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

Abstract

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), Clostenum sp. (Zygnematophyceae), Thalassiosira sp. (Bacillariophyceae), Cryptornonas sp. (Cryptophyceae), Ochromonas sp. (Chrysophyceae), and Amphidinium carterae (Dinophyceae), and the freshwater green alga Chlamydornonas reinhardtij. AOX inhibitor effects were higher in stationary phase (nutrient-limited) cultures compared to linearly growing cultures. With the exception of Clostenum, 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, Tlialassiosira, Cryptornonas, 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.

Comparing electron transport with gas exchange: parameterising exchange rates between alternative photosynthetic currencies for eukaryotic phytoplankton

“…A slope <1 would indicate a decline in O 2 uptake with increasing irradiance and a slope >1 would indicate an increase in O 2 uptake with increasing irradiance. Only 3 of the species (Prorocentrum minimum, Pycnococcus provasolii and Storeatula major) had slopes that were 20 to 30% >1, indicating light-stimulated O 2 uptake, which is consistent with other species from the same taxonomic groups (25 to 35%: Lewitus & Kana 1995, Eriksen & Lewitus 199935 to 50%: Suggett et al 2008). Curiously, we did not observe any light-stimulated O 2 uptake for Thalassiosira weissfloggi and Dunaliella tertiolecta, as might have been expected given previous studies of green algae and diatoms (Weger et al 1988, Luz et al 2002.…”

“…Generally, however, the effect of light on O 2 uptake over the range from dim irradiance to saturating irradiance for photosynthesis may range from close to nil to highly stimulatory, particularly at light-saturated photosynthesis. Indeed, it is common to observe O 2 uptake rates as high as 30 to 40% of the gross O 2 evolution rate in cultivated algae under saturating light (Weger et al 1988, Lewitus & Kana 1995, Eriksen & Lewitus 1999, Suggett et al 2008, which means that 30 to 40% of photosynthetic reductant is directed to O 2 reduction rather than to anabolism.…”

Alternative oxidase and plastoquinol terminal oxidase in marine prokaryotes of the Sargasso Sea

“…Interestingly, in both phytoplankton (Weger and Dasgupta, 1993;Eriksen and Lewitus, 1999) and higher plants (Vanlerberghe and Ordog, 2002), AOX can be strongly induced by nutrient limitation and may thus play an important role in such environments.…”

“…Studies which have been done suggest that AOX may indeed be widespread in aquatic environments. For example, a significant capacity for AOX respiration (i.e., O 2 uptake resistant to CN but sensitive to AOX inhibitors) has been found in members of the Euglenophyceae, Chlorophyceae, Rhodophyceae, Bacillariophyceae, Cryptophyceae, Chrysophyceae and Dinophyceae (Benichou et al, 1988;Weger and Dasgupta, 1993;Robaina et al, 1995;Eriksen and Lewitus, 1999). Also, isotope discrimination experiments (which use the difference in isotopic discrimination against 18 O by cyt oxidase and AOX to estimate the partitioning of electrons in respiration) in Lake Kinneret suggest a largescale uptake of O 2 by AOX in this environment (Luz et al, 2002).…”

Evaluation of community respiratory mechanisms with oxygen isotopes: A case study in Lake Kinneret

“…Recently, it has been shown that AOX is widespread and quantitatively important among diverse aquatic phytoplank-ton and can be stimulated by various environmental factors (McIntosh et al 1998;Eriksen and Lewitus 1999). In particular, Maxwell et al (1999) demonstrated the engagement of this mechanism as a response to the formation of reactive oxygen species that resulted in oxidative stress.…”