BOX 2-3

Community Robustness: The Case of Sludge

The removal of phosphorus from wastewater by microbes by a process known as enhanced biological phosphorus removal (EBPR) depends upon the stability and robustness of the microbial community responsible for phosphorus accumulation (Levantesi et al. 2002; Garcia Martin et al. 2006). A single organism, Candidatus Accumulibacter phosphatis, supplies all the required biochemical functions to remove phosphorus in many systems. However, although A. phosphatis can be enriched to high numbers in laboratory scale bioreactors, the organisms remain recalcitrant to growth in pure culture, and this suggests a role for additional community members in their maintenance.

Although EBPR is generally stable and was first used in full-scale waste-water treatment facilities over thirty years ago, these facilities must continue to maintain backup chemical phosphorus-removal systems to respond to periodic crashes of the biological systems. The cause of crashes is not well understood, but they are hypothesized to result from particular biological and environmental perturbations that destabilize the phosphorus-accumulating microbial community. In laboratory-scale reactors that mimic the wastewater treatment plant cycling, small perturbations in pH and the type of carbon supplied can stimulate the growth of competitors of the phosphorus accumulators and result in less efficient or completely abolished phosphorus removal. In addition, homogeneity of the population of A. phosphatis may leave the community vulnerable to infection by bacteriophage. Greater understanding of the interactions sustaining the EBPR microbial community will lead to more reliable phosphorus-removal systems.

in which they occur and on the cells that harbor them. Episodes of selection of favored mutants periodically purge populations of genetic and genomic diversity and maintain the cohesiveness (genome-to-genome and cell-to-cell similarity) that allows us to recognize and define species.

However, discoveries of the last decade indicate that gene transfer between similar but nonidentical genomes is, at least in some bacteria, more often the cause of genetic diversity than are new mutations in clones. Indeed, recombination may well be the principal generator of evolutionary novelty in such groups and has parallels to the role of sex in the evolution of animal species. But in other respects there are important differences between microbes and animals: the boundaries of cross-species homologous recombination may be much less distinct, and lateral gene transfer, almost by definition a transgressor of species boundaries, clearly is an important cause of divergence and adaptation in bacteria.

Debate will continue to rage over the frequency and evolutionary importance of such cross-species transfer. Metagenomics, by focusing on genes in an environmental rather than an organismal context, will recast the terms of the debate, as it will of the question “What is a species?” Under-



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