Description: During the discovery and description of seven New Zealand methane seep sites, an infaunal assemblage dominated by ampharetid polychaetes was found in association with high seabed methane emission. This ampharetid-bed assemblage had a mean density of 57,000 ± 7800 macrofaunal individuals m−2 and a maximum wet biomass of 274 g m−2, both being among the greatest recorded from deep-sea methane seeps. We investigated these questions: Does the species assemblage present within these ampharetid beds form a distinct seep community on the New Zealand margin? and What type of chemoautotrophic microbes fuel this heterotrophic community? Unlike the other macro-infaunal assemblages, the ampharetid-bed assemblage composition was homogeneous, independent of location. Based on a mixing model of species-specific mass and isotopic composition, combined with published respiration measurements, we estimated that this community consumes 29–90 mmol C m−2 d−1 of methane-fueled biomass; this is 〉 290 times the carbon fixed by anaerobic methane oxidizers in these ampharetid beds. A fatty acid biomarker approach supported the finding that this community, unlike those previously known, consumes primarily aerobic methanotrophic bacteria. Due to the novel microbial fueling and high methane flux rates, New Zealand's ampharetid beds provide a model system to study the influence of metazoan grazing on microbially mediated biogeochemical cycles, including those that involve greenhouse gas emissions

Description: The discovery that foraminifera are able to use nitrate instead of oxygen as energy source for their metabolism has challenged our understanding of nitrogen cycling in the ocean. It was evident before that only prokaryotes and fungi are able to denitrify. Rate 5 estimates of foraminiferal denitrification were very sparse on a regional scale. Here, we present estimates of benthic foraminiferal denitrification rates from six stations at intermediate water depths in and below the Peruvian oxygen minimum zone (OMZ). Foraminiferal denitrification rates were calculated from abundance and assemblage composition of the total living fauna in both, surface and subsurface sediments, 10 as well as from individual species specific denitrification rates. A comparison with total benthic denitrification rates as inferred by biogeochemical models revealed that benthic foraminifera account for the total denitrification on the shelf between 80 and 250m water depth. They are still important denitrifiers in the centre of the OMZ around 320m (29–56% of the benthic denitrification) but play only a minor role at the lower OMZ 15 boundary and below the OMZ between 465 and 700m (3–7% of total benthic denitrification). Furthermore, foraminiferal denitrification was compared to the total benthic nitrate loss measured during benthic chamber experiments. Foraminiferal denitrification contributes 1 to 50% to the total nitrate loss across a depth transect from 80 to 700 m, respectively. Flux rate estimates ranged from 0.01 to 1.3 mmolm−2 d−1. Fur20 thermore we show that the amount of nitrate stored in living benthic foraminifera (3 to 705 μmolL−1) can be higher by three orders of magnitude as compared to the ambient pore waters in near surface sediments sustaining an important nitrate reservoir in Peruvian OMZ sediments. The substantial contribution of foraminiferal nitrate respiration to total benthic nitrate loss at the Peruvian margin, which is one of the main nitrate sink 25 regions in the world oceans, underpins the importance of previously underestimated role of benthic foraminifera in global biochemical cycles.

Description: This study presents benthic data from 12 samplings from February to December 2010 in a 28 m deep channel in the southwest Baltic Sea. In winter, the distribution of solutes in the porewater was strongly modulated by bioirrigation which efficiently flushed the upper 10 cm of sediment, leading to concentrations which varied little from bottom water values. Solute pumping by bioirrigation fell sharply in the summer as the bottom waters became severely hypoxic (〈 2 μM O2). At this point the giant sulfide-oxidizing bacteria Beggiatoa was visible on surface sediments. Despite an increase in O2 following mixing of the water column in November, macrofauna remained absent until the end of the sampling. Contrary to expectations, metabolites such as dissolved inorganic carbon, ammonium and hydrogen sulfide did not accumulate in the upper 10 cm during the hypoxic period when bioirrigation was absent, but instead tended toward bottom water values. This was taken as evidence for episodic bubbling of methane gas out of the sediment acting as an abiogenic irrigation process. Porewater–seawater mixing by escaping bubbles provides a pathway for enhanced nutrient release to the bottom water and may exacerbate the feedback with hypoxia. Subsurface dissolved phosphate (TPO4) peaks in excess of 400 μM developed in autumn, resulting in a very large diffusive TPO4 flux to the water column of 0.7 ± 0.2 mmol m−2 d−1. The model was not able to simulate this TPO4 source as release of iron-bound P (Fe–P) or organic P. As an alternative hypothesis, the TPO4 peak was reproduced using new kinetic expressions that allow Beggiatoa to take up porewater TPO4 and accumulate an intracellular P pool during periods with oxic bottom waters. TPO4 is then released during hypoxia, as previous published results with sulfide-oxidizing bacteria indicate. The TPO4 added to the porewater over the year by organic P and Fe–P is recycled through Beggiatoa, meaning that no additional source of TPO4 is needed to explain the TPO4 peak. Further experimental studies are needed to strengthen this conclusion and rule out Fe–P and organic P as candidate sources of ephemeral TPO4 release. A measured C/P ratio of 〈 20 for the diffusive flux to the water column during hypoxia directly demonstrates preferential release of P relative to C under oxygen-deficient bottom waters. This coincides with a strong decrease in dissolved inorganic N/P ratios in the water column to ~ 1. Our results suggest that sulfide oxidizing bacteria could act as phosphorus capacitors in systems with oscillating redox conditions, releasing massive amounts of TPO4 in a short space of time and dramatically increasing the internal loading of TPO4 to the overlying water.

Description: A meaningful application of Mo as a paleo-redox proxy requires an understanding of Mo cycling in modern reducing environments. Stagnant euxinic basins such as the Black Sea are generally regarded as model systems for understanding euxinic systems during early Earth history. However, drawing direct parallels between the Black Sea and open-marine euxinic margins is somewhat complicated by differences in the seawater residence time between these two environments. We report sediment and pore water Mo, U, Mn and Fe data for a euxinic basin with a short seawater residence time; the weakly restricted Gotland Deep in the Baltic Sea. Here, prolonged periods of euxinia alternate with brief inflow events during which well-oxygenated, saline water penetrates into the basin. During these inflow events, dissolved Mn and Fe that has accumulated within the euxinic deep water can be oxidized and precipitated. Co-variations of Mo and U within the sediment suggest that these inflow and oxygenation events may favor Mo accumulation in the sediment through adsorption to freshly oxidized Mn and Fe solid phases. Once Mo is sequestered within the deeper euxinic water and sediments, Mo retention can be further facilitated by conversion to thiomolybdate species and interactions with organic matter and metal sulfides. By comparing our data with those from previous studies where a Mn and Fe “shuttle” for Mo has been demonstrated, we identify two prerequisites for the occurrence of this mechanism. First, there must be a water column oxic–anoxic redox-boundary; this provides a solubility contrast for Mn and Fe. Second, the residence time of seawater in the system has to be short (weeks to a few years). The latter criterion can be met through regular inflow in weakly restricted basins or upwelling in oxygen minimum zones at open-marine continental margins. Based on prior work, we suggest that similar conditions to those currently represented by the Gotland Deep may have prevailed at euxinic ocean margins during the Proterozoic. A boundary between euxinic and oxic water masses overlying the continental shelf may have resulted in accelerated Mo transport through the water column with Mn and Fe (oxyhydr)oxides. We propose that this mechanism, along with Mo isotope fractionation during adsorption, could contribute to the light Mo isotope composition observed in open-marine euxinic sediment facies of the Proterozoic.