Why Sequence Type I Accumulibacter?

Enhanced biological phosphorus removal (EBPR) is a wastewater treatment process used throughout the world to protect surface waters from accelerated eutrophication (overgrowth due to nutrient pollution). The microbial assemblages carrying out EBPR in lab-scale and full-scale wastewater treatment plants are ideal model systems in which to study the forces driving community, population, and genome dynamics, because they are moderately complex but easily manipulated. Many EBPR systems are dominated by a phylogenetically coherent group named Candidatus Accumulibacter phosphatis (CAP). Metagenomic analysis and functional-gene targeted studies have shown that two distinct “strains” of CAP, identified as Type I and Type II, are most often selected for in lab-scale EBPR bioreactors.

Aeration basin at Nine Springs Wastewaster Treatment Plant, Madison, Wisconsin. Photo courtesy Katherine D. McMahon.

An analysis of population dynamics over time in an EBPR bioreactor has shown that Types I and II often inversely correlate each other in abundance. These dynamics might be caused by competitive exclusion and/or strain-specific phage attacks. JGI previously sequenced EBPR metagenomes from bioreactors in two geographically remote locations (Wisconsin, USA and Queensland, Australia). The Type II strain dominated the microbial communities in both reactors at the time of sample collection, allowing us to assemble a nearly complete genome sequence for this strain. A comparative analysis of the two metagenome sequences suggested that global populations of Type II are essentially panmictic, given the degree of shared genome organization and identity. The few rapidly evolving regions of the CAP genome appear to be associated with phage defense and exopolymeric substance formation (which is thought to facilitate gravitational settling of cell aggregates – a feature that allows CAP to compete in the wastewater treatment process environment). Comparative genomics should allow researchers to find differences between the CAP Type I and Type II genomes that could explain the observed reciprocal population dynamics.

By learning more about the forces acting to shape the CAP genome, we will develop a better understanding of how community and population dynamics are related to community function (i.e. process performance), leading to better engineering design of these important environmental systems.

Principal Investigators: Katherine D. McMahon (Univ. of Wisconsin-Madison) and Philip Hugenholtz (JGI)