Light inhibition of respiration is due to a dual function of the cytochrome b6-f complex and the plastocyanin/cytochrome c-553 pool in Aphanocapsa.

Studies of the respiratory electron transport pathway in the blue-green alga, Aphanocapsa, demonstrated the presence of cytochrome oxidase and a cytochrome complex. The use of antimycin A showed only the occurrence of a plastidal type of cytochrome complex (the cytochrome b6-f complex), which is insensitive to this inhibitor. Determination of the extent of photooxidation of cytochromes c-553 and f-556 under conditions of high and low cytochrome oxidase activities indicated an electron flow through both cytochromes to cytochrome oxidase. Direct evidence for a common segment of photosynthetic and respiratory electron transport from plastoquinone via the cytochrome b6-f complex to the soluble plastocyanin/cytochrome c-553 pool, as well as a competition between cytochrome oxidase and Photosystem I for reductants in this pool in the light, was obtained by measurements of electron transport with suitable electron donors in this alga.