Determination of dissolved oxygen in photosynthetic systems by nitroxide spin-probe broadening

Concentrations of dissolved oxygen were monitored by following the width of the midfield line of the electron spin resonance spectrum of a nitroxide spin-probe. Measurements of peak-to-trough widths of first derivative spectra yielded accurate data over a high range of O2 concentrations (up to 5mM). Continuous traces of second harmonic line heights yielded similar results and proved to be advantageous for kinetic measurements, but were nonlinear and less sensitive at O2 levels above 2 mM. Photosynthetic oxygen evolution and respiratory oxygen uptake in the cyanobacterium Agmenellum quadruplicatum were thus examined over a broad range of oxygen concentrations. Upon prolonged (greater than 1 min) illumination, effects of photooxidative damage to both photosynthesis and respiration were demonstrated in the same experimental system. With the addition of an impermeable paramagnetic broadening agent, rapid transients in intracellular concentrations of dissolved O2 also could be measured.