Coral Endolithic Algae: Life in a Protected Environment!

Endolithic algae inhabiting skeletons of living corals appear to be adapted to an extreme environment created by the coral. However, measurements on three coral species from the genus Porites revealed that these corals provide several modes of protection to the algae as well. High concentrations of ultraviolet (UV)-absorbing compounds, mycosporine-like amino acids (MAAs), were found in the tissues of all corals examined, but they were not detected in extracts of the endolithic algae. Coral tissues and skeleton filter 93.98-99.5% of the ambient UV radiation and thus shade the endolithic algae from this potentially damaging radiation. In addition endolithic algae are largely relieved from grazing pressure by herbivorous fish, because only 4% of fish bites on Porites corals resulted in exposed endolithic algae. Thus, the coral skeleton provides a refuge to the endolithic algae from some of the environmental pressures normally experienced by free-living algae on the reef. created by a living coral. Among the corals hosting endolithic algae are several species from the genus Porites: P. compressa Dana (Shashar and Stambler 1992), P. evermanni Vaughan (N.S., pers. obs.), P. lobata Dana (MacIntyre and Town 1975, Patzol 1988), and P. lutea Edwards & Haime (Highsmith 1979, 1981). In the genus Porites, these algae may appear as dense green bands underneath the coral tissue, as in P. lobata and P. evermanni, or can be found throughout the coral skeleton, as in the branching P. compressa. Odum and Odum (1955) hypothesized a major contribution by the endolithic algae to the primary productivity ofthe reef. This suggestion was later challenged by Kanwisher and Wainwright (1967) and by Shashar and Stambler (1992), who reported a photosynthetic rate of 0.01 mg O2 . min-I. rn1 coral skeleton1 for P. compressa. This low rate of photosynthesis can be attributed to strong attenuation of solar radiation by the coral tissue, composed of cnidarian host and algal symbionts (HalldaI1968, Shibata and Haxo 1969), and by the inorganic coral skeleton (Kanwisher and Wainwright 1967). Coral reefs present various types of environmental pressures for algae growing on them. These include competition for substrate with other sessile species, grazing by numerous herbivorous fish, photoinhibitory and other damaging effects of solar radiation, both photosynthetically active radiation (PAR) and ultravi167 THE CALCIUM CARBONATE skeleton of living corals provides a unique habitat for both algae and bacteria (Odum and Odum 1955). These "endolithic" organisms have been described from numerous scleractinian corals (Shashar and Stambler 1992), as well as the hydrozoan Millepora tenella Ortmann (Bellamy and Risk 1982). In most cases, these endolithic organisms include filamentous algae, usually siphonaceous chlorophytes of the genus Ostreobium (Duerden 1902, Jeffrey 1968, Lukas 1974), which easily can be seen as a green band or zone when the coral is broken. According to Campion-Alsumard et al. (1995), Ostreobium quekettii Bornet & Flahault is the only chlorophyte species known to survive in the unique environment I Development of the coral-adapted spectrophotometer was supported by ONR grant NOOI4-92-J-1852 to Celia Smith and Cynthia Hunter. This research was supported by the E. W. Pauley Foundation, NSF grants BIR-937927 (to T. Cronin and N.S.), OCE-92l6307 and OCE-9496082 (to M.P.L.), and the U.S.-Israel Binational Science Foundation. Manuscript accepted 19 June 1996. 2 Marine Biology Laboratory, Woods Hole, Massachusetts 02543. 3 Department of Biological Sciences, University of California, Santa Barbara, California 93106. Current address: Smithsonian Environmental Research Center, P.O. Box 28, Edgewater, Maryland 21037. 4 Department of Zoology, University of New Hampshire, Durham, New Hampshire 03824. 5 Department of Life Sciences, Bar-Han University, Ramat Gan 52900, Israel.