title: Dissimilatory nitrate reduction by Aspergillus terreus isolated from the seasonal oxygen minimum zone in the Arabian Sea.
abstract: 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.
  
  
  RESULTS
  Axenic incubations of An-4 in the presence and absence of oxygen and nitrate revealed that this fungal isolate is capable of dissimilatory nitrate reduction to ammonium under anoxic conditions. A ¹⁵N-labeling experiment proved that An-4 produced and excreted ammonium through nitrate reduction at a rate of up to 175 nmol ¹⁵NH₄⁺ g⁻¹ protein h⁻¹. The products of dissimilatory nitrate reduction were ammonium (83%), nitrous oxide (15.5%), and nitrite (1.5%), while dinitrogen production was not observed. The process led to substantial cellular ATP production and biomass growth and also occurred when ammonium was added to suppress nitrate assimilation, stressing the dissimilatory nature of nitrate reduction. Interestingly, An-4 used intracellular nitrate stores (up to 6-8 μmol NO₃⁻ g⁻¹ protein) for dissimilatory nitrate reduction.
  
  
  CONCLUSIONS
  Our findings expand the short list of microbial eukaryotes that store nitrate intracellularly and carry out dissimilatory nitrate reduction when oxygen is absent. In the currently spreading oxygen-deficient zones in the ocean, an as yet unexplored diversity of fungi may recycle nitrate to ammonium and nitrite, the substrates of the major nitrogen loss process anaerobic ammonium oxidation, and the potent greenhouse gas nitrous oxide.
authors: Max Planck Institute for Marine Microbiology, Microsensor Group, Bremen, Germany. peterstief@biology.sdu.dk.
authors: Peter
authors: P
authors: Stief
authors: Silvia
authors: S
authors: Fuchs-Ocklenburg
authors: Anja
authors: A
authors: Kamp
authors: Cathrine-Sumathi
authors: CS
authors: Manohar
authors: Jos
authors: J
authors: Houbraken
authors: Teun
authors: T
authors: Boekhout
authors: Dirk
authors: D
authors: de Beer
authors: Thorsten
authors: T
authors: Stoeck
mesh_terms: D064751:Ammonium Compounds / Q000378:metabolism; D001230:Aspergillus / Q000302:isolation & purification / Q000378:metabolism*; D004269:DNA, Bacterial / Q000737:chemistry / Q000235:genetics; D019015:Geologic Sediments / Q000382:microbiology*; D000860:Hypoxia; D008969:Molecular Sequence Data; D009566:Nitrates / Q000378:metabolism*; D010084:Oxidation-Reduction; D010100:Oxygen / Q000378:metabolism; D017422:Sequence Analysis, DNA
chemical_list: D064751:Ammonium Compounds; D004269:DNA, Bacterial; D009566:Nitrates; D010100:Oxygen