  We study the velocity distribution of unicellular swimming algae Euglena
gracilis using optical microscopy and theory. To characterize a peculiar
feature of the experimentally observed distribution at small velocities we use
the concept of active fluctuations, which was recently proposed for the
description of stochastically self-propelled particles [Romanczuk, P. and
Schimansky-Geier, L., Phys. Rev. Lett. 106, 230601 (2011)]. In this concept,
the fluctuating forces arise due to internal random performance of the
propulsive motor. The fluctuating forces are directed in parallel to the
heading direction, in which the propulsion acts. In the theory, we introduce
the active motion via the depot model [Schweitzer et al., Phys. Rev. Lett. 80,
23, 5044 (1998)]. We demonstrate that the theoretical predictions based on the
depot model with active fluctuations are consistent with the experimentally
observed velocity distributions. In addition to the model with additive active
noise, we obtain theoretical results for a constant propulsion with
multiplicative noise.
