Due to the dense aggregation of cells, the rate of mass transfer of nutrients and metabolites between bulk medium and granular matrix may not be sufficient to ensure normal cell metabolism in the granule interior. The concentration of dissolved oxygen can drop to zero at some depth below the granule surface. This depth depends on the specific rate of oxygen consumption and also on the porosity and extent of channel structures in the granules. The typical depth of the aerobic zone in a thick microbial biofilm in the presence of aeration is between 50 and 200 |xm (Villaverde and Fernandez-Polanco, 1999; Gieseke et al., 2001). The Bacto1080 probe with the sequence 5/-GCACTTAAGCCGACACCT-3/ is specific for Bacteroides spp. (Sghir et al., 2000) and was labeled and used to detect obligate cells of anaerobic Bacteroides spp. It was demonstrated that obligate anaerobic bacteria can grow in the interior of aerobically grown granules (Tay et al., 2002a,b, 2003a). Optical and mechanical sectioning of the granules following FISH incubation showed that the typical structure of a granule could be described as a sack consisting of thick walls of active biomass. In the granules with the walls approximately 1000 |xm thick, the cells of the Bacteroides group were concentrated in a layer approximately 100 |xm thick. This layer was located at a depth of 800-850 |xm below the granule surface.
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