Photosynthetic Electron Transfer in Preparations of the Cyanobacterium Spirulina platensis

Publisher Website
Google Scholar
Abstract
Electron transfer activity in intact trichomes of S. platensis (Nordst.) Geitl. occurs with either CO2 or methylviologen as the Hill acceptor. Ferricyanide cannot penetrate the intact trichomes, but photoreduction of this oxidant occurs when mediated by lipophilic oxidants such as p-phenylenediamine or 2,5-dimethyl-p-benzoquinone. The insensitivity of these reactions to dibromothymoquinone indicates that they are due largely to the activity of photosystem II. Direct photoreduction of ferricyanide occurs in spheroplasts of Spirulina, indicating that such preparations have altered permeability properties when compared with intact trichomes. Preparations of these sphereoplasts, which are osmotically fragile, requires that intact trichomes be washed with KCl and EDTA to induce lysozyme sensitivity and allow digestion of the cell wall. The KCl/EDTA washing procedure used for spheroplast preparation alters the permeability of Spirulina trichomes, as evidenced by the ability of these preparations to photoreduce ferricyanide. This photoreduction reaction is insensitive to dibromothymoquinone and is stimulated by high concentrations of divalent cations. During assays, the reaction is inhibited by the inclusion of polyethyleneglycol as an osmotic protectant. Photoreduction of methylviologen and NADP+ occurs in the washed trichomes, with an endogenously catalyzed photoreduction of O2 to H2O2. Photophosphorylation cannot be observed in the washed preparations, but cyclic photophosphorylation with phenazinemethosulfate occurs after mild sonication. Evidently, KCl/EDTA-washed trichomes of S. platensis retain the full range of energy transducing capacities associated with thylakoid membranes of the intact trichomes; the washing procedure facilitates spheroplast formation and alters, but does not abolish, permeability barriers in these preparations.