The significance of pollution in eliciting the 'bleaching' response in symbiotic cnidarians

The loss of pigmentation, or 'bleaching', in tropical reef organisms such as corals and symbiotic sea anemones has been extensively reported around the world over the last 15 years. The widespread bleaching of corals has generally been associated with seawater warming; increased sea surface temperatures and irradiance both play a part in eliciting the bleaching response. At the same time, it is recognized that bleaching is a generalized stress response in symbiotic cnidarians, being elicited also by exposure to pollutants such as heavy metals, oil, pesticides and increased sedimentation. However, reported occurrences of bleaching in response to pollutants in the field are extremely limited. Such events, unlike seawater warming-induced bleaching, are usually extremely localized. Nevertheless, there exists the possibility that seawater warming and pollutant discharges may act additively or synergistically to produce severe bleaching. Recovery of coral communities following bleaching-induced mortality is affected by pollution, which may either delay (or halt) recovery and/or cause a shift in species dominance so that branching coral species become replaced by more physically rigorous massive corals, with resultant loss in ecosystem biodiversity. As a generalized stress response, bleaching has much to offer in the development of toxicological bioassays. Measurements of symbiotic algal loss, algal division rates and alterations in synchronous algal division patterns in selected hosts offer sensitive tools for the assessment of the effects of pollutants, which in some cases have shown greater resolution than molecular assays.