The Principal Investigator (PI) plans a 2-year program to determine the significance of previously identified mechanisms that may control atmospheric iron speciation before and after deposition into the ocean. Photochemical experiments will be carried out on previously collected marine aerosol samples (South Atlantic Ocean, 2005; Equatorial Pacific Ocean, 2006) in the presence of dimethyl sulfide (DMS) and isoprene oxidation products, as these compounds have been shown to increase iron bioavailability in laboratory simulations. By optimizing analytical capabilities for low concentrations of essential chemical constituents, results from this study will provide further evidence to support or refute the hypothesis that phytoplankton themselves are key protagonists in a feedback cycle to manipulate the form of iron in aerosol particles for their optimal metabolic use. Crustal aerosol particles deposited at sea constitute a significant source of scarce micronutrient iron that limits phytoplankton growth in remote open oceans. Phytoplankton, in turn, modulate global climate by accounting for half the earth's photosynthetic absorption of CO2, as well as by emission of climate forcing gases, such as DMS and nitrous oxide (N2O). The systematics of open-ocean aerosol iron delivery as well as the chemistry that controls iron bioavailability to phytoplankton, however, remains ill defined thus prohibiting any quantitative analysis of how phytoplankton affect global climate.
The work will be carried out by undergraduate and M.S. students at Central Washington University (CWU) with existing and new instrumentation, the latter of which will significantly increase the capacity of the PI's lab to investigate iron in the atmosphere-ocean context. Students involved in this research will obtain valuable hands-on experience in the field and the laboratory, and will interact with scientists from diverse research areas and countries, preparing them for the intricate nature of present and future scientific challenges. Outreach activities specifically designed for K-12 students that include global warming and its regulation by feedback mechanisms will be an integral part of this research program.
Iron is a limiting micronutrient for phytoplankton in many parts of the open ocean where the main delivery pathway is through the deposition of iron-containing aerosol particles of natural and anthropogenic origin. Despite the fact that this iron modulates the uptake and release rates of important climate forcing gases such as CO2, little is known about the physico-chemical processes that make this iron more bioavailable to phytoplankton assimilation. Previous results obtained in laboratory simulation experiments showed that iron bioavailability increased when iron oxides were exposed to solar irradiation and oxidation products of gases emitted by phytoplankton, including dimethyl sulfide (DMS) and isoprene. The major objective of the proposed project was to determine the significance of these chemical mechanisms under more realistic conditions. To that end, photochemical experiments were carried out on collected marine aerosol samples (South Atlantic Ocean, 2005; Equatorial Pacific Ocean, 2006) in the presence of DMS and isoprene as well as their oxidation products and under near-environmental conditions of remote oceans. One key analytical challenge in this experiment was to quantitate iron at the very low sub-nanomolar concentrations encountered in this setting. We were able to successfully optimize a chemiluminescence based technique that allowed us to quantify ferrous iron down to 0.1 nM concentrations continuously throughout dissolution experiments of real aerosol particles in real seawater. Addition of DMS and isoprene as well as their oxidation products increased ferrous iron concentrations in these experiments significantly, and we observed hydrogen peroxide formation as well. These results provide further evidence to support the proposal’s main hypothesis, namely that phytoplankton themselves are key protagonists in a feedback cycle to manipulate the form of iron in aerosol particles for their optimal metabolic use. As part of this project, 7 undergraduate and 2 graduate students received essential hands-on experiences in independent research and in disseminating their results in professional venues such as American Chemical Societies National Meetings.
