11 October 2010


(Phytoplankton. Wikimedia Commons.)
Swimming organisms might be responsible for as much as one-third of the mixing of the oceans by the force of their swimming alone. The other two known forces being wind and tides. 

This calculation was made by oceanographer William Dewar at Florida State University. His paper, Oceanography: A Fishy Mix, appeared in Nature in 2009. Here's what I wrote about it in DEEP BLUE HOME:
Oceanographer William Dewar of Florida State University calculates that the marine biosphere, starting at its broad base, the plankton, invests around one terawatt [1 terawatt equals 1 billion kilowatts] in the mechanical energy of swimming, effectively mixing the World Ocean from within. Amazingly, the swimmers of the deep blue home provide a third of the power required to blend the waters of the world from the abyss to the surface, with the remaining two-thirds powered by the more obvious forces of the wind and the motions of the tides. So it is that life itself rotates the great waterwheel driving the underwater rivers of the world and making our planet habitable.

(Fluid dynamics in action. Natural-color image of phytoplankton bloom in the Barents Sea, from August 31, 2010. Image courtesy Norman Kuring, NASA Ocean Color Group.)

Now two new papers in Physical Review Letters report groundbreaking measurements of the fluid dynamics produced by two species of tiny creatures that likely play a crucial role in this ocean mixing.

In one paper, researchers at the University of Cambridge discovered how the motion of an individual microbe affects its neighbors, and how those motions can then trigger collective motions of microorganism swarms. 

(Chlamydomonas reinhardtii. Source.)
The research suggests the cumulative effect of this microscopic swimming produces large scale stirring that distributes oxygen, chemicals, and nutrients in oceans and lakes.

The work was done by tracking the motion of tiny tracer beads suspended in the fluid. These movements revealed a record of the swimming motions of a species of small blue-green, Chlamydomonas reinhardtii (above), that swims by paddling a pair of whiplike flagella, as well as a larger spherical species of alga, Volvox carterii (below), that propels itself with thousands of flagella.  

(Volvox carterii. Source.)

The tracer beads showed that the two organisms generate very different flow patterns, both far more complex than anticipated.

(Image credit: K. Drescher, R. E. Goldstein, N. Michel, M. Polin, and I. Tuval, University of Cambridge.)

In the image above, a spherical Volvox is swimming from the bottom to the top of the image. The color map corresponds to the fluid velocity and streamlines appear as red curves. 

And, in a related study performed at Haverford College in Pennsylvania, researchers used a high speed camera to track the flow of tracer particles around Chlamydomonas in a thin 2-dimension film of fluid over the course of a single stroke of its flagella. 

You can see that and a whole lot more of their joint work in the video below.

(Mixing by Swimming Algae. Courtesy J. S. Guasto, K.C. Leptos, J. P. Gollub, A.I. Pesci, and R.E. Goldstein, Haverford College. From here.)

In an accompanying Viewpoint article in Physical Review Letters, David Saintillan at the University of Illinois at Urbana Champagne writes an overview of the two papers, concluding:

The studies by the Cambridge and Haverford groups, by providing the first detailed quantitative picture of the flow fields driven by freely swimming microalgae, demonstrate three important points: (i) the flows around swimming microorganisms can be quite complex, especially in the near field where the largest velocities arise; (ii) two distinct species are likely to drive qualitatively different disturbance flows, including in the far field, where it is often assumed that the flow fields can be universally described in terms of a stresslet; (iii) the representation of these flows in terms of time-averaged velocity fields is simplistic, as time fluctuations can be of the same order as the mean. The implications of these findings in terms of hydrodynamic interactions and mixing are far-reaching and will only be fully understood once the details of these flows are incorporated into mathematical models or computer simulations.


Finally, here are a few of the tiny players in the great waterwheel of life, swimming to Frédéric Chopin's Nocturne No. 2.

The papers:

William K. Dewar. Oceanography: A fishy mix. Nature 460 (2009) 581-582. DOI:10.1038/460581a

Knut Drescher, et al. Direct Measurement of the Flow Field around Swimming Microorganisms. Phys. Rev. Lett. 105, 168101 (2010) [4 pages]. DOI: 10.1103/PhysRevLett.105.168101

Jeffrey S. Guasto, et al. Oscillatory Flows Induced by Microorganisms Swimming in Two Dimensions. Phys. Rev. Lett. 105, 168102 (2010). DOI: 10.1103/PhysRevLett.105.168102

David Saintillan. A quantitative look into microorganism hydrodynamics. Physics 3, 84 (2010) DOI: 10.1103/Physics.3.84
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