Glycolate metabolism in algal chloroplasts: inhibition by salicylhydroxamic acid (SHAM)

Unicellular green algae such as Chlamydomonas and Dunaliella excrete small amounts of glycolate during active photosynthesis. This phenomenon has been explained by the fact that these algae do not have leaf-type peroxisomes and glycolate oxidase; instead, they have a limited capacity to metabolise glycolate in their mitochondria by a membrane-associated glycolate dehydrogenase. Salicylhydroxamic acid (SHAM), an inhibitor of alternative oxidase in plant and algal mitochondria, stimulates glycolate excretion by the algae or their isolated chloroplasts 5-fold. In the presence of SHAM, cells of Chlamydomonas or Dunaliella grown with high-CO₂ (5% CO₂ in air, v/v) or adapted with air levels of CO₂ excreted glycolate at a rate of about 14 µmol glycolate mg⁻¹ Chl h⁻¹. Aminooxyacetate (AOA), an inhibitor of aminotransferases, also increases glycolate excretion by the algal cells or chloroplasts but at a lower rate (about 50%) than SHAM. The algal, light dependent, SHAM-sensitive glycolate oxidizing system in the chloroplasts appears to be the primary site for glycolate oxidation, and it is different and more active then the minor mitochondrial glycolate dehydrogenase.