Physiological acclimation to gradients of solar irradiance within mats of the filamentous green macroalga Chaetomorpha linum from southern Spain

Abstract

Chaetomorpha linum (Mller) Ktzing is a dominant mat-forming green macroalgal species within eutrophic systems. Its structure, comprising thick mats of interwoven filaments, requires acclimation to either high or low irradiance along a steep gradient of light arising from selfshading. Occurring in high-light environments, such as that along the southern coast of Spain, C. linum mats represent an excellent model system for studying spatial differences in photoacclimation and also the interactive effects of photosynthetically active and ultraviolet radiation on photoinhibition. This study was conducted in eutrophic brine-pond systems along the Bay of Cdiz (southern Spain). For periods of up to 1 wk, C. linum mats were exposed to natural solar radiation under different cut-off filters to study the effects of different wavelength ranges on photosynthetic activity, pigment and protein composition, and the occurrence of oxidative stress and changes in enzymatic activity of superoxide dismutase (SOD). The results revealed a particularly high tolerance of C. linum photosynthesis to high light and UV exposure. Addition of the UV-B range (280 to 315 nm) to photosynthetically active radiation, resulted in inhibition of non-photochemical quenching, a delay in recovery from photoinhibition, and an increase in both lipid peroxidation and SOD activity. In contrast, pigment and protein data revealed acclimation to the high irradiances of photosynthetically active radiation, but no additional effect of UV radiation. The data suggest that even in a highradiation environment such as southern Spain, UV-B radiation plays a minor role in photoacclimation of C. linum, but is effective in increasing both the generation and scavenging of oxidative stress. The UV-induced increase in SOD activity represents an important physiological acclimation mechanism of C. linum, enabling it to thrive under high UV radiation and high-light conditions.