  We investigate how light polarization affects the motion of photo-responsive
algae, \textit{Euglena gracilis}. In a uniformly polarized field, cells swim
approximately perpendicular to the polarization direction and form a nematic
state with zero mean velocity. When light polarization varies spatially, cell
motion is modulated by local polarization. In such light fields, cells exhibit
complex spatial distribution and motion patterns which are controlled by
topological properties of the underlying fields; we further show that ordered
cell swimming can generate directed transporting fluid flow. Experimental
results are quantitatively reproduced by an active Brownian particle model in
which particle motion direction is nematically coupled to local light
polarization.
