Matching Different Inorganic Compounds as Mixture of Electron Donors to Improve CO<sub>2</sub> Fixation by Nonphotosynthetic Microbial Community without Hydrogen

The dominant bacteria in nonphotosynthetic microbial community (NPMC) isolated from the ocean were identified by PCR-DGGE. The results revealed that the dominant microorganisms in cultures of NPMC differed when Na<sub>2</sub>S, Na<sub>2</sub>S<sub>2</sub>O<sub>3</sub>, and NaNO<sub>2</sub> were used as the electron donor to reduce CO<sub>2</sub>. These findings implied that different microorganisms in the NPMC respond to different inorganic compound as suitable electron donor, indicating that matching of Na<sub>2</sub>S, Na<sub>2</sub>S<sub>2</sub>O<sub>3</sub>, and NaNO<sub>2</sub> may provide mixed electron donors that increase the ability of NPMC to fix CO<sub>2</sub>. Accordingly, the central composite response surface method (RSM) was used to predict the optimal concentration and match of Na<sub>2</sub>S, Na<sub>2</sub>S<sub>2</sub>O<sub>3</sub>, and NaNO<sub>2</sub> as mixed electron donors to improve CO<sub>2</sub> fixation efficiency under aerobic and anaerobic conditions without hydrogen. The results indicated that 0.46% NaNO<sub>2</sub>, 0.50% Na<sub>2</sub>S<sub>2</sub>O<sub>3</sub>, and 1.25% Na<sub>2</sub>S were the optimal match under aerobic conditions, while 1.04% NaNO<sub>2</sub>, 1.07% Na<sub>2</sub>S<sub>2</sub>O<sub>3</sub>, and 0.98% Na<sub>2</sub>S were the optimal match under anaerobic conditions. Under these conditions, the fixed CO<sub>2</sub> by NPMC was determined to be 387.51 and 512.57 mg/L, respectively, which obviously exceeded those values obtained prior to optimization (5.94 and 7.14 mg/L, respectively), as well as that obtained when hydrogen was used as the electron donor (91.60 mg/L).