Natural cycles of brominated methanes : macroalgal production and marine microbial degradation of bromoform and dibromomethane

Citation

Goodwin, Kelly D. (1996) Natural cycles of brominated methanes : macroalgal production and marine microbial degradation of bromoform and dibromomethane. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/3H8J-X847. https://resolver.caltech.edu/CaltechETD:etd-12142007-081155

Abstract

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. Two pieces of the bromine biogeochemical cycle were investigated - marine macroalgal production and microbial degradation of brominated methanes. Seawater concentrations of bromoform (CHBr3) and dibromomethane (CH2Br2), as well as methyl iodide, were measured from a variety of southern California coastal sites. Elevated concentrations were associated with the Giant Kelp, Macrocystis pyrifera. A strong cross-shore gradient was observed with highest halomethane concentrations in and around the kelp bed relative to surface waters 5 km offshore. Water exiting a productive estuary was also enriched with CH2Br2. Seawater adjacent to decaying macroalgae on the bottom of a submarine canyon was not enriched in these halomethanes relative to surface water, indicating that bacterial degradation of drift kelp was not a significant source of halomethanes in this environment. M. pyrifera produced CHBr3 and CH2Br2 during incubations of tissue disks and whole blades. Median production rates measured from whole blade incubations were 171 ng [...] (0.68 nmoles [...]) and 48 ng [...] (0.27 nmoles [...]), based on a 12 hr photoperiod (n = 19; 190 blades). Comparable production rates were measured from preliminary incubations in situ, supporting the validity of laboratory measurements. Light and algal photosynthetic activity affected CHBr3 and CH2Br2 release by M. pyrifera. These results suggest that environmental factors that influence kelp physiology (e.g., health, light, season, climate, etc.) may ultimately affect release of halomethanes into the atmosphere. M. pyrifera, the dominant macroalga in southern California, produces an estimated 3x10[superscript 6]g Br/yr from CHBr3, Ch2Br2, and methyl bromide (MeBr) in Orange and San Diego Counties. Bromoform contributes 77% of that bromine. Anthropogenic bromine emissions appear to dominate macroalgal emissions in this region because MeBr fumigation alone releases an estimated 10[superscript 8]g Br/yr. California accounts for approximately 10% of global MeBr use, thus this pattern of dominant MeBr emission is not expected for all coastal regions, but it may represent other areas with dense urban and agricultural development. Although both CHBr3 and CH2Br2 are released by macroalgae, only CH2Br2 was degraded by kelp-associated microorganisms in seawater enrichments. Dibromomethane degradation rates in seawater enrichments ranged from 0.11 to 73 nmoles [...] seawater, depending in part on the initial CH2Br2 concentration. Microbial degradation was observed only for dihalomethanes; Ch2Br2 and dichloromethane (Ch2Cl2) were degraded, but CHBr3 and MeBr were not. Dibromomethane degradation was associated with particles >1.2 [mu]m, supporting the hypothesis 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 rates were extrapolated to environmental conditions based on seawater CH2Br2 concentrations measured in a M. pyrifera canopy (0.0 18 nM). Microbial degradation at this Ch2Br2 concentration was estimated as 0.023 ng [...], at maximum; approximately 136 days would be required to deplete 0.018 nM CH2Br2 at this degradation rate. This result indicates that the rate of microbial degradation is slower than volatilization of Ch2Br2 from the ocean yet faster than hydrolysis or halide substitution. Macroalgal production was estimated as 19 ng [...] seawater based on whole blade incubations and M. pyrifera biomass density, which was 0.4 kg/m[superscript 3] (averaged over the water column) estimated from field and laboratory measurements. Microbial CH2Br2 degradation thus represents only 0.1% of Ch2Br2 production by M. pyrifera, well within the error of the production estimate itself. Although microbial degradation of Ch2Br2 occurs, it appears to be an insignificant water column sink within the kelp bed. Microbes attached to kelp surfaces, however, could encounter elevated CH2Br2 concentrations, and significant degradation might occur at the kelp surface. Macroalgal production measurements would thus reflect net production from kelp and associated Ch2Br2-degrading microbes, but production estimates themselves would remain unchanged.

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