3.2: Cell morphology and size
- Page ID
- 131120
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Many cell morphologies have been observed, including cells that look like lemons, teardrops, oblong spheroids, flat squares, beans, disks, and more (Young, 2006). However, the most common shapes are bacilli (rods), cocci (spherical), spirilla (curved rods), spirochetes (tightly coiled cells), and filamentous bacteria (long thin cells or chains of cells) (Fig. \(3.2\)). Selective forces that guide variation in microbial morphology are not well known, but possibilities include nutrient limitations, growth demands, attachment, dispersal effects, motility, and predation (Young, 2006).
Eukaryotic cells are generally much larger than bacterial and archaeal cells. Eukaryotic cells range from about 5 to 100 µm in diameter whereas bacterial and archaeal cell diameters typically range from about 0.5 to 2 µm (Konhauser, 2007). There are exceptions, such as the centimeter-long bacterial cells recently described by Volland et al. (2022). However, microorganisms are generally very small and unable to be seen with the naked eye.
The small size of microorganisms appears to reflect their dependence on diffusion. Virtually all bacteria, even those able to use a flagellum to move, depend on diffusion to bring required compounds to their surface and mix nutrients and other molecules within their cytoplasm (Young, 2006). Smaller cells tend to have larger surface area to volume ratios, allowing more efficient exchanges of nutrients and waste than larger cells. This characteristic can allow smaller cells to have more rapid growth rates than larger cells.
Although small size can provide an advantage, it has its limits. Cells need to be large enough to house the essential stuff the cell needs, such as nucleic acids, ribosomes, proteins, and more (Koch, 1996). Moreover, surface-to-volume ratio seems to be more important in low-nutrient environments (Young, 2006). Where nutrient availability is high, a higher diffusive flux has the potential to allow cells to form larger cells, despite the potential decrease in the surface area to volume ratio.
One environmental implication of the small size of microorganisms is that it makes it possible for them to dominate life in the subsurface. Microorganisms can exist anywhere that macroorganisms are found. But microbes rule in the subsurface, where space available for life is mostly limited to small spaces between mineral grains or along fractures.
