Effect of exogenous calcium on morphological development and biopolymer synthesis in the fungus Aureobasidium pullulans

The effect of adding exogenous Ca2+ to cultures of Aureobasidium pullulans which were initially entirely in the yeastlike form was examined in shake flasks and a stirred-tank reactor (STR). Since culture pH is known to affect Ca2+ availability, studies were performed at a range of starting pHs. In addition, in order to confirm the role of Ca2+, the chelating agent Ethylene Glycol-bis(beta-aminoethylether)-N,N,N',N-Tetraacetic Acid (EGTA) and an inhibitor of Ca2+ uptake were used. In shake flasks, addition of exogenous Ca2+ led to a decrease in the proportion of the culture in mycelial form and a marked increase in biopolymer synthesis. This morphological response was apparent even at the lowest level of added CaCl2 (0.01 kg m(-3)). At high added Ca2+ (0.3 kg m(-3)), extensive precipitation occurred and a pelleted morphology resulted. Restriction of Ca2+ availability by the use of Methyl Hydroxy Butyrate (MHB) or EGTA confirmed the role of Ca2+ in the observed morphological changes. At both low (2.5) and high (6.5) initial pHs, the effects of exogenous Ca2+ were much less pronounced. Culture pH also affected the molecular mass of the biopolymer formed. In the STR, the effects of Ca2+ nutrition on morphology were less pronounced, although an effect on the molecular mass of pullulan was apparent. The possible reasons for the differing results in different experimental systems are discussed. (C) 1997 Elsevier Science Inc.