What was done
The authors review our current understanding of the contributions of various biotic components and depth layers to respiration in the open ocean.  Specifically, they examine estimates of microplanktonic respiration near the surface of the seas, in the mesopelagic layer, the ocean interior, including the benthic communities on the ocean floor, as well as respiratory contributions from metazoan zooplankton and larger metazoans in the water column.

What was learned
The authors begin their review by noting, in the first sentence of their abstract, that "a key question when trying to understand the global carbon cycle is whether the oceans are net sources or sinks of carbon."  In attempting to answer this question in the course of their review, however, they are forced to conclude, as expressed in the last sentence of their abstract, that "whether the biota act as a net source or sink of carbon remains an open question."

What it means
The authors rightly state that "we cannot claim to grasp the global carbon cycle when we do not know whether the biota of the world's oceans is a net source or sink for carbon."  And if we do not understand the global carbon cycle, even qualitatively, how can we possibly presume to order up a proper prescription for fixing what we do not even know to be broken?