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The majority of bacterial growth and respiration in the upper ocean is thought to result from coupling between microheterotrophic populations and the reactive soluble components of planktonic primary and secondary production. However, we know little about the potential turnover of these components and the concomitant growth of bacteria under conditions of intermittent or transient inputs of natural dissolved organic matter (DOM) compared to quasi-steady state, low DOM conditions. The present study evaluated the short-term (-3 d) rates and net extents of utilization (as measured losses) of selected constituents of plankton-derived DOM (DOM PD ) by indigenous bacterioplankton populations in eastern North Pacific surface waters, and assessed bacterial growth efficiencies (BGE) during temporarily non-limiting DOM conditions. Approximately 28% of the starting dissolved organic carbon (DOC) and 34 % of the dissolved organic nitrogen (DON) in incubations supplemented with DOM PD could be characterized as dissolved free and combined amino acids (DFAA and DCAA, respectively) and monosaccharides (MCHO). Up to 31 % of the added DOC and 26 % of the added DON was utilized in +DOM PD incubations; however, BGE under supplemented conditions (∼4 to 5%) was similar to estimates for ambient oligotrophic waters. Of the net DOC consumed, 75 % was accounted for by DFAA (which alone was 61 % of the total), DCAA, and MCHO, while the remaining non-characterizable 25 % may represent an inherent or rapidly formed component of lower reactivity. In contrast to DOC, net DON utilization was supported entirely by DFAA and DCAA, with DFAA alone accounting for the vast majority (up to 99%). Together, DCAA and MCHO accounted for only -13 % of the DOC consumed and ≤∼5 % of the DON (i.e. as DCAA) utilized. These findings are consistent with bacterial growth in the open ocean being controlled predominantly by inputs of a small fraction of bulk DOM, and further suggest that bacteria may function primarily as remineralizers even during transient periods where labile DOC and DON is relatively available.

Bacterial utilization of transient plankton-derived dissolved organic carbon and nitrogen inputs in surface ocean waters