An anaerobic protozoon, with symbiotic methanogens, living in municipal landfill material

We have established that anaerobic protozoa do live in municipal landfill material although they probably spend much of the time encysted, especially in the drier (< 40% water) site. At least eight species were observed; they were readily isolated by adding anoxic water to dry landfill samples. The ciliate Metopus palaeformis was frequently i isolated; it appears to be ubiquitous in anaerobic landfills. It has a polymorphic life cycle, it is positive for hydrogenase, each ciliate contains about 500 bromoethanesulfonate‐sensitive methanogen symbionts (probably Methanobacterium formicicum ), and maximum cell densities in culture exceed 3000 per ml. The methanogens are not attached to the hydrogenosomes, neither do they undergo morphological transformation; the ciliate receives no measurable energetic advantage from its symbionts. The ciliate encysts in response to a shortage of food or water, and the methanogens remain viable within the cysts. When the protozoon excysts, the methanogens resume growth and cell division within the trophic form of the ciliate. Unlike free‐living methanogens, the M. palaeformis ‐methanogen consortium is not particularly sensitive to oxygen; the symbiotic methanogens remain viable following exposure of the consortium to atmospheric oxygen for several days. Dispersal of methanogen‐bearing protozoan cysts through oxygenated environments is a potential mechanism of transfer between landfill sites and other anaerobic environments. Anaerobic protozoan consortia are theoretically capable of making a significant contribution to methane generation from wet landfill sites.