Rands,-M.L.; Douglas,-A.E.; Loughman,-B.C.; Ratcliffe,-R.G. Avoidance of hypoxia in a cnidarian symbiosis by algal photosynthetic oxygen. BIOL.-BULL.-MAR.-BIOL.-LAB.- WOODS-HOLE. 1992. vol. 182, no. 1, pp. 159-162. The algal symbionts in a variety of invertebrates are widely believed to increase the oxygen tension in the animal tissue by producing photosynthetic oxygen. This could be advantageous to the animal by maintaining normoxia in anoxic waters. Equally, it could be detrimental, through hyperoxia and the generation of toxic oxygen radicals, and this may contribute to the recurrent incidence of mass bleaching in tropical cnidarian symbioses over the last decade. Here, we use in vivo super(31)P nuclear magnetic resonance spectroscopy to assess the effect of photosynthetic oxygen production by the symbiotic alga Symbiodinium sp. on the energy metabolism of a sea anemone, Anemonia viridis and, by using acidification of the tissues and elevated ADP/ATP ratios as linked indices of anaerobiosis, we show unequivocally that photosynthetic oxygen can protect an invertebrate from hypoxia. Illumination prevents the rapid acidification and reduction in ATP that occurs under hypoxic conditions in the dark, and we suggest that this effect could be partly responsible for algal enhancement of coral calcification.