Effects of acetate on facultative autotrophy in Chlamydomonas reinhardtii assessed by photosynthetic measurements and stable isotope analyses

The green alga Chlamydomonas reinhardtii can grow photoautotrophically util izing CO2, heterotrophically utilizing acetate, and mixotrophically utilizi ng both carbon sources. Growth of cells in increasing concentrations of ace tate plus 5% CO2 in liquid culture progressively reduced photosynthetic CO2 fixation and net O-2 evolution without effects on respiration, photosystem II efficiency (as measured by chlorophyll fluorescence), or growth. Using the technique of on-line oxygen isotope ratio mass spectrometry, we found t hat mixotrophic growth in acetate is not associated with activation of the cyanide-insensitive alternative oxidase pathway. The fraction of carbon bio mass resulting from photosynthesis, determined by stable carbon isotope rat io mass spectrometry, declined dramatically (about 50%) in cells grown in a cetate with saturating light and CO2. Under these conditions, photosyntheti c CO2 fixation and O-2 evolution were also reduced by about 50%. Some growt h conditions (e.g. limiting light, high acetate, solid medium in air) virtu ally abolished photosynthetic carbon gain. These effects of acetate were ex acerbated in mutants with slowed electron transfer through the D1 reaction center protein of photosystem II or impaired chloroplast protein synthesis. Therefore, in mixotrophically grown cells of C. reinhardtii, interpretatio ns of the effects of environmental or genetic manipulations of photosynthes is are likely to be confounded by acetate in the medium.