Molybdenum isotope fractionation by cyanobacterial assimilation during nitrate utilization and N₂ fixation

We measured the delta 98Mo of cells and media from molybdenum (Mo) assimilation experiments with the freshwater cyanobacterium Anabaena variabilis, grown with nitrate as a nitrogen (N) source or fixing atmospheric N-2. This organism uses a Mo-based nitrate reductase during nitrate utilization and a Mo-based dinitrogenase during N-2 fixation under culture conditions here. We also demonstrate that it has a high-affinity Mo uptake system (ModABC) similar to other cyanobacteria, including marine N-2-fixing strains. Anabaena variabilis preferentially assimilated light isotopes of Mo in all experiments, resulting in fractionations of -0.2 parts per thousand to -1.0 parts per thousand +/- 0.2 parts per thousand between cells and media (epsilon(cells-media)), extending the range of biological Mo fractionations previously reported. The fractionations were internally consistent within experiments, but varied with the N source utilized and for different growth phases sampled. During growth on nitrate, A. variabilis consistently produced fractionations of -0.3 +/- 0.1 parts per thousand (mean +/- standard deviation between experiments). When fixing N-2, A. variabilis produced fractionations of -0.9 +/- 0.1 parts per thousand during exponential growth, and -0.5 +/- 0.1 parts per thousand during stationary phase. This pattern is inconsistent with a simple kinetic isotope effect associated with Mo transport, because Mo is likely transported through the ModABC uptake system under all conditions studied. We present a reaction network model for Mo isotope fractionation that demonstrates how Mo transport and storage, coordination changes during enzymatic incorporation, and the distribution of Mo inside the cell could all contribute to the total biological fractionations. Additionally, we discuss the potential importance of biologically incorporated Mo to organic matter-bound Mo in marine sediments.