Cyanide-resistant respiration in diverse marine phytoplankton. Evidence for the widespread occurrence of the alternative oxidase

In higher plants, the alternative oxidase (AOX) is the terminal oxidase in a mitochondrial electron transport chain thought to allow carbon flow throu gh glycolysis and the citric acid cycle when cellular energy levels are hig h. In phytoplankton, information is lacking on the taxonomic distribution a nd metabolic properties of the AOX. We used cyanide-resistant respiration t o indicate the presence of the AOX, and the AOX inhibitors, salicylhydroxam ic acid (SHAM) and propyl gallate (PG), to estimate the relative activity a nd capacity of the AOX in axenic cultures of the marine phytoplankton, Chlo rella sp. (Chlorophyceae), Closterium sp. (Zygnematophyceae), Thalassiosira sp. (Bacillariophyceae), Cryptomonas sp. (Cryptophyceae), Ochromonas sp. ( Chrysophyceae), and Amphidinium carterae (Dinophyceae), and the freshwater green alga Chlamydomonas reinhardtii. AOX inhibitor effects were higher in stationary phase (nutrient-limited) cultures compared to linearly growing c ultures. With the exception of Closterium, in which respiration was almost completely inhibited by cyanide, estimates of AOX capacity, determined as t he effect of AOX inhibitors on cyanide-resistant respiration, were nearly i dentical using the 2 AOX inhibitors, and ranged from 46 to 113 % of the res piration rates of cultures not exposed to inhibitors. The presence of the A OX in 5 of the 6 marine phytoplankton species tested suggests that the AOX is widespread among phytoplankton. Furthermore, the pronounced inhibitory e ffect of SHAM and PG added alone to stationary phase cultures of Chlorella, Thalassiosira, Cryptomonas, and A. carterae (21 to 63 % of uninhibited res piration rates) implies that the AOX contributes substantially to oxygen an d carbon cycling in many species of phytoplankton during nutrient deficienc y.