Cells were grown in TAP medium for 24 h, washed and transferred to N- and S-deprived medium. Under N-deprived
conditions, a slightly increased H2 production was observed (Fig. 5A). Similar results were observed for cells under
S-deprived conditions. A combination of N- and S-deprived conditions resulted in about 1.5-fold increase in H2 production.
When cells were grown in TAP supplemented with 0.5 mM b-mercaptoethanol, washed and transferred to
a combination of N- and S-deprived medium, there was little or no effect of both N- and S-deprivation on H2 production
(Fig. 5B). Previous studies have shown an increased H2 production in cells cultivated in S-deprived conditions during
the production phase [24e29]. The stimulatory effect of N- and S-deprivation may have been masked by the increased H2
production due to the effect of b-mercaptoethanol. One possibility is that there was no more capacity to increase the
rate of electron supply to the hydrogenase.