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

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• 作者：N. E. Ostrom ; H. Gandhi ; G. Trubl and A. E. Murray
• 刊名：Geobiology
• 出版年：2016
• 出版时间：November 2016
• 年：2016
• 卷：14
• 期：6
• 页码：575-587
• 全文大小：1407K
• ISSN：1472-4669

文摘

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 O₂, 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 ¹⁷O anomaly was observed in N₂O, indicating that this gas was predominantly formed in the lake. In contrast, the ¹⁷O 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 ¹⁵N-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 ¹⁵N-enriched nitrite (NO2−), both N₂ and N₂O exhibited substantial ¹⁵N enrichments; however, isotopic enrichment declined with time, which is unexpected. Additions of ¹⁵N–NO2− alone and in the presence of HgCl₂ and ZnCl₂ to aged brine at −13 °C resulted in linear increases in the δ¹⁵N 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 ¹⁵N-enriched NO2− and ZnCl₂ and determined rates of N₂O and N₂ 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 465 years. Thus, chemodenitrification alone is sufficient to explain the high levels of N₂O present in Lake Vida.