Traits of selected Clostridium strains for syngas fermentation to ethanol

Abstract

Syngas fermentation is an anaerobic bioprocess that could become industrially relevant as a biorefinery platform for sustainable production of fuels and chemicals. An important prerequisite for commercialization is adequate performance of the biocatalyst (i.e., sufficiently high production rate, titer, selectivity, yield, and stability of the fermentation). Here, we compared the performance of three potential candidate Clostridium strains in syngas-to-ethanol conversion: Clostridium ljungdahlii PETC, C. ljungdahlii ERI-2, and Clostridium autoethanogenum JA1-1. Experiments were conducted in a two-stage, continuously fed syngas-fermentation system that had been optimized for stable ethanol production. The two C. ljungdahlii strains performed similar to each other but different from C. autoethanogenum. When the pH value was lowered from 5.5 to 4.5 to induce solventogenesis, the cell-specific carbon monoxide and hydrogen consumption (similar rate for all strains at pH 5.5), severely decreased in JA1-1, but hardly in PETC and ERI-2. Ethanol production in strains PETC and ERI-2 remained relatively stable while the rate of acetate production decreased, resulting in a high ethanol/acetate ratio, but lower overall productivities. With JA1-1, lowering the pH severely lowered rates of both ethanol and acetate production; and as a consequence, no pronounced shift to solventogenesis was observed. The highest overall ethanol production rate of 0.301 g · L(-1) · h(-1) was achieved with PETC at pH 4.5 with a corresponding 19 g/L (1.9% w/v) ethanol concentration and a 5.5:1 ethanol/acetate molar ratio. A comparison of the genes relevant for ethanol metabolism revealed differences between C. ljungdahlii and C. autoethanogenum that, however, did not conclusively explain the different phenotypes.

Continuous Ethanol Production from Synthesis Gas by Clostridium ragsdalei in a Trickle-Bed Reactor

“…However, ethanol productivity in the present study was lower than the 301 mg/L·h ethanol productivity reported for C. ljungdahlii in a two stage CSTR and bubble column with gas and cell recycling [23].…”

“…Ethanol produced in the present study (13.2 g/L) was higher than that reported in a CSTR with cell recycle using A. bacchi (6 g/L), a bubble column reactor (1.6 g/L) and a monolithic biofilm reactor (4.9 g/L) using C. carboxidivorans [1,5,28]. However, 19 g/L ethanol was reported in a two-stage continuous syngas fermentation in a CSTR followed by a bubble column with gas and cell recycling [23] and 48 g/L ethanol was reported in a CSTR with cell recycle using C. ljungdahlii [14]. Up to 24 g/L ethanol production was reported with hollow fiber membrane biofilm reactor (HFM-BR) using C. carboxidivorans [29].…”

“…Bioreactors such as air-lift reactors, continuous stirred tank reactors (CSTRs), trickle-bed reactors (TBRs) and hollow fiber membrane (HFM) reactors have been characterized for their capabilities for CO mass transfer into fermentation medium [12,[15][16][17][18][19]. Further, improved ethanol production over batch bottle fermentations was reported when fermentations were performed in various bioreactors that provided larger working volume, greater cell recycling, continuous addition of nutrients and syngas, and better control of operating parameters [1,16,[20][21][22][23]. For example, C. carboxidivorans produced only 0.9 g/L ethanol in batch bottles [24] compared to 1.6 g/L ethanol in bubble column [21].…”

Bioethanol Production via Syngas Fermentation

“…These strains are categorized into mesophiles as their optimum growth temperature is between 37 o C and 40 o C. C. ljungdahlii and C. autoethanogenum are the common model microorganism for basic research and commercial application [4]. From a comparison study of syngas fermentation between C. ijungdahlii and C. autoethanogenum strains, C. ljungdahlii is revealed to be a more superior ethanol producer than C. autoethanogenum by ethanol titter produced in low pH condition [11]. Furthermore, some studies also confirm that C. autoethanogenum is a low productivity bacteria in ethanol production compared to other carboxidotrophic strains of Clostridium sp.…”

A Prospective Study on the Fermentation Landscape of Gaseous Substrates to Biorenewables Using Methanosarcina acetivorans Metabolic Model

“…Acetogens has been the workhouse for gas fermentation in industry for over two decades. Anaerobic conversion of carbon monoxide into valuable products such as ethanol, acetate, and 2,3-butanediol at industrial scale has been pursued using different strains of Clostridium (Simpson et al, 2010; Köpke et al, 2011a,b; Tran and Simpson, 2015; Daniell et al, 2016; Martin et al, 2016). A recent study on the co-utilization of carbon dioxide and carbon monoxide or hydrogen to produce acetate using Moorella thermoacetica (Hu et al, 2016) further demonstrates the need for systematic study of co-utilization of various C 1 gases in other potential microbial hosts.…”