The Arabian Sea harbors two different O2-deficient conditions, which includes a seasonal OMZ along the continental shelf and an open-ocean, perennial OMZ [17]. The distribution of anaerobic nitrogen cycling in the Arabian Sea is patchy and covers areas with predominant

denitrification [18] or anammox activity [19]. The Arabian Sea is also a globally important site of N2O emission [17, 20, 21]. The oversaturation of the water column with this potent greenhouse gas is ascribed to denitrification activity [17]. Here, the ecophysiology of an A. terreus isolate (An-4) obtained from the seasonal OMZ in the Arabian Sea was studied. An-4 was enriched from coastal sediment sampled during a period of bottom-water anoxia using anoxic, -amended conditions. It was therefore hypothesized that An-4 is capable of dissimilatory NO3 – reduction. The role LY333531 chemical structure of O2 and availability in triggering dissimilatory NO3 – reduction was studied in axenic incubations.

In a dedicated 15N-labeling experiment, all environmentally relevant products of dissimilatory reduction were determined. Intracellular storage, a common trait of NO3 –respiring eukaryotes, SB202190 chemical structure was studied combining freeze-thaw cycles and ultrasonication for lysing -storing cells. Production of cellular energy and biomass enabled by dissimilatory reduction was assessed with ATP and protein measurements, respectively. Using these experimental strategies, we present the first evidence for dissimilatory reduction by an ascomycete fungus that is known from a broad range of habitats, but here was isolated from a marine environment. Results Aerobic and anaerobic nitrate and ammonium turnover Morin Hydrate The fate of added to the liquid media of axenic An-4 cultures (verified by microscopy and PCR screening, see Methods) was Mdivi1 in vivo followed during aerobic and anaerobic cultivation (Experiment 1), in a 15N-labeling experiment involving an oxic-anoxic shift (Experiment 2), and in a cultivation experiment that addressed the intracellular storage of (Experiment 3). Nitrate was generally consumed, irrespective of O2 availability (Figures  1A + B (Exp. 1),

2A (Exp. 2), and 3A + B (Exp. 3)). Under oxic conditions, concentrations in the liquid media exhibited sudden drops when high biomass production and/or depletion was noted in the culture flasks (Figures  1A and 3A). Under anoxic conditions, however, concentrations in the liquid media decreased steadily over the whole incubation period during which neither sudden increases in biomass production, nor depletion were noted (Figures  1B, 2A, and 3B). Figure 1 Time course of nitrate and ammonium concentrations during axenic cultivation of A. terreus isolate An-4 (Experiment 1). (A) Aerobic, (B) anaerobic cultivation. The liquid media were amended with nominally 50 μmol L-1 of NO3 – and NH4 + each at the beginning of cultivation. Means ± standard deviation (n = 3).