Résumé
Compelling evidence exists that the colorless algae of the genus
Polytomella arose from a green
Chlamydomonas-like ancestor by losing its functional photosynthetic apparatus. Due to the close relationship between the colorless and the green chlorophyte,
Polytomella sp. appeared as a useful indicative framework for structural studies of
Chlamydomonas reinhardtii mitochondria. However, comparative studies reported here unexpectedly revealed significant differences between the mitochondrial respiratory systems of the two algae. Two-dimensional blue native/SDS-PAGE of isolated mitochondria indicated that cytochrome-containing respiratory complexes III and IV in the two chlorophytes contrast in size, subunit composition and relative abundance. Complex IV in
Polytomella is smaller than its counterpart in
C. reinhardtii and occurs in two forms that differ presumably in the presence of subunit COXIII. The cytochrome
c and the iron–sulfur Rieske protein of both chlorophytes revealed structural differences on the amino acid sequence level. Under comparable culture conditions, the colorless alga exhibits lower levels of cytochrome
c and complex IV but a higher respiratory activity than the green alga. Cytochrome
c levels were also found to be differently regulated by the growth conditions in both algae. The divergence between the respiratory systems in the two related chlorophytes can be viewed as a consequence of the loss of photosynthetic activity and/or of the adaptation to the environment via the acquisition of a more flexible, heterotrophic metabolism. Our understanding of mitochondrial function and evolution is expected to be greatly enhanced via further parallel studies of photosynthetic/non-photosynthetic algae, for which this study forms an incentive.