BIOLOGICAL TRANSPORT OF NITRATE WITHIN SEDIMENTS UNDERLYING OXYGEN DEFICIENT ZONES: A NEW TWIST IN THE N CYCLE

Benthic denitrification appears to be responsible for most of the fixed inorganic nitrogen losses from the oceans, but contributing processes are neither well quantified nor understood. We have investigated benthic nitrogen cycling in three semi-enclosed basins of the California Borderlands: Santa Monica, San Pedro and Santa Barbara. Deep water in these basins is separated from the open ocean by sills of various depths, contributing to the low [O₂], 2 to10 uM.

A common feature observed in pore waters of each of these basins is a presence of a subsurface nitrate peak (with [NO₃] up to 300 uM) at depths up to 6 cm below the sediment-water interface. Mass balances between ammonium and oxygen fluxes exclude the possibility of in situ aerobic nitrification. Instead, we propose that the “peaks” represent an intracellular nitrate pool, released during the pore water extraction. The bacteria Thioploca and Beggiatoa found in this region are known to accumulate up to 500 mM of nitrate intracellularly. They transport nitrate into the sediments where it may be used for H₂S oxidation. Measured d¹⁵N and d¹⁸O of the “peak” nitrate increase from the bottom water values of 8‰ and 5‰ to 27‰ and 25‰ respectively. Concurrent increase of the two ratios implies an enzymatic transformation of the subsurface nitrate pool. We have also measured increases in pore water [N₂] of 25 to 55 uM above that of bottom water. Results of the reaction-diffusion model show that N₂ production at depth exceeds, by factors of 1.5 to 3, the maximum denitrification rate that could be supported by downward nitrate diffusive flux. The [N₂] increase is accompanied by an increase in the d¹⁵N-N₂ from ~0.6‰ to ~1.2‰ at depth. The d¹⁵N of the subsurface N₂ source is calculated to be 7‰ to 9‰, within the range of the bottom water nitrate d¹⁵N. Model calculations based on data show that 40% to 70% of the nitrate flux into the sediments may be due to a non-local bacterial transport rather than classic diffusion supported denitrification. The d¹⁵N of 15-18‰ of the pore water NH₄ (~ 7-10‰ heavier than in situ sedimentary OM) at the depth of the nitrate peaks and N₂ production suggests that some of the transported nitrate may be used in the Anammox reaction. This supports a previously proposed hypothesis of symbiosis between the nitrate transporting vacuolated organisms and the Anammox bacteria.