Reduced isotope fractionation by denitrification under conditions relevant to the ocean

Abstract

Experiments with two well-studied denitrifiers and one recently isolated marine suboxic zone denitrifier show that the cellular-level denitrification N isotope effect (15ε) is typically lower than the canonical value of ∼25‰ under many conditions prevalent in the ocean. Across all three strains, 15ε is 10–15‰ at cellular nitrate reduction rates that are more representative of the environment than the very high rates under which we and previous investigators measure 15ε to be 20–30‰. A sharp decrease in 15ε is also observed in individual nitrate drawdown assays as the extracellular nitrate concentrations approach 2–35μM and nitrate uptake becomes the rate-limiting step. On an apparently strain-specific basis, lower values of 15ε 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 (18ε/15ε=0.93±0.06 (1SD)) supports the interpretation that fractionation is imparted solely by the internal respiratory nitrate reductase, with the amplitude of 15ε varying with the proportional importance of cellular nitrate efflux relative to uptake. Aspects of the 15ε variation are unexpected; nevertheless, the occurrence of lower 15ε is robust. It is uncertain if our lower 15ε estimates apply to oceanic water column denitrification because field studies have generally yielded 15εwc between 20–30‰, 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 15ε of ∼10–15‰, it would remove an apparent imbalance between global ocean N inputs and outputs previously suggested by fixed N isotope budgeting.