Physiological and morphological acid resistance ofCandida boidinii
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The growth ofCandida boidinii strain 2 in a methanol-limited chemostat at a dilution rate of 0.1/h and a low extracellular pH (2.8–4.0) is characterized by a maximum yield coefficient referred to the methanol consumedY S of 0.4 g/g and a maximum cell content of nitrogenous compounds of 60%. The cell proteins are rich in essential amino acids. At pH<2.6 or >4.0 the cell concentration decreases due to lower growth rate, accompanied by increased metabolic quotientsQ S,Q CO2 andQ form, and increased activities of dissimilating dehydrogenases. The activity of alcohol oxidase (AO) in intact cells (0.54 IU/mg protein) was unaffected by pH 2.8–3.8 although in a cell-free extract the AO activity decreased at these low pH values after a 10-min incubation. The lower AO activity in cells at pH<2.8 and pH>3.8 brought about increased residual methanol levels in the medium, and also an increased level of riboflavin phosphate, arising probably by the release of FAD from active AO. Catalase activity was completely pH-independent. Cell morphology also showed no changes at pH 2.8–4.2, formation of cell chains being observed only at pH<2.8. However, the ultrastructure of cells grown in the chemostat at pH 2.6, however, did not evince any changes as compared with cells grown, at higher pH apart from a lag in cytokinesis. These findings, which point to acid resistance of strain 2, make it possible to produce biomass from methanol, with a high content of valuable proteins and AO, under nonsterile conditions.
KeywordsDilution Rate Alcohol Oxidase Methylotrophic Yeast Metabolic Quotient Formaldehyde Dehydrogenase
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