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

A mass spectrometer with a membrane inlet was used to monitor light-driven O 2 evolution, O 2 uptake, and CO 2 uptake in suspensions of algae ( Scenedesmus obliquus ). We observed the following. ( a ) The rate of O 2 uptake, which, in the presence of iodoacetamide, replaces the uptake of CO 2 , showed a distinct plateau (V max ) beyond ∼30% O 2 and was half-maximal at ∼8% O 2 . We concluded that this light-driven O 2 uptake process, which does not involve carbon compounds, is saturated at lower O 2 concentrations than are photorespiration and glycolate formation. ( b ) In the absence of inhibitor, O 2 evolution was relatively unaffected by the presence or absence of CO 2 . During the course of CO 2 depletion, electron flow to CO 2 was replaced by an equivalent flow to O 2 . ( c ) There was a distinct delay between the cessation of CO 2 uptake and the increase in O 2 uptake. We ascribe this delay to the transient utilization of another electron acceptor—possibly bicarbonate or another bound form of CO 2 .