Effect of irradiance on the emission of short-lived halocarbons from three common tropical marine microalgae

Marine algae have been reported as important sources of biogenic volatile halocarbons that are emitted into the atmosphere. These compounds are linked to destruction of the ozone layer, thus contributing to climate change. There may be mutual interactions between the halocarbon emission and the environment. In this study, the effect of irradiance on the emission of halocarbons from selected microalgae was investigated. Using controlled laboratory experiments, three tropical marine microalgae cultures, Synechococcus sp. UMACC 371 (cyanophyte), Parachlorella sp. UMACC 245 (chlorophyte) and Amphora sp. UMACC 370 (diatom) were exposed to irradiance of 0, 40 and 120 µmol photons m^-2s^-1. Stress in the microalgal cultures was indicated by the photosynthetic performance (F_v/F_m, maximum quantum yield). An increase in halocarbon emissions was observed at 120 µmol photons m^-2s^-1, together with a decrease in F_v/F_m. This was most evident in the release of CH₃I by Amphora sp. Synechococcus sp. was observed to be the most affected by irradiance as shown by the increase in emissions of most halocarbons except for CHBr₃ and CHBr₂Cl. High positive correlation between F_v/F_m and halocarbon emission rates was observed in Synechococcus sp. for CH₂Br₂. No clear trends in correlation could be observed for the other halocarbons in the other two microalgal species. This suggests that other mechanisms like mitochondria respiration may contribute to halocarbon production, in addition to photosynthetic performance.