The ecotoxicological effects of Photosystem II herbicides on corals

The recent discovery of contamination of the tropical marine environment by Photosystem II (PSII) herbicides used in agriculture and antifouling paints has led to concerns regarding the effects on corals and their symbiotic dinoflagellate algae. In reviewing the ecotoxicological studies conducted so far, PSII herbicides appear able to readily penetrate the coral tissues and rapidly (within minutes) reduce the photochemical efficiency of the intracellular algal symbionts. The dinoflagellates appear at least as sensitive to PSII herbicides as other phototrophs tested so far, with photosynthesis being affected at exceptionally low concentrations (i.e. in the ng l−1 range). At these levels and over short exposure periods, the effects can be fully reversible (i.e. when corals are returned to clean seawater) and vary according to type of herbicide; however, when exposed to higher concentrations in the light or over longer exposure periods, it results in a long-term sustained reduction of the photochemical efficiency of the algae (symptomatic of chronic photoinhibition). This can result in the dissociation of the symbiosis (bleaching) which is a common but nevertheless significant sub lethal stress response requiring many months to recover from. It is argued that the reliance of corals on an endosymbiotic photoautotrophic energy source, together with predilection for the symbiosis to dissociate when photosynthesis of the algae is affected, renders coral particularly susceptible to changes in environmental conditions—and especially phytotoxins such as PSII herbicides.