For nearly 50 years, marine chemists have generally accepted that when microscopic organic compounds in seawater, known as dissolved organic carbon (DOC), sank into the deep ocean below 1000 m in depth, they became resistant to microbial degradation (recalcitrant). Once in the deep ocean, DOC can remain there for hundreds or even thousands of years. As a result, while most organic carbon cycles rapidly through nature, DOC in the deep ocean accumulates and only slowly is recycled to living things, the soil, and the air. Why this deep-ocean DOC is so stable has been uncertain, but it seemed possible that marine microorganisms could not metabolize the chemical structures of the DOC. The observation led some scientists to propose methods for mitigating global warming by sequestering carbon as DOC in the deep ocean. However, recent research indicates that such efforts might not be very helpful. See also: Biogeochemistry; Ecosystem; Global warming; Marine ecology; Marine microbiology; Marine sediments; Ocean; Seawater

A team of scientists funded by the Spanish National Research Council (CSIC) reported in the journal Science (March 2015) that DOC in the deep ocean is not recalcitrant; instead, it consists of the leftovers from degradable materials at concentrations so low that the microorganisms in the deep ocean are unable to use them efficiently as an energy source. Samples of deep ocean water were collected in the Tropical Eastern Pacific, the Tropical Western Atlantic, and the North Atlantic at depths ranging from 1000 to 4200 m. As experiments on the samples demonstrated, the DOC concentrations at those depths were too dilute to support bacterial growth. When the DOC was concentrated (2–10 times greater than the ambient concentration), bacteria consumed it readily and multiplied. The results suggest that attempts to sequester carbon in the deep ocean as DOC could be self-defeating: as the concentration of DOC rose, deep sea bacteria would exploit it and return it to the organic carbon cycle.