To understand coral bleaching, first, we need to understand corals. Corals have a symbiotic relationship with Zooxanthellae, a photosynthetic algae-like organism. Coral provides a home to zooxanthellae, giving them protection and nutrients needed for photosynthesis, which in turn provides corals with as much as 90% of their food supply as well as their color.
Coral bleaching occurs when environmental factors cause corals to expel their zooxanthellae, leaving them colorless or “bleached.” Without the zooxanthellae, the corals have not only lost their color but have also lost their main source of food. Without food, the coral struggles to survive, becoming more susceptible to disease, and typically dies. There is a chance that zooxanthellae could return, but the corals must survive long enough for this to happen, which is unlikely in conditions that cause bleaching, to begin with.
Coral Bleaching is caused by a change in environmental conditions that makes life unsuitable for the zooxanthellae or stresses the corals, which then causes them to expel their zooxanthellae. This can happen due to a few different environmental conditions, one being temperature. When temperatures get too warm, exceeding averages for a region, corals become stressed and expel their zooxanthellae. Now stressed and lacking a major food source, the coral struggles to survive.
An increase in pollutants in the water can also cause coral bleaching. Between 6,000 and 14,000 tons of sunscreen wash off swimmers, scuba divers, and snorkelers into coral reef environments each year, making sunscreen a major potential pollutant for marine environments. Chemical sunscreens containing chemicals like oxybenzone and octinoxate are the most commonly bought and sold types of sunscreen, meaning they are the most common to wash into water. The inorganic UV filters in chemical sunscreens induce the viral lytic cycle in zooxanthellae, spreading infections through the colony and causing bleaching. Other sources of pollutants include runoff from local cities or rural areas. Different pollutants, from oils, pesticides, and fertilizers, to pet waste and even trash, can all be collected in runoff and deposited into the ocean. Runoff is the water that picks up pollutants as it runs across the land until it is deposited into the ocean.
Lowering tides are also a cause of coral bleaching. As tides lower, corals become exposed to a higher intensity of UV rays which can cause enough stress to expel their zooxanthellae. In extreme cases, corals exposed to airflow due to lowered tides are exposed to more solar radiance than they can handle, causing them extreme stress, and potentially even killing them. Lower tides can be naturally occurring, depending on where the coral is located, or they can be caused by external environmental factors such as droughts.
The images below provide a real-life look at what coral bleaching is and how fast it takes effect. The reefs appeared healthy or fairly healthy until 2004, when a major bleaching event occurred, and by 2014 there was only the calcium carbonate skeleton of the corals left. It is evident that coral bleaching is a major risk to coral. If the corals are going to recover from a bleaching event, they must survive long enough for the zooxanthellae to return, which can be very difficult in their weakened state. While it takes hundreds of years to grow into the coral reefs we see today, it can only take moments to kill them. That is why it is so important to protect these species!