Cells of Chlamydomonas reinhardtii Dangeard were synchronized under a 12 hour:12 hour light:dark regimen. Behavior of mitochondria in these cells was studied by fluorescence microscopy using a mitochondrial membrane-binding fluorescent dye, dimethylaminostyrylmethyl-pyridiniumiodine (DASMPI), as well as by electron microscopy. Following time courses of change in frequency of occurrence of five typical morphologies of mitochondria in synchronized cells, strikingly dynamic behavior of mitochondria was demonstrated. The five types are (A) a giant global mitochondrion with large matrix and peripherally localized cristae, a part of which is in close contact with the nucleus, (B) a mitochondrion composed of thick-corded bodies connected to each other, a part of which is in contact with the nucleus, (C) thin-corded forms with a few branches, (D) small lump forms scattered in the cytoplasm, and (E) stringy forms with intricate branchings extended throughout the cytoplasm. During the early half of the light period, changes of C-->B-->C-->D occur, while the inversely sequential changes of D-->C-->B-->C proceed during the later half of the light period. The appearance of the B-type mitochondrion is accompanied by a transient decrease of O2-uptake activity of cells. The early appearing B-type mitochondrion is temporarily turned into a giant A-type mitochondrion, concomitant with discharge of membranes into the cytoplasm and their retake by the A-type form in the process of reversion to B-type. In the reversion process, partitioning membranes are also formed in the large matrix of A-type mitochondrion.(ABSTRACT TRUNCATED AT 250 WORDS)

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