AN INVESTIGATION OF PEROXIDE, IRON AND IRON BIOAVAILABILITY IN IRRADIATED MARINE WATERS

Abstract

An investigation was completed which contributes to the understanding of chemical dynamics in the marine photic zone. The research was directed toward a description of natural photochemical processes which influence hydrogen peroxide (HOOH) distributions, the oxidation-reduction chemistry of iron in surface seawater, and the availability of iron to phytoplankton. An enzyme catalyzed fluorometric method based on the dimerization of phydroxyphenylacetic acid (POHP AA) was developed for analysis of HOOH in seawater. Preconcentration of the fluorescent product provided a limit of detection for HOOH of 0.2 nanomolar (nM). Observed changes in reaction stoichiometry were described with a kinetic competition model involving POHP AA and naturally occurring organic material, identifying a potential interference for all enzyme-based HOOH methods. The selective decomposition of HOOH by the enzyme catalase was studied using laboratory studies with peroxyacetic acid and a kinetic model of interactions among catalase, HOOH, and organic peroxides. Both approaches show significant organic peroxide decomposition due to interactive reactions with HOOH in mixed solutions. Field studies show peroxide in Narragansett Bay from 50 nM at 0800 hr LT to 200 nM at 1600 hr LT. Peroxide levels in the Sargasso Sea were above 100 nM with diel changes of 40 nM. Rain was a significant source of HOOH to the Sargasso Sea and vertical distributions of HOOH in coastal and open ocean waters reflect vertical stratification. Laboratory irradiations of coastal seawater resulted in linear HOOH accumulation ranging from 30 to 57 nM hr-1 and Fe(II) values from 0.5 to 10 nmole/kg depending on pH and total iron concentration. The apparent oxidation rates measured for photochemically produced Fe(II) were 2 to 7 times slower than expected. Kinetic modeling described the observed Fe(II) variations using pH dependent oxidation rates and Fe(II) photoproduction rates of about 6 nmole/kg hr-1 (pH= 6.5 to 8.0). Fe(II) oxidation by HOOH is substantial. The diatom Skeletonema costatum was cultured in previously irradiated seawater containing ferric colloids with different bioavailability. Irradiation of iron in seawater which previously supported only limited cell populations caused increased growth. Calculations suggest that most Fe(II) photoproduction in coastal waters represents cycling within a bioavailable pool.