The Marine Biogeochemistry of Molybdenum

Caroline Tuit, Ph.D., 2003
Gregory Ravizza, Advisor

Prevailing wisdom holds that the vertical distribution of molybdenum (Mo) in the open ocean is conservative, despite Mo's important biological role and association with Mn oxides and anoxic sediments. A high precision (±0.5%) isotope dilution ICP-MS method for measuring Mo was developed to re-evaluate the marine distribution of Mo in the dissolved and particulate phase. Dissolved Mo profiles of the Eastern Tropical Pacific showed both depletion and enrichment of dissolved Mo possibly associated with interaction of Mo with coastal sediments. Dissolved Mo profiles in several California Borderland Basins showed 1-2 nM Mo depletions below sill depth. Mn oxides were not found to significantly influence either the dissolved or SPM Mo distribution. Mo is used in both nitrogenase, the enzyme responsible for N2 fixation, and nitrate reductase, which catalyzes assimilatory and dissimilatory nitrate reduction. Experiments examining Mo cellular concentrations and Mo:C ratios in two N2 fixing cyanobacteria and a marine denitrifier showed a positive correlation with Mo-containing enzyme activity. Mo cellular concentrations in cultured organisms were too small to significantly influence dissolved Mo distributions, but may slightly affect Suspended Particulate Matter (SPM) distributions. Mean SPM Mo:C ratios were slightly elevated in regions of N2 fixation and denitrification.