The deep ocean remains one of the last frontiers of oceanographic exploration, and in particular, the microbial activity in deep ocean waters is perhaps one of the least understood aspects of the functioning of the biosphere. The existing data and information on dark ocean microbial metabolism are few and scattered, and more importantly, often contradictory.  A variety of approaches have been used to quantify deep-ocean metabolism. This includes the oxygen utilization rate (OUR), a biogeochemical estimate based on large-scale patterns in oxygen distribution, and a number of physiological assays that target either respiration, the electron transport system (ETS) or heterotrophic production of the microbial communities that dominate the overall metabolic activity in deep waters.

Here we analyzed an extensive dataset of published and unpublished data from the northern North Atlantic and critically compare these four approaches to conclude that OURs probably yield the most realistic estimates of remineralization in the deep-waters. In vitro oxygen consumption rates and ETS activities are consistently elevated over OURs, however, based on their relationship and also including particulate organic carbon we suggest that the rates are not an artifact but might indicate the potential of utilizing labile organic matter apparently present in the deep waters of the Atlantic. Alternatively, we hypothesize that the microbial respiration rates may be biased due to the decompression of the dark ocean seawater while sampling with conventional Niskin bottles.