Although microbes may adhere to and grow on a variety of objects on land, the focus in this section will deal with soil, as it is a critical component of terrestrial ecosystems. Extreme terrestrial habitats, such as volcanic vents and fissures, and the extremophiles that inhabit them are described in chapter 11.
■ extreme thermophiles, p. 293 ■ thermophilic extreme acidophiles, p. 294
Human interest in the microbiology of soil stems partly from the ability of microbes to synthesize a variety of useful chemicals. For example, over 500 different antibiotic substances are produced by Streptomyces species, at least 50 of which have useful applications in medicine, agriculture, and industry. The pharmaceutical industry has tested many thousands of soil microorganisms to find those that produce useful antibiotics. There is also an ongoing search to find organisms capable of synthesizing other useful compounds. In addition, soil microbes are being investigated for their ability to degrade toxic chemicals, an application of environmental microbiology called bioreme-diation, which will be discussed in chapter 31. Probably in no other habitat can one hope to find a greater range of biosynthetic and biodegradative capabilities than are represented in the soil.
Characteristics of Soil
Soil is composed of pulverized rock, decaying organic material, air, and water. It teems with life, including bacteria, fungi, algae, protozoa, worms, insects, and plant roots. Soil communities may contain more than 4,000 different species per gram of soil. The top 6 inches of fertile soil may harbor more than 2 tons of bacteria and fungi per acre! Soil represents an environment that can fluctuate abruptly and dramatically. Heavy rains, for example, can cause a soil to rapidly become waterlogged. Trees dropping their leaves can suddenly enrich the soil with organic nutrients. Farmers and gardeners can rapidly change the nutrient mix by applying fertilizers.
Soil forms as rock weathers. Water, temperature changes, wind-blown particles, and other physical forces gradually lead to cracking and fragmentation of the rock. Photosynthetic organisms including algae, mosses, and lichens growing on the surfaces of rocks synthesize organic compounds. Various chemoorganohet-erotrophic bacteria and fungi then use these compounds as carbon and energy sources. Their metabolism results in the production of acids and other chemicals, which gradually decompose the rocks. As soil slowly forms, some plants begin to grow. When these die and decay, the residual organic material functions as a sponge, retaining water and thus enabling more plants to grow. Over time, more organic compounds accumulate, forming a slowly degrading complex polymeric substance called humus. ■ lichens, p. 311 ■ chemoorganoheterotroph, p. 91
Soil has multiple layers, or horizons, each with distinct characteristics. Topsoil, or the A horizon, is the dark nutrient-rich uppermost layer that supports plant growth; its depth may vary from a few inches to several feet. Subsoil, or the B horizon, is an accumulation of clay, salts, and various nutrients leached from the upper layer. The next layer is the C horizon, which is partially weathered bedrock. The lowest layer is bedrock, or the R horizon, which is unweathered.
The texture of the soil influences its porosity, which in turn impacts the amount of air exchange and how much water can flow through (figure 30.8). Finely textured soils, such as clay soils, are more apt to become waterlogged and anaerobic. In contrast, sandy soils that dry quickly allow water to pass through and are generally aerobic.
The density and composition of the flora of the soil is dramatically affected by environmental conditions. Wet soils, for example, are unfavorable for aerobic microbes because the spaces in the soil fill up with water, thus diminishing the amount of air in the soil. When the water content of soil drops to a very low level, as during a drought or in a desert environment, the metabolic activity and number of soil microorganisms decrease. Many species of soil organisms produce survival forms such as endospores and cysts that are resistant to drying. Other environmental influences that impact soil microbes include acidity, temperature, and nutrient supply. For example, acidity suppresses the growth of bacteria, allowing fungi to thrive with less competition for nutrients. This is why mushrooms often
Concentration at cell surface
Figure 30.8 Texture of Soil The porosity of soil impacts the amount of air exchange and how much water can flow through, influencing the composition of microbial population.
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