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Sedimentary rocks and minerals formed during the final two-hundred million years of the Archean Eon (2.7 to 2.5 billion years ago, or Ga) are more depleted in 56Fe than at any other time in Earth’s past. Three hypotheses are proposed to explain these 56Fe depletions: (1) a very negative late-Archean seawater δ56Fe value, (2) “shuttling” of isotopically light Fe across the chemocline in redox-stratified settings, and (3) pyrite formation in an Fe(II)-rich ocean. Each of these scenarios has different implications for the initial oxidation of Earth’s surface, the climax of which – the Great Oxidation Event – immediately post-dates the appearance of these 56Fe depletions in the rock record. To help inform this debate, we measured the Fe isotope ratios of 120 shale and pyrite samples from Western Australia (Mt. McRae Shale and Jeerinah Formation) and South Africa (Klein Naute Formation) deposited between ∼2.65 Ga and ∼2.50 Ga. As in previous studies, we also find very strong sedimentary 56Fe depletions, to as low as δ56Fe = −2.06 ± 0.08‰ in bulk shales and δ56Fe = −2.31 ± 0.08‰ in pyrite. Some, but not all, of the severest 56Fe depletions appear alongside evidence of an Fe shuttle and local pyrite formation. These processes need not be mutually exclusive, and some combination of them likely played a partial, probably faciliatory role in driving some strong 56Fe depletions in our dataset. Most interestingly, and with little exception, the severest 56Fe depletions appear in samples deposited farther from shore under H2S-rich and anoxic (“euxinic”) conditions. We find it difficult to explain this connection without invoking the persistent presence of a very negative global seawater δ56Fe value during the latest Archean, one that was most consistently captured in sediments formed in distal euxinic settings. In order to impart this isotopic effect on seawater, the global seawater Fe(II) reservoir needed to have been partially oxidized during at least the final few hundreds of millions of years leading up to the Great Oxidation Event. Our new data add support to the idea that Earth’s initial oxidation was a long and protracted process rather than a rapid event.

Thallium andV isotopes in organic-rich sediments have recently been explored for reconstructing marine oxygenation by tracking the burial ofMn oxides either locally or globally. The ‘Fe and Mn shuttle’ is a well known mechanism that can transport elements associated with Fe and Mn oxide phases formed under mildly oxidizing shallow water environments both laterally and vertically to euxinic (sulfidic water column) deep waters. The Fe and Mn shuttle has been demonstrated to strongly affect Fe andMo isotope compositions of sediments accumulating beneath anoxic and euxinic settings. Similar to Mo, Tl and V are also enriched in oceanic ferromanganese crust and nodules, which impart the largest isotopic offsets from modern seawater for Tl and V. Thus, the mechanism of an Fe and Mn shuttle has the potential to, but currently unconstrained, affect Tl and V isotopic compositions (e205Tl and δ 51V) in organic-rich sediments in the modern and ancient records. We have used Black Sea sediments that are known to preserve enrichment and sedimentary isotope signatures for an Fe shuttle. This is due to an Fe transport from the more oxic margin to the deep euxinic basin and corresponding enrichment in the underlying sediments. These samples were used to test for a potential Fe and Mn shuttle effect on e 205Tl and δ 51V in sediments deposited under oxic, suboxic, and euxinic water columns. Authigenic e 205Tl from all three depositional settings is indistinguishable (–2.7 ± 0.3, –2.4 ± 0.5, and –2.4 ± 0.3), which is within error of the Black Sea surface seawater (–2.2 ± 0.3). In contrast, oxic and suboxic sediment have similar δ 51V values (–1.06 ± 0.33 ‰ and –1.04 ± 0.30 ‰), which are significantly more negative than euxinic sediments (–0.57 ± 0.06 ‰). All the sediments deposited under oxic, suboxic, and euxinic water columns in the Black Sea capture e 205Tl of the surface seawater because there is almost no permanent burial of Mn oxides in this basin. This finding is consistent with the indistinguishable e 205Tl between the surface seawater and the riverine input. Sedimentary δ 51V appears to be controlled predominantly by the redox state of the depositional environments with little effect from an Fe or Mn shuttle. The lack of Fe and Mn shuttle effect on e 205Tl and δ 51V in the Black Sea is likely due to the shallow (∼100 m) chemocline in the strongly, 2000-m deep redox-stratified water column of this basin and very slow deep-water renewal rate. Iron and Mn in oxides are not delivered to the sediments, instead, they are dissolved and captured in the water column as iron sulfide and reduced Mn mineral phases. Our results suggest that Fe and Mn shuttle transport mechanisms may not strongly affect Tl and V isotopes, at least in extreme cases of water column redox stratification. Further work is required to constrain the isotopic signals using other modern and, by inference, diverse ancient settings for additional shuttling mechanisms.

Earth’s Great Oxidation Event (GOE), ca. 2.5–2.0 Ga, was one of the most extreme environmental perturbations in the history of the planet. In addition to the first sustained accumulation of O2 in the atmosphere, the latter half of the GOE is associated with a very large positive carbon isotope excursion, both in terms of magnitude and inferred duration. The end of the GOE may have been associated with a decrease in atmospheric oxygen levels, although this transition remains poorly understood. We test if this suggested decline in atmospheric O2 is reflected in the molybdenum stable isotope compositions (δ98Mo) and Ce anomalies of a large number (N = 299) of carbonate sedimentary rocks collected from Finnish Lapland and the Canadian Labrador Trough and Belcher Group, which collectively span ca. 2.1–1.88 Ga. Clear evidence for a shift in redox conditions across the end-GOE is obscured by coupled stratigraphic variations in δ98Mo values, Ce anomalies, and Mn concentrations, suggesting local controls on these redox proxies as a function of depositional environment, likely as a result of particulate shuttling of Mo and Ce associated with Mn redox cycling across a chemocline. The most negative Ce anomalies recorded in the Belcher Group (

Encrusting foraminifers locally can play an essential reef-forming role such as in the Carnian fossiliferous Hallstatt Limestones of Austria where a dense succession of hardgrounds occurs with numerous small buildups (up to 2 cm high) composed of sessile foraminifers. Similar foraminiferal micro-buildups occur in a 10-cm-thick bed in the basinal facies in the basal Zechstein (Upper Permian) strata in the Radlin 60 borehole, in western Poland. Foraminiferal micro-buildups of presumably columnar habit account for up to a half of the volume. The growth of columns was interrupted by hiatal surfaces, although usually the growth of columns was restored afterwards. In the upper part of the bed with foraminiferal micro-buildups, the foraminiferal encrustations become visibly less abundant, and in most cases, they have a shape of less regular masses. The bed with foraminiferal micro-buildups abounds in closely spaced discontinuity (hiatal) surfaces, occasionally encrusted by foraminifers. Tubular foraminifers in the bed with foraminiferal micro-buildups are accompanied by various encrusting organisms, possibly microbes; they are grouped under the name Palaeonubecularia. In the intercolumnar spaces, common Midiella sp. occur. During deposition, the conditions were mostly suboxic, and various types of ferruginous morphs, common in the lower part of the bed, suggest that dysoxic-anoxic interfaces produced various microaerophilic environments for iron-bacteria. The environmental conditions during the deposition of the upper part of the bed were very similar to those characterizing the starved basin environment in which the bioclastic-peloidal and oncoidal sediments in the Zechstein Limestone have been formed. In turn, the lower part of the bed registers the transition from normal (= not starved) marine conditions, with a clear chemocline, to the starved basin conditions. The deposition of the bed was coeval with the Kupferschiefer deposition based on the correlation of δ13C curves, which implies that the previous stratigraphical concept of basal Zechstein strata has to be re-evaluated.

Subterranean estuaries (STEs), the zones in which seawater and subsurface groundwater mix, are recognized as hotspots for biogeochemical reactions; however, little is known of the microbial communities that control many of those reactions. This study investigated the potential functions of microbes inhabiting a cenote and an offshore submarine spring (Pargos) in the near-coastal waters of the Yucatan Peninsula, Mexico. The inland cenote (Cenote Siete Bocas; C7B) is characterized by a chemocline that is host to an array of physicochemical gradients associated with microbial activities. The chemocline includes an increasing gradient in sulfide concentrations with depth and a decreasing gradient in nitrate concentrations. The microbial community within the chemocline was dominated by Sulfurimonas and Sulfurovum of the Campylobacteria, which are likely responsible for sulfide oxidation coupled with nitrate reduction. Although C7B has not been directly connected with Pargos Spring, water discharging from the spring has physicochemical characteristics and microbial community structures similar to C7B, strongly suggesting biogeochemical processing in the STE impacts groundwater composition prior to discharge. This work yields insight into the microbial communities and biogeochemical reactions in STEs in karstic aquifers and provides evidence for the importance of Campylobacteria in controlling nitrate concentrations exported to marine springs.

Deep-sea hypersaline anoxic basins are polyextreme environments in the ocean’s interior characterized by the high density of brines that prevents mixing with the overlaying seawater, generating sharp chemoclines and redoxclines up to tens of meters thick that host a high concentration of microbial communities. Yet, a fundamental understanding of how such pycnoclines shape microbial life and the associated biogeochemical processes at a fine scale, remains elusive. Here, we applied high-precision sampling of the brine–seawater transition interface in the Suakin Deep, located at 2770 m in the central Red Sea, to reveal previously undocumented fine-scale community structuring and succession of metabolic groups along a salinity gradient only 1 m thick. Metagenomic profiling at a 10-cm-scale resolution highlighted spatial organization of key metabolic pathways and corresponding microbial functional units, emphasizing the prominent role and significance of salinity and oxygen in shaping their ecology. Nitrogen cycling processes are especially affected by the redoxcline with ammonia oxidation processes being taxa and layers specific, highlighting also the presence of novel microorganisms, such as novel Thaumarchaeota and anammox, adapted to the changing conditions of the chemocline. The findings render the transition zone as a critical niche for nitrogen cycling, with complementary metabolic networks, in turn underscoring the biogeochemical complexity of deep-sea brines., This research was supported by King Abdullah University of Science and Technology through baseline funding and Red Sea Research Center competitive fund to DD. We thank the crew of R/V Thuwal and CMOR for their support in sampling the brines, especially Brian Hession and Thomas Hoover. We also thank personal of the KAUST Bioscience Core Lab for their assistance with sequencing.

Background The Black Sea is the largest brackish water body in the world, although it is connected to the Mediterranean Sea and presents an upper water layer similar to some regions of the former, albeit with lower salinity and temperature. Despite its well-known hydrology and physicochemical features, this enormous water mass remains poorly studied at the microbial genomics level. Results We have sampled its different water masses and analyzed the microbiome by shotgun and genome-resolved metagenomics, generating a large number of metagenome-assembled genomes (MAGs) from them. We found various similarities with previously described Black Sea metagenomic datasets, that show remarkable stability in its microbiome. Our datasets are also comparable to other marine anoxic water columns like the Cariaco Basin. The oxic zone resembles to standard marine (e.g. Mediterranean) photic zones, with Cyanobacteria (Synechococcus but a conspicuously absent Prochlorococcus), and photoheterotrophs domination (largely again with marine relatives). The chemocline presents very different characteristics from the oxic surface with many examples of chemolithotrophic metabolism (Thioglobus) and facultatively anaerobic microbes. The euxinic anaerobic zone presents, as expected, features in common with the bottom of meromictic lakes with a massive dominance of sulfate reduction as energy-generating metabolism, a few (but detectable) methanogenesis marker genes, and a large number of “dark matter” streamlined genomes of largely unpredictable ecology. Conclusions The Black Sea oxic zone presents many similarities to the global ocean while the redoxcline and euxinic water masses have similarities to other similar aquatic environments of marine (Cariaco Basin or other Black Sea regions) or freshwater (meromictic monimolimnion strata) origin. The MAG collection represents very well the different types of metabolisms expected in this kind of environment. We are adding critical information about this unique and important ecosystem and its microbiome.

Rare earth elements (REE) in marine minerals have been widely used as proxies for the redox status of depositional and/or diagenetic environments. Phosphate nodules, which are thought to grow within decimetres below the sediment–water interface and to be able to scavenge REE from the ambient pore water, are potential archives of subtle changes in REE compositions. Whether their REE signals represent specific redox conditions or they can be used to track the overlying water chemistry is worth exploring. Through in situ laser ablation – inductively coupled plasma – mass spectrometry (LA-ICP-MS), we investigate the REE compositions of a drill-core-preserved phosphate nodule from the lower Cambrian Niutitang Formation in the Daotuo area, northeastern Guizhou Province, South China. REE distributions of the nodule show concentric layers with systematic decreases in Ce anomalies (Ce/Ce*) from the core to the rim. The lowest Ce/Ce* appears in the outer rim where REE concentrations are relatively high. These results are interpreted to reflect REE exchange with pore water at a very early stage or bathymetric variation during apatite precipitation. The origin of the shale-normalized middle REE (MREE) enrichment in our sample is less constrained. Possible driving factors include preferential MREE substitution for Ca in the apatite lattice, degradation of organic matter and deposition beneath a ferruginous zone. Although speculative, the last possibility is consistent with the chemically stratified model for early Cambrian oceans, in which dynamic fluctuations of the chemocline provided an ideal depositional context for phosphogenesis.

Rare earth elements (REEs) were measured in the water column of the highly stratified and euxinic Upper Basin of the Pettaquamscutt River estuary (Rhode Island) to investigate their cycling and evolution across the chemocline. The euxinic bottom waters of the estuary exhibit total dissolved sulfide (i.e., ΣS-II) concentrations up to 4 mmol kg−1, which exceeds values of the Black Sea by nearly 10-fold and makes this system ideal for study of redox-sensitive elements. The REE concentrations decrease with depth in the oxic surface waters, before substantially increasing across the chemocline, corresponding to redox cycling of manganese (Mn) and iron (Fe). For example, the mean ± 1σ neodymium (Nd) concentration in the oxic surface waters is 511 ± 184 pmol kg−1, but increases by approximately 5-fold across the chemocline reaching 2476 pmol kg−1 at 7.4 m depth. Concurrently, Mn and Fe concentrations increase by 5- and 11-fold, respectively, within the chemocline relative to their oxic surface water values. The pore water within the Upper Basin sediments exhibits REE concentrations that are on average 130-fold greater than their values in the overlying waters, driving an upward diffusive REE flux into the euxinic bottom waters. An outstanding feature of the REEs in this study is the preservation of negative cerium (Ce) anomalies across the chemocline into the highly euxinic bottom waters. Likewise, our reaction path model shows that the mass balance of the local water sources is the major driver of the REE compositions of the water column. Primarily, the negative Ce anomaly in the euxinic bottom waters highlights the influence of the Ce depleted sediment pore water on the REE composition and fractionation patterns of the euxinic bottom waters. This study underscores the importance of local sources and indicates that negative Ce anomalies can persist in highly euxinic waters that evolve from Ce depleted sources, further demonstrating that negative Ce anomalies are not necessarily a good proxy of the redox conditions of aqueous environments.

Two short-term summer cruises were performed in the vicinity of Rizhao Port, western Yellow Sea, to investigate summer water stratification and the phenomenon of marine sediment acidification. Results show that water stratification has formed as early as in May. The chemoclines of dissolved oxygen (DO) and pH usually occur deeper than the thermocline. Sometimes the chemoclines of DO and pH occur synchronously at the same depths, but sometimes they do not synchronize. Above the chemocline DO fluctuates markedly and maintains at a high level or even oversaturation in daytime, suggesting the far-field impact of vessel propulsion. Below the chemocline it invariably remains at a low and stable level and slowly drops downward. The pH values of surface sediments vary in the range of 6.3–7.6. A conspicuous sediment acidification is observed along the navigation region. This phenomenon is chiefly attributed to the intermittent disruption of summer stratification by shipping activity along the shipping channel and waterways. Water stratification precludes the penetration of DO down to the bottom sediments on fair ocean status, and maintains anoxia below the sediment–water interface, whereas the abrupt DO increase driven by propeller agitating is nearly restricted in the navigation area. Therefore, the aeration effect accelerating oxidation of reducing substances and organic matter is primarily responsible for enhanced acid accumulation in surface sediments. In addition, the suspension effect is identified by the negative offsets of zero point on pH–mV diagram. Most of the measured points are scattered correspondingly on three distinct regression lines, with similar slopes but different negative offsets of pH 6.53, 6.87, and 6.99, respectively. Their geographical distribution suggests that this effect has a positive relationship with the intensity of anthropogenic disturbance. Although the suspension effect is caused by the adsorption of charged particles at the pH glass electrode interface (Yang et al. in Part Sci Technol 7:139–152, 1989), its various intensity observed in practice may also reflect the inherent distinction of depositional environments in physicochemical conditions.

Brine beneath Taylor Glacier has been proposed to enter the proglacial west lobe of Lake Bonney (WLB) as well as from Blood Falls, a surface discharge point at the Taylor Glacier terminus. The brine strongly influences the geochemistry of the water column of WLB. Year-round measurements from this study are the first to definitively identify brine intrusions from a subglacial entry point into WLB. Furthermore, we excluded input from Blood Falls by focusing on winter dynamics when the absence of an open water moat prevents surface brine entry. Due to the extremely high salinities below the chemocline in WLB, density stratification is dominated by salinity, and temperature can be used as a passive tracer. Cold brine intrusions enter WLB at the glacier face and intrude into the water column at the depth of neutral buoyancy, where they can be identified by anomalously cold temperatures at that depth. High-resolution measurements also reveal under-ice internal waves associated with katabatic wind events, a novel finding that challenges long-held assumptions about the stability of the WLB water column.

Meromictic lakes are unique aquatic ecosystems that occur extremely rarely. The phenomenon of meromixis can result from both natural and anthropogenic factors. The aim of this study was to analyse thermal and chemical stratification in a small, deep (6 ha, H max = 24.5 m) lake. The evaluated lake had a typical summer thermal profile with a shallow epilimnion, a sharp thermocline, and a distinct monimolimnion layer in the hypolimnion, which was also maintained during circulation. The lake had a clinograde oxygen profile, with an oxygen deficit in the metalimnion and permanent anoxic conditions in the deeper layers, including during circulation. A redox zone was identified during summer stagnation. The monimolimnion formed a thermally isolated layer at a depth of around 15 m, and the chemocline was situated above the monimolimnion. In the chemocline, the EC gradient ranged from 61 to 77 μS·cm−1 per meter of depth in the summer and from 90 to 130 μS·cm−1 per meter of depth during circulation. EC was significantly correlated with Ca2+ concentration (r2 = 0.549). Chemical stratification, particularly with regard to organic matter distribution, was observed in the chemocline. The monimolimnion severely limited nutrient internal loading.

Lake Cadagno, a permanently stratified high-alpine lake with a persistent microbial bloom in its anoxic chemocline, has long been considered a model for the low-oxygen, high-sulfide Proterozoic ocean where early microbial life gave rise to Earth’s oxygenated atmosphere. Although the lake has been studied for over 25 years, the absence of concerted study of the bacteria, phytoplankton, and viruses, together with primary and secondary production, has hindered a comprehensive understanding of its microbial food web. Here, the identities, abundances, and productivity of microbes were evaluated in the context of Lake Cadagno biogeochemistry. Photo-synthetic pigments and chloroplast 16S rRNA gene phylogenies suggested high abundances of eukaryotic phytoplankton, primarilyChlorophyta, through the water column. Of these, a close relative ofAnkyra judayi, a high-alpine adapted chlorophyte, peaked with oxygen in the mixolimnion, whileClosteriopsis-related chlorophytes peaked in the chemocline and monimolimnion. Anoxygenic phototrophic sulfur bacteria, Chromatium, dominated the chemocline along withLentimicrobium, a newly observed genus of known fermenters. Secondary production peaked in the chemocline, suggesting anoxygenic primary producers depended on heterotrophic nutrient remineralization. Virus-to-microbe ratios spanned an order of magnitude, peaking with high phytoplankton abundances and at a minimum at the peak of Chromatium, dynamic trends that suggest viruses may play a role in the modulation of oxygenic and anoxygenic photo- and chemosynthesis in Lake Cadagno. Through the combined analysis of bacterial, eukaryotic, viral, and biogeochemical dynamics of Lake Cadagno, this study provides a new perspective on the biological and geochemical connections that comprised the food webs of the Proterozoic ocean.IMPORTANCEAs a window to the past, the study offers insights into the role of microbial guilds of Proterozoic ocean chemoclines in the production and recycling of organic matter of sulfur- and ammonia-containing ancient oceans. The new observations described here suggest that eukaryotic algae were persistent in the low oxygen upper-chemocline in association with purple and green sulfur bacteria in the lower half of the chemocline. Further, this study provides the first insights into Lake Cadagno viral ecology. High viral abundances suggested viruses may be essential components of the chemocline where their activity may result in the release and recycling of organic matter. The framework developed in this study through the integration of diverse geochemical and biological data types lays the foundation for future studies to quantitatively resolve the processes performed by discrete populations comprising the microbial loop in this early anoxic ocean analogue.

Meromictic Lake Cadagno, an ancient ocean analogue, is known for its permanent stratification and persistent anoxygenic microbial bloom within the chemocline. Although the anaerobic microbial ecology of the lake has been extensively studied for at least 25 years, a comprehensive picture of the microbial food web linking the bacterial layer to phytoplankton and viruses, with explicit measures of primary and secondary production, is still missing. This study sought to understand better the abundances and productivity of microbes in the context of nutrient biogeochemical cycling across the stratified zones of Lake Cadagno. Photosynthetic pigments and chloroplast 16S rRNA gene phylogenies suggested the presence of eukaryotic phytoplankton through the water column. Evidence supported high abundances of Ankyra judayi, a high-alpine adapted chlorophyte, in the oxic mixolimnion where oxygenic-primary production peaked. Through the low- and no-oxygen chemocline and monimolimnion, chlorophytes related to Closteriopsis acicularis, a known genus of meromictic lakes, and Parachlorella kessleri were observed. Chromatium, anoxygenic phototrophic sulfur bacteria, dominated the chemocline along with Lentimicrobium, a genus of known fermenters whose abundance was newly reported in Lake Cadagno. Secondary production peaked in the chemocline suggesting primary producers depend on heterotrophs for nutrient remineralization. As previously observed, sulfur-reducing bacteria (SRBs), especially Desulfocapsa and Desulfobulbus, were present in the chemocline and anoxic monimolimnion. Virus-to-microbe ratios (VMR) peaked in the zone of phytoplankton yet were at a minimum at the peak of Chromatium. These dynamic trends suggest viruses may play a role in the modulation of oxygenic and anoxygenic photo- and chemosynthesis in Lake Cadagno and other permanently stratified systems.ImportanceAs a window to the past, the study offers insights into the role of microbial guilds of Proterozoic ocean chemoclines in the production and recycling of organic matter of sulfur- and ammonia-containing ancient oceans. The new observations described here suggest that eukaryotic algae were persistent in the low oxygen upper-chemocline in association with purple and green sulfur bacteria in the lower half of the chemocline. Further, this study provides the first insights into Lake Cadagno viral ecology. High viral abundances suggested viruses may be essential components of the chemocline where their activity may result in the release and recycling of organic matter. The framework developed in this study through the integration of diverse geochemical and biological data types lays the foundation for future studies to quantitatively resolve the processes performed by discrete populations comprising the microbial loop in this early anoxic ocean analogue.

In order to better understand the biogeochemical cycle of nitrogen in meromictic lakes, which can serve as a model for past aquatic environments, we measured dissolved concentrations of nitrate, nitrite, ammonium and organic nitrogen in deep (39 m maximal depth) subarctic Lake Svetloe (NW Russia). The lake is a rare type of freshwater meromictic water boy with high concentrations of methane, ferrous iron, manganese and low concentrations of sulfates and sulfides in the monimolimnion. In the oligotrophic mixolimnion, the concentration of mineral forms of nitrogen decreased in summer compared to winter, likely due to phytoplankton bloom. The decomposition of the bulk of organic matter occurs under microaerophilic/anaerobic conditions of the chemocline and accompanied by the accumulation of nitrogen in the form of N-NH4 in the monimonimlion. We revealed a strong relationship between methane and nitrogen cycles in the chemocline and monimolimnion horizons. The nitrate concentrations in Lake Svetloe varied in the range from 9 to 13 μM throughout the water column. This fact is rare for meromictic lakes, where nitrate concentrations up to 13 µM are found in the monimolimnion zone down to the bottom layers. We hypothesize, in accord with available data for other stratified lakes, that under conditions of high concentrations of manganese and ammonium at the boundary of redox conditions and below, anaerobic nitrification with the formation of nitrates occurs. Overall, most of organic matter in Lake Svetloe undergoes biodegradation essentially under microaerophilic/anaerobic conditions of the chemocline and the monimolimnion. Consequently, the manifestation of the biogeochemical nitrogen cycle is expressed in these horizons in the most vivid and complex relationship with other cycles of elements.

Mono Lake is a closed‐basin, hypersaline, alkaline lake located in Eastern Sierra Nevada, California, that is dominated by microbial life. This unique ecosystem offers a natural laboratory for probing microbial community responses to environmental change. In 2017, a heavy snowpack and subsequent runoff led Mono Lake to transition from annually mixed (monomictic) to indefinitely stratified (meromictic). We followed microbial succession during this limnological shift, establishing a two‐year (2017–2018) water‐column time series of geochemical and microbiological data. Following meromictic conditions, anoxia persisted below the chemocline and reduced compounds such as sulfide and ammonium increased in concentration from near 0 to ~400 and ~150 µM, respectively, throughout 2018. We observed significant microbial succession, with trends varying by water depth. In the epilimnion (above the chemocline), aerobic heterotrophs were displaced by phototrophic genera when a large bloom of cyanobacteria appeared in fall 2018. Bacteria in the hypolimnion (below the chemocline) had a delayed, but systematic, response reflecting colonization by sediment “seed bank” communities. Phototrophic sulfide‐oxidizing bacteria appeared first in summer 2017, followed by microbes associated with anaerobic fermentation in spring 2018, and eventually sulfate‐reducing taxa by fall 2018. This slow shift indicated that multi‐year meromixis was required to establish a sulfate‐reducing community in Mono Lake, although sulfide oxidizers thrive throughout mixing regimes. The abundant green alga Picocystis remained the dominant primary producer during the meromixis event, abundant throughout the water column including in the hypolimnion despite the absence of light and prevalence of sulfide. Our study adds to the growing literature describing microbial resistance and resilience during lake mixing events related to climatic events and environmental change.

Fayetteville Green Lake (FGL) is a recognized, extensively studied present-day model of the stratified Proterozoic ocean. Nonetheless, biomass sedimentation in FGL remains hard to explain: while virtually all sediment pigments belong to photosynthetic sulfur bacteria from a chemocline, the isotopic carbon signature of the bulk organic matter suggests its epilimnetic phytoplankton origin. To explain the epilimnetic origin of sedimented carbon, we studied the dominant Synechococci, isolated from FGL. Here, we present experimental evidence that FGL Synechococci produce copious extracellular polysaccharides (EPS) especially when availability of inorganic carbon (Ci ) is high relative to availability of other macronutrients, for example phosphorus. The accumulating EPS become impregnated with calcium, magnesium, and sodium cations and are released to the environment as ballasted cell coverings. Sedimentation of these cell-free EPS can constitute the bulk of pigment-free organic material in FGL sediment. Because increased availability of Ci specifically stimulates production of EPS and the accumulated EPS adsorb cations and become ballasted, we propose the universal role of cyanobacterial EPS in biomass sedimentation in the high-Ci Paleoproterozoic ocean as well as in modern aquatic systems like FGL.

This study investigated the biogeochemistry of uranium (U), and the co-occurring elements nickel (Ni) and arsenic (As), in colloids and sediments from two meromictic mine pit lakes that have considerably different depths and geochemistry. In order to characterize the processes controlling metal speciation and cycling in the pits, the distribution and speciation of the elements in colloidal size fractions were analyzed using micro- and ultrafiltration in combination with transmission electron microscopy. Sediment traps collected fresh sediments over the course of one year below the chemocline of the pit lakes and were subsequently analyzed by scanning electron microscopy (SEM) for morphology and chemical digestions. The most common particles found in the shallower pit consisted of Ca-O and Fe-O colloids, while the particles in the deeper pit were composed of Ca-S-O. Filtration results showed a higher abundance of metals in larger colloidal fractions in aged samples, suggesting that colloids can act as metal accumulators. Sediment traps showed the formation of Fe-O, Fe-S, Al-Si, and Ce-P phases, which were observed to sorb U and Ni. The overall U removal was calculated to be 0.9 g/m2/year in both pits, despite considerably different geochemical conditions between the two, and the maximum removal rates for As (shallow pit) and Ni (deeper pit) were 4.7 g/m2/year and 0.6 g/m2/year, respectively. Bottom sediments were also collected from both pits, and characterized using sequential extractions, SEM, synchrotron-based X-ray absorption spectroscopy and Laue diffraction techniques. These techniques showed that the stability of metals in the sediment follows the order Ni