THERMAL STRESS & CORAL BLEACHING

Coral bleaching

Loss or removal of zooxanthellae from the coral host, often triggered by environmental stress.

Thermal stress

Temperature stress that can cause coral bleaching.

Where does the bleaching mechanism begin?

In the photosynthetic machinery of zooxanthellae.

Heat stress proteins

Proteins that repair dysfunctional enzymes or mark damaged enzymes for breakdown and replacement.

What happens to enzymes during thermal stress?

Enzymes become dysfunctional.

Effect of dysfunctional enzymes

Demand for photoprotective processes increases.

Effect of increased photoprotection demand

Photosystems begin to break down.

Effect of photosystem breakdown

Reactive oxygen species increase.

Reactive oxygen species (ROS)

Highly reactive oxygen-containing molecules that can damage cells.

Photo-oxidative stress

Cellular stress caused by increased reactive oxygen species.

What happens when ROS increase?

ROS can leak from the zooxanthellae into the coral host.

What happens when ROS enter the host?

The host removes dysfunctional zooxanthellae.

Thermal stress response sequence

Enzymes fail → need for photoprotection rises → photosystems break → ROS rise → ROS leak into host → zooxanthellae are removed.

Easy thermal stress sequence

Enzymes fail → protection ↑ → photosystems break → ROS ↑ → ROS leak → zooxanthellae OUT.

Why does heat damage cause bleaching?

Damaged zooxanthellae photosystems cannot process light properly, causing stress and removal of zooxanthellae.

Carbon limitation hypothesis

Under stress, changes in coral and zooxanthellae metabolism reduce the amount of carbon supplied to the coral host.

Coral holobiont respiration under stress

Respiration increases under stress.

Effect of increased respiration

CO2 and inorganic nitrogen availability for zooxanthellae increases.

Zooxanthellae response to increased CO2 and nitrogen

Zooxanthellae retain more photosynthates and nitrogen, and cell density increases.

Photosynthates

Energy-rich products made during photosynthesis.

Effect of zooxanthellae retaining more photosynthates

Less carbon is transferred to the coral host.

Zooxanthellae becoming parasitic

Under stress, zooxanthellae retain more resources while requiring more from the host.

Host response to parasitic zooxanthellae

The coral host removes zooxanthellae.

Carbon limitation hypothesis sequence

Stress → respiration ↑ → CO2 and inorganic N ↑ → zooxanthellae retain resources → less carbon to host → zooxanthellae become parasitic → host removes them.

Nitric oxide (NO)

Signaling molecule involved in the coral bleaching response.

Nitric oxide production during stress

NO production can trigger bleaching.

Role of NO in bleaching

NO plays an important role in host-cell signaling that triggers apoptosis.

Apoptosis

Programmed cell death triggered through cellular signaling.

Nitric oxide (NO)

Signaling molecule involved in the coral bleaching response.

Nitric oxide production during stress

NO production can trigger bleaching.

Role of NO in bleaching

NO plays an important role in host-cell signaling that triggers apoptosis.

Apoptosis

Programmed cell death triggered through cellular signaling.

What determines survival and recovery from bleaching?

Energy reserves, capacity for heterotrophy, and occurrence of additional stressors.

Energy reserves and bleaching

Availability of stored energy affects whether a coral can survive and recover.

Heterotrophy and bleaching recovery

Ability to obtain food externally can help support survival and recovery.

Additional stressors and bleaching

Other simultaneous stressors can reduce a coral's ability to survive and recover.