Of particular relevance to coral bleaching, evidence that ocean acidification may cause changes in symbiont densities of corals has emerged. Horwitz and Fine (2014) observed a 38-70%  decline in symbiont densities in corals exposed to 1800-4000 µatm pCO2. Tremblay et al. (2013) observed a 48% decline in symbiont density in corals exposed to seawater at pH 7.2. Muehllehner (2013) observed a decline in symbiont density with increasing pCO2 in Acropora cervicornis in the lab and Acropora formosa and Pocillopora damicornis at natural marine CO2 vents. Kaniewska et al. (2012) observed a 50%  in symbiont density in corals exposed to 1010-1350 ppm pCO2 after 28 days. Krief et al. (2010) observed a 25-30% decline in symbiont densities in corals exposed to 1908-3976 ppm pCO2. Whilst the CO2 levels of most of these studies are very high (reflecting scenarios at the end-of-century and beyond), they provide concrete evidence that high ocean acidification alone decreases the Symbiodinium densities of corals. However, these studies did not establish whether the decrease was detrimental to coral health, or just a shift to a new equilibrium symbiont density that had no detrimental effect on coral health.