| Abstract: | Oxygen inhibited the rate of light-saturated photosynthesis of the marine diatom Phaeodactylum tricornutum Bohlin. However, inhibition could only be detected with O2 concentrations approaching 100%. Atmospheric concentrations of O2 (21%) had little effect on photosynthesis. In this, Phaeodactylum more closely resembles the so-called C-4 plants which show low rates of photorespiration. The results presented here agree with others in showing increased O2 inhibition at reduced bicarbonate concentrations. The biochemical mechanism of photorespiration in Phaeodactylum appears to be similar to that reported for other photosynthetic systems. The activity of ribulose-1,5 diphosphate (RuDP) carboxylase in cell-free extracts was also inhibited, by oxygen. Inhibition by O2 was optimal at pH 9.2 as was the RuDP-dependent O2 uptake. RuDP carboxylase/oxygenase ratios decreased with increasing pH and were greater in cells grown at lower light intensities. Carboxylase levels were less affected by the light intensity for growth than were the levels of the oxygenase. Short-term incorporation of NaHCO3-14C by cells grown at high light intensities showed increased labelling of glycolate and glycine plus serine under O2 compared with nitrogen. There was a concomitant decrease in the radioactivity found in phosphoglyeric acid (PGA) and sugar phosphates in the presence of O2. The effects of O2 on the short-term pattern of photosynthesis were less marked when the alga was previously grown at low light intensities. |
