The effects of elevated CO 2 and temperature on photosynthesis and calcification in the calcifying algae Halimeda macroloba and Halimeda cylindracea and the symbiont-bearing benthic foraminifera Marginopora vertebralis were investigated through exposure to a combination of four temperatures (28°C, 30°C, 32°C, and 34°C) and four CO 2 levels (39, 61, 101, and 203 Pa; pH 8.1, 7.9, 7.7, and 7.4, respectively). Elevated CO 2 caused a profound decline in photosynthetic efficiency (F V: F M), calcification, and growth in all species. After five weeks at 34°C under all CO 2 levels, all species died. Chlorophyll (Chl) a and b concentration in Halimeda spp. Significantly decreased in 203 Pa, 32°C and 34°C treatments, but Chl a and Chl c 2 concentration in M. vertebralis was not affected by temperature alone, with significant declines in the 61, 101, and 203 Pa treatments at 28°C. Significant decreases in F V: F M in all species were found after 5 weeks of exposure to elevated CO 2 (203 Pa in all temperature treatments) and temperature (32°C and 34°C in all pH treatments). The rate of oxygen prod°Ction declined at 61, 101, and 203 Pa in all temperature treatments for all species. The elevated CO 2 and temperature treatments greatly reduced calcification (growth and crystal size) in M. vertebralis and, to a lesser extent, in Halimeda spp. These findings indicate that 32°C and 101 Pa CO 2, are the upper limits for survival of these species on Heron Island reef, and we conclude that these species will be highly vulnerable to the predicted future climate change scenarios of elevated temperature and ocean acidification.