Reduced isotope fractionation by denitrification under conditions relevant to the ocean

Experiments with two well-studied denitrifiers and one recently isolated marine suboxic zone denitrifier show that the cellular-level denitrification N isotope effect ((15)epsilon) is typically lower than the canonical value of similar to 25 parts per thousand under many conditions prevalent in the ocean. Across all three strains, (15)epsilon is 10-15 parts per thousand at cellular nitrate reduction rates that are more representative of the environment than the very high rates under which we and previous investigators measure (15)epsilon to be 20-30 parts per thousand. A sharp decrease in (15)epsilon is also observed in individual nitrate drawdown assays as the extracellular nitrate concentrations approach 2-35 mu M and nitrate uptake becomes the rate-limiting step. On an apparently strain-specific basis, lower values of (15)epsilon are observed under diverse conditions common in the natural environment: less reduced carbon sources, small inputs of oxygen, nutrient availability, agitation, and age of starter culture (i.e., initiation of assays with cells that had recently depleted a large previous nitrate amendment or were more recently in the exponential growth (bloom) phase). A conserved oxygen-to-nitrogen isotope relationship across the experiments for all three denitrifiers ((18)epsilon/(15)epsilon = 0.93 +/- 0.06 (1SD)) supports the interpretation that fractionation is imparted solely by the internal respiratory nitrate reductase, with the amplitude of (15)epsilon varying with the proportional importance of cellular nitrate efflux relative to uptake. Aspects of the (15)epsilon variation are unexpected; nevertheless, the occurrence of lower (15)epsilon is robust. It is uncertain if our lower (15)epsilon estimates apply to oceanic water column denitrification because field studies have generally yielded (15)epsilon(we) between 20-30 parts per thousand, more similar to previous culture estimates and our estimates at high cell specific nitrate reduction rates. If denitrification in the ocean's major suboxic zones does have an (15)epsilon of similar to 10-15 parts per thousand, it would remove an apparent imbalance between global ocean N inputs and outputs previously suggested by fixed N isotope budgeting. (C) 2012 Elsevier Ltd. All rights reserved.