Acetate mutants and obligate photoautotrophy in Chlamydomonas eugametos

Abstract

Incorporation of C-14 labelled sodium acetate by three species of Chlamydomonas: C. eugametos, C. reinhardii and C. moewusii indicates that all three species assimilate and metabolize exogenous acetate. All three species have higher growth rate in the light with acetate than without. C. eugametos and C. moewusii, being obligate photoautotrophs, are not able to grow heterotrophically in the dark. C. reinhardii, a facultative photoautotroph, can grow heterotrophically. C. reinhardii has radioactive sodium acetate uptake over ten times greater than that of C. eugametos or C. moewusii. This suggests that the former assimilates and metabolizes acetate more efficiently than the latter two. Adaptation from autotrophic to mixotrophic growth is accompanied in C. eugametos and C. reinhardii by their individual characteristic and different patterns of radioactive sodium acetate uptake, indicating that the two species have different regulatory systems for acetate metabolism. Comparative intermediary carbohydrate metabolism in terms of enzyme activities in the tricarboxylic acid cycle was investigated in C. eugametos and C. reinhardii. All the expected enzyme activities of this cycle are present in both species grown either autotrophically or mixotrophically with acetate. The patterns of enzyme activities in each species differ with different growth conditions. In C. reinhardii, concomitant presence of light and acetate leads to higher activities of succinate dehydrogenase, fumarase and malic enzyme; while condensing enzyme, aconitase, NADP specific isocitrate dehydrogenase, NAD specific isocitrate dehydrogenase and [alpha]-ketoglutarate dehydrogenase are acetate inducible and light independent. The situation in C. reinhardii just mentioned indicates that succinate dehydrogenase, fumarase and malic enzyme may be closely associated with photosynthesis. Ten acetate mutants requiring both acetate and light to survive were recovered after ultraviolet treatment and are designated: R-l, R-2, R-3, R-4, R-5, R-6, R-7, R-8, R-9 and R-10. Mutants R-4, R-5, R-6, R-8 and R-9 have Mendelian segregation, and mutant R-l is most likely nontransmissible. Mutant R-3 has all the expected enzyme activities in the tricarboxylic acid cycle. Mutants R-l, R-5 and R-7 have no succinate dehydrogenase. In mutant R-4, activity of condensing enzyme cannot be demonstrated.