  Self-organization of kinesin-driven, microtubule-based 3D active fluids
relies on the collective dynamics of single microtubules. However, the
connection between macroscopic fluid flows and microscopic motion of
microtubules remains unclear. In this work, the motion of single microtubules
was characterized by means of 2D gliding assays and compared with the flows of
3D active fluids. While the scales of the two systems differ by 1,000x, both
were driven by processive, non-processive or an equal mixture of both molecular
motor proteins. To search for the dynamic correlation between both systems, the
motor activities were tuned by varying temperature and ATP concentration, and
the changes in both systems were compared. Motor processivity played an
important role in active fluid flows but only when the fluids were nearly
motionless; otherwise, flows were dominated by hydrodynamic resistance
controlled by sample size. Furthermore, while the motors' thermal reaction led
active fluids to flow faster with increasing temperature, such temperature
dependence could be reversed by introducing temperature-varying depletants,
emphasizing the potential role of the depletant in designing an active fluid's
temperature response. The temperature response of active fluids was nearly
immediate (<10 sec). Such a characteristic enables active fluids to be
controlled with a temperature switch. Overall, this work not only clarifies the
role of temperature in active fluid activity, but also sheds light on the
underlying principles of the relationship between the collective dynamics of
active fluids and the dynamics of their constituent single microtubules.
