Photoacoustic spectroscopy of Anacystis nidulans. III. Detection of photosynthetic activities.

Photosynthetic activities of Anacystis nidulans can be detected by photoacoustic spectroscopy. Algae treated by a photosynthetic inhibitor are used to provide the signal from the photochemically inactive sample. The results of these measurements correspond well with the activities which can be monitored by conventional biochemical assays. Acoustic data from A. nidulans are used to obtain the action spectrum for photochemical energy storage. It is concluded that phycocyanin harvests light for both photoreactions but that chlorophyll alpha molecules convey most of their excitation energy to photoreaction I. As judged from the relationship between the modulation frequency and the acoustic signal intensity, at least 60% of the photons absorbed at 630 nm perform photochemical work and about half of the useful energy is stored at stable products. Although it cannot be separated from the purely thermal effect, the contribution of modulated oxygen evolution to the acoustic signal of algae is estimated to be relatively small. Due to structural peculiarities, the opposite situation predominates in low frequency measurements performed with leaves from Impatiens petersiana.