Coaggregation Facilitates Interspecies Hydrogen Transfer between Pelotomaculum thermopropionicum and Methanothermobacter thermautotrophicus
2005
Ishii, Shun'ichi | Kosaka, Tomoyuki | Hori, Katsutoshi | Hotta, Yasuaki | Watanabe, Kazuya
A thermophilic syntrophic bacterium, Pelotomaculum thermopropionicum strain SI, was grown in a monoculture or coculture with a hydrogenotrophic methanogen, Methanothermobacter thermautotrophicus strain [Delta]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 [Delta]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 [Delta]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 [micro]m, while the allowable distances for ethanol and propanol oxidation were 16 [micro]m and 32 [micro]m, respectively. Considering that the mean cell-to-cell distance in the randomly dispersed culture was approximately 30 [micro]m (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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