The occurrence and fine structural characterization of microbodies in the green alga Bracteococcus cinnabarinus grown heterotrophically
Hornung, Ronald L.
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Cultures of the soil and water green alga, Bracteococcus cinnabarinus, were grown photoheterotrophically and heterotrophically on a conventional culture medium containing sodium acetate, potassium acetate and glucose as carbon sources. Control cultures were grown photoautotrophically. Ultrathin sections of cells fixed with glutaraldehyde and osmium tetroxide were examined with the electron microscope to determine the fine structural changes that may have occurred due to the different substrates and light conditions. Several changes in the ultrastructure of Bracteococcus grown photoheterotrophically and heterotrophically were observed. Disruption of the lamellae in the chloroplast, showing very little organization along with increases in the size of starch granules, was observed in cells grown heterotrophically on the acetate media. Lipid bodies which were not observed in photoautotrophically grown cells were found in all cells grown either photoheterotrophically and heterotrophically. Circular to oval microbodies ranging from 0.3 to 0.7 microns in diameter with a single limiting membrane and a homogeneous electron- dense matrix were found in close association to lipid bodies in Bracteococcus cells grown heterotrophically on sodium acetate and potassium acetate, but none were positively identified in cells maintained on the three different growth media and grown in the with cytochemical analysis of staining with diaminobenzidine (DAB). Microbodies and cristae of some mitochondria showed staining with DAB, indicating catalase activity in the microbodies and cytochromeoxidase activity in the mitochondria. Aminotriazole inhibited the DAB reactivity. Evidence from this investigation suggests that Bracteococcus, when transferred from photoautotrophic to heterotrophic conditions on acetate, will probably take up the acetate, convert it to lipid, and store it as lipid droplets. Beta-oxidation enzymes located in the glyoxysomes, induced by these conditions, convert the lipid into acetyl-coenzyme A which may in turn be converted to succinate by glyoxylate cycle enzymes. The organism may convert succinate to carbohydrates, which may be taken up by the chloroplast and converted to starch or utilized for energy. This investigation confirms the theory that the substrate and light conditions play an important role in the metabolic activity of Bracteococcus cinnabarinus and possibly other members of the Chlorophyceae, via microbody function.