The quantum efficiency of photosynthesis in macroalgae and submerged angiosperms

Photon absorption and photosynthesis under conditions of light limitation were determined in six temperate marine macroalgae and eight submerged angiosperms. Photon absorption and photosynthetic efficiency based on incident light increased in proportion to chlorophyll density per area and approached saturation at the highest densities (∼300 mg chlorophyll m^-2) encountered. Absorption and photosynthetic efficiency were higher in brown and red algae than in green algae and angiosperms for the same chlorophyll density because of absorption by accessory pigments. Among thin macroalgae and submerged angiosperms chlorophyll variations directly influence light absorption and photosynthesis, whereas terrestrial leaves have chlorophyll in excess and thus there is only a minor influence of pigment variability on light-limited photosynthesis. The quantum efficiency of photosynthesis averaged 0.062±0.019 (±SD) mol O₂ mol^-1 photons absorbed for macroalgae and, significantly less, 0.049±0.016 mol O₂ mol^-1 photons for submerged angiosperms. Of the measurements 80% were between 0.037 and 0.079 mol O₂ mol^-1 photons. The results are lower than values given in the literature for unicellular algae and terrestrial C₃ species at around 0.1 mol O₂ mol^-1 photons, but resemble values for other marine macroalgae and terrestrial C₄ species. The reason for these differences remains unknown, but may be sought for in differential operation of cyclic photophosphorylation and photorespiration.