Microbial nitrous oxide production associated with aquatic invertebrates

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Animal guts provide distinct and abundant microenvironments for microbial nitrogen conversions, which calls for a conceptual and quantitative integration into the overall aquatic microbial nitrogen cycle. We recently discovered that a large variety of aquatic invertebrates emits substantial amounts of the greenhouse gas nitrous oxide (N2O) (Stief et al. 2009). Our initial findings suggested that the preferential emission of N2O instead of N2 was due to incomplete denitrification of ingested microbes in the anoxic animal gut. New findings, however, suggest that N2O production may also take place in microbial biofilms that grow on the exoskeleton of some species. Therefore, the proposed research will investigate the different mechanisms of N2O production associated with aquatic animals. 1) Molecular microbial analysis of the gut contents will test the hypothesis that the oxic-anoxic transition experienced by bacteria when they are eaten leads to the incomplete induction of denitrification. 2) Feeding experiments will reveal whether ingested NO3 --storing diatoms fuel the N2O production in the gut of aquatic invertebrates. 3) Tracer experiments will identify and quantify pathways of N2O production in exoskeletal biofilms. Taken together, these experiments will improve our understanding of a so far overlooked source of N2O from aquatic ecosystems.