Biogeochemical roles of planktonic archaea

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Archaea, and specifically the Crenarchaeota, dominate microbial assemblages in deep waters throughout virtually every ocean basin, but little is known of their biogeochemical roles. Recently, the presence of an archaeal-associated ammonia monooxygenase (amo) gene indicated that archaea may be capable of ammonia oxidation, the first step of chemoautotrophic nitrification. Cultivation of the first non-extremophilic marine Crearchaeote in 2005 confirmed this possibility, as Nitrosopumilus maritimus is capable of ammonia oxidation in culture and carries an archaeal amoA gene. Moreover, this gene appears to be widespread in the marine environment, suggesting many archaea are capable of ammonia oxidation. This project will evaluate these new insights with field observations and experimental approaches. Broadly, it will address the role of archaea in marine ammonia oxidation, or in nitrogen and carbon cycling in general. More specifically, it will examine whether all or most Crenarchaeota capable of this reaction and to what extent they are also heterotrophic or mixotrophic, will measure archaeal oxidation rates in field situations, and will examine population dynamic of archaea. The PI will combine a time series approach with molecular techniques and biogeochemical measurements and experiments. The underlying hypothesis is that a significant proportion of Crenarchaeota are capable of ammonia oxidation and perform the reaction when necessary substrates are available, but that many of them are also capable of utilizing dissolved organic matter, and are therefore mixotrophic. The study will be at the San Pedro Ocean Time-series (SPOT) site, where an ongoing independently-funded study already has 5 years of relevant data. By the time the project is completed, a 7+ year time series will be compared with accompanying biogeochemical data that are collected as part of SPOT. Broader impacts of the project include postdoctoral training, a significant connection to K-12 teaching through USC's COSEE West program, undergraduate teaching and research, and outreach that will include a website with sections for the general public.