Environment and Ecology Research 4(4): 231-236, 2016 http://www.hrpub.org DOI: 10.13189/eer.2016.040407 Autotrophic Growth of Paracoccus denitrificans in Aerobic Condition and the Accumulation of Biodegradable Plastics from CO 2 Kenji Tanaka 1,* , Shunya Mori 1 , Mai Hirata 1 , Hiromi Matsusaki 2 1 Department of Biological and Environmental Chemistry, Faculty of Humanity-Oriented Science and Engineering, Kindai University, Japan 2 Department of Food and Health Sciences, Faculty of Environmental and Symbiotic Sciences, Prefectural University of Kumamoto, Japan Copyright©2016 by authors, all rights reserved. Authors agree that this article remains permanently open access under the terms of the Creative Commons Attribution License 4.0 International License Abstract The cell growth on H 2 and O 2 as the energy source and CO 2 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 O 2 in particular the growth of P.pantotrophus was completely inhibited at the concentrations above 2% O 2 . P.denitrificans grew until 15% O 2 however the optimum O 2 concentration was 5%. The growth characteristic of P.denitrificans was investigated by pH-controlled batch culture with supplying the gas mixture of H 2 , O 2 and CO 2 . The specific growth rate was 0.12 h -1 at 5% O 2 , 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 NH 3 solution as the nitrogen source was fed sufficiently. Keywords Polyhydroxyalkanoates, Paracoccus, Hydrogen-oxidizing Bacterium 1. Background Paracoccus spp. is the gram-negative bacteria possessing the ability for denitrification (reduction of NO 3 - to N 2 ). They play an important role in the nature. Especially, P.denitrificans is employed in the biotreatment of waste water. Genome of this bacterium was sequenced in 2004 [1]. Paracoccus spp. accumulates poly-β-hydroxybutyrate (PHB) or the copolyester of β-hydroxybutyrate and other hydroxyalkanoate (PHA) in their cells and the polyester is expected as biodegradable plastics [2]. Some strains of Paracoccus spp. grow with CO 2 as carbon source, and H 2 or thiosulfate as energy source in autotrophic condition. Many researchers have been reported about Paracoccus spp., however in most of the researches for the autotrophic culture using the inorganic gasses as the substrate was studied only for denitrification in anaerobic condition, and the accumulation of PHB or PHAs was studied only in heterotrophic culture using organic compounds as the substrate, for example feeding mixed substrate of acetic acid and valeric acid [3], glycerol [4], methanol and n-amyl alcohol [5]. We studied the growth characteristics of Paracoccus spp. in the autotrophic condition using the gas mixture of H 2 , O 2 and CO 2 as the substrate, and high cell density culture and the accumulation of PHB from CO 2 . 2. Methods The bacteria used in this study were Paracoccus denitrificans NBRC13301, P.alcaliphilus JCM7364 T , P.pantotrophus NBRC102493, P.thiocyanatus NBRC14569, P.aminophilus NBRC16710, P.aminovorans NBRC 16711 and P.versutas NBRC14567. All the strains were purchased from NBRC (NITE Biological Resource Center, National Institute of Technology and Evaluation, Tokyo) or JCM (Japan Collection of Microorganisms, RIKEN, Tsukuba, Japan). These bacteria were subcultured every three weeks on BY medium or PYG medium agar plates. The composition of BY media was meat extract 5.0g, peptone 5.0g, NaCl 2.5g, K 2 HPO 4 0.1g, MgSO 4 ･7H 2 O 0.2g, yeast extract 5.0g and agar 15g per 1 L of distilled water (pH7.2); and that of PYG medium was peptone 5.0g, yeast extract 5.0g and glucose 5.0g per 1 L (pH7.0). Basic composition of mineral medium for autotrophic