Atmospheric carbon dioxide concentration is expected to exceed 500 parts per million and global temperatures to rise by at least 2°C by 2050 to 2100, values that significantly exceed those of at least the past 420,000 years during which most extant marine organisms evolved. Under conditions expected in the 21st century, global warming and ocean acidification will compromise carbonate accretion, with corals becoming increasingly rare on reef systems. The result will be less diverse reef communities and carbonate reef structures that fail to be maintained. Climate change also exacerbates local stresses from declining water quality and overexploitation of key species, driving reefs increasingly toward the tipping point for functional collapse. This review presents future scenarios for coral reefs that predict increasingly serious consequences for reef-associated fisheries, tourism, coastal protection, and people. As the International Year of the Reef 2008 begins, scaled-up management intervention and decisive action on global emissions are required if the loss of coral-dominated ecosystems is to be avoided.

Symbiotic reef corals occupy the entire photic zone; however, most species have distinct zonation patterns within the light intensity gradient. It is hypothesized that the presence of specific symbionts adapted to different light regimes may determine the vertical distribution of particular hosts. We have tested this hypothesis by genetic and in situ physiological analyses of the algal populations occupying two dominant eastern Pacific corals, over their vertical distribution in the Gulf of California. Our findings indicate that each coral species hosts a distinct algal taxon adapted to a particular light regime. The differential use of light by specific symbiotic dinoflagellates constitutes an important axis for niche diversification and is sufficient to explain the vertical distribution patterns of these two coral species.

The photosynthetic responses to variations in photon flux density were determined for Symbiodinium microadriaticum, the symbiont of the Caribbean jellyfish Cassiopeia xamachana, S. kawagutii, the symbiont of the Indo-Pacific stony coral Montipora verrucosa, and S. pilosum, the symbiont of the Caribbean zoanthld Zoanthus sociatus. Photosynthetic responses were characterized in terms of cellular pigment content, photosynthesis versus irradiance (P-I) relationships, and number and size of the photosynthetic unit (PSU). Analyses of the responses under 2 Light regimes of 40 and 250 p 0 1 quanta m-' S-' indicate that: (1) the 3 different species cultured under identical conditions possess different photosynthetic characteristics; (2) the 3 species acclimate to low photon flux density by simultaneously increasing the number and size of their PSU; (3) PSU characteristics were not correlated with the parameters of their respechve P-lcurves; (4) the 3 species analyzed have different photoacclimatory capabilities which can be correlated with their respective ecological distribution as endosymbionts. As different species demonstrate characteristic responses, photo-acclunatory adjustment in symbiotic dinoflagellates may be under genetic constraints, and thus may represent photo-adaptation (sensu 0. Bjorkman). As a whole, the results suggest that symbiont photo-adaptation may constitute an important axis of niche diversification for the intact associations.