Production of bromoform and dibromomethane by Giant Kelp: Factors affecting release and comparison to anthropogenic bromine sources

Macrocystis pyrifera (Giant Kelp), a dominant macroalgal species in southern California, produced I71 ng per g fresh wt (gfwt) per day of CHBr, and 48 ng gfwt-’ d-’ of CH,Br, during laboratory incubations of whole blades. Comparable rates were measured during in situ incubations 01 intact fronds. Release of CHBr, and CH,Br, by M. pyr$eru was affected by light and algal photosynthetic activity, suggesting that environmental factors influencing kelp physiology can affect halomethane release to the atmosphere. Data from H,O, additions suggest that brominated methane production during darkness is limited by bromide oxidant supply. A bromine budget constructed for a region of southern California indicated that bromine emitted from the use of CH,Br as a fumigant (1 X 1 Ox g BI yr -I) dominates macroalgal sources (3 X 10” g Br yr-I). Global projections, however, suggest that combined emissions of marine algae (including microalgac) contribute substantial amounts of bromine to the global cycle, perhaps on the same order of magnitude as anthropogenic sources. Investigations of brominated methane sources are motivated in part by concerns over the roles brominated species play in the atmosphere. Bromine delivered to the atmosphere from marine and terrestrial sources may participate in ozonedepleting chemistry (e.g. Yung et al. 1980). Changes in brominated methane concentrations also have the potential to affect hydroxyl radical (OH.) concentrations, leaving OH, unavailable to destroy greenhouse gases or gases involved in stratospheric ozone depletion. An accurate biogeochemical picture of brominated methanes is needed to predict their role in changing tropospheric and stratospheric ozone concentrations, which in turn are predicted to affect climate (Ramanathan et al. 1985; Lacis et al. 1990). In addition, quantifying the biogenic sources of brominated methanes provides a context in which to evaluate the impacts of anthropogenic emissions. Methyl bromide (CH,Br) is known for its role in atmospheric chemistry (e.g. Butler and Rodriguez 1996), and, in addition, bromoform (CHBr,) and dibromomethane (CH,Br,) are important to the bromine biogeochemical cycle (Reifenhguser and Heumann 1992). Marine biogenic sources of CHBr, and CH,Br, have been indicated from concentration patterns in air (Class and Ballschmiter 1988; Atlas et al. 1993) and seawater (e.g. Class and Ballschmiter 1988; Moore and Tokarczyk 1993). Specifically, seawater from macroalgal stands are associated with elevated concentraI Present address: U.S. Geological Survey, 345 Middlcfield Rd., MS 480, Menlo Park, California 94025. 2 Present address: University of Washington, Department of Chemical Engineering, Box 35 1750, Seattle, Washington 98 195-