ANACONDAS: Amazon iNfluence on the Atlantic: CarbOn export from Nitrogen fixation by DiAtom Symbioses

The Amazon River plume ecosystem, extending thousands of km from the river mouth, represents a potentially significant but poorly understood, climate-sensitive C sequestration pathway that seems to violate the expectation of an inefficient open-ocean biological pump. The ANACONDAS investigators hypothesize that large tropical river plumes with low N:P ratios provide an ideal niche for diatom-diazotroph assemblages (DDAs). They suggest that the ability of these organisms to fix N2 within the surface ocean is responsible for significant C export in the Amazon River plume. Since primary production fueled by allochthonous sources of N such as N2 fixation can drive a net, biologically mediated transfer of C from the atmosphere to the ocean, C sequestration by DDAs in the Amazon River plume is a regionally significant process. Because DDAs have been found in other tropical river systems, they may represent a globally significant, yet previously overlooked biological pump mechanism.