On the basis of in situ NO { 3 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 NO N 2 production was attributed to prokaryotic denitrification (59%), anammox (37%), and foraminifera-based denitrification (4%). Anammox thereby represented an important nutrient sink, but the N 2 production was dominated by denitrification. Despite the fact that NO { 3 stored inside foraminifera represented ,80% of the total benthic NO { 3 pool, the slow intracellular NO { 3 turnover that, on average, sustained foraminifera metabolism for 12-52 d, contributed only to a minor extent to the overall N 2 production. The microbial activity in the surface sediment is a net nutrient sink of ,1.1 mmol N m 22 d 21 , 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.

A laboratory-based optical pH sensor for 2-dimensional pH imaging at benthic interfaces is presented. The sensor consists of a single-layer hydrogel matrix embedding the fluorescent pH indicator 2′, 7′-dihexyl-5(6)-Noctadecyl-carboxamidofluorescein ethyl ester (DHFAE) and the phosphorescent ruthenium(II)-Tris-4,7-diphenyl-1,10-phenanthroline incorporated in nanoparticles, serving as an inert reference standard. The measuring principle is based on time domain dual-lifetime referencing (t-DLR). The fluorophore/phosphor couple is simultaneously excited by a green LED (λ max 530 nm) pulsed in the microsecond range, and the sensor emission is recorded by a fast-gateable CCD camera. For each pH image, intensity images in 2 time windows (1 during and 1 after the excitation phase) are taken. The ratio of these 2 images is proportional to the pH of the sample and not affected by the overall signal intensity. The sensor has a dynamic range suitable for marine conditions (pH 7.3 to 9.3) with an apparent pK a of 8.3. The sensor was long-term stable (months) when kept in darkness and had a response time of < 200 s when going from pH 8.3 to 7.6. Light proved to have a negative effect on the sensor performance due to photobleaching of the pH indicator, resulting in a negative drift in the signal ratio at higher pH after prolonged light exposure. The spatial resolution (83 by 83 μm/pixel) of the sensor was capable of resolving smallscale spatial variability in the pH at a heterogeneous sediment-water interface, and time series of calibrated pH images also expressed a marked temporal variability in the pH distribution across this interface. Hotspots with intensified microbial activity were observed, and pH minima along burrow walls of polychaetes indicated elevated diagenetic activity in these zones. Individually extracted profiles from the pH images agreed well with independently measured pH microelectrode profiles, confirming the robustness of the approach.

The deepest part of the global ocean, hadal trenches, are considered to act as depocenters for organic material. Relatively high microbial activity has been demonstrated in the deepest sections of some hadal trenches, but the deposition dynamics are thought to be spatially and temporally variable. Here, we explore sediment characteristics and in-situ benthic oxygen uptake along two trenches with contrasting surface primary productivity: the Kermadec and Atacama trenches. We find that benthic oxygen consumption varies by a factor of about 10 between hadal sites but is in all cases intensified relative to adjacent abyssal plains. The benthic oxygen uptake of the two trench regions reflects the difference in surface production, whereas variations within each trench are modulated by local deposition dynamics. Respiratory activity correlates with the sedimentary inventories of organic carbon and phytodetrital material. We argue that hadal trenches represent deep sea hotspots for early diagenesis and are more diverse and dynamic environments than previously recognized.

The environmental conditions inside the gut of Calanus hyperboreus and C. glacialis were measured with microelectrodes. An acidic potential hydrogen (pH) gradient was present in the gut of C. hyperboreus, and the lowest pH recorded was 5.40. The gut pH of a starved copepod decreased by 0.53 after the copepod resumed feeding for a few hours, indicating the secretion of acidic digestive fluid. A copepod feeding on Thalassiosira weissflogii (diatom) had slightly lower pH than that feeding on Rhodomonas salina (cryptophyte). Oxygen was undersaturated in the gut of both C. hyperboreus and C. glacialis, with a steep gradient from the anal opening to the metasome region. The central metasome region was completely anoxic. Food remains in the gut led to a lower oxygen level, and a diatom diet induced a stronger oxygen gradient than a cryptophyte diet. The acidic and suboxic-anoxic environments of the copepod gut may support iron dissolution and anaerobic microbial activities that otherwise are not favored in the well-buffered and oxygenated ambient ocean.

The oxygen flux between benthic systems and the water above is a widely used proxy for benthic primary production and organic carbon mineralization and is one of the most measured variables in marine and freshwater research. This presentation reviews the relatively new aquatic eddy covariance technique for measuring this flux. Because the approach relies on measurements that integrate over a large area (multiple m2) without disturbing the benthic system or the natural drivers of flux, it allows non‐invasive studies of whole‐system benthic metabolism that are not possible with any other approach. After summarizing the basic principles of eddy covariance, the instruments used, and key steps in the data evaluation, the rapidly growing body of research utilizing aquatic eddy covariance is presented with examples of the new kinds of insights that can be attained. These examples focus on benthic surfaces where traditional flux methods are difficult or problematic to apply and include highly dynamic benthic environments such as coral reefs, Arctic sediments, dense seagrass meadows, and permeable sands. Finally, future applications of the technique, new areas for development, and avenues to disseminate data and outcomes between new and experienced users are recommended.

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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, the relative contributions of the processes that underlie these compositional gradients have not been determined, and it is unknown whether microbial dispersal is indeed too slow to outpace burial. Here, we applied ecological statistical frameworks to 16S rRNA gene amplicon‐based community composition data from Atacama Trench sediments to investigate the links between biogeochemistry, burial, and microbial community assembly processes. We confirm that dispersal limitation affects microbial communities and find that gradual changes in community composition are driven by selective pressures that change abruptly across the discrete boundaries between redox zones rather than along continuous biogeochemical gradients, while selective pressures are uniform within each zone. The gradual changes in community composition over centimetres of depth within a zone hence reflects a decades‐long response to the abruptly changing selective pressures.

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