Although acetogens are able of utilizing CO and CO2/H2 as carbon and energy source, other constituents such as vitamins, trace metal elements, minerals and reducing agents are also required for maintenance of high metabolic activity [16, 113]. Studies indicated that forma­tion of ethanol in solventogenic Clostridiais non-growth associated and limitation of growth by reducing availability of carbon-, nitrogen — and phosphate — nutrients shift the balance from acidogenesis to solventogenesis [113, 200, 201]. Optimization of medium formulation for C. Ijungdahlii through reduction of B-vitamin concentrations and elimination of yeast ex­tract significantly enhanced the final ethanol yield to 48 g/l in a CSTR with cell recycling (23 g/l without cell recycling) [113]. Another study by Klasson et al. showed thatthe replacement of yeast extract with cellobiose not only increased maximum cell concentration, but also en­hanced ethanol yield by 4-fold [14]. Media formulation for C. autoethanogenum was investi­gated using Plackett-Burman and central composite designs, but only low ethanol yield was recorded overall [202]. In an attempt to reduce the cost of fermentation medium and im­prove process economics, 0.5 g/l of cotton seed extract without other nutrient supplementa­tion was shown to be a superior medium for C. carboxidivorans strain P7 in producing ethanol from syngas fermentation [203]. A recent study showed that increasing concentra­tions of trace metal ions such as Ni2+, Zn2+, SeO4-, WO4-, Fe2+ and elimination of Cu2+ from medium improved enzymatic activities (FDH, CODH, and hydrogenase), growth and etha­nol production in "C. ragsdalei" under autotrophic conditions [107].

A low redox potential is necessary for strict anaerobes to grow, hence reducing agents such as sodium thioglycolate, ascorbic acid, methyl viologen, benzyl viologen, titanium (III)—cit — rate, potassium ferricyanide, cysteine-HCl and sodium sulfide are commonly added to fer­mentation medium [14, 16, 204]. Furthermore, the addition of reducing agent directs the electron and carbon flow towards solventogenesis by enhancing the availability of reducing equivalents to form NADH for alcohol production [16, 205]. Excessive addition of reducing agents can cause slower microbial growth due to reduced ATP formation from acetogenesis so it is important to determine the optimum concentration of reducing agents [14, 16]. The sulfur containing gases (e. g. H2S) present in syngas are toxic to chemical catalysts but can be beneficial for microbial catalysts by reducing medium redox potential, stimulate redox sen­sitive enzymes such as CODH, and promote alcohol formation [206, 207].