This project evaluated macroalgal production and microbial degradation of brominated methanes in a marine environment; two pieces of a large and dynamic puzzle -- the bromine biogeochemical cycle. Elevated seawater concentrations of bromoform (CHBr3) and dibromomethane (CH2Br2), as well as methyl iodide, were associated with the Giant Kelp, Macrocystis pyrifera compared to several other coastal sites. M. pyrifera produced CHBr3 and CH2Br2 during incubations of tissue disks and whole blades. Release was affected by light and algal photosynthetic activity suggesting that environmental factors that influence kelp physiology may ultimately affect release of halomethanes into the atmosphere. Median rates measured from whole blade incubations were 165 ng CHBr3/gfwt/day and 48 ng CH2Br2/gfwt/day based on a 12-hr photoperiod (gfwt = g fresh weight). Comparable rates were measured in preliminary investigations of CHBr3 and CH2Br2 production in situ. M. pyrifera produces an estimated 3 x 106 g Br/yr from CHBr3, CH2Br2, and methyl bromide (MeBr) in Orange and San Diego Counties. Bromoform contributes 80% of that bromine. Anthropogenic bromine emissions appear to dominate macroalgal emissions in this region because MeBr fumigation alone releases an estimated 108 g Br/yr.

The dihalomethanes CH2Br2 and dichloromethane (CH2Cl2) were degraded by kelp-associated microorganisms in seawater enrichments, but CHBr3 and MeBr were not degraded, although both CHBr3 and CH2Br2 are released by macroalgae. Degradation rates ranged from 0.11 to 73 nmoles CH2Br2/day/L seawater, depending in part on initial concentration. Degradation was associated with particles >1.2 micrometer, supporting speculation that CH2Br2-degrading bacteria may be attached to kelp surfaces in the environment. Inhibitor studies indicated that eukaryotic organisms and a number of microbial processes, including methanotrophy, did not contribute to CH2Br2 degradation.

Laboratory degradation extrapolated to seawater concentrations of 18 pM CH2Br2 gives a potential degradation rate of 0.024 ng CH2Br2/day/L seawater. Macroalgal production was 19 ng CH2Br2/day/L seawater based on whole-blade incubations and M. pyrifera estimated average biomass density of 0.4 kg/m3. Microbial CH2Br2 degradation thus represents <1% of CH2Br2 macroalgal production. However, microbes attached to kelp surfaces could encounter elevated CH2Br2 concentrations, and significant degradation might occur at the kelp surface. Comparing time scales of various loss mechanisms indicates that microbial degradation is faster than hydrolysis or halide substitution, but volatilization will probably dominate in the shallow waters in and around a kelp bed.