Bacteriophages can impact bacterial communities, not just through predation, but also through the regulation of host genes that can improve biofilm development. Bacterial communities are essential in the function of most ecosystems including engineered systems such as activated floccular sludges of water reclamation plants. In these systems, bacteria are responsible for nutrient removal and flocculation. Activated floccular sludge is often used to cultivate aerobic granules and the formation of such granules is normally achieved by selection through physical parameters. Although granule formation has been optimised by controlling these physical factors, granule instability and the long initiation times for their formation remain a challenge for their implementation in full scale water reclamation plants. As granules represent a suspended biofilm, it has been hypothesized that bacteriophages are important in the formation of granules. However, few studies have characterized the abundance and diversity of bacteriophages during aerobic granulation.
In this study, four reactors were operated for simultaneous nitrification, denitrification and phosphorus removal, and a metagenomics approach was used to monitor the abundance of bacteriophages and the bacterial communities of the granulating sludge. It was observed that there was an increase in abundance of Inoviridae during the initiation of granulation suggesting a role of filamentous phages in the formation of granules. Furthermore, there was also a shift in the abundance of lytic phages during granule maturation, suggesting that lysis of certain bacterial genera may also be important in granulation. Lytic bacteriophage may have selected for bacteria that made up denser sludge, and filamentous bacteriophage may be part of the granule structure. The changes in the abundance of bacteria, especially that of ‘Candidatus Accumulibacter’, as well as physical factors such as settling time and biomass discharge are also important in this process.