Ocean acidification is one of obvious and serious environmental issues caused by increasing CO2 levels in the atmosphere.Seawater pH has decreased by about 0.1 units since the industrial revolution and the carbonate equilibrium has been changing to become detrimental to the health of coral reef ecosystems around the world. Recent studies revealed that ocean acidification has decreased the calcification rate of scleractinian corals and other reef-building organisms,while promoting the dissolution rate.Models based on the Intergovernmental Panel on Climate Change(IPCC) business-as-usual emission scenarios suggest that atmospheric CO2 levels could approach 800 μL/L near the end of this century.Under this scenario,carbonate ion concentrations and aragonite saturation in surface seawater will be substantially reduced,and coral reef ecosystems around the world would experience net dissolution.No obvious influences were detected on the photosynthesis of coral symbionts,but the symbiosis system of corals and zooxanthellae could be affected.Corals would be more susceptible to bleaching with further ocean acidification,especially with simultaneously increasing temperatures.Studies showed that ocean acidification led to serious problems with larval recruitment,and affected the metabolism and reproduction of coral-reef organisms.It is believed that early developmental and reproductive stages of calcifying organisms are the points within a life cycle most vulnerable to environmental change.Laboratory experiments revealed that ocean acidification has negative impacts on the fertilization,cleavage,larva,settlement and reproductive stages of several marine calcifiers.Therefore future ocean acidification will potentially impact the population size and dynamics,as well as the community structure of calcifiers,and will have negative impacts on coral reef ecosystems. However,ocean acidification often has non-lethal and species-specific impacts on various species of coral.It is necessary to distinguish different species and different processes of metabolism in corals,when considering the impacts of ocean acidification.The research on ocean acidification is relatively new and the mechanisms behind ocean acidification affecting reef ecosystems are poorly understood.Nevertheless,the impacts of ocean acidification on coral reef ecosystems are indisputable.With increasing pressures from global climate change and human activities,other stress factors,such as global warming,sedimentation and eutrophication emerged,and would have synergistic impacts with ocean acidification on coral reef ecosystems,driving reefs toward the tipping point for functional collapse.Scientific evidence indicates that coral reefs have deteriorated rapidly in the past few decades and the future prospects for coral reefs are poor. Despite an urgent need for action,our knowledge on the impacts of ocean acidification on coral reef ecosystems and dependent organisms is still very limited.In order to clearly understand the effects of ocean acidification on coral disease(such as coral bleaching),we suggest that a long-term and systematic monitoring system,including laboratory and in-situ experiments,should be established.Physiological,molecular and ecological methods should be utilized to study the coral reef ecosystem on individual,population,community and ecosystem levels.Studies should also focus on the adaptive capability of marine organisms,which will be important to forecast how coral reef ecosystems will respond to ocean acidification.