Properties of the primary electron acceptor complex of photosystem I in the blue green alga Chlorogloea fritschii.

Photooxidation of P700 at low temperatures in membrane fractions from the blue-green alga $\text{Chlorogloea fritschii}$ may be coupled irreversibly to the reduction of a bound ferredoxin. If this ferredoxin is reduced before freezing, P700 photooxidation at low temperatures becomes reversible. This reversible photooxidation is coupled to the reduction of a component with an EPR signal at g = 2.08, 1.88 and 1.78. A complete spectrum of this component has been obtained for the first time. We propose that as in higher plants this component is the primary electron acceptor of Photosystem I, the bound ferredoxin is a secondary electron acceptor. Using ⁵⁷Fe enriched preparations we have shown that the ERP signals attributed to the bound ferredoxin are due to iron containing centres. This experiment did not show the presence of iron in the primary electron acceptor.