9416634 Bronk The dissolved organic nitrogen (DON) pool is emerging as a dynamic component of marine nitrogen cycles. Though DON can represent the largest pool of fixed nitrogen in marine systems, the lack of suitable methods has kept information on its utilization largely limited to a few individual organic compounds such as amino acids or urea. Despite the lack of knowledge on utilization of the bulk of the undefined DON pool, bacteria are considered the primary users of DON in marine systems (with the exception of urea). Recent work in phytoplankton enzymology, however, suggests this assumption needs to be reevaluated. Some phytoplankton are now known to posses cell surface oxidases which allow them to cleave amino groups from amino acids, primary amines, and potentially other organic compounds. The phytoplankton known to posses these oxidases are smaller autotrophs which could potentially be included in the "bacterial" size fraction in traditional size fractionation experiments. This Row Planning Grant project will combine the powerful tools of flow cytometry and newly developed 15N tracer techniques to discriminate autotrophic from heterotrophic uptake of DON. The objectives of this Planning Grant are to (1) refine existing cell sorting techniques for separating autotrophs from heterotrophs in large enough numbers to allow the measurement of 15N atom % enrichments and thus allow the measurement of DON uptake rates by autotrophs, and (2) to obtain preliminary data on uptake of DON by autotrophs in the Eastern Tropical North Pacific to be used in support of a full scale NSF proposal. ***
