PHOTOINHIBITION OF PHOTOSYNTHESIS IN MARINE MACROALGAE

Photoinhibition should be considered as a mechanism that protects the photosynthetic apparatus. In algae exposed to high fluence rates dynam ic photoinhibition converts excess light energy into heat and prevents photodamage. As a result radiationless energy dissipation increases a nd variable fluorescence of photosystem II (photosynthetic efficiency) and photosynthetic capacity decrease. The decrease of the photosynthe tic parameters showed a linear correlation with the increase in zeaxan thin content in photosystem II antenna of marine brown algae. Field me asurements of variable fluorescence and oxygen production rate indicat e that the depression caused by dynamic photoinhibition follows a diur nal pattern with the lowest photosynthetic activity occuring usually b etween noon and the early afternoon. In the afternoon photosynthesis c ommences to recover, and is almost complete by the evening. However, t he diurnal pattern depends on the tidal level because the water column above the algal beds protects the algae from high fluence rates. Desi ccation of benthic marine algae during low tide showed different effec ts on the photoinhibitory level in different species. Photosynthetic m easurements made in the natural habitat of different latitudes clearly showed that dynamic photoinhibition occurs regardless of water temper ature. The Antarctic brown alga Adenocystis utricularis regulated phot osynthesis by dynamic photoinhibition even faster than brown macroalga e of the tropic or temperate zones. However, algal zonation is thought to depend in part on the capability for dynamic photoinhibition. Inve stigations of photoinhibition of photosynthesis usually use both fluor escence and oxygen measurements. However, recent studies indicate that the two methods can give different results if the rate of saturated o xygen production is compared to the variable fluorescence of photosyst em II in red algae.