Autotrophic Growth of Paracoccus denitrificans in Aerobic Condition and the Accumulation of Biodegradable Plastics from CO2

The cell growth on H2 and O2 as the energy source and CO2 as the sole carbon source in the autotrophic culture condition was tested for the gram-negative bacteria Paracoccus spp. The aerobic growth in the autotrophic condition was only observed in Paracoccus denitrificans NBRC13301 and P.pantotrophus NBRC102493. Both strains were sensitive to O2 in particular the growth of P.pantotrophus was completely inhibited at the concentrations above 2% O2. P.denitrificans grew until 15% O2 however the optimum O2 concentration was 5%. The growth characteristic of P.denitrificans was investigated by pH-controlled batch culture with supplying the gas mixture of H2, O2 and CO2. The specific growth rate was 0.12 h at 5% O2, 30°C and pH7.0. The growth was much slower than other hydrogen-oxidizing bacteria however P.denitrificans accumulated biodegradable plastic, polyhydroxybutyrate, PHB with the content of 57.3% w/w in the cell under nitrogen limitation. Under DO limitation, the cell concentration increased to 25 g/L without accumulating PHB if NH3 solution as the nitrogen source was fed sufficiently.