Characterization of extracellular oxygen consumption by the green alga Selenastrum minutum

Abstract

Cells of the green alga Selenastrum minutum display a high capacity for extra‐mitochondrial O2 consumption in the presence of effectors such as salicylhydroxamic acid and/or NADH. We provide evidence that this O2 consumption is mediated by extracellular peroxidase. Peroxidase capacity, measured as the potential for stimulation of O2 consumption by a combination of salicylhydroxamic acid and NADH, changed over a 10‐day time course. Maximal stimulation of O2 consumption occurred at day three, at which point the capacity for peroxidase‐mediated O2 consumption was three‐to four‐fold higher than that of the control O2 consumption rate. Peroxidase‐mediated O2 consumption was sensitive to inhibition by 50 mM ascorbate and by cyanide. Cyanide titration curves indicated that O2 consumption by peroxidase was much more sensitive to inhibition by cyanide than was O2 consumption by cytochrome oxidase (I50 < 1.6 μM and I50= 18.3 μM cyanide, respectively). By using evidence from a combination of cyanide titration curves and ascorbate inhibition, we concluded that despite a large capacity for peroxidase‐mediated O2 consumption, peroxidase did not measurably contribute to control rates of O2 consumption. In the absence of effectors, O2 consumption was mediated primarily by cytochrome oxidase.