TOPOGRAPHY OF METHANOGENIC SUBPOPULATIONS IN A MICROBIAL CONSORTIUM ADAPTING TO THERMOPHILIC CONDITIONS

The microanatomy of a granular, methanogenic, microbial consortium (granule) was investigated by applying histochemical techniques and light microscopy. The granules were obtained from a full-scale, mesophilic, upflow anaerobic-sludge blanket (UASB) bioreactor and were inoculated into laboratory-scale counterparts with a defined substrate in which they were maintained for several weeks before shifting to thermophilic conditions. Samples for analysis were collected from the original inoculum, and from the laboratory-scale bioreactors before and after the temperature shift. Four months after this shift, the basic architectural plan of the granules was found to comprise a core or medulla, and a peripheral zone or cortex bounded by denser layers, and a spongy matrix formed by microbes and intercellular material. Also, five methanogenic subpopulations known to occur in these 4-month thermophilic granules were mapped to elucidate their spatial arrangement using immunofluorescence with antibody probes of predefined specificity spectra on histologic thin sections. Each subpopulation showed a distinctive distribution pattern; e.g. elongated surface and inner colonies for the methanogen antigenically related to Methanobacterium thermoautotrophicum DELTA-H, packets and cellular cords for the methanosarcina related to Methanosarcina thermophila TM1, bundles for Methanothrix rods, and dense or sparse lawns for the methanogens antigenically related to Methanobrevibacter arboriphilus AZ and Methanobrevibacter smithii ALI, respectively. These features were absent from, or much less developed in, the original inoculum granules, and before the temperature shift or even 1 week thereafter. All granules showed the same basic architecture involving a cortex and a medulla, but the inner texture of the mesophilic granules was considerably more compact than that of the 4-month thermophilic granules. Thus, the topography of methanogenic subpopulations and some structural details of the 4-month thermophilic granules seem to be the result of adaptation to enrivonmental factors such as temperature, the same way as the array of methanogenic species and their numbers were found to be in previous work.