Sergei Katsev

Ferruginous (iron‐rich) conditions have been prominent in oceans throughout the Earth's geologic history but now are reliably found only in a handful of permanently stratified lakes. Microbially mediated iron reduction in such anoxic environments competes with sulfate reduction, which promotes euxinic (sulfide‐rich) conditions. Besides the shared demand for organic compounds, the competition is fostered by the produced hydrogen sulfide, which may reduce iron oxides abiotically or co‐precipitate with dissolved iron as iron sulfides. Understanding why some environments develop ferruginous rather than euxinic conditions (or vice versa), as well as the attendant effects on methanogenic fermentation, is key to understanding both modern and ancient anoxic ecosystems. Here, we reproduce biogeochemical distributions in multiple anoxic, low‐sulfate, meromictic lakes around the world using a biomass‐explicit reaction‐transport model with a fixed set of metabolism‐specific microbial parameters. The results suggest that sulfate reduction and methanogenesis are ubiquitous even in iron‐rich systems, and are reflected in microbial surveys. Ferruginous conditions typically develop for surface sulfate concentrations below ≃100 μM. Interestingly, there seems to be a dearth of stably stratified water bodies where sulfate concentrations can persist in the medium‐sulfate range of several hundred μM. Rather, when sulfur burial into the sediments becomes iron limited, sulfate tends to accumulate in the water column to much higher (mM) concentrations. A similar mechanism could be suggested to have operated in the variably sulfidic and ferruginous water columns of early oceans. Model simulations also reveal the previously underappreciated role of physical transport in shaping biogeochemical distributions, as minor variations in mixing rates can lead to large variations in microbial abundances. Model applicability across multiple lakes points to an encouraging possibility that geochemical patterns in complex biogeochemical systems may be described from a small number of thermodynamic and kinetic principles using a minimum of fitting parameters.

Tasik Biru is a ~70 m‐deep tropical lake in Malaysia, originating from a water‐filled open pit mine. We investigated the biogeochemistry and microbial community of the lake as a modern model habitat to the stratified ancient ocean. We found that a sharp redoxcline exists at around 50 m depth, related to the decrease of O ₂ and pH (7.2–6.8) going down into the monimolimnion. Despite being relatively sulfate‐rich (~320 μM), only a slight decrease of sulfate (to ~240 μM) was observed coupled with an increase of dissolved sulfide to 4 μM, attributed to microbial sulfate reduction in the monimolimnion. Comparatively, dissolved Fe and total Mn rose to ~50 μM in the anoxic layer with an unusual 1:1 concentration ratio. Other nutrients (PO ₄ ³⁻ , Si) and trace metal(loid)s (As, Mo, Sb, Co, U, and V) depth profiles increased or decreased across the chemocline, indicating controls via cycling of redox‐sensitive elements. Microbial community analysis based on 16S rRNA amplicon sequencing reflects various metabolisms, from aerobic metabolisms in the mixolimnion to putative nitrite‐dependent methane oxidation (e.g., by Methylomirabilis ) at the chemocline, to sulfate reduction, methanogenesis, and fermentation in the monimolimnion. Tasik Biru is not in steady‐state, and its anoxic water is predicted to shift from being Fe/Mn‐rich to sulfide‐rich, perhaps lending it as a model habitat to investigate biogeochemical changes from the metal‐rich Archean to the Proterozoic oceans with expanding zones of sulfide‐rich margins. An overview of the current biogeochemical cycles in the lake is presented, and open questions regarding partial sulfate consumption, methane, and Mn cycling and mineralogical distribution are highlighted to guide future studies.

Microbial metabolisms underpin geochemical cycling in nearly all of the Earth's biosphere. Anaerobic pathways of carbon mineralization by iron and sulfate reduction and methanogenesis, in particular, have existed over most of Earth history and have been central in shaping the chemistry of the oceans and atmosphere. The governing principles by which microbial metabolisms contribute to water-column chemistry, however, are incompletely understood in natural systems where microbes compete for resources. Contrary to common assumptions, thermodynamics often fails to reproduce the observed metabolic succession. Here, we analyze the coupled effects of thermodynamics, kinetics, population dynamics, and physical transport on the composition of microbial communities, their integrated rates of metabolic activity, and effects on the chemistry of aqueous environments. Together, these controls explain the microbiology and chemistry of modern lakes and seas, and constrain the expected variability in the Earth's oceans over geologic time. We find, in particular, that microbial sulfate reduction, despite being less thermodynamically favored than iron reduction, can account for a significant fraction of organic matter remineralization, even without producing measurable concentrations of aqueous sulfide. In ferruginous water columns, methanogenesis is viable under low (<~100 uM) sulfate conditions, but methane accumulation in early oceans was unlikely before a widespread oxygenation of the ocean surface. We also illustrate how processes in anoxic water columns become reflected in the deposition of iron sulfide minerals that are commonly used as proxies of paleoredox conditions.

Tropical lakes experience less thermal convection than temperate lakes and can be stably stratified by temperature gradients alone. Deep tropical lakes are often presumed meromictic, but conditions for sustained meromixis are poorly constrained. Here, we investigate the stratification stability of a deep (200 m) tropical Lake Towuti (Indonesia) and analyze its sensitivity to meteorological and climatic parameters using exploratory hydrodynamic simulations. The inferred effects on water chemistry are further evaluated with a reactive‐transport biogeochemical model. In contrast to the previous assumption of meromixis, our results suggest that in 2008 Lake Towuti experienced mixing over most of its water column, with the attendant weak oxygenation of the previously anoxic lower strata. The lake likely remained stratified afterwards, but the chemical distributions measured after the episode, particularly during the International Continental Scientific Drilling Program in 2015, were out of steady state, and may be still evolving in time. We find the thermal stratification to be most sensitive to average air temperatures and wind speeds and predict further stabilization of the lake by climate change. We also describe a negative feedback mechanism that can stabilize oligomictic tropical lakes for a number of years after the episodes of mixing and cooling, and infer that such episodes should be significantly less frequent in deeper lakes.

Polyphosphate (polyP) is important to phytoplankton ecology, but a unified view of its variability and roles in ecosystem‐scale phosphorus (P) cycling is lacking. We study polyP in the world's largest freshwater ecosystem, the Laurentian Great Lakes, covering pelagic to nearshore areas across a wide nutrient gradient. We show that polyP (average 10.99 ± 3.90 nmol L ⁻¹ ) constitutes 3.8–30.2% (average 18.1 ± 7.2%) of total particulate P (TPP). PolyP accumulation is higher in low‐P pelagic waters compared with more productive nearshore areas. PolyP is preferentially degraded in the water column of the Great Lakes, enhancing P recycling and relieving the nitrogen (N) : P imbalance. Our data enables a coherent large‐scale freshwater‐to‐oceanic comparison. We show that while different plankton groups accumulate different levels of polyP with smaller plankton accumulating more, P availability is the key driver of polyP variability within and across systems.

Organic sulfur plays a crucial role in the biogeochemistry of aquatic sediments, especially in low sulfate (< 500 μM) environments like freshwater lakes and the Earth's early oceans. To better understand organic sulfur cycling in these systems, we followed organic sulfur in the sulfate‐poor (< 40 μM) iron‐rich (30–80 μM) sediments of Lake Superior from source to sink. We identified microbial populations with shotgun metagenomic sequencing and characterized geochemical species in porewater and solid phases. In anoxic sediments, we found an active sulfur cycle fueled primarily by oxidized organic sulfur. Sediment incubations indicated a microbial capacity to hydrolyze sulfonates, sulfate esters, and sulfonic acids to sulfate. Gene abundances for dissimilatory sulfate reduction (dsrAB) increased with depth and coincided with sulfide maxima. Despite these indicators of sulfide formation, sulfide concentrations remain low (< 40 nM) due to both pyritization and organic matter sulfurization. Immediately below the oxycline, pyrite accounted for 13% of total sedimentary sulfur. Both free and intact lipids in this same interval accumulated disulfides, indicating rapid sulfurization even at low concentrations of sulfide. Our investigation revealed a new model of sulfur cycling in a low‐sulfate environment that likely extends to other modern lakes and possibly the ancient ocean, with organic sulfur both fueling sulfate reduction and consuming the resultant sulfide.

The dataset includes polyphosphate (polyP) data for particulate samples from 13 locations in the Great Lakes. The dataset also includes other related parameters, including particulate phosphorus (PP), chlorophyll-a (Chl-a), activities of alkaline phosphatase (APase), soluble reactive phosphorus (SRP), particulate carbon (PC), and particulate N (PN). Ratios of polyP:PP reported in the literature are also included for comparison. The supplement data include CTD data (temperature, oxygen, Chl-a, and light transmission) and the phytoplankton abundance data reported in the Great Lakes Environmental Database (GLENDA) collected by the US Environmental Protection Agency Great Lakes National Program Office (EPA GLNPO). The parameters and data collection and processing methods are described in the supplement files 'Parameter descriptions' and 'GreatLakes_PolyP_Methods'.