  Predators and their foraging strategies often determine ecosystem structure
and function. Yet, the role of protozoan predators in microbial soil ecosystems
remains elusive despite the importance of these ecosystems to global
biogeochemical cycles. In particular, amoebae -- the most abundant soil
protozoan predators of bacteria -- remineralize soil nutrients and shape the
bacterial community. However, their foraging strategies and their role as
microbial ecosystem engineers remain unknown. Here we present a multi-scale
approach, connecting microscopic single-cell analysis and macroscopic whole
ecosystem dynamics, to expose a phylogenetically widespread foraging strategy,
in which an amoeba population spontaneously partitions between cells with fast,
polarized movement and cells with slow, unpolarized movement. Such
differentiated motion gives rise to efficient colony expansion and consumption
of the bacterial substrate. From these insights we construct a theoretical
model that predicts how disturbances to amoeba growth rate and movement disrupt
their predation efficiency. These disturbances correspond to distinct classes
of bacterial defenses, which allows us to experimentally validate our
predictions. All considered, our characterization of amoeba foraging identifies
amoeba mobility, and not amoeba growth, as the core determinant of predation
efficiency and a key target for bacterial defense systems.
