Selection of sulphur sources for the growth of Butyribacterium methylotrophicum and Acetobacterium woodii

Homoacetogenesis and microbial community composition are shaped by pH and total sulfide concentration

Biological CO2 sequestration through acetogenesis with H2 as electron donor is a promising technology to reduce greenhouse gas emissions. Today, a major issue is the presence of impurities such as hydrogen sulfide (H2 S) in CO2 containing gases, as they are known to inhibit acetogenesis in CO2 -based fermentations. However, exact values of toxicity and inhibition are not well-defined. To tackle this uncertainty, a series of toxicity experiments were conducted, with a mixed homoacetogenic culture, total dissolved sulfide concentrations ([TDS]) varied between 0 and 5 mM and pH between 5 and 7. The extent of inhibition was evaluated based on acetate production rates and microbial growth. Maximum acetate production rates of 0.12, 0.09 and 0.04 mM h-1 were achieved in the controls without sulfide at pH 7, pH 6 and pH 5. The half-maximal inhibitory concentration (IC50 qAc ) was 0.86, 1.16 and 1.36 mM [TDS] for pH 7, pH 6 and pH 5. At [TDS] above 3.33 mM, acetate production and microbial growth were completely inhibited at all pHs. 16S rRNA gene amplicon sequencing revealed major community composition transitions that could be attributed to both pH and [TDS]. Based on the observed toxicity levels, treatment approaches for incoming industrial CO2 streams can be determined.

Nitrate-dependent regulation of acetate biosynthesis and nitrate respiration by Clostridium thermoaceticum

Nitrate has been shown to shunt the electron flow in Clostridium thermoaceticum from CO2 to nitrate, but it did not influence the levels of enzymes involved in the Wood-Ljungdahl pathway (J. M. Fröstl, C. Seifritz, and H. L. Drake, J. Bacteriol. 178:4597-4603, 1996). Here we show that under some growth conditions, nitrate does in fact repress proteins involved in the Wood-Ljungdahl pathway. The CO oxidation activity in crude extracts of nitrate (30 mM)-supplemented cultures was fivefold less than that of nitrate-free cultures, while the H2 oxidation activity was six- to sevenfold lower. The decrease in CO oxidation activity paralleled a decrease in CO dehydrogenase (CODH) protein level, as confirmed by Western blot analysis. Protein levels of CODH in nitrate-supplemented cultures were 50% lower than those in nitrate-free cultures. Western blots analyses showed that nitrate also decreased the levels of the corrinoid iron-sulfur protein (60%) and methyltransferase (70%). Surprisingly, the decrease in activity and protein levels upon nitrate supplementation was observed only when cultures were continuously sparged. Northern blot analysis indicates that the regulation of the proteins involved in the Wood-Ljungdahl pathway by nitrate is at the transcriptional level. At least a 10-fold decrease in levels of cytochrome b was observed with nitrate supplementation whether the cultures were sparged or stoppered. We also detected nitrate-inducible nitrate reductase activity (2 to 39 nmol min-1 mg-1) in crude extracts of C. thermoaceticum. Our results indicate that nitrate coordinately represses genes encoding enzymes and electron transport proteins in the Wood-Ljungdahl pathway and activates transcription of nitrate respiratory proteins. CO2 also appears to induce expression of the Wood-Ljungdahl pathway genes and repress nitrate reductase activity.

Effect of nitrate on the autotrophic metabolism of the acetogens Clostridium thermoautotrophicum and Clostridium thermoaceticum

Although nitrate stimulated the capacity of Clostridium thermoautotrophicum and Clostridium thermoaceticum to oxidize (utilize) substrates under heterotrophic conditions, it inhibited autotrophic H2-CO2-dependent growth. Under basal medium conditions, nitrate was also inhibitory to the use of one-carbon substrates (i.e., CO, formate, methanol, or the O-methyl groups of vanillate or syringate) as sole carbon energy sources. This inhibitory effect of nitrate was bypassed when both O-methyl groups and CO were provided concomitantly; H2-CO2 did not replace CO. These results indicated that nitrate blocked the reduction of CO2 to the methyl and carbonyl levels. On the basis of the inability of acetogenic cells (i.e., cells cultivated without nitrate) to consume or reduce nitrate in resting-cell assays, the capacity to dissimilate nitrate was not constitutive. Nitrate had no appreciable effect on the specific activities of enzymes central to the acetyl-coenzyme A (CoA) pathway. However, membranes obtained from cells cultivated under nitrate-dissimilating conditions were deficient in the b-type cytochrome that was typical of membranes from acetogenic cells, i.e., cells dependent upon the synthesis of acetate for the conservation of energy. Collectively, these findings indicated that (i) C. thermoautotrophicum and C. thermoaceticum cannot engage the carbon-fixing capacities of the acetyl-CoA pathway in the presence of nitrate and (ii) the nitrate block on the acetyl-CoA pathway occurs via an alteration in electron transport.

Nitrate as a preferred electron sink for the acetogen Clostridium thermoaceticum

Nitrate enhanced the vanillin- and vanillate-dependent growth of Clostridium thermoaceticum. Under nitrate-enriched conditions, these aromatic substrates were subject to O demethylation. However, acetate, the normal product obtained from O demethylation, was not detected. Acetate was also not detected when methanol and CO cultures were supplemented with nitrate; glucose cultures likewise produced approximately one-third less acetate when enriched with nitrate. Reductant derived from the oxidation of these substrates was recovered in nitrite and ammonia. With an ammonia-limited medium employed to evaluate N turnover, the following stoichiometry was observed concomitantly with the consumption of 2.0 mM O-methyl groups (the recovery of nitrate-derived N approximated 89%): 3.9 mM NO3(-)-->2.8 mM NO2- +0.7 mM NH3. The results demonstrated that (i) nitrate was preferentially used as an electron sink under conditions that were otherwise acetogenic, (ii) nitrate dissimilation was energy conserving and growth supportive, and (iii) nitrate-coupled utilization of O-methyl groups conserved more energy than acetogenic O demethylation.