Microorganisms can be categorized into two major types depending on their optimum growth temperature, namely, mesophilic and thermophilic. In general, optimum growth tem­perature for mesophilic microorganisms varies from 37 to 40 °C, whereas for thermophiles, the temperature range varies from 55 to 80 °C (Munasinghe and Khanal, 2010 (a)). Mesophilic microorganisms, for example, Clostridium aceticum, Acetobacterium woodii, C. carboxydivorans,

and C. Ijungdahlii, have been dominated in syngas fermentation with higher syngas to biofuel conversion efficiencies compared to the thermophilic counterpart (Henstra et al., 2007; Younesi et al., 2005).

C. Ijungdahlii, one of the most widely used homoacetogenic microorganism, has an opti­mum growth temperature between 37 and 40 °C, and a pH of 5.8-6.0 (Tanner et al., 1993). However, C. Ijungdahlii was reported producing an ethanol concentration as high as 48 g/L in a continuous stirred-tank reactor (CSTR) at a low pH of 4.0-4.5 in nutrient-limited culture media (Klasson et al., 1993). C. carboxydivorans (earlier referred to as bacterium strain P7) is another mesophilic organism which has an ability to grow on a gas mixture consists of CO, CO2, H2, and N2, and produces mainly ethanol and acetic acid. The ethanol yield obtained was 0.16% (by weight) during a 10-day fermentation experiment at pH of 5.75 and tempera­ture of 37 °C in a bubble column reactor (Rajagopalan et al., 2002). Later, the authors com­pared the ethanol yields of C. carboxydivorans and C. ljungdahlii and found out that the results were similar. In a separate study, Heiskanen et al. (2007) used B. methylotrophicum at 37 °C and a pH of 6.0-6.9 to convert syngas to acetic acid. The authors claimed a maximum acetic acid concentration of 1.3 g/L at a gas mixture of 40% H2,35% CO and 25% CO2 after 144 h of fermentation.

The major advantage of thermophilic microbes in syngas fermentation is their capability of fermenting syngas at relatively high temperatures (around 60 ° C) with higher conversion rates. Though the high temperature reduces the solubility of the component gases in the fer­mentation broth, benefit in product recovery improves the overall cost effectiveness of the process (Henstra et al., 2007). There were several attempts to utilize extreme thermophiles (optimum growth temperature >70 °C) in producing organic solvents. During the last decade, researchers were able to isolate several thermophilic microorganisms which were able to grow on CO as a substrate (Henstra et al., 2007). Desulfotomaculum carboxydivorans, Carboxydocella sporoproducens, Moorella thermoacetica, and M. thermoautotrophica are some examples of thermophiles with optimum temperature ranges of 55-58 °C.

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