Light-induced rapid absorption changes during photosynthesis VI. Complex reactions in some blue-green algae

Abstract Depending on the measuring wavelengths, light-induced complex absorption-change transients with single exponential or biphasic decay have been observed in a number of blue-green algae. Detailed study with Plectonema boryanum revealed four types of transients with characteristic rise and decay kinetics: 1. (1) The rapidly rising (rise-time ⩽ 50 μ sec) and slowly decaying (half-time 300 ± 50 msec) transients, or S-transients. 2. (2) The rapidly rising and rapidly decaying (half-time 7 msec) transients, or R-transients. 3. (3) The rapidly rising and rapidly decaying (half-time 1.5 msec) transients. 4. (4) The slowly rising (∼ 10 msec) and slowly decaying (150 ± 50 msec) transients. In fresh blue-green cells, the S-(type 1) and R-transients (type 2) are always observed and reproducible, and the ratio R/S is near unity. In some blue-green algae, the R-transients deteriorate upon storage. The R-transients appear only when the excitation-beam intensity exceeds a certain threshold, but is independent of the measuring-beam intensity. Transients of types (3) and (4) are observed only part of the time. The light- minus -dark difference spectrum suggests that the rapidly rising and slowly decaying S-transients (type 1) represent the photooxidation of a cytochrome and the chlorophyll complex P 700 located at or near pigment system 1. However, there is no apparent evidence of a sequential reaction among these two species. The chemical nature of the other three types of transients remains unclear at this time. Heating up to 50° for 5 min has no effect on the 430-mμ biphasic transient. Heating near 60° preferentially affects the R-portion of the biphasic transient. Above 62°, the S-portion is also largely eliminated. Sonicated cell fragments yield a composite signal almost identical to that of fresh cells, except the overall magnitude is slightly reduced. The signal is stable between pH 4 and 11.8. 3(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) has little effect on the composite signal at 430 mμ, except the R/S ratio becomes slightly smaller, and the decay of the S-transient becomes slower. Parallel experiments on heating, sonication or DCMU treatment with oxygen evolution yield no correlation between the latter and the complex absorption-change transient. Phenylmercuric acetate at 10 −4 M shifts the oxidation level of cytochrome and P 700 in pigment system 1, and subsequent illumination causes their reduction, as manifested in the reversal of sign of the S-portion of the composite transients at 430 and 700 mμ. The simultaneous presence of phenylmercuric acetate and DCMU eliminates the composite signal completely. Addition of 10 −2 M ascorbate has no effect in relieving the inhibition, but ascorbate plus trace amount of dichlorophenol-indophenol or di(or tetra)methyl- n -phenylene diamine restored the complete biphasic signal. The effectiveness spectra obtained with 10-mμ-wide flashes indicate that 680-mμ light is active in producing the S-transients, whereas the R-transients can be produced by far-red light absorbed by chlorophyll as well as by 620-mμ light absorbed by phycocyanin. Interaction between the two pigment systems is demonstrated by the acceleration of decay of the S-transient when 680- and 620-mμ flashes are applied simultaneously, and by the absence of such an acceleration when different junctions of the electron-transport chain are blocked by DCMU or 2- n -heptyl-hydroxyquinoline- N -oxide. Interaction between the two pigment systems is further demonstrated by using one light for steady background illumination and another light for flash excitation. For instance, under steady illumination with 680-mμ light, a 620-mμ flash reverses the sign of the S-transient, indicating that 680-mμ light generates oxidized cytochrome and P 700 from pigment system 1 and subsequent 620-mμ flash generates a reductant from pigment system 2.