Date of Award

2017

Document Type

Open Access Dissertation

Department

Chemistry and Biochemistry

Sub-Department

College of Arts and Sciences

First Advisor

Timothy Shaw

Abstract

Biogeochemical cycles in ecosystems regulate the flow of energy between reduced species (typically carbon compounds) and a variety of oxidants via both biotic and abiotic reactions. A key class of chemical compounds that can link these cycles through abiotic pathways are Reactive Oxygen Species (ROS). The production of ROS via photochemical pathways is well known. More recently, the non-photochemical production of ROS via the one electron oxidation of ferrous iron (Fe(II)) by dioxygen (O2) has been detected in a range of environments. The oxidation of Fe(II) initiates a pathway that generates an array of ROS as Fe is cycled between Fe(II) and Fe(III) oxidation states. This dissertation presents studies to investigate the oxidation kinetics of Fe(II) and its role in producing and maintaining ROS at oxic-anoxic boundaries.

The determination of the rate constant for the reaction of Fe(II) and dioxygen is a challenge due to the difficulty in isolating the reaction from an assortment of simultaneous reactions involved in the Fe cycling process. The second order reaction rate evaluated using a competition kinetics method against a series of Fe-binding ligands was determined to be within 7x108- 2x109 M-1s-1. This fast reaction kinetics suggest that in natural environments, oxic-anoxic interfaces can trigger the rapid generation of ROS.

Historically salt marshes are associated with rapid primary production of plant material but low subsequent decomposition due to physical limitation of oxygen availability. Oxygen transport into carbon rich sediments is thought to be limited by low vii permeability in the fine-grained marsh sediments. Physical characteristics arising from marsh grass rhizosphere and benthic burrows can greatly alter the flow dynamics enhancing the advective flow of oxygen rich water into marsh sediment containing reduced species such as Fe(II). Radium tracer studies based on 228Th/224Ra disequilibrium were employed to assess the water exchange through a coastal marsh system. The greater flow and heightened mixing efficiency promote the trapping of particle phases and the transport of oxygen and other terminal electron acceptors to aid the organic carbon oxidation.

The production of ROS in the absence of light was verified in the organic carbon rich sediment around the rhizophere of the common marsh grass, Spartina alterniflora. Metastable mixtures of Fe(II), O2 and ROS were measured over several seasons. This finding indicates an abiotic pathway for ROS generation and a subsequent ROS mediated mechanism for the degradation of organic carbon in aquatic environments. Ultimately, these processes affect the carbon burial capacity and the export of carbon flux to the oceans making these ecosystems key players in regulating global carbon budgets.

Rights

© 2017, Dewamunnage Muditha Chathurangani Dias

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