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

• Published in: Geobiology
• Main Authors: Ostrom, N. E., Gandhi, H., Trubl, G., Murray, A. E.
• Other Authors: National Science Foundation, Division of Antarctic Sciences, National Aeronautics and Space Administration
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2016
• Subjects: East Antarctica; Victoria Valley; Lake Vida; Antarc*; Antarctica
• Online Access: http://dx.doi.org/10.1111/gbi.12190; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1111%2Fgbi.12190; https://onlinelibrary.wiley.com/doi/pdf/10.1111/gbi.12190; https://onlinelibrary.wiley.com/doi/full-xml/10.1111/gbi.12190; https://onlinelibrary.wiley.com/doi/am-pdf/10.1111/gbi.12190

Abstract Lake Vida, in the Victoria Valley of East Antarctica, is frozen, yet harbors liquid brine (~20% salt, >6 times seawater) intercalated in the ice below 16 m. The brine has been isolated from the surface for several thousand years. The brine conditions (permanently dark, −13.4 °C, lack of O 2 , and pH of 6.2) and geochemistry are highly unusual. For example, nitrous oxide (N 2 O) is present at a concentration among the highest reported for an aquatic environment. Only a minor 17 O anomaly was observed in N 2 O, indicating that this gas was predominantly formed in the lake. In contrast, the 17 O anomaly in nitrate ( ) in Lake Vida brine indicates that approximately half or more of the present is derived from atmospheric deposition. Lake Vida brine was incubated in the presence of 15 N‐enriched substrates for 40 days. We did not detect microbial nitrification, dissimilatory reduction of to ammonium ( ), anaerobic ammonium oxidation, or denitrification of N 2 O under the conditions tested. In the presence of 15 N‐enriched nitrite ( ), both N 2 and N 2 O exhibited substantial 15 N enrichments; however, isotopic enrichment declined with time, which is unexpected. Additions of 15 N– alone and in the presence of HgCl 2 and ZnCl 2 to aged brine at −13 °C resulted in linear increases in the δ 15 N of N 2 O with time. As HgCl 2 and ZnCl 2 are effective biocides, we interpret N 2 O 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 15 N‐enriched and ZnCl 2 and determined rates of N 2 O 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 N 2 O in Lake Vida could have been reached between 6 and 465 years. Thus, chemodenitrification alone is sufficient to explain the high levels of N 2 O present in Lake Vida.