Effect of the availability of magnesium ions in κ-carrageenan gels on the formation of conidia by Coniothyrium minitans

Abstract

An isolate of Coniothyrium minitans did not sporulate on media to which 0‐05 mm magnesium was added when j-carrageenan was used to solidify the medium. Normal conidiation was observed when a technical grade agar or agar-agar was used as the gelling agent. The j-carrageenan and agars used in this study contained significant amounts of Mg. In the agar media all the Mg present was available to the fungus. By contrast, in the j-carrageenan gel a large portion of the Mg was bound by the gel and not available. Sporulation of C. minitans was only observed on media containing & 0‐17 mm of unbound Mg. The possible role of Mg in the initiation of conidium formation by C. minitans is discussed. Most microbial cultivations in industry are done in submerged culture. For some processes, however, cultivation on solid substrates in the absence of free-flowing water (solid-substrate cultivation) is advantageous. The production of high quantities of Coniothyrium minitans W. A. Campb. conidia, for instance, in stirred-tank reactors has heretofore not been reported. Numerous conidia, however, can easily be obtained when C. minitans is grown on various solid substrates (McQuilken & Whipps, 1995). These conidia are a promising biocontrol agent to treat Sclerotinia sclerotiorum contaminated soils and plants (Whipps & Gerlagh, 1992). The prospects of a biocontrol agent based on C. minitans are promising, provided that an ecient production process is developed. Optimization of the solid substrate with respect to spore formation is essential for the development of a cost-eective biocontrol agent. Quantification of biomass dry weight is important in such optimization studies. Direct gravimetric quantification of biomass is usually not possible when natural solid substrates are used, due to the close association of the microorganism with the substrate. During the development of an alternative model system based on j-carrageenan which facilitates the quantitative recovery of biomass dry matter (Weber, Tramper, & Rinzema, 1999), we found that magnesium is very important for sporulation. The present study evaluates the eect of several minerals in combination with various gelling agents on sporulation of C. minitans. We demonstrate that the amount of freely diusible magnesium strongly aects the spore yield. Furthermore, we demonstrate that the gelling agent used to mimic solid substrate cultivation may produce artefacts in medium optimization studies.