Long-range interactions dominate surface accumulation of Chlamydomonas reinhardtii

Swimming microorganisms accumulate near solid surfaces in their environment. This has implications for the process of surface colonisation. For extensile swimmers, long-range hydrodynamic interactions have been shown to play a role in governing this near-surface accumulation. Such interactions’ role in the accumulation of contractile swimmers is less clear. We investigate the roles of surface scattering and long-range interactions in the near-surface accumulation of a model contractile swimmer. A population of C. reinhardtii swimming within a large volume, bounded by two flat surfaces, was recorded simultaneously by four cameras, and a large sample of 3D cell trajectories triangulated. We derived statistics of the cells’ isotropically diffusive swimming kinematics as well as their surface scattering dynamics, and observe a long-range cell-surface interaction. These statistics were sampled to build an empirically-driven Markov Chain Monte-Carlo simulation. In this way, we directly link the population’s near-surface accumulation to the cells’ swimming, scattering and surface interaction dynamics. We find that the experimentally observed population distribution can only be accounted for by including, in the model, the long-range cell-surface interaction characterised experimentally.