Algae-coral symbiosis: fragility owing to anthropogenic activities and adaptive response to changing climatic trends

Humans have relied on oceans since time immemorial and have exploited them for natural resources and recreational activities. With the advancement of technologies, rapid population growth and land-use change, the dependability on marine ecosystems has increased tremendously. Anthropogenic activities are causing increased global temperatures, altered weather conditions, melting glaciers, rising sea levels, acidification etc. The biological processes of marine ecosystems are getting affected indirectly or directly from the molecular level to rock pools to ocean basins, thus impacting overall ecosystem services. Out of various disturbances being caused by humans, global warming has been one of the most threatening factors with other anthropogenic inputs such as nutrient enrichment, sewage and microplastics, thereby causing significant changes in the symbiotic relationship between algae and corals. The coral associations with algae, macroalgae and other groups play a very important role in the functioning of ocean ecosystems and are very sensitive to anthropogenic inputs. The symbiotic association of algae with corals results in enriched biodiversity as well as it maintains the biogeochemistry of oceans and open coastal areas. The review outlines the importance of algal associations towards the maintenance of coral reefs, along with discussing the ecological services offered by them. One of the sections also discusses the impact of anthropogenic activities on the association between corals and algae. Also, the potential adaptive response of corals to the changing climatic conditions, and conservation strategies for the conservation of coral ecosystems to ensure a sustainable environment have also been discussed. Graphical Abstract