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.
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.
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 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.
A new CO, microelectrode with a tip diameter of 10 pm and a response time (t,,) of -10 s is presented. The sensor allows CO, measurements with a detection limit of <3 PM. The microsensor was tested in experimental systems of increasing complexity. A diffusion-reaction simulation model was used to calculate CO, profiles in order to check the reliability of the measured profiles. Measured CO, and 0, profiles showed that, in highly active layers with photosynthetic and respiratory organisms, local equilibrium of the carbcnate system cannot be assumed. In such highly active systems, the CO, profiles were determined by the slow CO, hydration rate, the biological conversion rates, and the diffusion of all species of the carbonate system. We concluded that measured CO, profiles cannot easily be extrapolated to describe the total carbonate concentration profile, because CO, may not be in equilibrium with the rest of the carbonate system, and because a very accurate alignment of pH and CO, profiles is needed to calculate C,. However, the new CO, microelectrode is useful in research involving biological processes directly producing or consuming CO, such as photosynthesis or respiration.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.