Immunocytochemical Characterization of the Intrapyrenoid Thylakoids of Cryptomonads1

Thylakoid lamellae extend into the pyrenoids of only two genera of cryptomonad algae,ChroomonasandHemiselmis, We used immunoelectron microscopy to assess the photosynthetic competency of cryptomonad intrapyrenoid thylakoids. Intrapyrenoid thylakoids possess phycobiliproteins and the chlorophyll a/c2light‐harvesting complex, both of which are associated with photosystem (PS) II in a light‐harvesting capacity. In addition, thylakoids that extend into the pyrenoid ofHemiselmis brunnescenswere immunolabelled by anti‐PSI. These results indicate that cryptomonad intrapyrenoid thylakoids likely function in a manner analogous to thylakoids of the chloroplast stroma. Moreover, our observation that the Calvin cycle enzyme ribulose 1,5‐bisphosphate carboxylase/oxygenase (Rubisco) is pyrenoid‐localized in these two cryptophytes indicates that the processes of photosynthetic O2‐evolution and ribulose 1,5‐bisphosphate (RuBP) carboxylation/oxygenation are not spatially separated in these algae.