Climate change and coral health

Keshavmurthy Shashank

Coral reefs from an important part of coastal marine ecosystem.  Coral reefs are built over millions of years by sea anemone like animals known as coral.  Corals secrete calcium carbonate that helps in formation of structures known a reefs.  Reefs are known as rain forests of marine ecosystem since they support numerous other animals which include fishes, plants and invertebrates.  Coral reefs also support the economy of people of many tropical island nations by providing them with various resources for their livelihood.

Climate change 

Climate change is a broad term that includes all the changes that is happening to the climate of our planet Earth.  More often climate change is confused to global warming.  In fact, global warming is one of the effects of climate change.  While climate change is a result of cumulative changes in the earth’s environment (both as a result of natural and anthropogenic processes) resulting in alteration of various environmental conditions of which one is global warming.  Global warming is due to release of various gases and pollutants in to the atmosphere where they from a layer and trap the heat of sun which enter to the earth’s atmosphere.  In other words, global warming is like a condition in a green house.  As a result of climate change, mainly due to increased activity of humans (release of various gasses into the atmosphere, deforestation, pollution, coastal development etc) various effects like global warming / cooling, ultraviolet irradiation, increased natural calamities like flood, droughts, typhoons/cyclones has increased in recent years.

Health of coral 

Health of coral is dependent on its symbiotic association with a single celled plant cell known as zooxanthellae (Symbiodinium sp).  In this association, coral animal provides a home and protection and food to those tiny cells and in turn the plant cells which are algae, provide energy by photosynthesis and beautiful color through their pigments to coral animal.  This relationship is very important to the proper functioning and good health condition (Fig. 1) of coral animal.  This combined association forms what is known as holobiont (coral animal + zooxanthellae).  In recent years health of corals have been affected by various stressors as a result of natural (seawater temperature change, salinity, sedimentation, etc) and anthropogenic influences (pollution).  When corals become unhealthy (Fig. 2 and 4) due to effect of stress, the symbiotic relationship is affected and it becomes difficult for corals to survive without their partners. 

How corals respond to stress? 

Corals respond to stress somewhat similar to as we humans do.  They show a kind of immune response to stress by accumulating pigments at the site of injury (Fig. 3).  Corals that are effected by microorganisms like bacteria, virus or fungi show a change in color at the site of infection (Fig. 3) by accumulation of pigments or increased mucus secretion or complete death of coral tissue (Fig. 4).  This has been seen to occur both at the tissue and cellular level.  In this case, the coral may show a stress response either temporarily or until the effect of pathogenic bacteria persists.  Some coral species also secrete a layer of protective pigments that act as a sunscreen to ward of the effect of increased ultraviolet radiation.  The most common response, when corals are affected by stressors like increased temperature, increase/decrease in salinity,  pH changes of seawater, sedimentation or chemicals and pollutants, is bleaching (Fig. 2) or in other words loss of their color as a result of breakdown in the symbiotic relationship between the coral animal and zooxanthellae. When corals bleach, it is either due to loss of the presence of zooxanthellae cells inside the coral animal or due to reduction of the pigments of the zooxanthellae or both.  This mechanism of what exactly happens when corals bleach is still poorly understood.  
Many studies have shown that, there are many different biochemical and physiological responses that occur when coral is under stress including release of toxic oxygen both by coral animal and zooxanthellae cells, release of various kind of stress proteins as well as cell and tissue death.  Response of a coral species to stress also depends on the coral animal and the type of the zooxanthellae it is associated with.  Again, similar to humans, the effect of stressors is not the same for all the coral species.  It depends on various characteristics such as, sturdiness of coral animal as well as zooxanthellae, the way they respond to a particular stress, the type of surrounding environment in which they exist and speed at which a coral can respond to stress etc.  Two coral species living besides each other can show completely different response to stress and this aspect of what makes each coral species different is still not fully understood.  In fact, due to the symbiotic association between coral animal and zooxanthellae, the various processes undergoing inside a coral is so complex that, more often research on this aspect has been difficult to perform.

Climate change and coral health 

As discussed above, coral health is influenced by various natural and anthropogenic factors and climate change as a result of human activity is one of the most important factors that is affecting coral health and its proper functioning.  As health of a coral is influenced by environmental conditions, increasing seawater temperatures is taking its toll on the survival of corals.  Cumulative effect of various stressors will lead to reduced functioning of coral animal and in turn effect the total health of a coral reef.