Anaerobic oxidation of methane (AOM) is a potentially important sink for methane in freshwater sediments. Assessing the contribution of AOM to methane budgets requires an understanding of AOM process kinetics, yet such information remains scarce. In the present study, we experimentally quantified the kinetics of AOM in the sediments of the Danish Lake Ørn, where sulfate‐ and iron‐dependent AOM occur side by side. The process e...

Microbial communities in marine sediments are highly diverse, yet the processes that give rise to this complexity are unclear. It has been proposed that benthic microbial communities must be continuously re‐seeded from the water column because dispersal within the sediment is severely limited. Previous studies consistently report that the composition of the microbial community gradually changes with sediment depth. However, th...

Shallow coastal waters are dynamic environments that dominate global marine methane emissions. Particularly high methane concentrations are found in seasonally anoxic waters, which are spreading in eutrophic coastal systems, potentially leading to increased methane emissions to the atmosphere. Here we explore how the seasonal development of anoxia influenced methane concentrations, rates of methane oxidation, and the community...

Summary Coastal waters are a major source of marine methane to the atmosphere. Particularly high concentrations of this potent greenhouse gas are found in anoxic waters, but it remains unclear if and to what extent anaerobic methanotrophs mitigate the methane flux. Here we investigate the long‐term dynamics in methanotrophic activity and the methanotroph community in the coastal oxygen minimum zone (OMZ) of Golfo Dulce, Costa...

Anaerobic oxidation of methane (AOM) is a potentially important methane sink in lake sediments, but the biogeochemistry and microbial ecology of this process are understudied. Potential electron acceptors for AOM include Fe(III) and sulfate; however, it is not clear to which extent low sulfate concentrations constrain the coupling of AOM to sulfate reduction, nor if Fe(III) reduction drives AOM directly or via a cryptic sulfur...

The sulfur cycle is an important, although understudied facet of today's modern oxygen minimum zones (OMZs). Sulfur cycling is most active in highly productive coastal OMZs where sulfide‐rich sediments interact with the overlying water column, forming a tightly coupled benthic‐pelagic sulfur cycle. In such productive coastal systems, highly eutrophic and anoxic conditions can result in the benthic release of sulfide leading to...

Hadal trenches represent the deepest part of the ocean and are dynamic depocenters with intensified prokaryotic activity. Here, we explored the distribution and drivers of prokaryotic and viral abundance from the ocean surface and 40 cm into sediments in two hadal trench regions with contrasting surface productivity. In the water column, prokaryotic and viral abundance decreased with water depth before reaching a rather stable...

Coastal sediments are major contributors to global carbon (C) mineralization and nutrient cycling and are tightly linked to processes in the pelagic environment. In this study, we aimed to investigate the regulating potential of quantity and quality of planktonic organic matter (OM) deposition on benthic metabolism, with a particular focus on nitrogen (N) cycling processes. We simulated inputs of spring (C : N 10.9) and summer...

We investigated methane oxidation in the oxygen minimum zone (OMZ) of the eastern tropical North Pacific (ETNP) off central Mexico. Methane concentrations in the anoxic core of the OMZ reached ~ 20 nmol L −1 at off shelf sites and 34 nmol L −1 at a shelf site. Rates of methane oxidation were determined in ship‐board incubations with 3 H‐labeled methane at O 2 concentrations 0–75 nmol L −1 . In vertical profiles at off‐shelf st...

We investigated anaerobic nitrogen (N) cycling in the water column of Lake Kivu, a deep meromictic tropical lake in East Africa. Data were collected at one station in the Northern Basin and one in the Southern Basin, during two sampling campaigns (June 2011—dry season, and February 2012—rainy season). Short‐term incubations of sulfide‐free water with 15 N‐labeled substrates revealed high potential denitrification and dissimila...

Experimental determinations of nitrogen cycling in deep‐sea sediments are strongly underrepresented in the databases. To investigate the total N 2 production rates and relative contribution of denitrification and anaerobic ammonium oxidation (anammox) to benthic fixed‐N removal processes, we conducted 15 N isotope‐labeling incubation experiments in whole cores and slurries at nine stations across the continental margin from th...

The marine sediment hosts a mosaic of microhabitats. Recently it has been demonstrated that the settlement of phycodetrital aggregates can induce local changes in the benthic O 2 distribution due to confined enrichment of organic material and alteration of the diffusional transport. Here, we show how this microscale O 2 shift substantially affects benthic nitrogen cycling. In sediment incubations, the settlement of diatom‐aggr...

Summary Marinimicrobia bacteria are widespread in subeuphotic areas of the oceans and particularly abundant in oxygen minimum zones (OMZs). Information on Marinimicrobia metabolism is sparse, making the biogeochemical influence of this group challenging to predict. Here, metagenome‐assembled genomes representing Marinimicrobia subgroups PN262000N21 and ARCTIC96B‐7 were retrieved to near completion (97% and 94%) from OMZ metage...

Copepods are exposed to a high non‐predatory mortality and their decomposing carcasses act as microniches with intensified microbial activity. Sinking carcasses could thereby represent anoxic microenvironment sustaining anaerobic microbial pathways in otherwise oxic water columns. Using non‐invasive O 2 imaging, we document that carcasses of Calanus finmarchicus had an anoxic interior even at fully air‐saturated ambient O 2 le...

The Louisiana Shelf in the Gulf of Mexico experiences recurrent bottom water hypoxia under summertime eutrophic conditions. The onset, maintenance, and breakdown of hypoxia are associated with dynamic microbial biogeochemical cycles. However, the distribution of microbial taxa and metabolisms across Shelf oxygen gradients remains under‐characterized. We combined biogeochemical analyses of nitrogen (N) distributions and metabol...

Coastal upwelling areas are highly productive marine systems in which the development of oxygen‐depleted conditions and the availability of diverse electron donors (e.g., organic matter, , H 2 S) favor the processes involved in nitrogen (N) loss. We characterize the temporal and vertical variability of anammox and denitrification over the continental shelf off central Chile (36.5°S), through 15 N and 13 C tracer experiments, i...

Summary We investigated anammox, denitrification and dissimilatory reduction of nitrite to ammonium ( DNRA ) activity in the E astern T ropical S outh P acific oxygen minimum zone ( OMZ ) off northern C hile, at high‐depth resolution through the oxycline into the anoxic OMZ core. This was accompanied by high‐resolution nutrient and oxygen profiles to link changes in nitrogen transformation rates to physicochemical characterist...

Summary While most oxygenic phototrophs harvest light only in the visible range (400–700 nm, VIS ), anoxygenic phototrophs can harvest near infrared light (> 700 nm, NIR ). To study interactions between the photosynthetic guilds we used microsensors to measure oxygen and gross oxygenic photosynthesis ( gOP ) in a hypersaline microbial mat under full ( VIS + NIR ) and VIS illumination. Under normal dissolved inorganic carbon (...

Freshwater systems are identified as one of the main natural methane sources, but little is known about the importance of anaerobic oxidation of methane (AOM) in these systems. We investigated AOM in a lake sediment characterized by a high reactive iron content, normal sulfate concentrations in the bottom water (∼ 250 µmol L −1 ), and a relatively deep sulfate penetration of ∼ 14 cm, which facilitated the spatial resolution of...

We quantified the removal of fixed nitrogen as N 2 production by anammox and N 2 and N 2 O production by denitrification over a distance of 1900 km along the coasts of Chile and Peru, using short‐term incubations with 15 N‐labeled substrates. The eastern South Pacific contains an oxygen minimum zone (OMZ) characterized by an anoxic, nitrate‐ and nitrite‐rich layer of ∼ 200‐m thickness below 30–90 m of oxic water. Anammox and d...