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 through glycolysis and the citric acid cycle when cellular energy levels are high. In phytoplankton, information is lacking on the taxonomic distribution and metabolic properties of the AOX. We used cyanide-resistant respiration to indicate the presence of the AOX, and the AOX inhibitors, salicylhydroxamic acid (SHAM) and propyl gallate (PG), to estimate the relative activity and capacity of the AOX in axenic cultures of the marine phytoplankton, Chlorella 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 cultures. With the exception of Closterium, in which respiration was almost completely inhibited by cyanide, estimates of AOX capacity, determined as the effect of AOX inhibitors on cyanide-resistant respiration, were nearly identical using the 2 AOX inhibitors, and ranged from 46 to 113 % of the respiration rates of cultures not exposed to inhibitors. The presence of the AOX in 5 of the 6 marine phytoplankton species tested suggests that the AOX is widespread among phytoplankton. Furthermore, the pronounced inhibitory effect of SHAM and PG added alone to stationary phase cultures of Chlorella, Thalassiosira, Cryptomonas, and A. carterae (21 to 63 % of uninhibited respiration rates) implies that the AOX contributes substantially to oxygen and carbon cycling in many species of phytoplankton during nutrient deficiency.