Chronological transition of mitochondrial morphology in Chlamydomonas reinhardtii (Chlorophyceae) poststationary phase growth(1).

Abstract

In Chlamydomonas reinhardtii P. A. Dangeard, mitochondrial morphology has been observed during asexual cell division cycle, gamete and zygote formation, zygote maturation, and meiotic stages. However, the chronological transition of mitochondrial morphology after the stationary phase of vegetative growth, defined as the poststationary phase, remains unknown. Here, we examined the mitochondrial morphology in cells cultured for 4months on agar plates to study mitochondrial dynamics in the poststationary phase. Fluorescence microscopy showed that the intricate thread-like structure of mitochondria gradually changed into a granular structure via fragmentation after the stationary phase in cultures of about 1week of age. The number of mitochondrial nucleoids decreased from about 30 per cell at 1week to about five per cell after 4months of culture. The mitochondrial oxygen consumption decreased exponentially, but the mitochondria retained their membrane potential. The total quantity of mitochondrial DNA (mtDNA) of cells at 4months decreased to 20% of that at 1week. However, the mitochondrial genomic DNA length was unchanged, as intermediate lengths were not detected. In cells in which the total mtDNA amount was reduced artificially to 16% after treatment with 5-fluoro-2-deoxyuridine (FdUrd) for 1week, the mitochondria remained as thread-like structures. The oxygen consumption rate of these cells corresponded to that of untreated cells at 1week of culture. This suggests that a decrease in mtDNA does not directly induce the fragmentation of mitochondria. The results suggest that during the late poststationary phase, mitochondria converge to a minimum unit of a granular structure with a mitochondrial nucleoid.