Contribution of Carbon Fixation Toward Carbon Sink in the Ocean Twilight Zone

Dark carbon (C) fixation in the ocean twilight zone plays a crucial role toward C sink, but its potential has not been tested sufficiently in experiments. Here we analyzed dark C fixation in the twilight zone of the Arabian Sea along with the primary production measurements in the euphotic zone. The average dark C fixation rates in the suboxic oxygen minimum zone (OMZ) waters were higher than that in the hypoxic OMZ waters, which could be explained by the preferential existence of chemoautotrophic ammonium oxidizers and anammox bacteria owing to NO<inf>2</inf>⁻ maxima in the suboxic OMZ waters. This study supports a previous hypothesis of significant contribution of dark C fixation to sinking C fluxes in the OMZ of the Arabian Sea. Extrapolation of the measured dark C fixation rates to the global ocean ranged up to 7.4 Pg C y⁻¹; amounting to ∼15% of the global ocean primary production.