phototrophic endoliths inhabiting the skeleton of Montipora monasteriata, growing at Heron Island, on the southern Great Barrier Reef. Endoliths that were exposed to sun after tissue removal were by far more susceptible to thermal photoinhibition and photo-damage than endoliths

Corals, the framework builders of reefs, live in close association with a range of phototrophic species including intracellular dinoflagellates and skeletal-dwelling endolithic microalgae and cyanobacteria. Microendoliths and especially the siphonaceous green algae Ostreobium Bornet and Flahault (Jeffrey, 1968; Lukas, 1974; Le Campion-Alsumard et al., 1995) inhabit the calcareous skeleton of many reef-building corals (Highsmith, 1981; Le Campion-Alsumard et al., 1995). These endoliths lie under the coral tissue and appear as dense green bands or zones within the external skeleton (Highsmith, 1981). The endolithic community lives in a protected environment within the coral skeleton (Shashar et al., 1997) where less than 5% of the photosynthetically active radiation (PAR) may be able to penetrate and reach these zones (Halldal, 1968; Shibata and Haxo, 1969; Schlichter et al., 1997) because of to its absorbance by the unicellular endosymbiotic algae (zooxanthellae) and the inorganic skeleton itself (Kanwisher and Wainwright, 1967). Coral bleaching involves the disruption of the association between coral hosts and their endosymbiotic photosynthetic algae. It is a stress response that results in abrupt decreases in the population density of symbiotic dinoflagellates after changes to the physical and chemical environment surrounding corals. These changes may include low salinity (Goreau, 1964; Egana and DiSalvo, 1982); low or high photosynthetic radiation (Vaughan, 1914; Yonge and Nichols, 1931; HoeghGuldberg and Smith, 1989; Jones et al., 1998); elevated ultraviolet radiation (Gleason and Wellington, 1993; Lesser et al., 1990); toxins, e.g. cyanide (Jones and Hoegh-Guldberg, 1999), copper ions (Jones, 1997), diuron and atrazine (Jones et al., 2003); microbial infection, e.g. Vibrio (Kushmaro et al., 1996) and temperature (high: Jokiel and Coles, 1977, 1990; Coles and Jokiel, 1978; Hoegh-Guldberg and Smith, 1989; Glynn and D’Croz, 1990; low: Saxby et al., 2003). More recently, global episodes of mass coral bleaching have occurred that are linked to elevated seawater temperature (Goreau and Hayes, 1994; Glynn, 1991, 1993; Brown, 1997; Hoegh-Guldberg, 1999). These responses are exacerbated by The Journal of Experimental Biology 208, 75-81 Published by The Company of Biologists 2005 doi:10.1242/jeb.01381