Although mechanisms and models for anaerobic granulation are available abundantly in the literature, none of them could provide a complete description for anaerobic granulation process. Intercellular communication and multicellular coordination have been known as an effective way for bacteria to achieve an organized spatial structure. It has been shown that quorum sensing is a prominent example of social behavior in bacteria, as signal exchange among individual cells allows the entire population to choose an optimal way of interaction with the environment.
The cellular automaton model shows that biofilm structure is determined by localized substrate concentration (Wimpenny and Colasanti, 1997), however it has been found that a cell indeed can read its position in a concentration gradient of an extracellular signal factor, and to determine its developmental fate accordingly (Gurdon and Bourillot, 2001). Based on recent research findings on cell-to-cell communication (Davies et al., 1998; Pratt and Kolter, 1999; Ben-Jacob et al., 2000), it can be predicted that cell-to-cell signaling mechanisms are effective in developing anaerobic granules and organizing the spatial structure of granule-associated bacteria in response to environmental stresses. In fact, larger-scale organization had been observed in the distribution of distinct species and of distinct metabolic processes within the UASB granules (Shapiro, 1998).
A number of different groups of bacteria are involved in carrying out sequential metabolic processes in anaerobic granules. In order to efficiently utilize a target organics, the bacteria need to be spatially organized. As summarized by Shapiro (1998), the benefits of an organized microbial structure include more efficient proliferation; access to resource and niches that cannot be utilized by isolated cells; collective defense against antagonists that eliminate isolated cells, and optimization of population survival by differentiation into distinct cell types. These are strongly supported by experimental evidence that UASB granules are much more resistant than suspended sludge to toxicity of hydrogen sulfide, heavy metals, and aromatic pollutants in wastewater (Bae et al., 2000; Fang, 2000; Tay et al., 2000a,b).
It has been generally observed in UASB reactors that a change in wastewater composition could result in a washout of the granular sludge within a short period of time. This phenomenon can be reasonably explained by the cell-to-cell communication mechanism. As pointed out earlier, the bacteria in a UASB granule are not randomly distributed but rather organized to best meet the needs of each species for a defined organic substrate. In fact, spatial organization of UASB granules is developed to cope with the constraints imposed by the substrate and corresponding metabolic processes. When the composition of wastewater is changed, the granule-associated bacteria would respond by re-organizing microbial spatial distribution and structure, in order to adapt to new metabolic processes required for the oxidation of present organic substrate.
Structure changes induced by a substrate shift have been reported in biofilm culture processes (Wolfaardt et al., 1994; Tolker-Nielsen and Molin, 2000). The substrate change-induced structural re-organization would result in a partial or complete breakup of the granules developed from the previous substrate. The observed washout of sludge blanket from UASB reactors is thought to be resulted from the substrate change-caused granule breakup. It appears from the cell-to-cell communication model that organized bacterial community, such as biofilms or granules, is not simply a scaled-up version of individual bacteria. Further research is required to refine the cell-to-cell communication-based mechanism for anaerobic granulation.
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