Influence of water activity and nutrients on growth and production of squalestatin S1 by a Phoma sp.

This study investigated the effects of temperature, nutrient status and water activity (aW) on the production of squalestatin S1 by a Phoma sp. The fungus was grown on malt extract (MEA), wheat extract (WEA), oat extract (OEA) and oil seed rape extract (OSREA) agars at 15, 20 and 25 degrees C and 0.998, 0.995, 0.990, 0.980 and 0.960 aW levels. The growth rate and secondary metabolite formation were followed over a total of 30 d. The maximum growth rate was observed at 25 degrees C and 0.998-0.990 aW for all media types, which was significantly reduced (P = 0.05) for most media at 0.96 aw. The growth rate was greatest for WEA and OEA but the growth form was an effuse exploitative type compared with the dense assimilative type on the richer MEA. The lipid-based OSREA appeared to be a poor growth substrate for this fungus. In contrast to the growth rate data, squalestatin S1 production was maximal for all media types at slightly reduced aw in the range 0.990-0.980. There was greater production of the secondary metabolite under significant water stress (0.960 aW) compared with that with freely available water (0.998 aW). Maximum production was observed in WEA. Production began earlier in WEA and OEA compared with MEA. Squalestatin S1 production was not significantly affected by incubation temperature (P = 0.05). This study has shown that nutritionally depleted substrates may be usefully employed in the production of squalestatin S1 and perhaps also for other secondary metabolites.