Chemodenitrification in the cryoecosystem of Lake Vida, Victoria Valley, Antarctica

Lake Vida, in the Victoria Valley of East Antarctica, is frozen, yet harbors liquid brine (similar to 20% salt, >6 times seawater) intercalated in the ice below 16m. The brine has been isolated from the surface for several thousand years. The brine conditions (permanently dark, -13.4 degrees C, lack of O-2, and pH of 6.2) and geochemistry are highly unusual. For example, nitrous oxide (N2O) is present at a concentration among the highest reported for an aquatic environment. Only a minor O-17 anomaly was observed in N2O, indicating that this gas was predominantly formed in the lake. In contrast, the O-17 anomaly in nitrate (NO3-) in Lake Vida brine indicates that approximately half or more of the NO3- present is derived from atmospheric deposition. Lake Vida brine was incubated in the presence of N-15-enriched substrates for 40 days. We did not detect microbial nitrification, dissimilatory reduction of NO3- to ammonium (NH4+), anaerobic ammonium oxidation, or denitrification of N2O under the conditions tested. In the presence of N-15-enriched nitrite (NO2-), both N-2 and N2O exhibited substantial N-15 enrichments; however, isotopic enrichment declined with time, which is unexpected. Additions of N-15-NO2- alone and in the presence of HgCl2 and ZnCl2 to aged brine at -13 degrees C resulted in linear increases in the N-15 of N2O with time. As HgCl2 and ZnCl2 are effective biocides, we interpret N2O production in the aged brine to be the result of chemodenitrification. With this understanding, we interpret our results from the field incubations as the result of chemodenitrification stimulated by the addition of N-15-enriched NO2- and ZnCl2 and determined rates of N2O and N-2 production of 4.11-41.18 and 0.55-1.75 nmol L(-1)day(-1), respectively. If these rates are representative of natural production, the current concentration of N2O in Lake Vida could have been reached between 6 and 465years. Thus, chemodenitrification alone is sufficient to explain the high levels of N2O present in Lake Vida.