Compensation Point is usually defined as the minimum amount of light required for oxygen production to meet the zooxanthellae/coral host respiratory requirements. Corals have the ability to absorb oxygen from the surrounding water (as they do in darkness); however, insufficient light energy may also result in low production of photosynthetic lipids. During periods of prolonged darkness (or inadequate light) zooxanthellae will then use their energy reserves until they are depleted and a sort of starvation occurs, usually resulting in irreversible damage or death. Compensation points vary from specimen to specimen and often depend upon their light history. Compensation points in low light adapted corals may be just a few µMols·m2·sec or much higher in high light adapted corals (350 µMols·m2·sec or ~17,500 lux; see Kirk, 1983). It should be understood that light intensity should exceed the zooxanthella’s compensation point.
Saturation Point Photosynthetic rates are proportional to light intensity only to a certain point. The Saturation Point has been met when photosynthesis is at a maximum, and increasing light will no longer increase the rate of photosynthesis. Saturation occurs when the photosynthesis electron transport systems are operating at full capacity. Exceeding the saturation point is pointless, and from a practical standpoint, results in needlessly high electric bills. If light energy greatly exceeds the saturation point, Photoinhibition may occur.
Photoinhibition is generally defined as any occurrence interrupting the normal electron flow in photosynthesis. There are two types of photoinhibition – dynamic and chronic. The first is chronic photoinhibition that involves irreversible damage to Photosystem II and were synthesis of new “photosynthetic proteins” must occur before normal photochemistry may resume (Brown et al, 1999). Dynamic photoinhibition involves reversible photochemical reactions that divert excess light energy away from Photosystem II through thermal dissipation. This “quenching” of photosynthesis involves reversible changes in xanthophylls diadinoxanthin and diatoxanthin. Dynamic photoinhibition protects the zooxanthellae (through absorption of violet through yellow-green wavelengths of 400-550 nm) from high levels of photosynthetically produced oxygen radicals, including hydrogen peroxide. Not all strains of zooxanthellae have the ability to produce xanthophylls and therefore may have little resistance to the effects of high light intensity.
