Description: N2 and C fixation rates, particulate organic carbon (POC) and particulate organic nitrogen (PON) from the top 500 m of the water column, cruise ORV Sagar Kanya to the BoB during the winter monsoon in January 2014.

Description: The presence of surface methanogenesis, located within the sulfate-reducing zone (0-30 centimeters below seafloor, cmbsf), was investigated in sediments of the seasonally hypoxic Eckernförde Bay, southwestern Baltic Sea. Water column parameters like oxygen, temperature and salinity together with porewater geochemistry and benthic methanogenesis rates were determined in the sampling area 'Boknis Eck' quarterly from March 2013 to September 2014, to investigate the effect of seasonal environmental changes on the rate and distribution of surface methanogenesis and to estimate its potential contribution to benthic methane emissions. Water column parameters where determined via CTD (temperature, salinity, pressure), as well as gas chromatography (methane) and fluorometric methods (chlorophyll a). For porewater and sediment geochemistry various method were used including photometry (sulfide), ion chromatography (sulfate), N/C Analysis (DIC), Carbo-Elba element analysis (POC, C/N), gas chromatography (methane). Sediment net methanogenesis rates were determined via the methane increase (measured with gas chromatography) over time in sediment slurry batch incubations. Sediment hydrogenotrophic methanogenesis was measured by adding radiotracer (14C-bicarbonate) to sediment samples and measuring the production of 14C-methane (via scintillation counting) after a specific period of time. For further details (sample preparation and analysis) see the related publication (Maltby et al., 2017, Biogeosciences Discussions)

Description: Nitrogen fixation, the biological reduction of dinitrogen gas (N2) to ammonium (NH4+), is quantitatively the most important external source of new nitrogen (N) to the open ocean. Classically, the ecological niche of oceanic N2 fixers (diazotrophs) is ascribed to tropical oligotrophic surface waters, often depleted in fixed N, with a diazotrophic community dominated by cyanobacteria. Although this applies for large areas of the ocean, biogeochemical models and phylogenetic studies suggest that the oceanic diazotrophic niche may be much broader than previously considered, resulting in major implications for the global N-budget. Here, we report on the composition, distribution and abundance of nifH, the functional gene marker for N2 fixation. Our results show the presence of eight clades of diazotrophs in the oxygen minimum zone (OMZ) off Peru. Although proteobacterial clades dominated overall, two clusters affiliated to spirochaeta and archaea were identified. N2 fixation was detected within OMZ waters and was stimulated by the addition of organic carbon sources supporting the view that non-phototrophic diazotrophs were actively fixing dinitrogen. The observed co-occurrence of key functional genes for N2 fixation, nitrification, anammox and denitrification suggests that a close spatial coupling of N-input and N-loss processes exists in the OMZ off Peru. The wide distribution of diazotrophs throughout the water column adds to the emerging view that the habitat of marine diazotrophs can be extended to low oxygen/high nitrate areas. Furthermore, our statistical analysis suggests that NO2- and PO43- are the major factors affecting diazotrophic distribution throughout the OMZ. In view of the predicted increase in ocean deoxygenation resulting from global warming, our findings indicate that the importance of OMZs as niches for N2 fixation may increase in the futur

Description: Oxygen minimum zones are expanding globally, and at present account for around 20-40% of oceanic nitrogen loss. Heterotrophic denitrification and anammox-anaerobic ammonium oxidation with nitrite-are responsible for most nitrogen loss in these low-oxygen waters. Anammox is particularly significant in the eastern tropical South Pacific, one of the largest oxygen minimum zones globally. However, the factors that regulate anammox-driven nitrogen loss have remained unclear. Here, we present a comprehensive nitrogen budget for the eastern tropical South Pacific oxygen minimum zone, using measurements of nutrient concentrations, experimentally determined rates of nitrogen transformation and a numerical model of export production. Anammox was the dominant mode of nitrogen loss at the time of sampling. Rates of anammox, and related nitrogen transformations, were greatest in the productive shelf waters, and tailed off with distance from the coast. Within the shelf region, anammox activity peaked in both upper and bottom waters. Overall, rates of nitrogen transformation, including anammox, were strongly correlated with the export of organic matter. We suggest that the sinking of organic matter, and thus the release of ammonium into the water column, together with benthic ammonium release, fuel nitrogen loss from oxygen minimum zones.

Description: N2O production rates from ammonium, nitrite and nitrate and nitrate reduction rates and ammonium oxidation rates from the top 400 m water depth off the coast of Peru sampled from R/V Meteor during M138 in June 2017.

Description: Microbial dinitrogen (N2) fixation, the nitrogenase enzyme-catalysed reduction of N2 gas into biologically available ammonia, is the main source of new nitrogen (N) in the ocean. For more than 50 years, oceanic N2 fixation has mainly been attributed to the activity of the colonial cyanobacterium Trichodesmium. Other smaller N2-fixing microorganisms (diazotrophs)--in particular the unicellular cyanobacteria group A (UCYN-A)--are, however, abundant enough to potentially contribute significantly to N2 fixation in the surface waters of the oceans. Despite their abundance, the contribution of UCYN-A to oceanic N2 fixation has so far not been directly quantified. Here, we show that in one of the main areas of oceanic N2 fixation, the tropical North Atlantic7, the symbiotic cyanobacterium UCYN-A contributed to N2 fixation similarly to Trichodesmium. Two types of UCYN-A, UCYN-A1 and -A2, were observed to live in symbioses with specific eukaryotic algae. Single-cell analyses showed that both algae-UCYN-A symbioses actively fixed N2, contributing ~20% to N2 fixation in the tropical North Atlantic, revealing their significance in this region. These symbioses had growth rates five to ten times higher than Trichodesmium, implying a rapid transfer of UCYN-A-fixed N into the food web that might significantly raise their actual contribution to N2 fixation. Our analysis of global 16S rRNA gene databases showed that UCYN-A occurs in surface waters from the Arctic to the Antarctic Circle and thus probably contributes to N2 fixation in a much larger oceanic area than previously thought. Based on their high rates of N2 fixation and cosmopolitan distribution, we hypothesize that UCYN-A plays a major, but currently overlooked role in the oceanic N cycle.