Low-potential cytochrome c that plays a role in the oxygen-evolving complex of photosystem II (PSII). Unlike Synechococcus vulcanus it does not bind by itself to PSII, but requires all extrinsic members of the OEC. Binds 1 heme c group covalently per subunit. The oxygen-evolving complex in red algae is composed of psbO (OEC33), psbQ', cytochrome c-550 and psbU. Associated with photosystem II at the lumenal side of the thylakoid membrane. Belongs to the cytochrome c family. PsbV subfamily.