Influence of some environmental factors on Rhizopus nigricans submerged growth in the form of pellets

Morphological changes of a steroid transforming filamentous fungus Rhizopus nigricans were studied by altering submerged growth conditions at inoculum sizes previously determined to favor pelleted growth. Beside the main clas sification between pelleted and clumpy growth forms, the size, concentratio n and structure of pellets were characterized at different cultivating temp erature, initial pH value, medium composition, additives, and aeration cond itions. Initial pH below 4 and above 7, the presence of Ca2+ and Tween-80 g ave rise to the clumpy growth, otherwise pelleted growth prevailed. Among t ested factors the pellet size was mainly influenced by the inoculum size an d the presence of baffles and Ca2+ in cultivation medium. The formation of smooth pellets, prerequisite for further application in the process of ster oid biotransformation, resulted in cultivations at lower temperature, high agitation rates in shaken cultures without baffles and at high nitrogen con centration.