Coral Bleaching

What are zooxanthellae?

To understand coral bleaching, first, we need to understand corals and their symbiotic relationship with zooxanthellae. Zooxanthellae are single-celled, algae-like organisms that can photosynthesize. Corals and zooxanthellae have a mutualistic symbiotic relationship, meaning they help each other survive. Corals provide a home for zooxanthellae to live in, and zooxanthellae return the favor by providing corals with as much as 90% of their food supply (via photosynthesis), as well as their color.

What is bleaching?

Coral bleaching occurs when environmental factors cause corals to become stressed and as a result, expel their zooxanthellae. When this happens, they lose the two things zooxanthellae give them: their food and color. When they lose their color, it allows you to see through the now transparent coral polyps to the white skeleton underneath (hence “beaching”). They have also now lost their main source of food. A bleached coral is NOT a dead coral, but without their food, corals struggle to survive. They become starving, weak, and more susceptible to disease, which very commonly leads to death.

Causes of Coral Bleaching

One of the most commonly discussed of cause of coral bleaching is temperature change. Corals prefer temperatures of ~73-84 degrees Fahrenheit. When temperatures get too warm or too cold outside that range, the zooxanthellae have a reaction to the temperature change, causing them to produce a harmful toxin called a “reactive oxygen species” that can harm both themselves and the coral. As a result, the corals expel the zooxanthellae to protect themselves from the toxin. Now stressed and lacking a major food source, the coral is starving, weakened, and less able to fight off any disease that comes its way.

Temperature is not the only cause of coral bleaching. An increase in pollutants in the water can also cause cause stress to the corals, resulting in 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 increasing instances of bleaching by lowering the temperature at which bleaching occurs.

Other sources of pollutants include stormwater runoff from local cities or rural areas. Stormwater runoff is the water that washes across the land until it is deposited into the ocean, picking up pollutants along the way. Different pollutants, from oils, pesticides, fertilizers, pet waste, and even trash, can all be collected in stormwater runoff and deposited into the ocean. Pollutants that make the ocean less healthy, decreasing the health and well being of the organisms within it.

Can Corals Recover from Coral Bleaching?

Yes, corals can survive from coral bleaching, but only if the stressors are reduced within a short enough time period for zooxanthellae to recover. You’ll know a coral is actually dead once there is algae growing over the coral skeleton. This happens when the polyp has died and only the rock skeleton is left behind, allowing algae to grow. It is evident that coral bleaching is a major risk to coral. While it takes hundreds of years to grow into the corals we see today, it can only take a short period of stress to kill them. That is why it is so important to protect corals in any way we can, including what sunscreen we use!