In batch experiments, we studied the isotope fractionation in N and O of dissolved nitrate during dentrification. Denitrifying strains Thauera aromatica and “Aromatoleum aromaticum strain EbN1” were grown under strictly anaerobic conditions with acetate, benzoate, and toluene as carbon sources. 18O-labeled water and 18O-labeled nitrite were added to the microcosm experiments to study the effect of putative backward reactions of nitrite to nitrate on the stable isotope fractionation. We found no evidence for a reverse reaction. Significant variations of the stable isotope enrichment factor ε were observed depending on the type of carbon source used. For toluene (ε15N, −18.1 ± 0.6‰ to −7.3 ± 1.4‰; ε18O, −16.5 ± 0.6‰ to −16.1 ± 1.5‰) and benzoate (ε15N, −18.9 ± 1.3‰; ε18O, −15.9 ± 1.1‰) less negative isotope enrichment factors were calculated compared to those derived from acetate (ε15N, −23.5 ± 1.9‰ to −22.1 ± 0.8‰; ε18O, −23.7 ± 1.8‰ to −19.9 ± 0.8‰). The observed isotope effects did not depend on the growth kinetics which were similar for the three types of electron donors. We suggest that different carbon sources change the observed isotope enrichment factors by changing the relative kinetics of nitrate transport across the cell wall compared to the kinetics of the intracellular nitrate reduction step of microbial denitrification.

We analysed carbon (δ 13 C) and nitrogen (δ 15 N) isotope ratios of organisms and biogenic tissues from Comau Fjord (southern Chile) to characterise benthic food webs and spatial isotope variability in this ecosystem. These values were intended to serve as a baseline for detecting anthropogenic impacts on Patagonian marine fjord ecosystems in later studies. Benthic macroalgae and invertebrate suspension feeders were primarily considered, with some supplementary data from cyanobacteria, plankton, fish, and coastal vertebrates. Six depth transects typified the lateral salinity gradients from the innermost part of the fjord to its mouth, as well as the vertical density gradients caused by freshwater inflow. Carbon isotope signatures indicated predominant consumption of either CO 2 or HCO 3 -for benthic macroalgal. All CO 2 users belonged to rhodophytes. The δ 15N values of benthic macrophytes decreased with decreasing salinity, both vertically and along the fjord axis. This implies the influence of 15 N-poor terrestrial dissolved inorganic nitrogen (DIN) at these sites. Enhanced influence of freshwater influx also lowered N contents and increased C/N ratios in algal tissues. Exceptionally high macroalgae δ Mar Ecol Prog Ser 428: [89][90][91][92][93][94][95][96][97][98][99][100][101][102][103][104] 2011 2007, van Ofwegen et al. 2006, Esteves et al. 2007, Sinniger & Häussermann 2009). The driving forces behind these patterns are not yet fully understood (Fernandez et al. 2000, Escribano et al. 2003. Possible environmental factors affecting biodiversity are specific physical and chemical gradients or highly dynamic nutrient and organic matter fluxes. However, few oceanographic data are available (e.g. Pickard 1971, Silva et al. 1995, Strub et al. 1998). An understanding of the functioning of the Chilean fjord ecosystem is a prerequisite in order to assess the consequences of the intensified exploitation of marine resources and of rapidly expanding aquaculture, the latter adding additional nutrient sources to benthic ecosystems. In addition to species inventories, a deeper understanding of the biogeochemical cycles in Patagonian fjords is the basis for environmental monitoring.Stable isotope methods are frequently applied to characterise marine food webs (Minagawa & Wada 1984, Schoeninger & DeNiro 1984, Bergmann et al. 2009, Steinarsdóttir et al. 2009). The nitrogen and carbon isotopic compositions of natural samples are given as delta values (δ), representing the isotope ratio of the heavy to the light isotope (‰) relative to an international standard and is defined as: δ y X = (R sample / R standard -1) × 1000, with R being the ratio of the heavier to the lighter isotope ( The carbon isotope composition (δ 13 C) of an organism can be used to distinguish isotopically different sources in food webs (DeNiro & Epstein 1978). Minor δ 13 C differences of 0 to 1 ‰ are observed between animals and their food (Peterson & Fry 1987). Thus, the much larger carbon isotope discrepancies between certain groups of prim...

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Globally occurring nitrate pollution in groundwater is harming the environment and human health. In situ hydrogen addition to stimulate denitrification has been proposed as a remediation strategy. However, observed nitrite accumulation and incomplete denitrification are severe drawbacks that possibly stem from the specific microbial community composition. We set up a microcosm experiment comprising sediment and groundwater from a nitrate polluted oxic oligotrophic aquifer. After the microcosms were sparged with hydrogen gas, samples were taken regularly within 122 h for nitrate and nitrite measurements, community composition analysis via 16S rRNA gene amplicon sequencing and gene and transcript quantification via qPCR of reductase genes essential for complete denitrification. The highest nitrate reduction rates and greatest increase in bacterial abundance coincided with a 15.3-fold increase in relative abundance of Rhodocyclaceae, specifically six ASVs that are closely related to the genus Dechloromonas. The denitrification reductase genes napA, nirS and clade I nosZ also increased significantly over the observation period. We conclude that taxa of the genus Dechloromonas are the prevailing hydrogenotrophic denitrifiers in this nitrate polluted aquifer and the ability of hydrogenotrophic denitrification under the given conditions is species-specific.