The analysis involved 50 amino acid sequences. All ambiguous

positions were removed for each sequence pair. There were a total of 863 positions find more in the final dataset. Evolutionary analyses were conducted in MEGA5 [20]. Thermodynamic calculations were performed using values provided by Thauer et al.[21] and the CRC Handbook of Chemistry and Physics [21, 22]. BioEdit v.7.0.9.0 [23] was used to perform sequence alignments. Results and discussion Survey of End-product yields A literature survey of end-product yields (normalized to mol end-product per mol hexose equivalent) of the species surveyed in this study is summarized in Table 2. While it is difficult to perform a direct comparison of end-product yields from available literature due to different growth conditions employed (ex. growth substrate, carbon loading, reactor conditions, etc.), and further difficult to validate these data due to incomplete end-product quantifications

and lack of corresponding carbon balances and oxidation/reduction (O/R) ratios, it still provides a good approximation of molar end-product yields based on substrate utilization. Calculated end-product yields reveal that the Caldicellulosiruptor, Pyrococcus, Thermococcus, and Thermotoga species surveyed, produced, in most cases, near-maximal H2 yields with Anlotinib concomitant CO2 and acetate production, and little or no ethanol, formate, and lactate [24–40]. It is important to note that while some studies [29–31, CYTH4 34, 35, 39] report lower overall end-product yields, likely due ASK inhibitor to a large amount of carbon flux being directed towards biomass production under a given growth condition, H2:ethanol ratios remain high. Cal. subterraneus subsp. tengcongensis, E. harbinense, and Clostridium species displayed mixed end-product fermentation patterns, with comparatively lower H2, CO2, and acetate yields, higher ethanol yields, and generally low formate and lactate yields [10, 41–47].

Ta. pseudethanolicus produced the highest ethanol yields of the organisms surveyed with little concomitant H2, acetate, and lactate production, and no formate synthesis [48–50]. G. thermoglucosidasius and B. cereus produced the highest lactate and formate yields, moderate ethanol and acetate yields, and low H2 and CO2 yields [51, 52]. Table 2 Summary of end-product yields, optimal growth temperatures, total molar reduction values of H 2   + ethanol ( RV EP ), and growth conditions employed Organism Growth temp (°C) End products (mol/mol hexose equivalent)   Growth condition Ref     H2 CO2 Acetate Ethanol Formate Lactate RV EP     Ca. saccharolyticus DSM 8903 70 4.0 1.8 NR ND ND ND 4.0 Cont., 1.1 g l-1 glucose (D = 0.09 h-1) [24]     3.6 1.5 1.6 ND ND ND 3.6 Cont., 4.1 g l-1 glucose (D = 0.1 h-1) [24]     3.5 NR 2.1 NR NR NR 3.5 Batch, 10 g l-1 sucrose [25]     2.5 1.4 1.4 ND ND 0.1 2.5 Batch, 10 g l-1 glucose [26] Ca.