Photosynthesis in Marine Macroalgae

Abstract

Marine macrophytes live in a highly variable environment in respect to light, {UV-radiation}, temperature and also salinity. During evolution their metabolism has adapted to these strongly changing conditions, especially in the upper littoral zone. In the lower subtidal, the photosynthetic apparatus has adapted to low light conditions, to absorb maximally the incident photons and to utilize the absorbed energy with high efficiency. In the intertidal and upper sublittoral zone the photosynthetic apparatus is exposed to strong light stress when the absorbed amount of light energy is higher than the organism can use for its metabolism. This happens generally when plants are exposed to irradiances, which are above the mean level of irradiation occurring at their growth sites (e.g., light flecks or low tide around noon). Under light stress the amount of thermal energy dissipation increases and the energy flowing into photochemistry decreases also. Photoinhibition causes a decrease of the photosynthetic quantum yield (dynamic inhibition) and at much higher fiuence rates or after a longer duration of high irradiation, also a decrease of the photosynthetic capacity (chronic inhibition). The absorption cross- section of the chromatophores can be diminished by chloroplast displacement or shrinking, especially in brown algae, which decreases the rate of photodamage to the light-absorbing apparatus. Irradiances, which exceed the protective capacities of these mechanisms, cause irreversible damage to the photosynthetic apparatus. Normally, the latter does not occur for long at the natural growth sites.