The enigmatic nitrogen biogeochemistry of Lake Vida, Antarctica: an isolated brine cryoecosystem
Abstract
Lake Vida, located in Victoria Valley, East Antarctica, harbors ice-entrained brine (20 % salt, > 6 times seawater) that has been isolated from the surface for more than 2,800 years. The brine conditions (permanently dark, temperature of -13.4 °C, lack of O2, and pH of 6.2) and geochemistry are highly unusual. As an example, the brine contains the highest concentrations of N2O reported for any aquatic ecosystem (86.6 μM) and exceptionally high levels of NH4+ (3.6 mM), NO3- (1.1 mM) and NO2- (27.3 μM). Though this cryoecosystem appears to be relatively inhospitable, microbial life is abundant (>10^7 cell-like particles per mL), is capable of protein production at in situ temperatures, and harbors an assemblage of bacterial phylotypes spanning at least eight phyla. To assess present and past microbial activity in situ and test hypotheses concerning energy generation in the brine cryoecosystem, the stable isotope signatures of nitrogen, oxygen, and hydrogen have been characterized in liquid and dissolved gas phases. The isotopic compositions of nitrate (δ15N = -7.9 ‰, δ18O = 31.7 ‰), ammonium (δ15N = -4.8 ‰) and dinitrogen (δ15N = 0.3 ‰) are all consistent with an atmospheric origin. Given that nearly all non-atmospheric sources of nitrate lack a 17O anomaly, a Δ17O value of nitrate in Lake Vida of 15.9 ‰ suggests that approximately half of the nitrate present is derived from atmospheric deposition. A 17O anomaly was not observed in N2O indicating that this gas was likely formed in the lake. While the bulk δ15N (-22.2 ‰) and site preference (SP; -3.6 ‰) values for N2O are consistent with a microbial origin, the δ18O value (3.0 ‰) is markedly depleted in 18O relative to the vast majority of published values. The soils surrounding Don Juan Pond, which is in a neighboring valley, have been shown to produce N2O with variable δ15N, δ18O and SP values of -45.4 to -34.5 ‰, 50.5 to 76.7 ‰, and -45.2 to 4.1 ‰, respectively, that may reflect inorganic production by chemodenitrification (Samarkin et al., 2010; Nature Geosci 3: 341-344). To address pathways of nitrogen cycling, samples of brine were incubated in the presence of 15N enriched ammonium, nitrite or N2O at a range of temperatures from -13 to +4 oC for up to 40 days. We found no evidence of nitrification, dissimilatory nitrate reduction to ammonium, or anaerobic ammonium oxidation. In the presence of 15N enriched nitrite, both N2 and N2O exhibited initial 15N enrichments followed by subsequently decrease of isotope values. These results are consistent with (1) an initial reduction of nitrite followed by the generation of both N2 and N2O from an unlabeled source; presumably nitrate. The lack of N2 production from 15N-N2O suggests the absence of microbial denitrification. Thus, while a total of 56 bacterial strains were isolated from Lake Vida brine and 9 strains have been shown to be capable of denitrification, our results indicate that inorganic processes largely control the geochemistry of Lake Vida. As our understanding of the Lake Vida brine cryoecosystem improves, we will develop a better understanding of the possibilities for life in encapsulated and icy ecosystems elsewhere.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.B21F..02O
- Keywords:
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- 0456 BIOGEOSCIENCES Life in extreme environments;
- 0406 BIOGEOSCIENCES Astrobiology and extraterrestrial materials;
- 0448 BIOGEOSCIENCES Geomicrobiology;
- 0454 BIOGEOSCIENCES Isotopic composition and chemistry