title: Flux measurements and maintenance energy for carbon dioxide utilization by Methanococcus maripaludis.
abstract: BACKGROUND
  The rapidly growing mesophilic methanogen Methanococcus maripaludis S2 has a unique ability to consume both CO2 and N2, the main components of a flue gas, and produce methane with H2 as the electron donor. The existing literature lacks experimental measurements of CO2 and H2 uptake rates and CH4 production rates on M. maripaludis. Furthermore, it lacks estimates of maintenance energies for use with genome-scale models. In this paper, we performed batch culture experiments on M. maripaludis S2 using CO2 as the sole carbon substrate to quantify three key extracellular fluxes (CO2, H2, and CH4) along with specific growth rates. For precise computation of these fluxes from experimental measurements, we developed a systematic process simulation approach. Then, using an existing genome-scale model, we proposed an optimization procedure to estimate maintenance energy parameters: growth associated maintenance (GAM) and non-growth associated maintenance (NGAM).
  
  
  RESULTS
  The measured extracellular fluxes for M. maripaludis showed excellent agreement with in silico predictions from a validated genome-scale model (iMM518) for NGAM = 7.836 mmol/gDCW/h and GAM = 27.14 mmol/gDCW. M. maripaludis achieved a CO2 to CH4 conversion yield of 70-95 % and a growth yield of 3.549 ± 0.149 g DCW/mol CH4 during the exponential phase. The ATP gain of 0.35 molATP/molCH4 for M. maripaludis, computed using NGAM, is in the acceptable range of 0.3-0.7 mol ATP/molCH4 reported for methanogens. Interestingly, the uptake distribution of amino acids, quantified using iMM518, confirmed alanine to be the most preferred amino acids for growth and methanogenesis.
  
  
  CONCLUSIONS
  This is the first study to report experimental gas consumption and production rates for the growth of M. maripaludis on CO2 and H2 in minimal media. A systematic process simulation and optimization procedure was successfully developed to precisely quantify extracellular fluxes along with cell growth and maintenance energy parameters. Our growth yields, ATP gain, and energy parameters fall within acceptable ranges known in the literature for hydrogenotrophic methanogens.
authors.affiliation: Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore, 117585, Singapore. nishu@u.nus.edu.
authors.forename: Nishu
authors.initials: N
authors.lastname: Goyal
authors.affiliation: Department of Civil and Environmental Engineering, National University of Singapore, 1 Engineering Drive 2, Singapore, 117576, Singapore. mp@u.nus.edu.
authors.forename: Mrutyunjay
authors.initials: M
authors.lastname: Padhiary
authors.affiliation: Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore, 117585, Singapore. cheiak@nus.edu.sg.
authors.forename: Iftekhar A
authors.initials: IA
authors.lastname: Karimi
authors.affiliation: Department of Civil and Environmental Engineering, National University of Singapore, 1 Engineering Drive 2, Singapore, 117576, Singapore. zhizhou@purdue.edu.|Division of Environmental and Ecological Engineering and School of Civil Engineering, Purdue University, 550 Stadium Mall Drive, West Lafayette, IN, 47907, USA. zhizhou@purdue.edu.
authors.forename: Zhi
authors.initials: Z
authors.lastname: Zhou
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D061249:Batch Cell Culture Techniques; D002245:Carbon Dioxide / Q000378:metabolism*; D004734:Energy Metabolism; D006859:Hydrogen / Q000378:metabolism; D008697:Methane / Q000378:metabolism; D017017:Methanococcus / Q000254:growth & development / Q000378:metabolism*
chemical_list: D002245:Carbon Dioxide; D006859:Hydrogen; D000255:Adenosine Triphosphate; D008697:Methane