The physical-chemical environment of microbes

Many physical-chemical properties affecting microbes are familiar to ecologists examining large organisms in our visible world. This chapter starts by reviewing the basics of these properties, such as the importance of water for microbes in soils and temperature in all environments. Another important property, pH, has direct effects on organisms and indirect effects via how hydrogen ions determine the chemical form of key molecules and compounds in nature. Oxygen content is also critical, as it is essential to the survival of all but a few eukaryotes. Light is used as an energy source by phototrophs, but it can have deleterious effects on microbes. In addition to these familiar factors, the small size of microbes sets limits on their physical world. Microbes are said to live in a “low Reynolds number environment”. When the Reynolds number is smaller than about one, viscous forces dominate over inertial forces. For a macroscopic organism like us, moving in a low Reynolds number environment would seem like swimming in molasses. Microbes in both aquatic and terrestrial habitats live in a low Reynolds number world, one of many similarities between the two environments at the microbial scale. Most notably, even soil microbes live in an aqueous world, albeit a thin film of water on soil particles. But the soil environment is much more heterogeneous than water, with profound consequences for biogeochemical processes and interactions among microbes. The chapter ends with a discussion of how the physical-chemical environment of microbes in biofilms is quite different from that of free-living organisms.