Bacteroids from soybean root nodules: accumulation of poly- β -hydroxybutyrate during supply of malate and succinate in relation to N 2 fixation in flow-chamber reactions

Abstract

A liquid reaction medium containing dissolved air and oxyleghaemoglobin was supplied to bacteroids confined in a stirred reaction chamber with no gas phase. The relative oxygenation of leghaemoglobin, dissolved CO 2 and NH 3 from N 2 fixation were monitored before, during (when N 2 fixation was diminished) and after the supply of [ 14 C]malate and succinate. The amounts and location of 14 C-labelled products were determined in bacteroids recovered from the chamber and 14 CO 2 was measured in the effluent. During the supply of [ 14 C]succinate, the bacteroids accumulated most 14 C from C 2 and C 3 whereas 14 CO 2 arose primarily from C 1 and C 4 . This suggested that production of pyruvate from malate via malic enzyme was a central feature. For up to 80 min after removal of [ 14 C]malate and succinate, CO 2 arose from stored carbon and N 2 fixation was enhanced; during this period 14 C continued to accumulate in poly-β-hydroxybutyrate, until this accounted for about 90% of bacteroid 14 C. Thereafter CO 2 was evolved with steady, low radioactivity and enhanced N 2 fixation continued. These results supported the proposal that poly-β-hydroxybutyrate, which is present in large amounts (50-70% of dry mass) in these bacteroids, is a mobilizable, energy-yielding reserve that provides endogenous substrates for support of N 2 fixation when exogenous substrates are not available.