Coaggregation facilitates interspecies hydrogen transfer between Pelotomaculum thermopropionicum and Methanothermobacter thermautotrophicus

Abstract

A thermophilic syntrophic bacterium, Pelotomaculum thermopropionicum strain SI, was grown in a monoculture or coculture with a hydrogenotrophic methanogen, Methanothermobacter thermautotrophicus strain DeltaH. 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 DeltaH 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 DeltaH 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 mum, while the allowable distances for ethanol and propanol oxidation were 16 mum and 32 mum, respectively. Considering that the mean cell-to-cell distance in the randomly dispersed culture was approximately 30 mum (at a concentration in the mid-exponential growth phase of the coculture of 5 x 10(7) cells ml(-1)), it is obvious that close physical contact of these organisms by coaggregation is indispensable for efficient syntrophic propionate oxidation.

FIG. 1.

Syntrophic oxidation of ethanol (A), propionate (B), and 1-propanol (C) by P. thermopropionicum SI in coculture with M. thermautotrophicus ΔH. The methane concentration was expressed as “mM equivalents” (mM eq.) by assuming that all methane was present in the aqueous phase. The data are means ± standard deviations obtained from three independent cultures.

FIG. 2.

Phase-contrast micrographs of an SI monoculture grown on fumarate (A), a ΔH monoculture grown on H₂ plus CO₂ (B), and a coculture of strains SI and ΔH grown on propionate (C). All micrographs were taken in the stationary phase (a phase in which the OD₆₀₀ was no longer elevated). Bars, 25 μm.

FIG. 3.

Representative FE-SEM images of a monoculture of strain SI grown on fumarate (A), a monoculture of strain ΔH grown on H₂ plus CO₂ (B), and a coculture of SI and ΔH cells grown on propionate (C to F). Bars, 1 μm.

FIG. 4.

Determination of the maximum H₂ partial pressures for ethanol (A), propionate (B), and 1-propanol (C) oxidation. Hydrogen consumption by ΔH cells was inhibited by adding 2-BES. The data are means ± standard deviations obtained from three independent cultures.