Testing the priming effect as a mechanism explaining DOC drawdown in the deep ocean

The deep ocean dissolved organic carbon (DOC) pool represents one of the largest reservoirs of fixed carbon (C) on the planet. With an average radiocarbon age of several thousand years, the DOC pool appears to be mostly inert. Yet, there is a concentration gradient across ocean basins that cannot be explained with a slow and steady removal process, suggesting a localized drawdown of DOC by microbial populations. Here, we used Antarctic circumpolar deepwater to test the hypothesis that episodic addition of labile DOC “primes” microbial community uptake, resulting in deep sea DOC drawdown. We conducted bottle incubations using an array of different organic compounds, including organic N- or P-containing substrates, to determine if the presence of nitrogen or phosphorus could further stimulate DOC consumption. We monitored DOC, dissolved organic N and dissolved organic P concentrations over 2 months to assess if a priming effect could be detected and how it would impact dissolved organic matter stoichiometry. Simultaneously, we tracked cell density and taxonomic composition to detect whether addition of priming substrates induced shifts in microbial community structure. We observed differential responses in the microbial communities and DOC degradation dynamics, but no clear indication of a priming effect. Our results suggest that deep ocean DOC degradation is unlikely to be primed by labile compounds and that other mechanisms are required to explain DOC drawdown.

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

Authors:

Richard LaBrie, McGill University  (richard.labrie90@gmail.com)

Corday Selden, Rutgers University  (crselden@marine.rutgers.edu)

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