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The Arabian Sea harbours one of the three major oxygen minimum zones (OMZs) in the world's oceans, and it alone is estimated to account for ~10–20 % of global oceanic nitrogen (N) loss. While actual rate measurements have been few, the consistently high accumulation of nitrite (NO2−) coinciding with suboxic conditions in the central-northeastern part of the Arabian Sea has led to the general belief that this is the region where active N-loss takes place. Most subsequent field studies on N-loss have thus been drawn almost exclusively to the central-NE. However, a recent study measured only low to undetectable N-loss activities in this region, compared to orders of magnitude higher rates measured towards the Omani Shelf where little NO2− accumulated (Jensen et al., 2011). In this paper, we further explore this discrepancy by comparing the NO2−-producing and consuming processes, and examining the relationship between the overall NO2− balance and active N-loss in the Arabian Sea. Based on a combination of 15N-incubation experiments, functional gene expression analyses, nutrient profiling and flux modeling, our results showed that NO2− accumulated in the central-NE Arabian Sea due to a net production via primarily active nitrate (NO3−) reduction and to a certain extent ammonia oxidation. Meanwhile, NO2− consumption via anammox, denitrification and dissimilatory nitrate/nitrite reduction to ammonium (NH4+) were hardly detectable in this region, though some loss to NO2− oxidation was predicted from modeled NO3− changes. No significant correlation was found between NO2− and N-loss rates (p>0.05). This discrepancy between NO2− accumulation and lack of active N-loss in the central-NE Arabian Sea is best explained by the deficiency of labile organic matter that is directly needed for further NO2− reduction to N2O, N2 and NH4+, and indirectly for the remineralized NH4+ required by anammox. Altogether, our data do not support the long-held view that NO2− accumulation is a direct

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We studied microbial N2 production via anammox and denitrification in the anoxic water column of a restored mining pit lake in Germany over an annual cycle. We obtained high-resolution hydrochemical profiles using a continuous pumping sampler. Lake Rassnitzer is permanently stratified at ca. 29 m depth, entraining anoxic water below a saline density gradient. Mixed-layer nitrate concentrations averaged ca. 200 µmol L-1, but decreased to zero in the anoxic bottom waters. In contrast, ammonium was

Contains fulltext : 35120.pdf (publisher's version ) (Closed access), Laboratory and field studies have indicated that anaerobic ammonium oxidation (anammox) is an important process in the marine nitrogen cycle. In this study 11 additional anoxic marine sediment and water column samples were studied to substantiate this claim. In a combined approach using the molecular methods, polymerase chain reaction (PCR), qualitative and quantitative fluorescence in situ hybridization (FISH), as well as (15)N stable isotope activity measurements, it was shown that anammox bacteria were present and active in all samples investigated. The anammox activity measured in the sediment samples ranged from 0.08 fmol cell(-1) day(-1) N(2) in the Golfo Dulce (Pacific Ocean, Costa Rica) sediment to 0.98 fmol cell(-1) day(-1) N(2) in the Gullmarsfjorden (North Sea, Sweden) sediment. The percentage of anammox cell of the total population (stained with DAPI) as assessed by quantitative FISH was highest in the Barents Sea (9% +/- 4%) and in most of the samples well over 2%. Fluorescence in situ hybridization and phylogenetic analysis of the PCR products derived from the marine samples indicated the exclusive presence of members of the Candidatus'Scalindua' genus. This study showed the ubiquitous presence of anammox bacteria in anoxic marine ecosystems, supporting previous observations on the importance of anammox for N cycling in marine environments.

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Filtration of planktonic algal cells from the water column into permeable sublittoral sediment and the fate of the cells in the shallow sands were studied during a red tide produced by the dinoflagellate Peridinella catenata at Hel Peninsula/Baltic in May 2004. Advective porewater flows associated with ripple topography of the bed caused rapid transport of algal cells down to 5 cm sediment depth. Sedimentary concentrations of algal cells mirrored algal concentrations in the overlying water column with increases and decreases within the upper 3 cm of the bed occurring within a few hours. Sedimentary algal uptake and release significantly differed between stations only 15 m laterally apart. Laboratory sediment-column experiments with 13C-labeled algal cells revealed algal decomposition at rates of up to 0.2% 13C h–1 in percolated sands originating from the study site. This was 2 orders of magnitude lower than observed decreases in sediment algal cell C abundance of up to 23% C h–1 after a drop in cell concentrations in the water column. Because bioturbation and ripple migration were negligible, we conclude that advective flushing of the uppermost sediment layer could rapidly remove cells from the sediment. Our results demonstrate close spatial and temporal coupling between algal cell concentrations in the boundary layer and those in the upper 6 cm of permeable sand sediment, and suggest that permeable beds can act as short-term storage buffer for phytoplankton. During passage through the sediment, planktonic algae may benefit from the higher nutrient concentrations available in the porewater.

Filtration of planktonic algal cells from the water column into permeable sublittoral sediment and the fate of the cells in the shallow sands were studied during a red tide produced by the dinoflagellate Peridinella catenata at Hel Peninsula/Baltic in May 2004. Advective porewater flows associated with ripple topography of the bed caused rapid transport of algal cells down to 5 cm sediment depth. Sedimentary concentrations of algal cells mirrored algal concentrations in the overlying water column with increases and decreases within the upper 3 cm of the bed occurring within a few hours. Sedimentary algal uptake and release significantly differed between stations only 15 m laterally apart. Laboratory sediment-column experiments with 13C-labeled algal cells revealed algal decomposition at rates of up to 0.2% 13C h–1 in percolated sands originating from the study site. This was 2 orders of magnitude lower than observed decreases in sediment algal cell C abundance of up to 23% C h–1 after a drop in cell concentrations in the water column. Because bioturbation and ripple migration were negligible, we conclude that advective flushing of the uppermost sediment layer could rapidly remove cells from the sediment. Our results demonstrate close spatial and temporal coupling between algal cell concentrations in the boundary layer and those in the upper 6 cm of permeable sand sediment, and suggest that permeable beds can act as short-term storage buffer for phytoplankton. During passage through the sediment, planktonic algae may benefit from the higher nutrient concentrations available in the porewater., Filtration of planktonic algal cells from the water column into permeable sublittoral sediment and the fate of the cells in the shallow sands were studied during a red tide produced by the dinoflagellate Peridinella catenata at Hel Peninsula/Baltic in May 2004. Advective porewater flows associated with ripple topography of the bed caused rapid transport of algal cells down to 5 cm sediment depth. Sedimentary concentrations of algal cells mirrored algal concentrations in the overlying water column with increases and decreases within the upper 3 cm of the bed occurring within a few hours. Sedimentary algal uptake and release significantly differed between stations only 15 m laterally apart. Laboratory sediment-column experiments with 13C-labeled algal cells revealed algal decomposition at rates of up to 0.2% 13C h–1 in percolated sands originating from the study site. This was 2 orders of magnitude lower than observed decreases in sediment algal cell C abundance of up to 23% C h–1 after a drop in cell concentrations in the water column. Because bioturbation and ripple migration were negligible, we conclude that advective flushing of the uppermost sediment layer could rapidly remove cells from the sediment. Our results demonstrate close spatial and temporal coupling between algal cell concentrations in the boundary layer and those in the upper 6 cm of permeable sand sediment, and suggest that permeable beds can act as short-term storage buffer for phytoplankton. During passage through the sediment, planktonic algae may benefit from the higher nutrient concentrations available in the porewater.

The aim of this study was to investigate the supposed vertical diel migration and the accompanying physiology of Beggiatoa bacteria from hypersaline microbial mats. We combined microsensor, stable-isotope, and molecular techniques to clarify the phylogeny and physiology of the most dominant species inhabiting mats of the natural hypersaline Lake Chiprana, Spain. The most dominant morphotype had a filament diameter of 6 to 8 µm and a length varying from 1 to >10 mm. Phylogenetic analysis by 16S rRNA gene comparison revealed that this type appeared to be most closely related (91% sequence identity) to the narrow (4-µm diameter) nonvacuolated marine strain MS-81-6. Stable-isotope analysis showed that the Lake Chiprana species could store nitrate intracellularly to 40 mM. The presence of large intracellular vacuoles was confirmed by fluorescein isothiocyanate staining and subsequent confocal microscopy. In illuminated mats, their highest abundance was found at a depth of 8 mm, where oxygen and sulfide co-occurred. However, in the dark, the highest Beggiatoa densities occurred at 7 mm, and the whole population was present in the anoxic zone of the mat. Our findings suggest that hypersaline Beggiatoa bacteria oxidize sulfide with oxygen under light conditions and with internally stored nitrate under dark conditions. It was concluded that nitrate storage by Beggiatoa is an optimal strategy to both occupy the suboxic zones in sulfidic sediments and survive the dark periods in phototrophic mats.

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The 14N/J5N ratio were measured on sinking matter, atmospheric dust and sediments in the equatorial and eastern South Atlantic. To determine the main controls on the nitrogen composition of the marine sediment in this region for the present situation, sediment traps and surface sediments were analysed and compared to other parameters and independent oceanographic and climatic data. Additionally, to better estimate the possible impact from the terrigenous input on the deep sea sediments, shipboard collected dust samples were analysed for their isotopic composition. For reconstructions of past changes in the high production zone in the Benguela upwelling regime, a eore transect off Namibia, South West Africa, were analysed for downcore variations in the q15N signal.

The 14N/J5N ratio were measured on sinking matter, atmospheric dust and sediments in the equatorial and eastern South Atlantic. To determine the main controls on the nitrogen composition of the marine sediment in this region for the present situation, sediment traps and surface sediments were analysed and compared to other parameters and independent oceanographic and climatic data. Additionally, to better estimate the possible impact from the terrigenous input on the deep sea sediments, shipboard collected dust samples were analysed for their isotopic composition. For reconstructions of past changes in the high production zone in the Benguela upwelling regime, a eore transect off Namibia, South West Africa, were analysed for downcore variations in the q15N signal.

The 14N/J5N ratio were measured on sinking matter, atmospheric dust and sediments in the equatorial and eastern South Atlantic. To determine the main controls on the nitrogen composition of the marine sediment in this region for the present situation, sediment traps and surface sediments were analysed and compared to other parameters and independent oceanographic and climatic data. Additionally, to better estimate the possible impact from the terrigenous input on the deep sea sediments, shipboard collected dust samples were analysed for their isotopic composition. For reconstructions of past changes in the high production zone in the Benguela upwelling regime, a eore transect off Namibia, South West Africa, were analysed for downcore variations in the q15N signal.

The 14N/J5N ratio were measured on sinking matter, atmospheric dust and sediments in the equatorial and eastern South Atlantic. To determine the main controls on the nitrogen composition of the marine sediment in this region for the present situation, sediment traps and surface sediments were analysed and compared to other parameters and independent oceanographic and climatic data. Additionally, to better estimate the possible impact from the terrigenous input on the deep sea sediments, shipboard collected dust samples were analysed for their isotopic composition. For reconstructions of past changes in the high production zone in the Benguela upwelling regime, a eore transect off Namibia, South West Africa, were analysed for downcore variations in the q15N signal.