Abstract:
Planktonic protozoan grazers have the potential to significantly affect the chemistry of
particle-associated trace metals. This is due both to the importance of protists as
consumers of bacterial-sized particles, and to the unique low-pH, enzyme-rich
microenvironment of the grazer food vacuole. This thesis examines the role of protozoan
grazers in the marine geochemistry of strongly hydrolyzed, particle-reactive trace metals,
in particular Th and Fe.
A series of tracer experiments was carried out in model systems in order to determine
the effect of grazer-mediated transformations on the chemical speciation and partitioning
of radioisotopes C9Fe, 234Th, 51Cr) associated with prey cells. Results indicate that
protozoan grazers are equally able to mobilize intracellular and extracellular trace metals.
In some cases, protozoan regeneration of trace metals appears to lead to the formation of
metal-organic complexes. Protozoan grazing may generate colloidal material that can
scavenge trace metals and, via aggregation, lead to an increase in the metal/organic
carbon ratio of aggregated particles.
Model system experiments were also conducted in order to determine the effect of
grazers on mineral phases, specifically colloidal iron oxide (ferrihydrite). Several
independent techniques were employed, including size fractionation ors9Fe-labeled
colloids, competitive ligand exchange, and iron-limited diatoms as "probes" for
bioavailable Fe. Experimental evidence strongly suggests that protozoan grazing can
affect the surface chemistry and increase the dissolution rate of iron oxide phases through
phagotrophic ingestion. In further work on protozoan-mediated dissolution of colloidal Fe oxides, a novel
tracer technique was developed based on the synthesis of colloidal ferrihydrite
impregnated with 133Ba as an inert tracer. This technique was shown to be a sensitive,
quantitative indicator for the extent of ferrihydrite dissolution/alteration by a variety of
mechanisms, including photochemical reduction and ligand-mediated dissolution. In
field experiments using this technique, grazing by naturally occuring protistan
assemblages was shown to significantly enhance the dissolution rate of colloidal
ferrihydrite over that in non-grazing controls. Laboratory and field results indicate that,
when integrated temporally over the entire euphotic zone, protozoan grazing may equal or
exceed photoreduction as a pathway for the dissolution of iron oxides.
