[1] Emissions of CH 3 Cl, CH 3 Br and CH 3 I were measured biweekly for 12-to 24-month periods between March 2002 and March 2005 from monospecific stands of four dominant southern California coastal salt marsh plants. These measurements revealed large inherent differences between species and more detailed patterns of seasonal production than previously reported. Marsh plants displayed intrinsic abilities to produce methyl halides. Salt marsh plants produced 92% of CH 3 Cl and 90% of CH 3 Br emitted and only 41% of the emitted CH 3 I. Unvegetated areas emitted 7.9% of CH 3 Cl, 9.9% CH 3 Br, and 59% of the emitted CH 3 I. The accuracy of the estimated methyl halide emissions from a coastal marsh and probably other ecosystems can be dramatically improved with increasing the number of species being measured and including emission from barren (mudflats and soil) areas. Estimates of global salt marsh emissions based on vegetated and barren area are 130, 21, 5.5 (mg m À2 yr À1 ) for CH 3 Cl, CH 3 Br, and CH 3 I, respectively, or 1.2, 3.9, and 0.8% of total global fluxes of these gases.

An in situ incubation assay measuring the bromination and iodination of phenol red was developed to detect the release of reactive bromine and iodine (primarily hypobromous acid [HOBr] and hypoiodous acid [HOI], respectively) from a putative extracellular bromoperoxidase of marine diatoms. Six of 11 species showed significant release compared to controls. Polar species were particularly active, releasing 0.6-180 fmol HOBr cell 21 h 21 (0.04-1.8 mmol HOBr [mg total chlorophyll] 21 h 21 ; at the seawater bromide concentration, 840 mmol L 21 ) and 1.9-271 fmol HOI cell 21 h 21 (0.02-2.7 mmol HOI [mg total chlorophyll] 21 h 21 , at 100 mmol L 21 iodide). Porosira glacialis consistently showed the highest rates of release. Several temperate diatoms, including Achnanthes cf longipes, known to have a bromide-sensitive peroxidase involved in stalk formation, and warmwater species also showed the ability to release reactive bromine and iodine. This release was influenced by light, temperature, bromide (and iodide) concentration, H 2 O 2 concentration, and pH. The rate of HOBr release by polar diatoms was much greater than bromoform emissions measured by others from laboratory cultures and seaice algae in the field. This indicates that most of the HOBr released may react with dissolved organic matter (DOM) to form nonvolatile bromine organics. Some fraction of diatom-produced HOBr and HOI may also form volatile Br 2 and I 2 , which could transfer to the polar troposphere. The reaction of diatom-released reactive bromine and iodine with seawater DOM may represent the major mechanism in the formation of oceanic polybromo-and polyiodo-methanes.

Production of methyl chloride (CH3Cl), methyl bromide (CH3Br), and methyl iodide (CH3I) was measured from short‐term incubations (light and dark) of Macrocystis pyrifera (kelp) blade tissue and from long‐term culture of Macrocystis blades. Seawater from within a natural Macrocystis bed had elevated CH3X concentrations compared to seawater from outside the kelp bed. Highest concentrations were associated with regions of high biomass density: surface canopy and sporophyll/juvenile frond zone. Rates of methyl halide production (normalized to kelp biomass) were determined from the short‐term incubation, blade culture, and natural population, the latter based on a simple hydrodynamic model. Estimates of global CH3X production based on these production rates and published estimates of global kelp standing stock suggest that kelps (and other marine macroalgae) are not a significant source of global CH3X.

Production rates of bromoform (CHBr,), methylene bromide (CH,Br,), and methyl iodide (CH31) were measured in the laboratory for 11 species of marine macroalgae. Production rates of the volatile bromomethanes extrapolated to a global scale suggest that marine macroalgae produce 2 x 10" g Br yr-I (1 x lo9 mol Br yr I), 98% of which is bromoform. Laminarians (kelps) produce 61% of this organic Br. These calculations suggest that marine macroalgae are important in the biogeochemical cycling of Br. Seawater concentrations of CHBr,, CH,Br,, and CH,I were determined from various southern California coastal locales. High concentrations were measured in seawater from the canopy and the bottom of a dense bed of Macrocystis as compared to other sites. Surface seawater concentrations of these halomethanes showed a strong cross-shore gradient with the highest concentration in the kelp canopy and the lowest at 5 km offshore. Seawater adjacent to decaying macroalgae on the bottom of a submarine canyon was not enriched in halomethanes relative to-surface water. Water exiting a productive estuary was enriched only with CH,Br,, although two algal species that are abundant there (U/vu and Enteromorpha) showed high laboratory production rates ofboth CHBr, and CH,Br,.

Unialgal cultures of 15 species of marine phytoplankton were examined for the production of methyl iodide (CHJ). CH,I production was quantified during log and stationary phases of phytoplankton growth in f/2 medium and compared to CHJ levels in control vessels. Significant increases in CHJ production were seen in five cultures. Phytoplankton CHJ production varied from a low of lop4 amol CHJ cell-l d I to a high of 8X 10 1 amol CHJ cell-l d-l. In three diatom cultures, the increase in total CHJ and cell-based CH,I production rates were much higher during the log phase than during the stationary phase, even though the bacterial numbers continued to increase, supporting the premise of phytoplankton CHJ production. In two cultures, it was more difficult to distinguish between phytoplankton and bacterial production. Additional biotic and abiotic sources and sinks of CHJ were considered in the analysis, including bacterial consumption, chemical removal, and photochemical production. The only significant CHJ sink during the course of the experiment was its nucleophilic substitution with Cl-.