Halocarbon emissions by selected tropical seaweeds exposed to different temperatures

Four tropical seaweeds, Gracilaria manilaensis Yamamoto & Trono, Ulva reticulata Forsskål, Kappaphycus alvarezii (Doty) L.M.Liao and Turbinaria conoides (J.Agardh) Kützing, collected from various habitats throughout Malaysia, were subjected to temperatures of 40, 35, 30, 25 and 20 °C in the laboratory. An exposure range of 21-38 °C is reported for Malaysian waters. The effect of the temperature exposures on the halocarbon emissions of the seaweeds were determined 4 and 28 h after treatment. The emission rates for a suite of six halocarbons commonly emitted by seaweeds, bromoform (CHBr₃), dibromomethane (CH₂Br₂), diiodomethane (CH₂I₂), iodomethane (CH₃I), dibromochloromethane (CHBr₂Cl) and dichlorobromomethane (CHBrCl₂), were measured using a cryogenic purge-and-trap sample preparation system coupled to a gas chromatography-mass spectrometry. The emission rate of CHBr₃ was the highest of the six halocarbons for all the seaweeds under all the temperatures tested, followed by CH₂Br₂, and CH₂I₂. The emission rates were affected by temperature change and exposure duration, but overall responses were unique to each seaweed species. Larger decreases in the emissions of CHBr₃, CH₂Br₂, CH₂I₂ and CHBr₂Cl were found for K. alvarezii and T. conoides after 4 h at 40 °C. In both cases there was a >90% (p < 0.05) reduction in the F_v/F_m value suggesting that photosynthetic actitivity was severely compromised. After a 28 h exposure period, strong negative correlations (r = -0.69 to -0.95; p < 0.01) were observed between temperature and the emission of CHBr₃, CH₂Br₂ and CH₂I₂ for U. reticulata, K. alvarezii and T. conoides. This suggests a potential decrease in the halocarbon emissions from these tropical seaweeds, especially where the temperature increase is a prolonged event. Strong correlations were also seen between seaweed chlorophyll and carotenoid pigment contents and the emission rates for CHBr₃, CH₂Br₂ and CH₂I₂ (r = 0.48 to 0.96 and -0.49 to -0.96; p < 0.05). These results suggest that the regulation of halocarbon production versus reactive oxygen species production in seaweeds is an area worthy of further exploration.