Salicylhydroxamic acid (SHAM) inhibition of the dissolved inorganic carbon concentrating process in unicellular green algae

Abstract

Rates of photosynthetic O2 evolution, for measuring K0.5(CO2 + HCO3-) at pH 7, upon addition of 50 micromolar HCO3- to airadapted Chlamydomonas, Dunaliella, or Scenedesmus cells, were inhibited up to 90% by the addition of 1.5 to 4.0 millimolar salicylhydroxamic acid (SHAM) to the aqueous medium. The apparent Ki(SHAM) for Chlamydomonas cells was about 2.5 millimolar, but due to low solubility in water effective concentrations would be lower. Salicylhydroxamic acid did not inhibit oxygen evolution or accumulation of bicarbonate by Scenedesmus cells between pH 8 to 11 or by isolated intact chloroplasts from Dunaliella. Thus, salicylhydroxamic acid appears to inhibit CO2 uptake, whereas previous results indicate that vanadate inhibits bicarbonate uptake. These conclusions were confirmed by three test procedures with three air-adapted algae at pH 7. Salicylhydroxamic acid inhibited the cellular accumulation of dissolved inorganic carbon, the rate of photosynthetic O2 evolution dependent on low levels of dissolved inorganic carbon (50 micromolar Na-HCO3), and the rate of 14CO2 fixation with 100 micromolar [14C] HCO3-. Salicylhydroxamic acid inhibition of O2 evolution and 14CO2-fixation was reversed by higher levels of NaHCO3. Thus, salicylhydroxamic acid inhibition was apparently not affecting steps of photosynthesis other than CO2 accumulation. Although salicylhydroxamic acid is an inhibitor of alternative respiration in algae, it is not known whether the two processes are related.