| Popis: |
The thylakoid lamellae which traverse the py- renoid of the unicellular red alga Porphyridium cruentum (Agardh) Nfigeli appear to lack phycobilisomes. We have confirmed by immuno-electron microscopy that phy- coerythrin (PE), an important structural component of the phycobilisomes of red algae, is absent from the pyre- noid. To characterize pyrenoid thylakoids further, elec- tron-microscopic cytochemical methods were employed to detect photosystem activity. Photosystem (PS) I activ- ity was demonstrated in both stromal and pyrenoid thy- lakoids by the photooxidation of 3,3'-diaminobenzidine. In contrast, the localization of photoreduced distyryl ni- troblue tetrazolium demonstrated that PSII activity was restricted to stromal thylakoids. The observed partition- ing of PE and PSII activity within the plastid may be related to another observation, that being the localiza- tion of nearly all ribulose-l,5-bisphosphate carboxylase/ oxygenase (EC 4.1.1.39) within the pyrenoid of this alga. It is possible that the pyrenoid of P. cruentum functions as a specific metabolic compartment where CO2 fixation is enhanced by the absence of photosynthetic 02 evolu- tion. |
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