What causes coral bleaching?

Coral bleaching primarily occurs due to changes in environmental conditions that lead to the expulsion of symbiotic algae known as zooxanthellae from coral tissues. These algae are crucial for coral health as they provide essential nutrients through photosynthesis and contribute to the vibrant colors of healthy corals. When corals experience stress from factors such as elevated sea temperatures, changes in salinity, nutrient overloads, or excessive UV radiation, they may expel these algae, resulting in a loss of color and vitality—a phenomenon referred to as coral bleaching. Without the algae, the corals struggle to survive, as they depend on these organisms for nourishment.

The other options, while they may relate to coral ecosystems in various ways, do not directly describe the primary mechanism behind coral bleaching. Increased algae growth due to pollution can affect water quality but is not the root cause of bleaching itself. Excessive fishing could disrupt coral habitats, but it does not cause bleaching directly. Similarly, natural predator population increases might influence some aspects of coral health, but they are not linked to the bleaching process itself. Therefore, the focus on environmental changes leading to the expulsion of algae captures the essence of what triggers coral bleaching.