FORMATION OF FATTY ACID-DEGRADING, ANAEROBIC GRANULES BY DEFINED SPECIES

An endospore-forming, butyrate-degrading bacterium (strain BH) was gro wn on butyrate in monoxenic coculture with a methanogen. The culture f ormed dense aggregates when Methanobacterium formicicum was the methan ogenic partner, but the culture was turbid when Methanospirillum hunga tei was the partner, In contrast, a propionate-degrading, lemon-shaped bacterium (strain PT) did not form aggregates with Methanobacterium f ormicicum unless an acetate-degrading Methanosaeta sp, was also includ ed in the culture, Fatty acid-degrading methanogenic granules were for med in a laboratory-scale upflow reactor at 35 degrees C fed with a me dium containing a mixture of acetate, propionate, and butyrate by usin g defined cultures of Methanobacterium formicicum T1N, Methanosaeta sp , strain M7, Methanosarcina mazei T18, propionate-degrading strain PT, and butyrate-degrading strain BH, The maximum substrate conversion ra tes of these granules for acetate, propionate, and butyrate were 43, 9 , and 17 mmol/g (dry weight)/day, respectively, The average size of th e granules was about 1 mm, Electron microscopic observation of the gra nules revealed that the cells of Methanobacterium formicicum, Methanos aeta sp,, butyrate-degrading, and propionate-degrading bacteria were d ispersed in the granules, Methanosarcina mazei existed inside the gran ules as aggregates of its own cells, which were associated with the bu lk of the granules, The interaction of different species in aggregate formation and granule formation is discussed in relation to polymer fo rmation of the cell surface.