Exploring light-dependent manganese oxide formation in a meromictic metal-rich pond

Woods Hole Oceanographic Institution, Woods Hole MA

Investigators

Abstract

The metal manganese (Mn) is found widely throughout the environment where it is closely tied to the health and function of living organisms. Within some environments, in the presence of oxygen, Mn can form Mn oxide minerals. These minerals are extremely reactive, and they have an important role in controlling the fate of nutrients, contaminants, and carbon at the Earth’s surface. Despite their environmental importance, the controls on natural Mn oxide formation remain poorly understood; recent discoveries by the PI and others have put forth evidence that new pathways for light promoted Mn oxide formation exist. The goal of this project is to determine the processes controlling Mn oxide mineral formation in surface sun-lit environments. The research team will study a local pond on Cape Cod where Mn oxides have been found to form and persist in the presence of light. Waters from the pond will be collected at various sites and depths and reacted in the laboratory under different environmental and light conditions to identify the chemical and biological pathways allowing Mn oxides to form. Mn oxides formed via different light and environmental conditions will be characterized to determine their formation pathways and reactivity. These findings will improve our understanding of the Mn cycle and associated elements on Earth and other planets, such as Mars. This project will also contribute to the training of several undergraduate and graduate students. Further, outreach efforts associated with this research will involve lectures each year at a local high school and mentoring of high school science fair projects that will introduce students to Earth science. Recent lab and field investigations point to light-mediated reactions as underappreciated components of the manganese (Mn) cycle within some water bodies. Further, the PI’s preliminary data from a local meromictic metal-rich pond (Siders Pond, Cape Cod, USA) suggest that light plays a key role in the formation of Mn oxides in the surface waters, despite presumed photoreductive processes occurring. Accordingly, here the PIs propose a systematic field and laboratory interrogation of the rates, products, and mechanisms of Mn oxidation and Mn oxide formation within Siders Pond. The overall goal of the research is to determine the conditions allowing for light-induced formation of Mn oxides within the site, which has broad implications for understanding and predicting light-driven Mn oxide formation on Earth at present and in the past. Objectives guiding this research are (1) to quantify the distribution of Mn species and rates of Mn(II) and Mn(III)-L oxidation and Mn(III)-L and Mn oxide reduction along the redoxcline over space and time, (2) to identify the underlying (a)biotic mechanisms of Mn(II) oxidation and Mn oxide formation within the photic zone, and (3) to characterize the composition, structure, and reactivity of Mn oxides as a function of depth and transition across the redoxcline. The objectives will be addressed via a combination of field measurements, laboratory incubations, and microbial culturing. Waters from the pond will be collected at various sites and depths and reacted in the laboratory under different environmental and light conditions to identify the chemical and biological pathways allowing Mn oxides to form. Mn oxides formed via different light and environmental conditions will be characterized to determine their formation pathways and reactivity. Identification of the underlying biogeochemical processes allowing for light dependent Mn oxide formation will improve modern biogeochemical models and paleorecord interpretations. These light-mediated reactions may also help explain previous observations of particulate Mn and Mn oxides in surface sun-lit waters. As part of this project, the PI and PhD student will contribute to high school education, mentor students in middle/high school science fair projects, and support research experiences for summer undergraduate fellows. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.