Comparative organic nutrition of an obligately and a facultatively autotrophic thiobacillus : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Microbiology at Massey University

Abstract

The organic nutrition of a facultatively autotrophic Thiobacillus (T. novellus) and an obligately autotrophic Thiobacillus (T.thioparus) were compared. A wide range of organic compounds as potential growth substrates and as effectors of growth were tested in agar plates and in liquid culture. The uptake and metabolism of 14C-labelled glucose, glutamate and acetate was examined in greater detail. (i) T. novellus used a wide range of organic compounds, including amino-acids, sugars, organic acids and alcohols, as substrates for heterotrophic growth. T. thioparas used no organic compounds as a sole energy or carbon source. (ii) Of the tested organic compounds most, but not all, were inhibitory to T. thioparus, but very few to T. novellus. Some similarities in the shape of the growth curves for the two species were evident in the presence of inhibitors, and possible mechanisms for this are discussed. The compounds capable of reversing threonine inhibition of T. thioparus are biosynthetically related. (iii) A very low rate of uptake of 14C-labelled glucose and glutamate (supplied at μ Molar levels) was found in T. thioparus, compared to the high rate of uptake in T. novellus. Acetate was rapidly taken up by both species. The assimilated compounds were largely oxidized to CO2 by T. novellus, both hoterotrophically- and autotrophically- grown, but in T. thioparus most of the 14C taken up was incorporated into cell constituents. (iv) The metabolism of both acetate and glutamate by T. thioparus was restricted by an incomplete TCA cycle: the resultant distribution of 14C can be correlated with a lack of α-ketoglutarate dehydrogenase. The TCA cycle was apparently complete in T. novellus. (v) The inhibitory effect of 16 mM succinate on T. thioparus was investigated. In liquid culture growth and consequent thiosulphate consumption and pH changes were inhibited. In cell suspensions the rate of acid production and of 14 CO2 assimilation was unaffected. The uptake and metabolism of acetate was unaffected. The uptake of 14C— glucose was markedly depressed, and possible mechanisms for this are discussed. No basis for the inhibitory effect of succinate on growth was found in these experiments. The findings are discussed in relation to the current theories of obligate autotrophy.