelf break-off and how they will affect ice-associated, pelagic, hard-bottom and soft-sediment substrates. We also present a view on future research approaches to address gaps in our knowledge and advance our understanding of the widespread effects of ice-shelf break-off and collapse.

Trace elements are central components of enzymes that catalyze many of the essential reactions mediated by life. The redox sensitive nature of trace elements also permits their use as a record of ancient ocean conditions preserved in the geologic record. Trace element geochemistry in modern stratified systems is often used as a proxy for the redox state of the ancient oceans, which are thought to have been largely anoxic. In the present study, we examined trace element behavior of simultaneously collected samples at a heretofore unprecedented depth resolution (1–0.25m intervals) throughout the redox-stratified water column of Fayetteville Green Lake, N.Y. (FGL), a 53m deep meromictic lake under euxinic conditions similar to those thought to have been prevalent in Proterozoic oceans. Among characterized Proterozoic ocean analogs, FGL represents an understudied proxy in terms of trace elements, with characteristics of low salinity and high sulfate. In the FGL water column, spikes in the concentration of dissolved Mn, Fe and Co are coincident with the transition from oxic to euxinic conditions, and are associated with a decrease in dissolved Mo concentration. In contrast, the concentration of dissolved Ni did not vary across this transition despite the dramatic shift in redox state. From these data we present a one dimensional model for element transport and cycling through the water column to the sediments. Collectively, this comprehensive analysis of water column geochemistry provides insight into the effects of biogeochemical cycling in stratified systems on dissolved trace element concentrations in the water column. This study, in concert with characterization of other early Earth analogs, will greatly enhance the use of trace elements in interpreting the geologic record.

Environmental context Concerns about carbon-driven climate change make it critical to better understand how carbon interacts with soils. We examined whether carbon compounds commonly found in soils affect the behaviour of soil iron minerals, and found that iron minerals remain dynamic and mix with the surrounding water, despite the presence of carbon in the water. Our findings suggest that both carbon and trace elements in the minerals may be more mobile and less stable than we previously thought. Abstract The close association of organic carbon and Fe oxides has been recognised for decades and recently interest in the role of Fe oxides in organic C sequestration has increased. Here we explore the effect of natural organic C on electron transfer and exchange of aqueous Fe-II and Fe in the bulk structure of goethite and magnetite. Using Fe-57 isotope experiments coupled with Fe-57 Mossbauer spectroscopy it is found that electron transfer occurs between aqueous Fe-II and structural Fe-III in both goethite and magnetite in the presence of most organic C compounds, including natural organic matter (NOM), extracellular polysaccharides (EPS), and cell materials. Only a long-chain (39-C) phospholipid containing organic C significantly inhibited electron transfer. Despite evidence for Fe-II-Fe-oxide(III) electron transfer in the presence of NOM, exchange of Fe between the aqueous phase and bulk Fe was less than observed in the absence of NOM. Our findings suggest that electron transfer between aqueous Fe-II and bulk structural Fe in goethite and magnetite is a robust process likely to occur in a variety of organic-rich biogeochemical environments but that Fe exchange may be inhibited by the presence of some C compounds.

Piscidins 1 (P1) and 3 (P3) are cationic α-helical membrane active antimicrobial peptides originally isolated from the hybrid striped seabass. The peptides, which possess broad-spectrum activity against bacteria, fungi, and viruses, bind to anionic membranes and lyse them once a sufficient concentration has been reached. In this study, we used confocal microscopy and FITC-labeled piscidins to show that at sub-inhibitory concentrations (0.75 μM), P1 and P3 are able to translocate across the membrane of Gram-positive and -negative bacteria. The FITC signal of the peptides was co-localized with the DAPI signal of the nucleoid region of the bacteria. CD using a 15 base pair piece of DNA and a gel retardation assay using a 1782 base pair piece of DNA confirmed that P1 and P3 could bind to DNA in vitro. It appears that P3 binding was more disruptive to the structure of DNA, and this may be due to the extra arginine residue in P3's sequence. However, these results did not indicate whether the peptides translocated through the membrane or entered the cells via pores formed in the membrane. To probe this question, we used the bacterial strain E. coli ML35 in a permeabilization assay to determine at which peptide concentration the membrane became permeabilized. Our data indicate that at the lowest concentration tested (0.1 μM) the membrane of ML35 cells became permeabilized by P1 and P3. Our results agree with prior findings that permeabilization of bacterial cell membranes can occur prior to lysis and cell death. The implications of piscidin's ability to translocate into bacterial cells and bind to intracellular targets at sub-lethal concentrations will be discussed in terms of its mechanism of action, which was previously believed to be based on its membrane activity.

Piscidins were the first antimicrobial peptides discovered in the mast cells of vertebrates. While two family members, piscidin 1 (p1) and piscidin 3 (p3), have highly similar sequences and α-helical structures when bound to model membranes, p1 generally exhibits stronger antimicrobial and hemolytic activity than p3 for reasons that remain elusive. In this study, we combine activity assays and biophysical methods to investigate the mechanisms underlying the cellular function and differing biological potencies of these peptides, and report findings spanning three major facets. First, added to Gram-positive (Bacillus megaterium) and Gram-negative (Escherichia coli) bacteria at sublethal concentrations and imaged by confocal microscopy, both p1 and p3 translocate across cell membranes and colocalize with nucleoids. In E. coli, translocation is accompanied by nonlethal permeabilization that features more pronounced leakage for p1. Second, p1 is also more disruptive than p3 to bacterial model membranes, as quantified by a dye-leakage assay and 2H solid-state NMR-monitored lipid acyl chain order parameters. Oriented CD studies in the same bilayers show that, beyond a critical peptide concentration, both peptides transition from a surface-bound state to a tilted orientation. Third, gel retardation experiments and CD-monitored titrations on isolated DNA demonstrate that both peptides bind DNA but p3 has stronger condensing effects. Notably, solid-state NMR reveals that the peptides are α-helical when bound to DNA. Overall, these studies identify two polyreactive piscidin isoforms that bind phosphate-containing targets in a poised amphipathic α-helical conformation, disrupt bacterial membranes, and access the intracellular constituents of target cells. Remarkably, the two isoforms have complementary effects; p1 is more membrane active, while p3 has stronger DNA-condensing effects. Subtle differences in their physicochemical properties are highlighted to help explain their contrasting activities.