Ecological significance of blue light stimulation of photosynthetic capacity in Laminaria spp. and other brown algae

Abstract

The transient stimulation of light-saturated photosynthesis In Laminaria digitata (Huds.) Lan~our, and L. saccharina (L.) Lamour., which has been observed following pulses of blue light, was found to persist when low irradiances of continuous blue light were given as a supplement to saturating irradiances of red or yellow light. The degree of stimulation was directly proportional to the logarithm of the irradiance of blue light, with a 50% response at 0.28 1.1mol m-' S-' and saturation above 1 pm01 m-' S-'. These irradiances represented about 0.2 % and 0.5 %, respectively, of the total irradiance incident on the plants. In natural underwater light fields, such low proportions of blue wavelengths would be found only close to, or below, the lower depth limit for Laminaria spp., where photosynthesis, if it occurred at all, would be light-limited and, therefore, not subject to blue light stimulation. Irradiances of blue light measured in the Laminaria zone during periods when the total irradiance was high enough to saturate photosynthesis were always higher than 1 pm01 m-' S-', and photosynthesis by Laminaria spp. in simulated underwater light fields in the laboratory was not affected by additional blue light. Unlike Laminaria, other brown algae (e.g. Asperococcus. Ectocarpus) exhibited stimulation by blue light in irradiances of red light which are limiting for photosynthesis, and their photosynthetic rates can, therefore, be limited when blue light is present as a higher proportion of the total irradiance than for Lam~naria. However, these plants are mostly found in the littoral zone, and will rarely experience low blue light environments. The stimulation of photosynthetic capacity by blue light in brown algae occurs at such low irradiances of blue light that photosynthesis by these plants, in their natural habitats, is unlikely ever to be limited by a shortage of blue light.