Debaryomyces 1. In yeasts, the physiological purpose of

Debaryomyces hansenii, a halotolerant and oleaginous yeast, owns a branched mitochondrial respiratory chain (Breuer U et al., 2006).

Besides the canonical pathway composes by complexes I-IV, the electron flux can be driven through alternative routes that include a cyanide-resistant alternative oxidase (AOX), an external NADH dehydrogenase (NDH2e) and a glycerol-phosphate dehydrogenase (MitGPDH)(Cabrera-Orefice A. et al., 2014), Fig. 1. In yeasts, the physiological purpose of alternative oxidoreductases is not entirely known but probably avoids electron flux saturation on the mitochondrial cytochrome pathway (Guerrero-Castillo et al., 2011) establishing a Radical Oxygen Species-preventing mechanism (El-Khoury R.

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et al., 2013). Protection from ROS could be explained due to this redox enzymes maintain a high oxygen consumption without a proton pumping activity, uncoupling the oxidative phosphorylation despite a raised transmembrane potential (Joseph-Horne et al., 2001). In contrast to NDH2e and MitGPDH constitutive expression (Cabrera-Orefice et al., 2014), AOX appears at the stationary growth phase (Veiga et al.

, 2003). Nevertheless, data from our group suggest that the expression of AOX may also depend on the carbon source added to the culture medium. Therefore, we analysed cyanide-resistant respiration (CRR) in intact yeast cells cultivated on different carbon sources. Here, we studied both cytochrome- and alternative-pathway related oxygen consumption differences in intact yeast cells grown on different carbon sources.

In lactate- or succinate-containing media, AOX was fully active in all growth phases. By contrast, in glucose- or galactose-containing media, AOX activity was detected until the late exponential phase. Since both lactate and succinate are considered as substrates which promote respiration, their donated electrons may saturate cytochrome route, and AOX serves as an alternative route. However, in glycerol-containing media, AOX activity was poorly detected in either growth phase. This result was unanticipated because glycerol is amply used in respiratory culture media and a lactate/succinate-grown cells behaviour will be expected. Besides, glucose-grown cells exhibited the most significant biomass yield. Nevertheless, biomass yield in glycerol-grown cells was higher than in the other three carbon sources.

This indicates that a considerable amount of glycerol was used in biosynthetic pathways. In this condition, D. hansenii probably did not express AOX to keep the Oxidative Phosphorylation system in a highly coupled mode and support ATP needs for glycerol-promoted lipid biosynthesis. This finding result attractive as D. hansenii would be useful for biotechnological targets such as biodiesel production.