Conserved proteins across oxygenic phototrophic organisms are important players in photosynthetic acclimation, yet many remain poorly or completely uncharacterized. In the green algaChlamydomonas reinhardtiiwe identified Cre15.g636050, previously annotated asCPLD31, that encodes such an uncharacterized, conserved, thylakoid lumenal protein belonging to the “TPM” domain family (PF04536). TPM refers to a protein fold named after the three founding proteins of this family, TLP18.3, Psb32, and MOLO-1, with the first two being Photosystem II-related proteins. We also found a paralogue ofCPLD31, Cre03.g154600, previously annotated asTLP15.2. In this work, we generated knock-out mutants of these genes in Chlamydomonas and produced the recombinant proteins. We showed both proteins are regulated by light and phototrophy, and while the former binds manganese in a TPM-domain metal binding site, the latter does not. Therefore, we reannotated them asLumenalMn-bindingTPM-domainProtein1(LMTP1) andThylakoidLumenalProtein26kDa (TLP26), respectively.