Rare earth ion Nd3+ promotes production of cellulose ethanol by Clostridium thermocellum ATCC 27405

Abstract

Clostridium thermocellum can directly produce ethanol from cellulose by consolidated bioprocessing. We studied how rare earth ions impact the ethanol titer and found that Nd3+ can significantly increase the production of ethanol from cellulose by C. thermocellum ATCC 27405. With the addition of 10−6 M Nd3+ in the initial incubation, the ethanol titer increased significantly, but the cellulose degradation ratio and cell growth did not change significantly. The intracellular distribution of Nd3+ was determined. Most Nd3+ was located on the surface of the cell wall. Nd3+ increased cell permeability, which was one of the reasons for the ethanol titer increased. We further explored the underlying mechanism from the perspective of key enzymes in the ethanol synthesis pathway. The enzyme activity and enzyme expression level of alcohol dehydrogenase (ADH) of strain C1 with added Nd3+ were lower than those of the original strain C0. However, both the enzyme activity and enzyme expression levels of acetaldehyde dehydrogenase (ALDH), pyruvate-ferredoxin oxidoreductase (PFO) and 6-phosphofructokinase (PFK) were increased. This rule was still valid for the sixth-generation strain C2 with Nd3+ removed. It was verified that the increase in the expression level of ALDH was not caused by the mutation of the ALDH promoter sequence. This research explored the key factors that limited the titer of cellulose ethanol in consolidated bioprocessing from an unconventional perspective and provided a new strategy for improving cellulose ethanol production.