Potential foraminiferal nitrate transport in sediments in contact with oxic overlying water

Abstract

AbstractSedimentary denitrification accounts for more than half of the global marine fixed nitrogen loss, but this removal is often restricted by nitrate diffusive supply in sediments. The results of this study showed that foraminiferal nitrate transport may supply nitrate below the nitrate penetration depth and promote nitrogen removal in sediments in contact with oxic overlying water. Six dominant foraminiferal species in the Yellow Sea sediments accumulated high concentrations of intracellular nitrate (25–68 mM), which is 1000 times higher than the maximum pore water nitrate concentration. Notably, this is the first time that Protelphidium tuberculatum, the most dominant species at almost all stations, is reported to have elevated intracellular nitrate. P. tuberculatum contributes 45% to 92% of the foraminiferal intracellular nitrate pools and plays an important role in foraminiferal nitrate transport. The potential foraminiferal denitrification rates below the nitrate penetration depth induced by foraminiferal nitrate transport ranged from 4 to 117 μmol N m−2 d−1 and contributed 3% to 81% of sedimentary denitrification. The intensity of foraminiferal nitrate transport showed a negative correlation with the nitrate penetration depth and positively correlated with the chlorophyll a (Chl a) and chloroplastic pigment equivalent concentrations, which indicate that shallower redox depth (nitrate penetration depth) and increased food supply (Chl a, chloroplastic pigment equivalent) may promote foraminiferal nitrate transport. The trophic oxygen model was modified by adding the pore water nitrate to explain foraminiferal nitrate transport in various marine environments, and this modified trophic oxygen model predicts the observed high foraminiferal nitrate transport intensity.