Bleaching Biology

illustration, healthy to dead coral, from GBRMPA

Source: Great Barrier Reef Marine Park Authority (GBRMPA)

Elevated seawater temperatures in combination with strong sunlight cause thermal stress in corals. This stress can cause disruption of normal photosynthetic processes in the coral’s zooxanthellae which leads to coral bleaching.

Role of Temperature and Light

The main trigger of large-scale bleaching events is an increase in water temperatures above the normal summer maximum. At elevated temperatures, the photosynthetic system of zooxanthellae is easily overwhelmed by incoming light leading to production of reactive oxygen species. These are a source of oxidative stress in the coral’s tissue, causing the coral to expel zooxanthellae to avoid further tissue damage. While increased temperatures are the trigger for bleaching, light is also an important factor. Increased irradiance can exacerbate bleaching risk, while corals that are partially shaded can tolerate higher temperatures before bleaching.

Recovery from Bleaching

Zooxanthellae is the name given to a wide array of different algae of the genus Symbiodinium. These zooxanthellae are all very different, but share the same spherical form that enables them to live within corals and many other tropical marine creatures in mutually beneficial relationships. Other than hard corals, zooxanthellae are also found in species of soft-corals, sea anemones, gorgonians, giant clams (Tridacna spp.), and many nudibranchs. The zooxanthellae fix the sun's light energy, benefiting both themselves and the animals in which they live, receiving, in turn, safe harbor and exchanging metabolic products in a remarkably efficient mutual relationship. This relationship has persisted for more than 200 million years, and is one of the most important for the existence of coral reefs. Photo © M. Takabayashi

Zooxanthellae is the name given to a wide array of different algae of the genus Symbiodinium. These zooxanthellae are all very different, but share the same spherical form that enables them to live within corals and many other tropical marine creatures in mutually beneficial relationships. Other than hard corals, zooxanthellae are also found in species of soft-corals, sea anemones, gorgonians, giant clams (Tridacna spp.), and many nudibranchs. The zooxanthellae fix the sun’s light energy, benefiting both themselves and the animals in which they live, receiving, in turn, safe harbor and exchanging metabolic products in a remarkably efficient mutual relationship. This relationship has persisted for more than 200 million years, and is one of the most important for the existence of coral reefs. Photo © M. Takabayashi

Without the zooxanthellae to support their metabolic processes, corals begin to starve. Should water temperatures return to normal conditions soon enough, corals can survive a bleaching event. Where bleaching is not too severe, the zooxanthellae can repopulate from the small numbers remaining in the coral’s tissue, returning the coral to normal color over a period of weeks to months. Some corals, like many branching corals, cannot survive for more than 10 days without zooxanthellae. Others, such as some massive corals, are capable heterotrophs and can survive for weeks or even months in a bleached state by feeding on plankton. Even corals that survive are likely to experience reduced growth rates, decreased reproductive capacity, and increased susceptibility to diseases.

Variations in Bleaching Susceptibility

Corals vary in their susceptibility to bleaching. Consistent patterns of susceptibility can be seen among coral species, with a general trend of higher susceptibility in more intricate, branching forms and lower susceptibility in massive species, especially those with fleshy polyps.

Corals can also acquire a greater tolerance to bleaching stresses if they are constantly exposed to higher temperatures or greater irradiance. Corals on reef flats, for example, will often be able to tolerate much higher water temperatures than colonies of the same species inhabiting reef slopes.

Video
Heat Resistant Zooxanthellae (2:54)

Andrew Baker discusses heat resistant zooxanthellae.

The type of zooxanthellae can also influence bleaching susceptibility. There are at least nine groups (called clades) of zooxanthellae currently recognized, and there may be many species within these groups. Zooxanthellae clades vary in their ability to tolerate elevated temperatures, and some corals have heat-resistant clades, and are therefore, more resistant to bleaching. However, corals with heat-resistant clades tend to grow more slowly, creating evolutionary trade-offs in the symbiotic relationship that maintain a diversity of clade-coral relationships.

Resources

A Reef Managers Guide to Coral Bleaching (pdf, 5.4M)

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Last updated July 8, 2015

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