A biologeochemical and molecular ecological study of aerobic methane oxidation and nitrification in Mono Lake California, USA
Carini, Stephen A.
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Methane (CH4) and ammonia (NH3) oxidation play key roles in the biogeochemical cycles of carbon (C) and nitrogen (N). In this dissertation, biogeochemical and molecular analyses were combined to elucidate environmental and microbiological controls on these processes and link specific activity with microbial community composition in Mono Lake, CA, a hypersaline, alkaline lake located just east of the Sierra Nevada range in northern California. Aerobic CH4 oxidation varied spatially and temporally coincident with a shifting microaerophilic zone associated with seasonal stratification. Sequence analysis indicated slight shifts in methanotroph community composition and stable absolute cell numbers. Variable CH4 oxidation rates in the presence of a relatively stable methanotroph population suggested that zones of high CH4 oxidation resulted from an increase in activity of a sub-set of the existing methanotroph population. Nitrification activity and NH3 oxidizer community composition were also assessed. Geochemical profiles and rate measurements indicated nitrifying activity. Bacterial NH3 oxidizer abundance varied seasonally but absolute cell numbers remained consistent over depth for each sample date while Crenarchaeota abundance was strongly correlated between with rate measurements. However, the lack of verifiable archaeal amoA gene copies and the presence of sufficient AOB cell numbers to physiologically account for all measured nitrification preclude specific assignment of nitrifying activity to Mono Lake Crenarchaeota, although a potential AOA contribution to nitrification in Mono Lake cannot be ruled out. Enrichment experiments identified interactions between methane oxidation and nitrification. The highest CH4 oxidation rates occurred in the NH4++CH4 enrichment, inferring that methanotrophs benefited from nitrogen (N) addition. FISH and DGGE analysis demonstrated that the methanotrophs in the CH4-only enrichment were able to take advantage of elevated CH4 through community composition adaptation. Crenarchaeota abundance correlated with NOX accumulation and nitrification activity while AOB abundance decreased, suggesting that ammonia-oxidizing archaea (AOA) may have contributed to nitrification in the NH4+- only enrichment. In contrast, there was a decrease in crenarchaeal abundance and an increase in AOB abundance in the CH4+NH4+ enrichment that correlated with increased nitrification activity. The reversal in the correlation between AOB and nitrification relative to that observed in the NH4+- only enrichment plus the significant decline in crenarchaeal abundance in the CH4+NH4+ enrichment suggests that AOB were likely more active in nitrification in the presence of CH4 and/or methanotrophs.