Dynamics of ammonium oxidizer activity and nitrous oxide (N20) within and beneath Antarctic sea ice

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Abstract
Nitrapyrin, an inhibitor of NH4+ oxidizing bacteria, was used to estimate the activity of NH4+ oxidizing bacteria in the bottom 1 to 15 cm of annual sea ice and in the water column at various locations in McMurdo Sound and along the Ross Ice Shelf (RIS). Nitrapyrin significantly inhibited dark 14C-HCO3- uptake in virtually all sea-ice samples, indicating the presence on NH4+ oxidizing bacteria. Inorganic carbon fixation by sea ice NH4+ oxidizers was only a small fraction of that fixed by sea-ice photoautotrophs, both on an hourly and annual basis. Despite their relative lack of importance to inorganic carbon fixation, NH4+ oxidizing bacteria may have an important role in the N dynamics within the biogenic layer of annual sea ice both in terms of NH4+ utilization and eventual NO3- production. Inorganic carbon fixation in the water column beneath sea ice was generally not inhibited significantly by nitrapyrin. NH4+ oxidizer activity was also not detectable in deep water flowing beneath (southward) or from under (northward) the RIS. N2O (a by-product of NH4+ oxidation) levels in pelagic samples were always near 100% of saturation with respect to the air above the sea surface corroborating the low levels of NH4+ oxidizer activity found in the water column.
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