Escape by dilution

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Science  17 Apr 2015:
Vol. 348, Issue 6232, pp. 290
DOI: 10.1126/science.aaa9852

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Earth's oceans contain as much carbon in the form of dissolved organic matter (DOM) as does the biosphere, and more than 200 times that of living marine biomass. Most of the DOM is in the deep sea below 1000 m. Radiocarbon data show that the bulk of the DOM is thousands of years old (1). This long residence of DOM in the deep ocean is intriguing: Prokaryotes (bacteria and archaea) are abundant and active in the deep ocean, and many of them require DOM for energy and carbon. Moreover, molecular biology data show high metabolic diversity in the deep ocean (2). Why does some DOM escape degradation in the deep sea? Nutrient limitation of consumer biomass may explain underutilization of resources in nutrient-low surface waters (3), but this does not apply to the nutrient-rich deep sea. On page 331 of this issue, Arrieta et al. (4) show that DOM is too dilute to be consumed.