A2 was grown in chemostat culture under four distinct types of substrate limitation: chemolithoautotrophically with limitation by thiosulphate or CO; heterotrophically with limitation by glucose; and mixotrophically with dual limitation by both thiosulphate and glucose. Under mixotrophic conditions energy was obtained from the oxidation of both thiosulphate and glucose, and carbon was derived both from CO fixation by the Calvin cycle and from glucose. Ribulosebisphosphate carboxylase (RuBP carboxylase) activity was negligible and chemolithotrophic thiosulphate oxidation and autotrophic CO fixation were apparently repressed in bacteria which had been grown heterotrophically. Conversely, under autotrophic conditions the ability to oxidize glucose was repressed. Growth yields from mixotrophic cultures were the sum of those obtained under single substrate limitation. Intermediate activities of RuBP carboxylase were detected in mixotrophic cultures, but more glucose was assimilated mixotrophically than heterotrophically. Glucose was metabolized by the Entner-Doudoroff (85 to 90%) and pentose phosphate (10 to 15%) pathways under both heterotrophic and mixotrophic conditions, with slight involvement also of the Embden-Meyerhof pathway (<9%) heterotrophically. RuBP carboxylase activity in autotrophic cultures was enhanced four- or fivefold by CO limitation. Repression of RuBP carboxylase activity and thiosulphate-oxidizing ability during the transition from autotrophy to heterotrophy and the activities of carbohydrate-metabolizing enzymes in autotrophic, heterotrophic and mixotrophic cultures are described.


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