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Coaggregation Facilitates Interspecies Hydrogen Transfer between Pelotomaculum thermopropionicum and Methanothermobacter thermautotrophicus

Marine Biotechnology Institute, Heita, Kamaishi, Iwate 026-0001,¹ Department of Materials Science and Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555,² Central Research Institute of Oral Science, School of Dentistry, Asahi University, Hozumi, Mizuho, Gifu 501-0296, Japan³

Received 14 June 2005/ Accepted 11 August 2005

A thermophilic syntrophic bacterium, Pelotomaculum thermopropionicum strain SI, was grown in a monoculture or coculture with a hydrogenotrophic methanogen, Methanothermobacter thermautotrophicus strain H. Microscopic observation revealed that cells of each organism were dispersed in a monoculture independent of the growth substrate. In a coculture, however, these organisms coaggregated to different degrees depending on the substrate; namely, a large fraction of the cells coaggregated when they were grown on propionate, but relatively few cells coaggregated when they were grown on ethanol or 1-propanol. Field emission-scanning electron microscopy revealed that flagellum-like filaments of SI cells played a role in making contact with H cells. Microscopic observation of aggregates also showed that extracellular polymeric substance-like structures were present in intercellular spaces. In order to evaluate the importance of coaggregation for syntrophic propionate oxidation, allowable average distances between SI and H cells for accomplishing efficient interspecies hydrogen transfer were calculated by using Fick's diffusion law. The allowable distance for syntrophic propionate oxidation was estimated to be approximately 2 µm, while the allowable distances for ethanol and propanol oxidation were 16 µm and 32 µm, respectively. Considering that the mean cell-to-cell distance in the randomly dispersed culture was approximately 30 µm (at a concentration in the mid-exponential growth phase of the coculture of 5 x 10⁷ cells ml^–1), it is obvious that close physical contact of these organisms by coaggregation is indispensable for efficient syntrophic propionate oxidation.

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