Flocculation in methanogens, a comparative study of Methanosarcina barken strains Jülich and Fusaro

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Abstract
Two strains of Methanosarcina barkeri grown on methanol as substrate were investigated for their ability to aggregate: the new, flocculent strain Jülich formed stable flocs of several millimetres in diameter during rapid growth on a methanol-containing medium ("bread-crumb growth"). When observed with an electron microscope, the Jülich strain showed a unique parenchymatic texture with thick-walled cells inside a floc, and coccoidlike cells on the periphery. In contrast to the Jülich strain, the strain Fusaro grew on methanol in dispersed form, and generated macroscopically visible clumps only under poor growth conditions, induced, for example, by calcium deficiency. The formation of large cell aggregations of the Fusaro strain could also be induced during growth in the presence of 0.01% of the stain Calcofluor which is known to interact specifically with β-1,4 and β-1,3 glucan moieties. Sugar analyses revealed a different pattern for both strains: the exopolymer of the flocculent strain Jülich contained half the rhamnose, a third the fucose, equal amounts of mannose and glucose, but four times more glucuronic acid and arabinose than the strain Fusaro grown in the dispersed form. The high proportion of glucuronic acid in the exopolymer of the flocculating form is of particular interest because of the role of uronic acids in adhesion.