Testing the priming effect in the deep ocean: are microorganisms too starved to consume recalcitrant organic carbon?

Deep ocean dissolved organic carbon (DOC) is one of the largest pools of reduced carbon on Earth. Many DOC compounds escape microbial degradation and persist for thousands of years in the ocean. Although many hypotheses have been proposed, the mechanisms responsible for this long-term stability remain unresolved. Heterotrophic microorganisms in the deep ocean are energetically starved and exhibit low metabolic activity. Here, we investigated whether the severe energy limitation in deep-sea environments acts as a barrier to microbial degradation of DOC. We hypothesized that alleviating this energetic barrier through the addition of labile compounds (i.e., the priming effect) could stimulate microbial consumption of DOC. We conducted 62-day bottle incubations with deep seawater from the Southern Ocean that were amended with simple organic carbon, nitrogen, and/or phosphorus-containing compounds. We tracked DOC concentration, cell abundance, and microbial community structure over the course of the experiment. Our results show no evidence of a priming effect regardless of the priming compound. Interestingly, priming compounds were selected for distinct microbial populations even when the compounds were chemically similar.