Eucheuma denticulatum has been shown to release bromoform, diiodomethane, dibromochloromethane, perchloroethylene, chloroiodomethane, chloroform, sec-butyl iodide, methyl iodide, methylchloroform, carbon tetrachloride, trichloroethylene and butyl iodide into its growth media, with bromoform (310 4-25 #g kg DW -~ h -1) and diiodomethane (182 q-9 #g kg DW -~ h -I) being the dominant volatile hatocarbons (VHCs). The production of VHCs was always higher at a photon flux density of 1500 than at 400 #tool photon m -2 s -1. The influence of pH was minimal at 400 #tool photon m -2 s -1. The addition of azide decreased mean VHC production at pH 8"2 4-0"2 and 8"8. Algae kept in media where extracellular hydrogen peroxide was decomposed by addition of manganese dioxide showed a decrease in VHC production at pH 8"2 q-0"2 and an increase at pH 8"8. We suggest that high light intensity and carbon dioxide deficiency caused by high pH in E. denticulatum promote VHC production through induction of hydrogen peroxide synthesis.Key words: bromoform, Eucheuma denticulatum, halocarbons, hydrogen peroxide, red algae, sodium azide. IntroductionRecent Arctic field studies, especially during spring, have established that there are changes in the ozone composition of the lower Arctic atmosphere (0-2 km) resulting from an increased tropospheric halogen content, notably of bromine, with algal volatile halocarbons (VHCs) being suspected to be a contributory source (Barrie et al., 1988;Sturges et al., 1992). Depending upon the atmospheric lifetime of a particular VHC released into the troposphere, photolysis can produce halogen atoms which catalyse stratospheric ozone removal (Molina & Rowland, 1974). Bromine is considered to be a more efficient catalyst than chlorine in destroying ozone (Prather et al., 1984), and consequently the marine organisms contributing to the production of bromo-compounds are thought to have an important potential influence on the composition of the stratosphere.Halo-peroxidases are known to catalyse the synthesis of halogenated compounds (Krenn et al., 1989; Wever et al., 1991). The synthesis occurs in the presence of hydrogen peroxide (HzO2), the respective halide ion and a suitable acceptor (for example, fl-keto acids, cyclic fldiketones and substituted phenols: Hager et al., 1966 that VHCs are cleavage products of halogenated C3 or C4 ketones. Such polyhaloketones are widely accepted to be toxic, and their presence has also been reported in some prolific VHC producers (Fenical, 1974). The bromoperoxidase from the red alga Bonnemaisonia hamifera Harlot (Bonnemaisoniaceae) can, for instance, produce dibromomethane, bromoform and pentylbromide in vitro from bromide ions, H20 2 and 3-oxooctanoic acid (Theiler et aL, 1978).For a metabolite system to be effective for defence purposes or in an allelopathic interaction, the distribution of producer enzymes, metabolite precursors or the metabolites themselves should ensure that metabolites can easily reach the grazers, parasites or competitors. From the literature on alg...

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Marine algae -a source of trichloroethylene and perchloroethylene Abstract-Our results show the natural production of two olefins, trichloroethylene and perchloroethylene, by various marine macroalgae and a microalga. We found significant difference in the ability of the algae to produce these compounds. The production rates for trichloroethylene varied between 0.022 and 3,400 ng g-l fresh wt (FW) h-l and were generally higher than those for perchloroethylene (0.0026-8.2 ng g-l FW h-l). The two subtropical algae, Asparagopsis taxiformis and Falkenbergia hillebrandii, showed the highest formation rates. One axenic marine red microalga, Porphyridium purpureum, was also tested and it could also produce trichloroethylene and perchloroethylene. The measured rates suggest that the emission of trichloroethylene and perchloroethylene from the oceans to the atmosphere may be of such a magnitude that it cannot be neglected in the global atmospheric chlorine budget.

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