Sedimentological, mineralogical, chronological and basic biogeochemistry document that the Aleutian Trench region consists of four distinct regions, each with their respective benthic habitat, deposition dynamics, and material sourcing: the Northern Aleutian Slope (NS), Aleutian Trench – Axis (AT-A), Aleutian Trench – South Slope (AT-SS), and Southern Aleutian Slope (SS). The carbon pool is characterized by pyrolysis derived hydrogen and oxygen indices (HI and OI), radionuclides (Δ14C) and stable isotopes (δ13C-values), clearly varying among the respective regions. The NS has elevated total organic carbon (TOC) content, including distinct imprint of fresh, marine-derived organics originating from upwelling in the Gulf of Alaska with subsequent westward transport by the Alaskan Stream, sustaining high remineralization rates, with well-defined infaunal burrow structures in the sediment. Stations at the AT-A have a 2.5–24.1 cm thick, glacially derived fluid mud layer (FML) deprived of infauna, blanketing the original sediment surface. Chronological markers document that the infilling of the FML happened since the mid 1950′s and now extends to at least one fifth of the entire Aleutian Trench axis. The FML contains a mixture of fresh, marine-derived and recalcitrant terrigenous organics, with elevated microbial mineralization rates, presumably enhanced by microbial priming and efficient microbial degradation of otherwise recalcitrant organics. The AT-SS and SS have low TOC contents, exhibit very low microbial remineralization rates, with only sparse infaunal imprints. Our investigations document that the Aleutian Trench forms a unique hadal environment, while simultaneously confirming that hadal trenches generally act as depocenters with intensified microbial activity, sustained by various sources of organics.

Marine snow forms at the ocean surface, sinks to depth, and ultimately enables carbon sequestration in the seabed. Fast-sinking marine snow particles, such as diatom aggregates, encounter a rapid increase in hydrostatic pressure during their descent. Using incubations in rotating pressure tanks, we found that pressure levels corresponding to 2- to 6-kilometer water depth induce leakage of dissolved organic matter (DOM) from diatom aggregates equivalent to ~50% of their initial carbon contents. The leaked DOM proved to be diatom-derived and changed the amount and composition of DOM in the surrounding seawater substantially. Ultrahigh-resolution mass spectrometry, high protein-like fluorescence, and low carbon:nitrogen ratios classified the leaked DOM as labile. The bioavailability of leaked DOM was demonstrated by its rapid utilization by a pelagic microbial community, leaving mainly recalcitrant DOM behind. Pressure-induced DOM leakage likely weakens the gravitational "biological carbon pump" and supplies labile DOM to the pelagic microbiome of the deep ocean.

The remineralization of organic matter by benthic bacteria is an essential process in the marine carbon cycle. In polar regions, strong variation in daylength causes pronounced seasonality in primary productivity, but the responses of sedimentary bacteria to these fluctuations are not well understood. We investigated the seasonal dynamics of benthic bacterial communities from an Arctic fjord and found a partitioning of the communities into seasonally responsive and stable guilds. We separately analyzed the fractions of cells in the porewater and those loosely and firmly attached to sand grains through 16S ribosomal RNA gene sequencing, cell counting, rate measurements, and geochemical analyses. The porewater and loosely attached bacterial communities showed a dynamic response in composition and activity, suggesting that they play a central role in benthic-pelagic coupling by responding rapidly to seasonal fluctuations in organic matter availability. In contrast, the majority of the firmly attached cells showed a more buffered response, as reflected, e.g. in the consistently high cell numbers of Woeseiaceae. This fraction is potentially key to maintaining baseline remineralization processes throughout the year, independent of fresh organic matter input. These findings provide a new mechanistic understanding of carbon cycling in Arctic surface sediments that may also apply beyond polar regions.

Fast-sinking zooplankton carcasses and fecal pellets appear to contribute significantly to the vertical transport of particulate organic carbon (POC), partly because of low temperature that decreases microbial degradation during the descent into the deep ocean. Increasing hydrostatic pressure could further reduce the degradation efficiency of sinking POC, but this effect remains unexplored. Here, the degradation of carcasses and fecal pellets of the abundant marine copepod Calanus finmarchicus was experimentally studied as a function of pressure (0.1-100 MPa). Samples were either exposed to elevated pressure in short 1-day incubations or a gradual pressure increase, simulating continuous particle sinking during a 20-day incubation. Both experiments revealed gradual inhibition of microbial respiration in the pressure range of 20-100 MPa, corresponding to 2-10-km depth. This suggests that hydrostatic pressure impedes carbon mineralization of fast-sinking carcasses and fecal pellets and enhances the deep-sea deposition rate of zooplankton-derived organic material.

In the hadal zone of the ocean (6–11 km), the characteristics of sinking marine snow particles and their attached microbial communities remain elusive, despite their potential importance for benthic life thriving at extreme pressures (60–110 MPa). Here, we used simulation experiments to explore how increasing pressure levels modify the microbial degradation, organic matter composition, and microbiome of sinking diatom aggregates. Individual aggregates were incubated in rotating tanks in which pressure was incrementally increased to simulate a descent from surface to hadal depth within 20 days. Incubations at atmospheric pressure served as controls. With increasing pressure, microbial respiration and diatom degradation decreased gradually and ceased completely at 60 MPa. Dissolved organic carbon leaked substantially from the aggregates at ≥40 MPa, while diatom lipid and pigment contents decreased moderately. Bacterial abundance remained stable at >40 MPa, but bacterial community composition changed significantly at 60–100 MPa. Thus, pressure exposure reduces microbial degradation and transforms both organic matter composition and microbiomes of sinking particles, which may seed hadal sediments with relatively fresh particulate organic matter and putative pressure-tolerant microbes.

Krill represent a major link between primary producers and higher trophic levels in polar marine food webs. Potential links to lower trophic levels, such as heterotrophic microorganisms, are less well documented. Here, we studied the kinetics of microbial degradation of sinking carcasses of two dominant krill species Thysanoessa raschii and Meganyctiphanes norvegica from Southwest Greenland. Degradation experiments under oxic conditions showed that 6.0-9.1% of carbon and 6.4-7.1% of nitrogen were lost from the carcasses after one week. Aerobic microbial respiration and the release of dissolved organic carbon were the main pathways of carbon loss from the carcasses. Ammonium release generally contributed the most to carcass nitrogen loss. Oxygen micro profiling revealed anoxic conditions inside krill carcasses/specimens, allowing anaerobic nitrogen cycling through denitrification and dissimilatory nitrate reduction to ammonium (DNRA). Denitrification rates were up to 5.3 and 127.7 nmol N carcass-1 d-1 for T. raschii and M. norvegica, respectively, making krill carcasses hotspots of nitrogen loss in the oxygenated water column of the fjord. Carcass-associated DNRA rates were up to 4-fold higher than denitrification rates, but the combined activity of these two anaerobic respiration processes did not contribute significantly to carbon loss from the carcasses. Living krill specimens did not harbor any significant denitrification and DNRA activity despite having an anoxic gut as revealed by micro profiling. The investigated krill carcasses sink fast (1500-3000 m d-1) and our data show that only a small fraction of the associated carbon is lost during descent. Based on data on krill distribution, our findings are used to discuss the potential importance of sinking krill carcasses for sustaining benthic food webs in the Arctic.

Identifying and quantifying nitrogen pools is essential for understanding the nitrogen cycle in aquatic ecosystems. The ubiquitous diatoms represent an overlooked nitrate pool as they can accumulate nitrate intracellularly and utilize it for nitrogen assimilation, dissipation of excess photosynthetic energy, and Dissimilatory Nitrate Reduction to Ammonium (DNRA). Here, we document the global co-occurrence of diatoms and intracellular nitrate in phototrophic microbial communities in freshwater ( n = 69), coastal ( n = 44), and open marine ( n = 4) habitats. Diatom abundance and total intracellular nitrate contents in water columns, sediments, microbial mats, and epilithic biofilms were highly significantly correlated. In contrast, diatom community composition had only a marginal influence on total intracellular nitrate contents. Nitrate concentrations inside diatom cells exceeded ambient nitrate concentrations ∼100–4000-fold. The collective intracellular nitrate pool of the diatom community accounted for <1% of total nitrate in pelagic habitats and 65–95% in benthic habitats. Accordingly, nitrate-storing diatoms are emerging as significant contributors to benthic nitrogen cycling, in particular through Dissimilatory Nitrate Reduction to Ammonium activity under anoxic conditions.

Arctic sea ice has alarmingly high concentrations of microplastics (MPs). Additionally, sea ice reduction in the Arctic is opening new opportunities for the oil and maritime industries, which could increase oil pollution in the region. Yet knowledge of the effects of co-exposure to MPs and crude oil on Arctic zooplankton is lacking. We tested the influence of MPs (polyethylene, 20.7 mu m) on polycyclic aromatic hydrocarbon (PAH) bioaccumulation and oil toxicity in the key arctic copepod Calanus hyperboreus after exposure to oil with and without dispersant. Up to 30% of the copepods stopped feeding and fecal pellet production rates were reduced after co-exposure to oil (1 mu L L-1) and MPs (20 MPs mL(-1)). The PAH body burdenwas similar to 3 times higher in feeding than in non-feeding copepods. Copepods ingested both MPs and crude oil droplets. MPs did not influence bioaccumulation of PAHs in copepods or their fecal pellets, but chemical dispersant increased bioaccumulation, especially of >= 4 ring-PAHs. Our results suggest thatMPs do not act as vectors of PAHs in Arctic marine foodwebs after oil spills, but, at high concentrations (20 MPs mL(-1)), MPs can trigger behavioral stress responses (e.g., feeding suppression) to oil pollution in zooplankton. (C) 2020 Elsevier B.V. All rights reserved.

Organic matter produced in surface waters is the primary food supply to deep-sea ecosystems. In contrast to phytoplankton aggregates and zooplankton fecal pellets, sinking zooplankton carcasses have rarely been considered as important agents of organic matter export, despite their prevalence in the water column. The efficiency of organic matter export to the deep sea depends on how fast zooplankton carcasses sink in relation to how fast they are degraded by microbial activities. Here, we studied the degradation kinetics of sinking carcasses of the small, cosmopolitan copepod Acartia tonsa and the two large krill species Thysanoessa raschii and Meganyctiphanes norvegica from the Arctic. Zooplankton carcasses are exposed to regionally varying surface temperatures and sink through a vertical gradient of temperature in the water column. Quantification of aerobic microbial respiration, as a proxy of carbon mineralization, indicated that copepod carcasses are degraded in time scales of days to weeks at high temperatures (16-20°C) and weeks to months at low temperatures (4-8°C). Taking the relatively slow sinking rates of copepod carcasses into account, their microbial degradation is likely constrained to the surface ocean in warmer regions. In contrast, copepod carcasses can reach the deep ocean in colder regions and thereby contribute to organic matter export. The rapidly sinking krill carcasses can reach the deep sea and even the hadal realm within 2-4 days. Pyrolytic analysis of krill

Microbially-produced enzymes catalyze the first step of organic matter degradation, but the extent to which their activities are affected by high hydrostatic pressures (HP) that characterize much of the oceans remain little understood. Understanding the effect of HP on enzymatic activities is equally relevant to some shallow marine-derived microorganisms— including those that live associated with sinking particles or those that live in shallow sediments that seed deep-sea environments by down-slope dispersal. Using a recently developed rotating pressure chamber, we quantified the effect of increasing levels of HP on the enzymatic activities of several microbial communities from coastal surface waters and sediments, and communities associated with copepod and phytoplankton cultures. Enzymatic activities assayed include leucine aminopeptidase, β -glucosidase, chitinase, laminarinase, pullulanase, xylanase, and chondroitin hydrolase. Rates were simultaneously measured at HP and ambient/atmospheric pressure (AP), the ratio of which signifies the pressure effect (PE = Rate HP /Rate AP ). We tested the following hypotheses: 1) PE would vary widely across different enzymes, and 2) PE < 1 for microbial communities sourced from shallow environments, indicating pressure-sensitivity. We measured varying PE for different enzymes, but values were generally < 1. However, low-to-moderate levels of HP do not lead to complete inhibition for a subset of enzymatic activities, suggesting differing degrees of pressure tolerance among surface-derived microbial communities. Future studies will examine enzymatic activities of HP-adapted microbial communities, which are more likely to yield values of PE > 1. Systematic and widespread measurements of PE

Phytoplankton aggregates forming in the sunlit layers of the ocean are densely colonized by bacteria and other microbes that exploit the entrapped organic matter while the aggregates sink to the deep ocean. Diatom-dominated aggregates are particularly fast-sinking due to the ballasting effect of the silica shells. Hence, microbes attached to diatom aggregates are quickly exposed to high levels of hydrostatic pressure, which significantly slows down microbial activity and thus aggregate degradation. We hypothesize that the partial inhibition of overall microbial activity is explained by the suppression of piezo-sensitive bacteria attached to the aggregates. Using 16S rDNA amplicon sequencing, we show that the pressure-induced decrease in microbial activity was accompanied by substantial changes in bacterial community composition during 1-day incubations in a rotating pressure tank. Bacterial groups apparently not adapted to the fast increase in pressure level were eliminated from the community, while a core bacterial community remained present up to the highest pressure levels tested (i.e., 100 MPa corresponding to 10 km water depth). Pressure exposure promoted also the leakage of dissolved organic carbon (DOC) from the diatom cells. It can be speculated that the ample supply of DOC inside the aggregates was exploited by piezo-tolerant or piezo-philic bacteria that endured the high pressure levels. In conclusion, (1) the partial inhibition of aggregate degradation by pressure will enhance the amount of labile organic matter that is deposited on the seabed, while (2) the suppression or promotion of different bacterial groups by pressure will re-shape the microbial communities that make it from surface waters to the deep sea.

Copepod carcasses are prevalent in marine ecosystems and might represent an important component of the sinking flux of particulate organic carbon in the ocean. The extent to which copepod carcasses contribute to the biological carbon pump is controlled by different environmental factors, including temperature. However, the effect of temperature on the longer-term kinetics of carbon mineralization of copepod carcasses is not well-studied. We conducted laboratory experiments to quantify the carbon mineralization associated with sinking carcasses of the cosmopolitan copepod Acartia tonsa through aerobic microbial respiration at 5 temperatures (20, 16, 12, 8, and 4°C). Microbial respiration rates associated with the carcasses were positively correlated with temperature and characterized by an initial short lag-phase, a rapid increase to a maximum rate, and a subsequent gradual decline in the rate of degradation. On average, 50% of the total carbon of the carcasses was mineralized within 6-12 d at 20°C, versus >60 d at 4°C. During the incubations, most carbon mineralization occurred in the ambient seawater, likely fueled by dissolved organic carbon leaking from the carcasses into the surrounding seawater. Extrapolating measured carbon turnover and sinking rates suggests that at 20°C, the mineralization of sinking copepod carcasses is constrained to the surface ocean. In contrast, at 4°C, sinking copepod carcasses can reach the deep ocean before they have been completely degraded. Hence, in low-temperature regions, copepod carcasses may represent an important agent for carbon export through the biological carbon pump.

It is generally anticipated that particulate organic carbon (POC) for most part is degraded by attached microorganisms during the descent of "marine snow" aggregates toward the deep sea. There is, however, increasing evidence that fresh aggregates can reach great depth and sustain relatively high biological activity in the deep sea. Using a novel high-pressure setup, we tested the hypothesis that increasing levels of hydrostatic pressure inhibit POC degradation in aggregates rapidly sinking to the ocean interior. Respiration activity, a proxy for POC degradation, was measured directly and continuously at up to 100 MPa (corresponding to 10 km water depth) in a rotating pressure tank that keeps the aggregates in a sinking mode. Model diatom-bacteria aggregates, cultures of the aggregate-forming diatom Skeletonema marinoi, and seawater microbial communities devoid of diatoms showed incomplete and complete inhibition of respiration activity when exposed to pressure levels of 10-50 and 60-100 MPa, respectively. This implies reduced POC degradation and hence enhanced POC export to hadal trenches through fast-sinking, pressure-exposed aggregates. Notably, continuous respiration measurements at >= 50 MPa revealed curved instead of linear oxygen time series whenever S. marinoi was present, which was not captured by discrete respiration measurements. These curvatures correspond to alternating phases of high and low respiration activity likely connected to pressure effects on unidentified metabolic processes in S. marinoi.

The copepod Calanus hyperboreus, a key species of Arctic marine ecosystems, has a partially anoxic gut that is suspected to host anaerobic microbial activities. So far, however, only dead specimens have been studied in which gut processes are quickly abolished by microbial carcass degradation. Here, live specimens were exposed to different feeding regimes and ambient oxygen levels to study the controls on copepod-associated microbial nitrogen cycling in 15N-enrichment experiments. Algae-fed copepods exhibited consistently high rates of denitrification and low or variable rates of dissimilatory nitrate reduction to ammonium (or nitrite), nitrification, and nitrous oxide release. Thus, live C. hyperboreus contribute to pelagic fixed-nitrogen loss through denitrification that is otherwise absent from oxygenated Arctic surface waters. The feeding-related controls on copepod-associated anaerobic nitrogen cycling are supported by significantly reduced rates in starved copepods. In addition, freshly released fecal pellets showed similar patterns of nitrogen cycling as fed copepods. Unlike in previous studies on copepod carcasses, low ambient oxygen levels did not stimulate anaerobic nitrogen cycling in live specimens, supporting that the C. hyperboreus gut is anoxic, irrespective of ambient oxygen levels. Fecal pellets and carcasses retained denitrification activity for 4 d of simulated sinking. We estimate that during the time of the Arctic spring bloom, pelagic denitrification associated with live specimens, fecal pellets, and carcasses of C. hyperboreus is equivalent to ~12% of the benthic fixed-nitrogen loss. This estimate increases to ~28%, provided that the abundant sibling species C. glacialis and C. finmarchicus exhibit denitrification activities in proportion to their smaller body size. A bottle full of copepods dominated by Calanus hyperbo reus specimens (collected in Disko Bay, West Greenland) in which microbial denitrification activity was found. Photo: Peter Stief OPEN ACCESS

A considerable fraction of freshwater zooplankton was recently found to consist of dead specimens that sink to the lake bottom. Such carcasses host intense microbial activities that may promote oxygen depletion at the microscale. Therefore, we tested the hypothesis that sinking zooplankton carcasses are microsites of anaerobic nitrogen cycling that contribute to pelagic fixed-nitrogen loss even in the presence of ambient oxygen. Incubation experiments were performed with the ubiquitous copepods Eudiaptomus sp. and Megacyclops gigas at different ambient oxygen levels that sinking carcasses encounter during their descent in stratified lakes. N-15-stable-isotope incubations revealed intense carcass-associated anaerobic nitrogen cycling only at low ambient oxygen levels (<25% air saturation). Dissimilatory nitrate reduction to ammonium (DNRA) dominated over denitrification and thus the potential for fixed-nitrogen loss was low. Consistent with this partitioning of anaerobic nitrogen cycling, the relative abundance of the carcass-associated marker gene for DNRA (nrfA) was similar to 20-400 times higher than that for denitrification (nirS). Additionally, the relative nrfA and nirS abundances were similar to 90-180 times higher on copepod carcasses than in lake water. This functional distinctiveness of carcass-associated bacterial communities was further substantiated by 16S rDNA-based fingerprinting. We conclude that the unique bacterial communities and microenvironments provided by zooplankton carcasses influence pelagic nitrogen cycling in lakes, but mainly at seasonally low ambient O-2 levels in the bottom water.

Intracellular nitrate is an important electron acceptor in oxygen-deficient aquatic environments, either for the nitrate-storing microbes themselves, or for ambient microbial communities through nitrate leakage. This study links the spatial distribution of intracellular nitrate with the abundance and identity of nitrate-storing microbes in sediments of the Bornholm Basin, an environmental showcase for severe hypoxia. Intracellular nitrate (up to 270 nmol cm-3 sediment) was detected at all 18 stations along a 35-km transect through the basin and typically extended as deep as 1.6 cm into the sediment. Intracellular nitrate contents were particularly high at stations where chlorophyll contents suggested high settling rates of pelagic primary production. The depth distribution of intracellular nitrate matched that of the diatom-specific photopigment fucoxanthin in the upper 1.6 cm and calculations support that diatoms are the major nitrate-storing microbes in these sediments. In contrast, other known nitrate-storing microbes, such as sulfide-oxidizing bacteria and foraminifers, played only a minor role, if any. Strikingly, 18S rRNA gene sequencing revealed that the majority of the diatoms in the sediment were pelagic species. We conclude that intracellular nitrate stored by pelagic diatoms is transported to the seafloor by settling phytoplankton blooms, implying a so far overlooked 'biological nitrate pump'.

The copepod Calanus hyperboreus, a key species of Arctic marine ecosystems, has a partially anoxic gut that is suspected to host anaerobic microbial activities. So far, however, only dead specimens have been studied in which gut processes are quickly abolished by microbial carcass degradation. Here, live specimens were exposed to different feeding regimes and ambient oxygen levels to study the controls on copepod-associated microbial nitrogen cycling in N-15-enrichment experiments. Algae-fed copepods exhibited consistently high rates of denitrification and low or variable rates of dissimilatory nitrate reduction to ammonium (or nitrite), nitrification, and nitrous oxide release. Thus, live C. hyperboreus contribute to pelagic fixed-nitrogen loss through denitrification that is otherwise absent from oxygenated Arctic surface waters. The feeding-related controls on copepod-associated anaerobic nitrogen cycling are supported by significantly reduced rates in starved copepods. In addition, freshly released fecal pellets showed similar patterns of nitrogen cycling as fed copepods. Unlike in previous studies on copepod carcasses, low ambient oxygen levels did not stimulate anaerobic nitrogen cycling in live specimens, supporting that the C. hyperboreus gut is anoxic, irrespective of ambient oxygen levels. Fecal pellets and carcasses retained denitrification activity for 4 d of simulated sinking. We estimate that during the time of the Arctic spring bloom, pelagic denitrification associated with live specimens, fecal pellets, and carcasses of C. hyperboreus is equivalent to similar to 12% of the benthic fixed-nitrogen loss. This estimate increases to similar to 28%, provided that the abundant sibling species C. glacialis and C. finmarchicus exhibit denitrification activities in proportion to their smaller body size.

Sinking phycodetrital aggregates can contribute to anaerobic nitrogen turnover as they may represent oxygen-depleted microbial hot spots in otherwise oxygenated waters. However, the dynamics of anaerobic nitrogen cycling during the long descent of aggregates through oxic or hypoxic waters are unknown. Thus, model aggregates prepared from the diatom Skeletonema marinoi were allowed to age for 4 d at high and low ambient O-2 levels (70 and 15% air saturation, respectively), and changes in nitrogen transformations and microbial community structure were followed. At both O-2 levels, denitrification and dissimilatory NO3- reduction to NO2 (DNRN) were the most important processes of aggregate-associated anaerobic nitrogen cycling. However, at 70% air saturation, rates of anaerobic N cycling were lower and decayed towards 0 after an early rise, whereas at 15% air saturation, they remained constantly high at average production rates of 0.66 nmol N-2-N aggregate (aggr.)(-1) h(-1) and 0.26 nmol NO2- aggr.(-1) h(-1). At both O-2 levels, but more pronouncedly at 70% air saturation, the microbial community underwent succession as expressed by an increase in (1) relative abundance of specific bacterial taxonomic units; (2) bacterial diversity; and (3) prokaryotic abundance. Probably, a higher carbon oxidation rate at high ambient O-2 level progressively selected for microbes capable of using complex carbon polymers. Taken together, the occurrence of anoxic aggregate centers may be ephemeral at high ambient O-2 levels, but persistent at low ambient O-2 levels, indicating that sinking phycodetrital aggregates can remain sinks for bioavailable N in the oceans for several days, especially in O-2-depleted settings.