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Dissimilatory nitrate reduction by Aspergillus terreus isolated from the seasonal oxygen minimum zone in the Arabian Sea.

Stief P, Fuchs-Ocklenburg S, Kamp A, Manohar CS, Houbraken J, Boekhout T, de Beer D, Stoeck T - BMC Microbiol. (2014)

Bottom Line: 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 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.Our findings expand the short list of microbial eukaryotes that store nitrate intracellularly and carry out dissimilatory nitrate reduction when oxygen is absent.

View Article: PubMed Central - HTML - PubMed

Affiliation: Max Planck Institute for Marine Microbiology, Microsensor Group, Bremen, Germany. peterstief@biology.sdu.dk.

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.

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Time course of intracellular nitrate contents (ICNO3) and extracellular nitrate concentrations (ECNO3) (Experiment 3). A. terreus isolate An-4 was cultivated under (A) oxic and (B) anoxic conditions. ICNO3 contents are expressed per g protein of the fungal biomass. Means ± standard deviation (n = 3).
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Figure 3: Time course of intracellular nitrate contents (ICNO3) and extracellular nitrate concentrations (ECNO3) (Experiment 3). A. terreus isolate An-4 was cultivated under (A) oxic and (B) anoxic conditions. ICNO3 contents are expressed per g protein of the fungal biomass. Means ± standard deviation (n = 3).

Mentions: The fate of added to the liquid media of axenic An-4 cultures (verified by microscopy and PCR screening, see Methods) was 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), and3A + 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 and3A). Under anoxic conditions, however, concentrations in the liquid media decreased steadily over the whole incubation period during which neither sudden increases in biomass production, nordepletion were noted (Figures 1B,2A, and3B).


Dissimilatory nitrate reduction by Aspergillus terreus isolated from the seasonal oxygen minimum zone in the Arabian Sea.

Stief P, Fuchs-Ocklenburg S, Kamp A, Manohar CS, Houbraken J, Boekhout T, de Beer D, Stoeck T - BMC Microbiol. (2014)

Time course of intracellular nitrate contents (ICNO3) and extracellular nitrate concentrations (ECNO3) (Experiment 3). A. terreus isolate An-4 was cultivated under (A) oxic and (B) anoxic conditions. ICNO3 contents are expressed per g protein of the fungal biomass. Means ± standard deviation (n = 3).
© Copyright Policy - open-access
Related In: Results  -  Collection

License 1 - License 2
Show All Figures
getmorefigures.php?uid=PMC3928326&req=5

Figure 3: Time course of intracellular nitrate contents (ICNO3) and extracellular nitrate concentrations (ECNO3) (Experiment 3). A. terreus isolate An-4 was cultivated under (A) oxic and (B) anoxic conditions. ICNO3 contents are expressed per g protein of the fungal biomass. Means ± standard deviation (n = 3).
Mentions: The fate of added to the liquid media of axenic An-4 cultures (verified by microscopy and PCR screening, see Methods) was 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), and3A + 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 and3A). Under anoxic conditions, however, concentrations in the liquid media decreased steadily over the whole incubation period during which neither sudden increases in biomass production, nordepletion were noted (Figures 1B,2A, and3B).

Bottom Line: 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 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.Our findings expand the short list of microbial eukaryotes that store nitrate intracellularly and carry out dissimilatory nitrate reduction when oxygen is absent.

View Article: PubMed Central - HTML - PubMed

Affiliation: Max Planck Institute for Marine Microbiology, Microsensor Group, Bremen, Germany. peterstief@biology.sdu.dk.

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.

Show MeSH
Related in: MedlinePlus