Phagocytosis without mitochondria is a lineage trait of planctomycetal bacteria of prey

Phagocytosis is considered energetically out of reach for prokaryotes. Mitochondria-first models of eukaryogenesis therefore suggest a non-phagocytic archaeal host. Yet two recently described planctomycetes lack mitochondria and engulf entire bacteria. Whether this mode of predation is an oddity of two isolates or the hallmark of a bacterial lineage remained unresolved. Here, we report eight additional predatory Planctomycetota from three aquatic habitats. The ten cultured strains now span two classes of the phylum Planctomycetota,"Candidatus Uabimicrobiia" and one newly proposed class. Using engineered fluorescent prey, we show that nine strains internalize prey into fully sealed compartments whereas one digests its prey externally, a non-engulfing predator for comparison. The strains additionally divide by two distinct modes, however, not correlating with their feeding behavior. Sequence- and structure-based searches detect no eukaryotic phagocytosis protein conserved across the engulfing strains. The actin homologue previously proposed to mediate uptake occurs only in three engulfing strains. Routine engulfment by aerobic bacteria without mitochondria thus unsettles a premise of mitochondria-first models and reopens the question of which cell plans could have hosted the mitochondrial endosymbiont.