Several aspects of photosynthesis in vivo (CO2 and O2 exchange, chlorophyll fluorescence and its quenching components) were measured in Spirulina platensis after filtering suspensions of this cyanobacteria through fibre glass filters. This simple procedure allowed the use of methods which previously could be applied only to leaves. Fluorescence characteristics displayed by Spirulina in suspension or collected on the solid support were similar, indicating that filtration and assay did not by themselves affect the photosynthetic process. When oxygen evolution and carbon dioxide uptake were followed simultaneously their kinetics differed, and the rate of O2 evolution was often higher than the rate of CO2 uptake. Slow kinetics and other fluorescence characteristics were strongly modified in aged cultures or when nutrient supply during growth was not optimal, or after exposure to photoinhibitory photon flux densities. Photoinhibition and recovery were studied in two strains differing in their sensitivity to high photon flux densities. During illumination using high photon flux densities two phases could be determined. After a few minutes variable fluorescence decreased and ‘dark fluorescence’(Fo) was unchanged, but after prolonged illumination Fo increased. The latter phase appeared earlier in the more sensitive strain and could be brought forward by increasing photon flux densities or, in some circumstances, by increasing [O2].
Results are discussed in terms of the underlying relationships between electron transport and photosynthetic carbon assimilation and of the use of chlorophyll fluorescence as a diagnostic probe. It is suggested that measurement of chlorophyll fluorescence (by itself or in conjunction with quantum yield) on cyanobacteria or algae collected on solid support could provide a convenient screening procedure to assess sensitivity to photoinhibition and to other kinds of stress likely to be encountered in large-scale culture.