Coral Symbiosis and Bleaching Mechanisms

Overview

• Lecture explains coral biology, symbiosis with photosynthetic dinoflagellates, and causes/mechanism of coral bleaching.
• Emphasis on cellular-level processes (chloroplast damage) leading to whole-colony bleaching and mortality.

Coral Structure and Organization

• Corals are animals that build large reef structures visible from space.
• Reefs consist of individual coral colonies located along reef edges.
• Colonies are composed of many genetically identical polyps.
• Each polyp has animal structures: mouth, tentacles, gonads, and can grow and reproduce.
• Coral tissue has two cell layers: epidermis (outer) and gastrodermis (inner).

Symbiosis With Dinoflagellates

• Coral color originates from internal symbionts, not coral animal itself.
• Symbionts are dinoflagellates living inside coral gastrodermal cells.
• Dinoflagellates are photosynthetic and contain chloroplasts with distinctive shapes.
• Symbionts provide the majority of the coral’s energy needs.

Photosynthesis, Chloroplasts, and Damage Mechanism

• Chloroplasts contain thylakoid membranes which host photosystems.
• Photosystems capture sunlight (photons) and convert light into chemical energy.
• High temperature and elevated light stress overdrive photosystems.
• Excess energy causes photosystems to break and generate reactive oxygen molecules.
• Reactive oxygen damages symbiont cells and coral host cells.

Bleaching Process

• Damaged coral cells expel symbionts (the “spitting” of symbionts).
• Loss of symbionts by individual cells is normal; colony-wide expulsion causes bleaching.
• Bleaching appears as whitening of colonies as symbiont pigments are lost.
• Symbionts supply about 75–80% of coral energy.
• Without symbionts, corals cannot sustain skeleton production and have reduced survival.
• Many bleached corals eventually die if symbiosis is not re-established.

Key Terms and Definitions

Consequences and Importance

• Bleaching reduces coral energy supply and impairs growth and skeleton formation.
• Mass bleaching events can lead to widespread coral mortality.
• Understanding the cellular cause (photosystem damage -> reactive oxygen) links environmental stress to reef decline.

Action Items / Next Steps (if studying)

• Review photosynthesis basics: photosystems, electron transport, and reactive oxygen species.
• Memorize the role and location of symbionts (gastrodermis) and percent energy supplied (75–80%).
• Study environmental factors that increase temperature and light stress on reefs.