Effect of a non-methanogenic symbiont and exogenous stigmasterol on the viability and tetrahymanol content of the anaerobic ciliate Trimyema compressum

Abstract

The effect of a non-methanogenic symbiont on the viability of Trimyema compressum strain N, cultivated monoxenically on a Bacteroides strain, was studied by elimination of the symbiont. Aposymbiotic cells and symbiont-bearing cells reached identical yield in short term experiments. After some weeks aposymbiotic cells had lost the ability to divide and exogenous stigmasterol was necessary for growth as observed for T. compressum strain K. Though with lower yields, some hopanoids were able to replace stigmasterol. The effect of stigmasterol on the generation time of T. compressum strain N and persistency of the non-methanogenic symbiont was studied. Both, higher temperature and stigmasterol supply, stimulated growth of the ciliate but, beside treatment with penicillin, only stigmasterol resulted in a loss of the non-methanogenic symbiont by statistical outgrowth. Triterpenoids were extracted from T. compressum strain N and from T. compressum strain K cultivated with optimal and suboptimal stigmasterol supply. Symbiont-bearing cells of T. compressum strain N and symbiont-free cells of T. compressum strain K grown under conditions of suboptimal stigmasterol supply contained tetrahymanol. The amount of tetrahymanol extracted per mg protein of the ciliate biomass indicated that biosynthesis of tetrahymanol in T. compressum is inhibited by exogenous stigmasterol. Tetrahymanol is not a product of the symbiont's metabolism. Stigmasterol has an effect on growth of the ciliate which appears to be similar to that of the non-methanogenic symbiont.