Ultrafast signaling among aquatic single-celled organisms
Group behavior in many species arises from cascading signals transmitted by individual organisms. For aquatic microorganisms, signaling often takes the form of flows generated by swimming; an individual may sense the movement of fluid from a neighbor and adjust its behavior. But such signals are generally slow and decay with distance. For example, some bacteria have to be packed into groups of 108 cells per cubic centimeter to coordinate their movements from hydrodynamic signals.
Now Arnold Mathijssen and Manu Prakash from Stanford University, along with Joshua Culver and Saad Bhamla of Georgia Tech, have discovered a new form of rapid, long-range signaling among single-celled organisms. They found it in the marsh-dwelling Spirostomum ambiguum (illustrated above), an exceptionally large millimeters-long organism that’s long been known to coil its body in an extremely rapid contraction and shrink to less than half its normal size in just 5 ms. The researchers found that the contraction shoots surrounding water into a rapid jetlike flow. The flow triggers another cell, sometimes centimeters away from its neighbor, to contract and generate its own flow. In a chain reaction, a hydrodynamic wave propagates through the colony hundreds of times faster than the cells’ swimming speed.
Zoologists suspect that S. ambiguum‘s contractions are the result of its being startled, perhaps by a predator. In a lab culture of the cells, electrical pulses or centrifugation can trigger the behavior. To investigate further, Prakash and Mathijssen designed an experiment that mimicked a predator’s filter-feeding action. They suctioned liquid out of a small hole in a pair of slides containing a single S. ambiguum in water. As the suction drew the cell closer to the hole, its body became stretched. When it reached a critical strain threshold, the cell contracted, propagating a flow that then signaled its neighbors, as shown below.
The researchers found that the strain required to trigger contraction matched the threshold for opening ion channels on S. ambiguum‘s outer membrane. Once the channels are open, ions can enter the cell and engage filaments that make the cell contract. Additionally, upon mechanical or electrical stimulation S. ambiguum releases toxins from pockets fixed to the cell membrane. The results suggest that an individual instigates coordinated contractions as a reaction to danger. Synchronized toxin release may help the colony deter predators like Stenostomum sphagnetorum, which learns to avoid the elusive prey. (A. J. T. M. Mathijssen et al., Nature 571, 560, 2019 .)
