The Neglected Role of Heterotrophic Protists in Engineered Water Systems

Heterotrophic protists can be considered the dark matter of microbial communities in engineered water systems. They are ubiquitous and ecologically significant yet remain largely overlooked. Although a growing body of research demonstrates their pivotal roles (e.g., predation, symbiosis, and nutrient cycling) in microbial communities in natural ecosystems, their functions in engineered water systems are poorly characterized, and heterotrophic protists are frequently excluded from microbial analyses. This is largely due to methodological constraints that have only recently been overcome. Recent advances in imaging, high-throughput sequencing, and meta-omics approaches, combined with expanding reference databases, have revolutionized studies of protist diversity and functions in a wide range of natural environments. Drawing on research from the fields of protistology, microbial ecology, and environmental microbiology, this review explores how the well-documented ecological roles of heterotrophic protists in natural environments translate to engineered ecosystems, offering insights into their functions in water treatment. We critically evaluate recent literature to synthesize both beneficial roles and potential risks of heterotrophic protists in various water treatment systems, while identifying key knowledge gaps and proposing directions for future research. We advocate for a shift in perspective that recognizes heterotrophic protists as important players and call for their integration into microbial community characterization and ecological frameworks in microbial ecology studies of engineered water systems. This integration will transform our understanding of microbial communities in engineered water systems, ultimately enabling novel, mechanistic, and ecologically informed management strategies.