  As the closest unicellular relatives of animals, choanoflagellates serve as
useful model organisms for understanding the evolution of animal
multicellularity. An important factor in animal evolution was the increasing
ocean oxygen levels in the Precambrian, which are thought to have influenced
the emergence of complex multicellular life. As a first step in addressing
these conditions, we study here the response of the colony-forming
choanoflagellate $Salpingoeca~rosetta$ to oxygen gradients. Using a
microfluidic device that allows spatio-temporal variations in oxygen
concentrations, we report the discovery that $S.~rosetta$ display positive
aerotaxis. Analysis of the spatial population distributions provides evidence
for logarithmic sensing of oxygen, which enhances sensing in low oxygen
neighborhoods. Analysis of search strategy models on the experimental colony
trajectories finds that choanoflagellate aerotaxis is consistent with
stochastic navigation, the statistics of which are captured using an effective
continuous version based on classical run-and-tumble chemotaxis.
