Save the date for Julie Rakes’s PhD defense on January 20th, 2022 at 8:00 AM MST. Julie is a graduate student in the Ferran Garcia-Pichel Lab at Arizona State University.
Title: Discovery, Characterization and Ecological Impact of a Predatory Bacterium of Cyanobacteria Responsible for Epidemics in Biocrusts
Abstract: Predatory bacteria are a guild of heterotrophs that feed directly on other living bacteria. Its members belong to phylogenetically unrelated groups that evolved this mode of life independently and occur in many microbiomes and environments. Current knowledge of predatory bacteria is, however, based on culture studies and simple detection in natural systems. The ecological consequences of their activity, unlike those of other populational loss factors like viral infection or grazing by protists, is yet to be assessed. During large-scale cultivation of mixed biological soil crusts intended for arid soil rehabilitation, I encountered episodes of catastrophic failure in cyanobacterial growth that could be ascribed to the action of an unknown predatory bacterium using bioassays. Its presence could be traced back to natural biocrust communities, where it formed cm-sized clearings (plaques) visible to the naked eye. Enrichment cultivation and purification by cell-sorting was used to obtain co-cultures of the predator with its cyanobacterial prey, as well as to identify and characterize it genomically, physiologically and ultrastructurally. A Bacteroidetes bacterium, unrelated to any known isolate at the family level, this predator is endobiotic, non-motile, obligately predatory, displays a complex life cycle and very unusual ultrastructure. Extracellular propagules are small (0.8-1.0 µm) Gram-negative cocci with internal two-membrane-bound compartmentalization. It gains entry to the prey likely using a rich suite of hydrolytic enzymes, localizing to the cyanobacterial cytoplasm, where growth begins into non-compartmentalized pseudofilaments that undergo excretion of vesicles and simultaneous multiple division to yield new propagules. I formally describe it as candidatus Cyanoraptor togatus. Its prey range is restricted to biocrust-forming, filamentous, non-heterocystous, gliding, bundle-making cyanobacteria. Molecular meta-analyses showed its worldwide distribution in biocrusts. Biogeochemical analyses of Cyanoraptor plaques revealed that it causes a complete loss of primary productivity, and significant decreases in other biocrusts properties like water-retention and dust-trapping capacity. Extensive field surveys in the US Southwest revealed its ubiquity and its dispersal-limited, aggregated spatial distribution and incidence. Overall, its activity tithes 10% of biocrust productivity at the ecosystem scale. My research points to predatory bacteria as a significant, but overlooked, ecological force in shaping soil microbiomes.