Recent studies have shown that methyl halides (methyl chloride-CH3Cl and methyl bromide-CH3Br) are produced in significant quantities by phytoplankton and by photochemical oxidation of dissolved organic matter (DOM). However, little is known about the mechanisms responsible for the photochemical production of methyl halides and the factors which affect the microbial formation and consumption in the surface waters. Scientists from Portland State University will carry out laboratory experiments to determine the mechanisms responsible for the photochemical and microbial formation and consumption of CH3Cl and CH3Br in the Everglades, a coastal wetland. Specifically, the study will focus on (1) quantifying and characterizing the temporal and spatial variability of methyl halide fluxes from a coastal wetland; (2) identifying the mechanisms responsible for the photochemical production of methyl halides; (3) examining the role of algal mats on the formation and consumption of methyl halides; (4) using stable isotopes to identify the sources and sinks for methyl halides within various ecotones of the Everglades and coastal waters; and (5) delineating sources and sinks using stable isotope analysis.
This research will yield new insights on the biological and photochemical production of methyl chloride and methyl bromide in a coastal wetland which may bring this science community one step closer to balancing the budget for these two methyl halides. One graduate and one undergraduate student will be supported and trained as part of this project.
Methyl halides (methyl chloride-CH3Cl and methyl bromide-CH3Br) in the atmosphere are a major contributor to stratospheric ozone loss. Recent studies have shown that CH3Cl and CH3Br are produced in significant quantities in surface waters by phytoplankton and by photochemical transformation of dissolved organic matter (DOM). However, little is known about the mechanisms responsible for the photochemical production of methyl halides and the factors which affect the microbial formation and consumption in the surface waters. Scientists from Portland State University have conducted laboratory experiments to obtain a better understanding of the photochemical and microbial formation and consumption of CH3Cl and CH3Br in the coastal waters of the Florida Everglades. Specifically, the study focused on (1) quantifying the production and decay of CH3Cl and CH3Br from photochemical DOM transformation; (2) examining the role of periphyton (algal) mats on the formation of CH3Cl and CH3Br; and (3) using stable isotopes to characterize the isotope signature of photochemically produced CH3Cl. Time course experiments with periphyton incubated at different salinities (freshwater to estuarine to seawater) demonstrated that increased salinity has a significant positive effect on the production of CH3Cl and CH3Br from periphyton mats. These results are important in areas that may be prone to salt water intrusions or from rising sea levels due to global climate change. Salinity was also an important factor when quantifying the production of methyl halides from photochemical transformation of DOM. The photochemical production of CH3Cl from coastal waters exposed to simulated sunlight was greatest at a salinity of 10‰. Coastal waters samples with low salinity (< 2‰) and high DOM concentration and coastal water samples with high salinity (~30‰) but low DOM concentration were an order of magnitude lower. Estuarine and coastal waters are thus ideal environments for the photochemical production of CH3Cl, having optimal concentrations of DOM and salinity. We also found that photochemically produced CH3Cl from sites that are terrestrially influenced is isotopically light relative to CH3Cl farther away from freshwater terrestrial DOM sources. We suggest two possible hypotheses for the observed trend. Methyl chloride produced from the photolysis of algal derived DOM and not from terrestrial sources is isotopically enriched. Alternatively, greater sunlight availability in the low DOM waters results in accelerated decomposition and isotopically enriched DOM and CH3Cl. Further experiments are required to elucidate the exact mechanism responsible and to better delineate the photochemical source of CH3Cl. Over the course of the project one graduate student, two undergraduate students, one high school student and one summer intern obtained valuable laboratory and field sampling skills and scientific literacy and presentation skills. This research has yielded new insights on the biological and photochemical production of methyl halides in a coastal wetland which may bring this science community one step closer to understanding and balancing the budget for methyl halides.
