ACTION SPECTRA OF MICROALGAL PHOTOSYNTHESIS AND DEPTH DISTRIBUTION OFSPECTRAL SCALAR IRRADIANCE IN A COASTAL MARINE SEDIMENT OF LIMFJORDEN, DENMARK

The role of complementary spectral utilization of light for the zonati on of different groups of oxygenic phototrophic organisms in sediments was studied. The marine sediment was covered by a dense population of diatoms with an underlying population of cyanobacteria. Action spectr a for photosynthesis and spectral scalar irradiance, E0, were measured directly in the sediment at a spatial resolution of 0.1 mm by the use of oxygen and light microsensors. The action spectrum for the diatoms was similar to the attenuation spectrum of the scalar irradiance, K0, in the diatom layer with Chl.a. and carotenoids being the major photo synthetic pigments. The action spectrum of the cyanobacteria showed ph otosynthesis maxima at the absorption regions of Chl.a. and phycocyani n. The measured depth distribution of spectral scalar irradiance and t he action spectra of diatoms and cyanobacteria were used to calculate the spectral quality for photosynthesis of the 400-700 nm light to whi ch the two populations were exposed. This spectral quality was compare d to that of the light incident on the sediment surface. Due to prefer ential extinction of wavelengths, at which their photosynthetically ac tive pigments had maximal absorption, the relative light quality for d iatoms was reduced to 85% of the quality of incident light at a simila r total quantum flux. This effect was partly due to spectral alteratio ns of light backscattered from the underlying sediment with cyanobacte ria. The cyanobacteria at the bottom of the euphotic zone, in contrast , experienced a light spectrum which was favorably altered, to 107% in quality, due to absorption by the overlying diatoms. It was concluded that these changes in spectral light quality can be considered as onl y one of more factors explaining the zonation of the two phototrophic populations, and that total light intensity and the chemical microenvi ronment are probably more important factors.