Photoinhibition in the Mediterranean green alga Acetabularia mediterranea measured in the field under solar irradiation

Abstract

Photoinhibition of photosynthesis was investigated in the Mediterranean green macroalga Acetabularia mediterranea using pulse amplitude modulated (PAM) chlorophyll fluorescence and oxygen evolution measurements in situ under solar radiation. Fluorescence parameters measured during the course of the day showed that moderate photoinhibition occured even in the natural environment of this alga when the sun was at high angles, but photosynthetic capacity was almost fully restored until sunset. A drastic decline in effective quantum yield was monitored when plants collected at 5 m depth were exposed to high irradiance close to the surface. Removal of the short wavelength band from solar radiation using cut-off filters revealed that UV has an overproportional inhibitory effect on photosynthesis. Especially the UV-B part of the spectrum contributes to photoinhibition whereas UV-A seems to be less effective in A. mediterranea. Total recovery of PAM chlorophyll fluorescence was obtained after 2 h of shading in all samples, indicating reversible photoinhibition rather than non-reversible photodamage. Oxygen evolution showed slower kinetics of inhibition.