Spinning carbon and sinking phosphorus: Misaligned cycles in the sea

Each year, the ocean absorbs more than 2 Pg of anthropogenic CO2, roughly a third of human emissions (1), but the climatic value of this uptake depends on how long that carbon remains isolated from the atmosphere. Proposals to fertilize large ocean regions aim to enhance photosynthesis and transfer newly formed organic matter into the interior sea. A central metric for judging the effectiveness of such strategies is therefore the duration of storage (2). Yet, export is sustained, and ultimately limited, by essential nutrients that are recycled by marine organisms and later returned by circulation. How quickly these nutrients are returned sets the pace at which additional carbon can be captured again. Sullivan et al. (3) use an inverse biogeochemical ocean circulation model to examine time scales of carbon and phosphorus cycling, finding that carbon is remineralized and returned to the surface ocean faster than the essential nutrient phosphorus. Misalignment in timing of these cycles has important potential consequences for fertilization-based marine CO2 removal proposals.

The ocean functions as a massive bioreactor where diverse microorganisms catalyze the flow of energy and the cycling of matter. In the sunlit upper ocean, photosynthetic microbes assimilate CO2 together with inorganic nutrients (e.g., nitrogen, phosphorus), producing ~55 Pg C yr−1 of organic matter, nearly half of global net primary production. Most of the recently produced organic matter is consumed through upper ocean food webs, leaving a relatively small fraction (5 to 20%) available for export into the interior ocean. Export can occur through various mechanisms, including sinking particles, vertically migrating animals, and transport by physical circulation (4). Once in the interior waters, exported organic matter fuels the metabolism of diverse consumers, recycling CO2 and nutrients. These recycled byproducts are eventually returned to the upper ocean by circulation.

Time scales of ocean carbon storage depend critically on the form of organic matter produced. While material packaged into particles can settle rapidly to depth via gravitational sinking, dissolved carbon is transported by circulation. A large fraction (50% or more) of newly produced primary production flows through highly reactive dissolved organic matter pool, supporting the metabolism of abundant and diverse heterotrophic bacteria (5). Although only a modest fraction of primary production is exported via sinking particles, particles transport this carbon into reservoirs that are isolated from atmospheric exchange for decades or longer. The larger share of primary production routed into dissolved organic matter gets remineralized at depths where waters reconnect with the surface on far shorter timescales (<1 y). Hence, biology is doing more than just moving carbon out of atmosphere and into the interior ocean: It is also sorting organic matter into different pools, each with distinct lifetimes.