Migrant solution to the anammox mystery.

Every night a massive migration takes place in ocean waters. Zooplankton and micronekton, a diverse group of organisms <1 mm to 10 cm in size, swim up from midwater depths (200–700 m) to the surface ocean to feed and return down to resting depths before daybreak. The importance of this diel vertical migration to marine biogeochemical cycles has been recognized for some time. Research has primarily focused on the migrant-mediated movement of elements including nitrogen (N) from the surface ocean to midwaters. Such an “active transport” involves feeding at night in the upper ocean and the consequent excretion of ammonium (1) and dissolved organic N (2, 3) at midwater depths during the day. However, within the midwater realm, the effect of such migratory behavior on elemental cycles is less well understood. This “twilight zone” hosts many of the reactions that drive global biogeochemical cycles, including the remineralization of particulate organic matter into nutrients such as nitrate and the production of dinitrogen (N2) gas. In PNAS, Bianchi et al. (4) significantly advance our knowledge of the role of animals in midwater biogeochemistry by linking the activity of migrant zooplankton and micronekton to an important N transformation pathway, anammox.