Physiological constraints on bromoform (CHBr3) production by Ulva lactuca (Chlorophyta)

Abstract

Physiological factors affecting bromoform (CHBr3) production by Ulva lactuca were investigated using metabolic inhibitors and presumed substrates of bromoperoxidase (BrPO). The metabolic inhibitors were used at a verified physiologically active concentration. Bromoform production was nearly tripled in the light (376 ± 92 pg cm−2 h−1) compared to the dark (114 ± 70 pg cm−2 h−1), was inhibited in the light in the presence of the photosynthetic inhibitor DCMU, and was inhibited in the dark in the presence of mitochondrial respiratory inhibitor rotenone. Removal of H2O2 from seawater (treatment with catalase) decreased CHBr3 production in the light and dark. Addition of H2O2 to incubations at either 1.0 mM or 100 µM significantly decreased CHBr3 production in the light and inhibited photosynthesis. In the dark, CHBr3 production was decreased and respiration inhibited in the presence of 1 mM H2O2; CHBr3 production was enhanced and respiration was not affected in the presence of 100 µM H2O2. Removal of dissolved organic matter (DOM) from seawater decreased CHBr3 production, as did the addition of alternative BrPO substrates. These results suggest the presence of an extracellular and intracellular BrPO that protects the alga from both internally produced and externally present H2O2. The results show that H2O2 produced as a result of photosynthetic and respiratory electron transport, presumably by superoxide dismutase, is available to BrPO for bromination, and that the bromination of an unidentified metabolite (presumably Βߚketo acids) and a component of DOM leads to the production of volatile polybromomethanes.