Oxygen dynamics in shelf seas sediments incorporating seasonal variability

Shelf sediments play a vital role in global biogeochemical cycling and are particularly important areas of oxygen consumption and carbon mineralisation. Total benthic oxygen uptake, the sum of diffusive and faunal mediated uptake, is a robust proxy to quantify carbon mineralisation. However, oxygen uptake rates are dynamic, due to the diagenetic processes within the sediment, and can be spatially and temporally variable. Four benthic sites in the Celtic Sea, encompassing gradients of cohesive to permeable sediments, were sampled over four cruises to capture seasonal and spatial changes in oxygen dynamics. Total oxygen uptake (TOU) rates were measured through a suite of incubation experiments and oxygen microelectrode profiles were taken across all four benthic sites to provide the oxygen penetration depth and diffusive oxygen uptake (DOU) rates. The difference between TOU and DOU allowed for quantification of the fauna mediated oxygen uptake and diffusive uptake. High resolution measurements showed clear seasonal and spatial trends, with higher oxygen uptake rates measured in cohesive sediments compared to the permeable sediment. The significant differences in oxygen dynamics between the sediment types were consistent between seasons, with increasing oxygen consumption during and after the phytoplankton bloom. Carbon mineralisation in shelf sediments is strongly influenced by sediment type and seasonality

Seafloor oxygen consumption fuelled by methane from cold seeps

The leakage of cold, methane-rich fluids from subsurface reservoirs to the sea floor at specific sites on continental slopes, termed cold seeps, sustains some of the richest ecosystems on the sea bed. These seep-fuelled communities utilize around two orders of magnitude more oxygen per unit area than non-seep seafloor communities. Much of the oxygen is consumed by microbes and animal-microbe symbioses that use methane as an energy source. The proportion of methane consumed varies with fluid flow rate, ranging from 80% in seeps with slow fluid flow to less than 20% in seeps where fluid flow is high. Assuming the presence of a few tens of thousands of active cold seep systems on continental slopes worldwide, we estimate that the total efflux of methane to the overlying ocean could reach 0.02 Gt of carbon annually. As much more methane is lost from continental slopes, be it through emission to the hydrosphere or consumption by microbes, than can be produced, we suggest that a substantial fraction of the methane that fuels seep ecosystems is sourced from deep carbon buried kilometres under the sea floor

Relative abundances of methane- and sulphur-oxidising symbionts in the gills of a cold seep mussel and link to their potential energy sources

Bathymodiolus mussels are key species in many deep-sea chemosynthetic ecosystems. They often harbour two types of endosymbiotic bacteria in their gills, sulphur- and methane oxidisers. These bacteria take up sulphide and methane from the environment and provide energy to their hosts, supporting some of the most prolific ecosystems in the sea. In this study, we tested whether symbiont relative abundances in Bathymodiolus gills reflect variations in the highly spatially dynamic chemical environment of cold seep mussels. Samples of Bathymodiolus aff. boomerang were obtained from two cold seeps of the deep Gulf of Guinea, REGAB (5°47.86S, 9°42.69E, 3170 m depth) and DIAPIR (6°41.58S, 10°20.94E, 2700 m depth). Relative abundances of both symbiont types were measured by means of 3D fluorescence in situ hybridisation and image analysis and compared considering the local sulphide and methane concentrations and fluxes assessed via benthic chamber incubations. Specimens inhabiting areas with highest methane content displayed higher relative abundances of methane oxidisers. The bacterial abundances correlated also with carbon stable isotope signatures in the mussel tissue, suggesting a higher contribution of methane-derived carbon to the biomass of mussels harbouring higher densities of methane-oxidising symbionts. A dynamic adaptation of abundances of methanotrophs and thiotrophs in the gill could be a key factor optimising the energy yield for the symbiotic system and could explain the success of dual symbiotic mussels at many cold seeps and hydrothermal vents of the Atlantic and Gulf of Mexico

Niche differentiation among mat-forming, sulfide-oxidizing bacteria at cold seeps of the Nile Deep Sea Fan (Eastern Mediterranean Sea)

Sulfidic muds of cold seeps on the Nile Deep Sea Fan (NDSF) are populated by different types of mat-forming sulfide-oxidizing bacteria. The predominant sulfide oxidizers of three different mats were identified by microscopic and phylogenetic analyses as (i) Arcobacter species producing cotton-ball-like sulfur precipitates, (ii) large filamentous sulfur bacteria including Beggiatoa species, and (iii) single, spherical Thiomargarita species. High resolution in situ microprofiles revealed different geochemical settings selecting for the different mat types. Arcobacter mats occurred where oxygen and sulfide overlapped above the seafloor in the bottom water interface. Filamentous sulfide oxidizers were associated with steep gradients of oxygen and sulfide in the sediment. A dense population of Thiomargarita was favored by temporarily changing supplies of oxygen and sulfide in the bottom water. These results indicate that the decisive factors in selecting for different mat-forming bacteria within one deep-sea province are spatial or temporal variations in energy supply. Furthermore, the occurrence of Arcobacter spp.-related 16S rRNA genes in the sediments below all three types of mats, as well as on top of brine lakes of the NDSF, indicates that this group of sulfide oxidizers can switch between different life modes depending on the geobiochemical habitat setting

Nitrogen cycling in a deep ocean margin sediment (Sagami Bay, Japan)

On the basis of in situ NO3−1 microprofiles and chamber incubations complemented by laboratory‐based assessments of anammox and denitrification we evaluate the nitrogen turnover of an ocean margin sediment at 1450‐m water depth. In situ NO3−1 profiles horizontally separated by 12 mm reflected highly variable NO3−1 penetration depths, NO3−1 consumption rates, and nitrification. On average the turnover time of the pore‐water NO3−1 pool was ~0.2 d. Net release of NH4+ during mineralization (0.95 mmol m−2 d−1) sustained a net efflux of ammonia (53%), nitrification (24%), and anammox activity (23%). The sediment had a relatively high in situ net influx of NO3−1 (1.44 mmol m−2 d−1) that balanced the N2 production as assessed by onboard tracer experiments. N2 production was attributed to prokaryotic denitrification (59%), anammox (37%), and foraminifera‐based denitrification (4%). Anammox thereby represented an important nutrient sink, but the N2 production was dominated by denitrification. Despite the fact that NO3−1 stored inside foraminifera represented ~80% of the total benthic NO3−1 pool, the slow intracellular NO3−1 turnover that, on average, sustained foraminifera metabolism for 12‐52 d, contributed only to a minor extent to the overall N2 production. The microbial activity in the surface sediment is a net nutrient sink of ~1.1 mmol N m−2 d−1, which aligns with many studies performed in coastal and shelf environments. Continental margin areas can act as significant N sinks and play an important role in regional N budgets