Temperature stress-induced bleaching of the coralline alga Corallina officinalis: a role for the enzyme bromoperoxidase

Coralline algae are important components of coral reefs and are involved in reef building via calcification, cementation, the synthesis of anti-fouling compounds and the synthesis of allochemicals to aid recruitment, settling and metamorphosis of reefs species. Using Corallina officinalis we have shown that these algae undergo temperature-related bleaching at similar temperatures to those known to cause bleaching in corals. The bleaching appears to be associated with considerable increases in the vanadium-containing enzyme bromoperoxidase (VBPO). This enzyme is involved in hydrogen peroxide (H2O2) elimination and generates the powerful brominating/oxidizing agent hypobromous acid (HOBr, probably present as Brþ). This is used to synthesize volatile halogenated organic compounds (VHOCs) from a pool of organic acceptor molecules. Earlier in vitro work has shown this enzyme to be effective in bleaching the phycobilin photosynthetic accessory pigments and to be partly located in chloroplasts. The data presented here supports the suggestion that increases in temperature lead to an increase in the cellular production of H2O2 and other reactive oxygen species that result in an increase in VBPO, a subsequent increase in HOBr/Brþ followed by pigment bleaching when the capacity to produce VHOCs has been exceeded. Addition of the exogenous antioxidant mannitol decreases both pigment bleaching and VBPO induction. A scheme is presented to illustrate proposals for the involvement of VBPO in the bleaching of coralline algae such as C. officinalis. The importance of these species in reef building and rebuilding is discussed.