MICROBIAL COMMUNITIES ACROSS OCEANIC DEPTHS PRODUCE STABLE DISSOLVED ORGANIC NITROGEN

Dissolved organic nitrogen (DON) represents the largest reservoir of fixed nitrogen (N) in the surface ocean. DON is critical for the growth and productivity of marine microorganisms, but also comprises compounds that are unreactive, representing a long-term storage of N. It has been hypothesized that more stable forms of DON (i.e., refractory) may originate from successive microbial reprocessing of bioavailable compounds, a “microbial nitrogen pump” (MNP). However, it is still unknown whether microbial communities from different oceanic depths are equally efficient in producing refractory DON. Using bottle incubations, we tested the MNP by mixing surface dissolved organic matter (DOM) with prokaryotic communities from the epi-, meso- and bathypelagic regions of the Labrador Sea, and tracked changes in DON concentration and composition over time. Our results suggest that DON accumulation was a two-step process, beginning with a conversion of inorganic N to organic N, followed by DON reprocessing into less reactive compounds. This was apparent in the short-term (1 month) and long-term (18 months) production/consumption cycles in fluorescent DOM, as well as in amino acids indices showing an accumulation of labile material before its progressive transformation info more recalcitrant DOM. This first empirical evidence of the MNP using unamended seawater showed that changes in DON composition were consistent in all treatments. Our results suggest that microbial communities across oceanic depths share similar metabolic pathways required for long-term DON sequestration.

Primary Presenter: Richard LaBrie, McGIll (richard.labrie@mcgill.ca)

Authors:

Richard LaBrie, McGill University  (richard.labrie@mcgill.ca)

Nagissa Mahmoudi, McGill University  (nagissa.mahmoudi@mcgill.ca)

Luc Tremblay, Université de Moncton  (luc.tremblay@umoncton.ca)

Jennifer Cherrier, Brooklyn College, City University of NewYork  (Jennifer.Cherrier18@brooklyn.cuny.edu)

Jean-Eric Tremblay, Université Laval  (jean-eric.tremblay@bio.ulaval.ca)

Roxane Maranger, Université de Montréal  (r.maranger@umontreal.ca)