Kinetics and Apparent Km of Oxygen Cycle under Conditions of Limiting Carbon Dioxide Fixation

Abstract

A mass spectrometer with a membrane inlet was used to monitor light-driven O2 evolution, O2 uptake and CO2 uptake in suspensions of algae (Scenedesmus obliquus). The rate of O2 uptake, which, in the presence of iodoacetamide, replaces the uptake of CO2, showed a distinct plateau (Vmax) beyond .apprx. 30% O2 and was half-maximal at .apprx. 8% O2. Apparently this light-driven O2 uptake process, which does not involve carbon compounds, is saturated at lower O2 concentrations than photorespiration and glycolate formation. In the absence of inhibitor, O2 evolution was relatively unaffected by the presence or absence of CO2. During the course of CO2 depletion, electron flow to CO2 was replaced by an equivalent flow to O2. There was a distinct delay between the cessation of CO2 uptake and the increase in O2 uptake. This delay was ascribed to the transient utilization of another electron acceptor, possibly bicarbonate or another bound form of CO2.