Hydrostatic pressure impedes the degradation of sinking copepod carcasses and fecal pellets.

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 was experimentally studied as a function of pressure (0.

Respiration by "marine snow" at high hydrostatic pressure: Insights from continuous oxygen measurements in a rotating pressure tank.

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.

Freshwater copepod carcasses as pelagic microsites of dissimilatory nitrate reduction to ammonium.

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.

Intracellular nitrate in sediments of an oxygen-deficient marine basin is linked to pelagic diatoms.

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.

Effect of settled diatom-aggregates on benthic nitrogen cycling.

The marine sediment hosts a mosaic of microhabitats. Recently it has been demonstrated that the settlement of phycodetrital aggregates can induce local changes in the benthic O distribution due to confined enrichment of organic material and alteration of the diffusional transport. Here, we show how this microscale O shift substantially affects benthic nitrogen cycling.

Intracellular Nitrate of Marine Diatoms as a Driver of Anaerobic Nitrogen Cycling in Sinking Aggregates.

Diatom-bacteria aggregates are key for the vertical transport of organic carbon in the ocean. Sinking aggregates also represent pelagic microniches with intensified microbial activity, oxygen depletion in the center, and anaerobic nitrogen cycling. Since some of the aggregate-forming diatom species store nitrate intracellularly, we explored the fate of intracellular nitrate and its availability for microbial metabolism within anoxic diatom-bacteria aggregates.

Direct Nitrous Oxide Emission from the Aquacultured Pacific White Shrimp (Litopenaeus vannamei).

Unlabelled: The Pacific white shrimp (Litopenaeus vannamei) is widely used in aquaculture, where it is reared at high stocking densities, temperatures, and nutrient concentrations. Here we report that adult L. vannamei shrimp emit the greenhouse gas nitrous oxide (N2O) at an average rate of 4.

Anaerobic Nitrogen Turnover by Sinking Diatom Aggregates at Varying Ambient Oxygen Levels.

In the world's oceans, even relatively low oxygen levels inhibit anaerobic nitrogen cycling by free-living microbes. Sinking organic aggregates, however, might provide oxygen-depleted microbial hotspots in otherwise oxygenated surface waters. Here, we show that sinking diatom aggregates can host anaerobic nitrogen cycling at ambient oxygen levels well above the hypoxic threshold.

Nitrate Storage and Dissimilatory Nitrate Reduction by Eukaryotic Microbes.

The microbial nitrogen cycle is one of the most complex and environmentally important element cycles on Earth and has long been thought to be mediated exclusively by prokaryotic microbes. Rather recently, it was discovered that certain eukaryotic microbes are able to store nitrate intracellularly and use it for dissimilatory nitrate reduction in the absence of oxygen. The paradigm shift that this entailed is ecologically significant because the eukaryotes in question comprise global players like diatoms, foraminifers, and fungi.

Anaerobic methane oxidation coupled to denitrification is the dominant methane sink in a deep lake.

Anaerobic methane oxidation coupled to denitrification, also known as "nitrate/nitrite-dependent anaerobic methane oxidation" (n-damo), was discovered in 2006. Since then, only a few studies have identified this process and the associated microorganisms in natural environments. In aquatic sediments, the close proximity of oxygen- and nitrate-consumption zones can mask n-damo as aerobic methane oxidation.

Effect of high electron donor supply on dissimilatory nitrate reduction pathways in a bioreactor for nitrate removal.

The possible shift of a bioreactor for NO3(-) removal from predominantly denitrification (DEN) to dissimilatory nitrate reduction to ammonium (DNRA) by elevated electron donor supply was investigated. By increasing the C/NO3(-) ratio in one of two initially identical reactors, the production of high sulfide concentrations was induced. The response of the dissimilatory NO3(-) reduction processes to the increased availability of organic carbon and sulfide was monitored in a batch incubation system.

High spatial resolution of distribution and interconnections between Fe- and N-redox processes in profundal lake sediments.

The Fe and N biogeochemical cycles play key roles in freshwater environments. We aimed to determine the spatial positioning and interconnections of the N and Fe cycles in profundal lake sediments. The gradients of O2, NO3(-), NH4(+), pH, Eh, Fe(II) and Fe(III) were determined and the distribution of microorganisms was assessed by most probable numbers and quantitative polymerase chain reaction.

The Marine Microbial Eukaryote Transcriptome Sequencing Project (MMETSP): illuminating the functional diversity of eukaryotic life in the oceans through transcriptome sequencing.

Current sampling of genomic sequence data from eukaryotes is relatively poor, biased, and inadequate to address important questions about their biology, evolution, and ecology; this Community Page describes a resource of 700 transcriptomes from marine microbial eukaryotes to help understand their role in the world's oceans.

Microscopic oxygen imaging based on fluorescein bleaching efficiency measurements.

Photobleaching of the fluorophore fluorescein in an aqueous solution is dependent on the oxygen concentration. Therefore, the time-dependent bleaching behavior can be used to measure of dissolved oxygen concentrations. The method can be combined with epi-fluorescence microscopy.

Dissimilatory nitrate reduction by Aspergillus terreus isolated from the seasonal oxygen minimum zone in the Arabian Sea.

Background: A wealth of microbial eukaryotes is adapted to life in oxygen-deficient marine environments. Evidence is accumulating that some of these eukaryotes survive anoxia by employing dissimilatory nitrate reduction, a strategy that otherwise is widespread in prokaryotes. Here, we report on the anaerobic nitrate metabolism of the fungus Aspergillus terreus (isolate An-4) that was obtained from sediment in the seasonal oxygen minimum zone in the Arabian Sea, a globally important site of oceanic nitrogen loss and nitrous oxide emission.

Response of the ubiquitous pelagic diatom Thalassiosira weissflogii to darkness and anoxia.

Thalassiosira weissflogii, an abundant, nitrate-storing, bloom-forming diatom in the world's oceans, can use its intracellular nitrate pool for dissimilatory nitrate reduction to ammonium (DNRA) after sudden shifts to darkness and anoxia, most likely as a survival mechanism. T. weissflogii cells that stored 4 mM (15)N-nitrate consumed 1.

Chironomus plumosus larvae increase fluxes of denitrification products and diversity of nitrate-reducing bacteria in freshwater sediment.

Benthic invertebrates affect microbial processes and communities in freshwater sediment by enhancing sediment-water solute fluxes and by grazing on bacteria. Using microcosms, the effects of larvae of the widespread midge Chironomus plumosus on the efflux of denitrification products (N2O and N2+N2O) and the diversity and abundance of nitrate- and nitrous-oxide-reducing bacteria were investigated. Additionally, the diversity of actively nitrate- and nitrous-oxide-reducing bacteria was analyzed in the larval gut.

Role of diatoms in the spatial-temporal distribution of intracellular nitrate in intertidal sediment.

Intracellular nitrate storage allows microorganisms to survive fluctuating nutrient availability and anoxic conditions in aquatic ecosystems. Here we show that diatoms, ubiquitous and highly abundant microalgae, represent major cellular reservoirs of nitrate in an intertidal flat of the German Wadden Sea and are potentially involved in anaerobic nitrate respiration. Intracellular nitrate (ICNO3) was present year-round in the sediment and was spatially and temporally correlated with fucoxanthin, the marker photopigment of diatoms.

High rates of denitrification and nitrous oxide emission in arid biological soil crusts from the Sultanate of Oman.

Using a combination of process rate determination, microsensor profiling and molecular techniques, we demonstrated that denitrification, and not anaerobic ammonium oxidation (anammox), is the major nitrogen loss process in biological soil crusts from Oman. Potential denitrification rates were 584±101 and 58±20 μmol N m(-2) h(-1) for cyanobacterial and lichen crust, respectively. Complete denitrification to N2 was further confirmed by an (15)NO3(-) tracer experiment with intact crust pieces that proceeded at rates of 103±19 and 27±8 μmol N m(-2) h(-1) for cyanobacterial and lichen crust, respectively.

Higher nitrate-reducer diversity in macrophyte-colonized compared to unvegetated freshwater sediment.

Freshwater macrophytes stimulate rhizosphere-associated coupled nitrification-denitrification and are therefore likely to influence the community composition and abundance of rhizosphere-associated denitrifiers and nitrate reducers. Using the narG gene, which encodes the catalytic subunit of the membrane-bound nitrate reductase, as a molecular marker, the community composition and relative abundance of nitrate-reducing bacteria were compared in the rhizosphere of the freshwater macrophyte species Littorella uniflora and Myriophyllum alterniflorum to nitrate-reducing communities in unvegetated sediment. Microsensor analysis indicated a higher availability of oxygen in the rhizosphere compared to unvegetated sediment, with a stronger release of oxygen from the roots of L.

Characterization of denitrifying granular sludge with and without the addition of external carbon source.

In this study granular sludge taken from a denitrifying upflow sludge reactor was characterized. Denitrification rates were determined in batch tests with and without external carbon source addition and pH microprofiles of the granules were studied. The microbial community structure was also determined.

Shell biofilm-associated nitrous oxide production in marine molluscs: processes, precursors and relative importance.

Emission of the greenhouse gas nitrous oxide (N2 O) from freshwater and terrestrial invertebrates has exclusively been ascribed to N2 O production by ingested denitrifying bacteria in the anoxic gut of the animals. Our study of marine molluscs now shows that also microbial biofilms on shell surfaces are important sites of N2 O production. The shell biofilms of Mytilus edulis, Littorina littorea and Hinia reticulata contributed 18-94% to the total animal-associated N2 O emission.

Shell biofilm nitrification and gut denitrification contribute to emission of nitrous oxide by the invasive freshwater mussel Dreissena polymorpha (zebra mussel).

Nitrification in shell biofilms and denitrification in the gut of the animal accounted for N(2)O emission by Dreissena polymorpha (Bivalvia), as shown by gas chromatography and gene expression analysis. The mussel's ammonium excretion was sufficient to sustain N(2)O production and thus potentially uncouples invertebrate N(2)O production from environmental N concentrations.

Diatoms respire nitrate to survive dark and anoxic conditions.

Diatoms survive in dark, anoxic sediment layers for months to decades. Our investigation reveals a correlation between the dark survival potential of marine diatoms and their ability to accumulate NO(3)(-) intracellularly. Axenic strains of benthic and pelagic diatoms that stored 11-274 mM NO(3)(-) in their cells survived for 6-28 wk.