Insights into the origin of N2O in Lake Vida brine
AuthorTrubl, Gareth G.
AdvisorMurray, Alison E.
Natural Resources and Environmental Science
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Lake Vida is a briny, frigid, inhospitable anoxic lake, and sealed beneath 16m of ice, there is an 18 to 21% NaCl based brine with a high concentration of N2O. Biological and abiotic denitrification pathways of N2O production were evaluated to determine the origin(s) of the high concentration of N2O in Lake Vida brine. Sixteen bacterial isolates from Lake Vida brine spanning six genera (Marinobacter, Psychrobacter, Exiguobacterium, Arthrobacter, Microbacterium, and Kocuria) were tested for the capability of incomplete or complete denitrification. Four of the sixteen isolates evaluated showed evidence of N2 production. The isolate from each genus that produced the most N2O was further tested to see if total protein production correlated with N2O production. Marinobacter sp. strain LV411 had the highest N2O production rate of 60 ± 6µM day−1 and a generation time of 30 ± 0 hours grown at 10°C. Isolate LV411 produced nearly 8-fold more N2O, than the next highest N2O producer (Exiguobacterium sp. strain LV05br1). Lake Vida bacterial isolates demonstrated the ability to denitrify, in which four of them showed the potential for complete denitrification. The potential, therefore exists that these isolates, Marinobacter sp., in particular, could influence the Lake Vida brine nitrogen biogeochemistry. The total N2O produced by Marinobacter sp. strain LV411 and Psychrobacter sp. strain LV181 each had a SP value around 0 / that is similar to that reported previously for production by denitrifying bacteria. The net isotope effect values for δ15N, δ18O, and SP for LV411 and LV181 were different from each other, suggesting different fractionation factors along the denitrification pathway. The potential for abiotic denitrification in Lake Vida brine was also demonstrated. The addition of ferrous iron (Fe2+), nitrite (NO2−), or nitrate (NO3−) stimulated N2O production in mercuric chloride (HgCl2) amended Lake Vida brine. A 15N tracer experiment, using Lake Vida brine, revealed a decrease in N2O concentration in several amendments with a decrease in δ15N bulk isotope values, confirming N2O production and suggesting further reduction of N2O to N2. Lake Vida brine treated with 10mM FeSO4 and 10mM NaNO2 had a N2O production rate of 10µM day−1, a final SP value of −1.5 ± 0.5 /, a final δ15N value of 24.9 ± 0.4 /, and a final δ18O value of −14.5 ± 0.1 /. The SP value is within the range of SP values reported from chemodenitrification. The addition of Fe2+, NO2−, or NO3− also stimulated CO2 production in HgCl2 amended Lake Vida brine, likely from acidification of the large reservoir of DIC in Lake Vida brine. The N2O SP values from biological and abiotic sources in Lake Vida were not significantly different and therefore cannot be used to discern a difference between the two sources (Marinobacter sp. strain LV411 was 1.2 ± 0.7 /, Psychrobacter sp. strain LV181 was −1.9 ± 0.8 / and abiotic treatment with 10mM FeSO4 + 10mM NaNO2 was −1.5 ± 0.5 /). A comparison of N2O SP values, along with evidence of possible biological and abiotic N2O production, suggests the source of N2O in Lake Vida brine could be the result of either process or a combination of both biological and abiotic sources.