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1 December 2010 Living at Micro Scale: The Unexpected Physics of Being Small.
Howard A. Stone
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Our everyday experiences help us develop intuition for basic physical and dynamical processes. For example, when swimming, we take it for granted that we can glide significant distances after pushing off from the pool wall. However, when we then try to apply our intuition to the world of motile microorganisms we run into surprises. If microorganisms attempted to glide through water, they would come to rest after moving only about an Angstrom; they would never get anywhere! The book Living at Micro Scale: The Unexpected Physics of Being Small summarizes the basic mechanical features of life at the scale of microorganisms and includes a broad range of fascinating topics that are discussed qualitatively and quantitatively. The world of motion is dominated by the viscous (or frictional) features of any flow—which engineers, mathematicians, and physicists refer to as “low Reynolds number flows”—and encompasses the dynamics of bacteria, phytoplankton, algae, and many other kinds of single cells or cell clusters.

Living at Micro Scale covers many topics, and is written in a wellorganized and refreshing style, so there will be new ideas for physical, mathematical, and biological scientists to explore. A distinguishing feature of the book is the author's continual attempt to provide quantitative guidelines to basic aspects of size, shape, type of locomotion, and chemical uptake rates. Author David B. Dusenbery has written broadly on quantitative features of the dynamics of microorganisms over many years. In this book he synthesizes his work and that of many other scientists who have been captivated by the physical world of living microorganisms. I believe that some aspects of the book will be of interest to almost any reader, though some comfort with quantitative discussions is required.

The book is structured into four main parts: physical ideas for thinking about mechanics of size and shape at low Reynolds numbers, physical ideas that affect various modes of locomotion through fluids, orientation to different gradients of stimulants, and finally, the various modes of interactions between microorganisms. The writing is clear and the book is logically organized. Occasional historical remarks serve to remind the reader of important landmarks in our understanding and discovery of the world around us. One of the book's strengths is that Dusenbery makes continual use of quantitative, mostly algebraic relationships, giving the book a distinct place in the literature. Even nonquantitative readers are likely to appreciate conclusions extracted from Dusenbery's arguments, but I did wonder whether some other format of presenting the main results would have been useful for readers less comfortable with the physical and mathematical principles.

In such a wide-ranging discussion, incorporating a large set of topics from classical physics, it is inevitable that a specialist reader would find some of the quantitative arguments in this book misleading. I found myself making a number of remarks of that type when reading about fluid mechanics in the text, and I imagine the knowledgeable microbiologist might feel similarly about some features of the biological discussion. However, the main message in every section is generally clear; Dusenbery has a gentle style for presenting an argument aimed at establishing quantitative trends and relationships as well as for identifying questions involving “optimization.” As such, the book will provide a wide and rich set of examples for most any type of biomechanics course. It has much the spirit of the biomechanics books wonderfully written by Steven Vogel, though Dusenbery goes further in bringing in quantitative arguments.

As I read Living at Micro Scale, I recognized that in some sense, there are two distinct audiences for this book: (1) physical scientists interested in quantitative aspects of biofluid dynamics in general and swimming microorganisms in particular, and (2) biological scientists comfortable with algebraic arguments who seek a mechanistic understanding of the microbiological world at the scale of individual cells. It is precisely a result of these two distinct audiences that I think many of the quantitative arguments would have benefited from figures and pictures and a more transparent definition of variables; nonquantitative readers may find the lack of diagrams a significant hurdle to understanding the text.

A second, small point of confusion results from several instances in the book where Dusenbery uses the term “relative velocity” or “relative speed” (together with the symbol U, commonly used for velocities) to refer to a speed per unit length, which a physical scientist would refer to as a (shear) rate. Additionally, one major shortcoming of the book is the lack of high-quality (color) images of actual microorganisms. Surely, many readers would find the diversity of microbial life, including various sizes, shapes, and appendages, to be captivating and motivating. Moreover, such images would nicely emphasize important themes raised in the book, such as convergent evolution.

I should also note that there are a number of appendixes. Perhaps a couple of them will be useful to a reader, though I fail to see how a twopage description of calculus (without a figure illustrating the derivative as the rate of change) is useful to anyone, or how a reader could benefit from a three-page description of the Navier-Stokes equation, which applies to any Reynolds number (and not simply to “high” Reynolds numbers, as the title of the appendix implies). It did not help that in the latter appendix, the only two numbered equations (A. 11.1 and A. 11.2) both had misprints.

A number of topics involving the mechanical world of microorganisms are not mentioned at any length in the book—biofilm formation; adhesion of bacteria to surfaces; “gyrotaxis,” which refers to the swimming of bottom-heavy algae that are able to orient in a flow; and the movements of multicellular Volvox—though Dusenbery does discuss simple orientations by distributed mass density. Nevertheless, the range of coverage and topics that are included is very large, and the author provides plenty of material for a curious reader or a university course. I am confident that readers interested in biomechanical themes will enjoy Living at Micro Scale because of the elegance with which Dusenbery weaves the concepts of classical physics into a quantitative characterization of the world of microorganisms.

Howard A. Stone "Living at Micro Scale: The Unexpected Physics of Being Small.," BioScience 60(11), 950-951, (1 December 2010).
Published: 1 December 2010

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