Microbial growth and the oxidation of inorganic sulphur compounds

Abstract

The work reported in this thesis is aimed at providing a broader basis for the understanding of the growth, physiology and ecology of bacteria that oxidise inorganic sulphur compounds either as a source of energy for growth or during heterotrophic metabolism. Using chemostat culture techniques, observed yields (Y) , true molar growth yields (Y max) and maintenance coefficients were obtained for Thiobacillus versutus T.neopolitanus, T. acidophilus and Thermothrix thiopara under various conditions. This was the first report of the ability of T. acidophilus to grow on soluble inorganic sulphur compounds, and of T. neapolitanus and Thermothrlx thiopnra to grow in continuous culture on tetrathionate or trithionate. The Y max values in this study were analysed in terms of 'biochemical efficiency' of the organisms. Previously only two distinct classes of inorganic sulphur oxidisers were identified on the basis of Y values. This study reassesses these findings as it is shown that a range of Y max values on thiosulphate exist. This may mean a greater" 'biochemical diversity' in terms of carbon metabolism, inorganic sulphur compound oxidation and/or energy conservation. The kinetics of growth of Thermothrix thlonara. Thiobacillus versutus and T. neopolitanus were studied at various temperatures in thiosulphate- limited culture . In T. versutus no significant difference was seen in Y max and m at growth temperatures of 17°-35⁰C. In T. neagolitanus . yield varied little with temperature, falling slightly at 10⁰ C above the optimum. Thermothrix thionara had similar Y max values at 65 and 72⁰ C. The biochemical implications of the high Y max values obtained for Thermothrix thiopara are discussed in detail. Using steady state pH values between pH 5.5-8.A, no significant difference in Y max was found for T. versutus and T . neapolitanus. The importance of heterotrophic and facultative inorganic sulphur oxidisers is discussed. Various enrichment and isolation techniques are described for obtaining novel facultative organisms. The isolation and study of a heterotrophic inorganic sulphur oxidiser led to a subsequent study to assess the ability of known heterotrophs to oxidise thiosulphate. Batch culture experiments revealed a rarity of organisms able to oxidise thiosulphate to sulphate but a common ability to oxidise this substrate quantitatively to tetrathionate. The organisms did not appear to obtain any useful energy from the oxidation. The ability of T. acldorhllus to growmixotrophicallv was investigated. In batch culture , tetrathionate and glucose were used simul taneously without diauxie, but yields and growth rates were lower mixotrophically than heterotrophically. In chemostat culture, dual limitation by glucose and tetrathionate resulted in biomass production greater than the. sum on the two substrates separately. Ribulose 1,5-bisphosphatecarboxylase activity was monitored during transitions from heterotrophic to mixotrophic to autotrophic growth. The behaviour of T. acidophilus was compared and contrasted with other facultative inorganic sulphur oxidisers