  Active matter comprises individually driven units that convert locally stored
energy into mechanical motion. Interactions between driven units lead to a
variety of non-equilibrium collective phenomena in active matter. One of such
phenomena is anomalously large density fluctuations, which have been observed
in both experiments and theories. Here we show that, on the contrary, density
fluctuations in active matter can also be greatly suppressed. Our experiments
are carried out with marine algae ($\it{Effrenium\ voratum}$) which swim in
circles at the air-liquid interfaces with two different eukaryotic flagella.
Cell swimming generates fluid flow which leads to effective repulsions between
cells in the far field. Long-range nature of such repulsive interactions
suppresses density fluctuations and generates disordered hyperuniform states
under a wide range of density conditions. Emergence of hyperuniformity and
associated scaling exponent are quantitatively reproduced in a numerical model
whose main ingredients are effective hydrodynamic interactions and uncorrelated
random cell motion. Our results demonstrate a new form of collective state in
active matter and suggest the possibility to use hydrodynamic flow for
self-assembly in active matter.
