A carbon for every nitrogen.

Biology and the environment interact, one shaping the other (1). In the oceans, the chemistry of seawater and the chemistry of life are intimately linked (2). In 1958, Alfred Redfield (3) noted that the microscopic plankton of the surface ocean contain carbon, nitrogen, and phosphorous atoms in a stoichiometry of ∼105:16:1 and that as these organisms sink and decay, the deep waters of the ocean become enriched in carbon, nitrogen, and phosphorous at the same ratio. This marked the beginnings of ecological stoichiometry, a growing field that is providing novel insight into the ecology and elemental cycles of the planet (4). A study in PNAS provides a new stoichiometric link, reporting that for every nitrogen consumed in the surface Atlantic Ocean, 1.12 carbons are converted from CO2 to dissolved organic carbon (DOC) (5). Carbon sits at the center of the elemental cycles. It is the backbone of the organic molecules that are the principal currency and building blocks of life. Carbon dioxide is the main anthropogenic greenhouse gas responsible for climate change (6). As phytoplankton grow in sunlit surface waters, they incorporate inorganic carbonate from seawater into organic molecules. Much of the organic carbon produced during photosynthesis is rapidly returned to the inorganic pool via respiration, with a small fraction accumulating as net community production (NCP). The carbonates incorporated by phytoplankton are the dissolved equivalent of atmospheric CO2, and, as carbonates are depleted in seawater, they are replenished by inputs of CO2 from the atmosphere. When the organic carbon produced by NCP is transported into the depths of the ocean, it provides a sink for atmospheric CO2 that is termed the biological carbon pump (7). Export to depth in the oceans can be in the form of particulate organic carbon or DOC. Organic particles that sink either are remineralized as they pass through the abyss, relinquishing their carbon back to the water, or survive to be buried at the ocean floor, potentially locking away carbon from the atmosphere for millennia. Other organic molecules dissolve into seawater and are collectively quantified as DOC. The diverse molecules that comprise the DOC pool provide sustenance for microbes and constitute significant global stores of carbon in the deep ocean (8). With respect to the latter, radiocarbon dating, natural isotopes, and chemical fingerprinting indicate that the pool of DOC within the ocean represents the NCP of ancient phytoplankton, which, over thousands of years, has accumulated to represent one of the largest organic carbon stores on Earth (9). The organic molecules composing the DOC pool have a C:N of 14 (10), compared with a Redfield C:N of 6.6 for plankton (3). Thus, the C:N of dissolved organics is approximately double the C:N of plankton, making DOC a nitrogenefficient means to sequester carbon in the deep ocean. In the past, changes in the size of the deep ocean DOC store may have driven changes in atmospheric CO2 and paleoclimate (11, 12). Today, the deep ocean DOC store and the atmospheric load of CO2 are of similar magnitude (13). The supply of carbon to the deep relies on phytoplankton and sunlight at the surface, where changes to ocean ecology could influence DOC production, export, and storage over timeframes of relevance to contemporary climate change. However, assessingwhether marine DOC will act as a sink for atmospheric CO2 has been hampered by the lack ofmechanisticmodels that predict DOC accumulation in surface waters. Fig. 1. New production in the surface ocean leads to the accumulation of 1.12 moles of dissolved organic carbon per mole of nitrate utilized.