Preying through: co-existence, oscillations and specificity sustain stability in predatory interactions between bacteria

A fundamental question in community ecology is the role of predator-prey interactions in food-web stability and species coexistence. In microbial ecosystems, protists, and phages largely contribute to bacterial mortality and turnover but the ecological role of Bdellovibrio and like organisms (BALOs), the only bacteria known to be obligate predators of bacteria is much less appreciated. We thus aimed at characterizing the BALO communities and their predatory dynamics in natural systems. Here, we tracked interacting BALOs and their gram negative prey in three wastewater treatments plants using high throughput 16S rRNA sequencing to identify and quantify predators and prey from sympatric floc and liquor fractions, in a year-long time series. We show that BALO diversity is far larger than previously known; that BALOs greatly vary in prey range, creating niche partitions that may explain co-existence of the predators. As BALO prey ranges varied from very restricted to semi-generalist, local control over specific bacterial populations led to population shifts in dominant as well as in rarer prey clades, forming complex predatory networks. It was then shown that while BALOs form quantitatively stable communities, individual populations fluctuate with time, being affected by temperature on the long term and by prey availability on the short term, the latter fitting "kill-the-winner" like dynamics. A cell sorting analysis combined with fluorescent in situ hybridization and community sequencing validated the computational approach, confirming predator-prey identification. This novel approach enables detailed reconstructions of trophic networks, including predator-driven cascades, uncovering hitherto hidden controls on bacterial populations.