Proteomic assessment of nitrogen assimilation in the North Pacific Subtropical Gyre

Diverse heterotrophic bacteria, cyanobacteria, and eukaryotic picophytoplankton compete for available nitrogen (N) in the oligotrophic surface ocean but differ in their preferences for and responses to various N substrates. Despite the important role that N limitation has in driving physiological adaptions to oligotrophic conditions, the cellular fate of newly assimilated N in these regions remains a key open question. We used 15N isotopic labeling and novel proteomics methods to track the assimilation of multiple organic and inorganic nitrogen substrates into specific proteins and taxa of the surface microbial community at Station ALOHA in the North Pacific Subtropical Gyre during short-term incubations. We found substantial incorporation of label into peptides from all three domains within 11 hours of incubation at in situ conditions. 15N-labeled peptides mapped to proteins representing a broad range of COG functional categories, with proteins related to amino acid and carbohydrate transport and metabolism having the highest levels of incorporation. Individual taxa showed a spectrum of responses with regard to substrate use from specialized to generalized and allocation to diverse metabolic functions. These findings provide direct connections between chemically distinct N substrates and cellular allocation to metabolic processes in specific members of the microbial community in the oligotrophic ocean.