Abstract:
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. Mo is used in both nitrogenase, the enzyme
responsible for Nz fixation, and nitrate reductase, which catalyzes assimilatory and
dissimilatory nitrate reduction. Laboratory culture work on two Nz fixing marine
cyanobacteria, Trichodesmium and Crocosphaera, and a marine facultative denitrifier,
Marinobacter hydrocarbanoclasticus, showed that Mo cell quotas in these organisms
were positively correlated with Mo-containing enzyme activity. Mo concentrations in
Crocosphaera dropped almost to blank levels when not fixing Nz suggesting daily
synthesis and destruction of the entire nitrogenase enzyme and release of Mo.
Trichodesmium cultues, however, retained a pool of cellular Mo even when not fixing
Nz. Colonies of Trichodesmium collected in the field have Mo:C tenfold higher than seen
in culture, these Mo:C ratios were reflected in SPM samples from the same region. Fe:C
ratios for Trichodesmium were between 12-160 μmol:mol in field and cultured samples.
The Fe:C ratio of Crocosphaera was established to be 15.8±11:3 under Nz fixing
conditions. 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 Nz fixation and denitrification.
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. Mn oxides were not found to significantly influence either the dissolved or SPM
Mo distribution. Dissolved Mo profiles from the Sargasso and Arabian Sea were
conservative. However, dissolved Mo profiles from 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. A more focused study of water
column response to sediment fluxes using the high precision Mo analyses is necessary to
determine whether these phenomena are related.
