Prof. Benjamin Friedrich: Sperm chemotaxis in turbulent flow

Portrait Benjamin Friedrich
© cfaed, Katharina Knaut

American Physical Society March Meeting:

"Sperm chemotaxis in marine species is optimal at physiological flow rates according theory of filament surfing" by Prof. Benjamin Friedrich and Steffen Lange

Biological cells can navigate in concentration gradients of signaling molecules. They can "smell" the diretion to go, a process called chemotaxis, which has been studied by many researchers, but mostly for idealized conditions of still water only. However, natural gradients are subject to many distortions, e.g., by turbulent flows in the ocean. Understanding cellular navigation in external flows is important for the ecology of foraging marine bacteria, the design of microswimmers for envisioned biomedical applications, and to predict fertilization in marine invertebrates that spawn sperm and eggs directly into the ocean.

As a case study of chemotaxis in such physiological environments, we developed a theory of sperm chemotaxis in marine invertebrates in small-scale turbulent flow. Surprisingly, a little bit of flow helps: Our theory explains the existence of an optimal turbulence strength at which fertilization is maximal. In short, externals flows shape concentration filaments into long filaments, along which sperm cells can "surf" to the egg. Stronger flows make these filaments longer, which is beneficial as the chances to hit the filament increase, but stronger flows also make these filaments thinner, which is disadvantageous, because sperm cells may not get captured even if they hit the filament, or it may take them to much time to reach the egg before intermittent turbulent flow erase the concentration field again.
Hence, the optimal flow strength, which in fact agrees with the strength of flow in typical habitats of sea urchins and sea snails. Moreover our theory agrees with previous fertilization experiments in controlled shear flow. 

Please find additional information on the website of the Biological Algorithms research group.

Information on the APS Meeting can be found on the website of the APS.