Microorganisms and Environmental Changes

1. External and internal sources of environmental fluctuations are common. As a result, enzymes may be induced, mutants may be selected, or other species may become dominant. (Figure 30.4)

Microbial Communities

1. Microorganisms most often grow in communities attached to a solid substrate or at air-water interfaces.

2. A microbial mat is a thick, dense, highly organized biofilm composed of distinct layers, often green, pink, and black. (Figure 30.5)

Studying Microbial Ecology

1. Microbial ecology has been difficult to study because so few environmental prokaryotes can be successfully grown in the laboratory.

2. Molecular techniques, including fluorescence in situ hybridization, polymerase chain reaction (PCR), denaturing gradient gel electrophoresis (DGGE), and DNA sequencing are enabling researchers to better understand complex microbial communities.

782 Chapter 30 Microbial Ecology

30.2 Aquatic Habitats

1. Oligotrophic waters are nutrient poor; eutrophic waters are nutrient rich. (Figure 30.6)

2. Excessive growth of aerobic heterotrophs may cause an aquatic environment to become hypoxic, resulting in the death of fish and other aquatic animals.

Marine Environments

1. Ocean waters are generally oligotrophic and aerobic, but inshore areas can be dramatically affected by nutrient-rich runoff.

Freshwater Environments

1. Oligotrophic lakes may have anaerobic layers due to thermal stratification. (Figure 30.7) Shallow, turbulent streams are generally aerobic.

Specialized Aquatic Environments

1. Salt lakes and mineral-rich springs support the growth of microbes specifically adapted to thrive in these specialized environments.

30.3 Terrestrial Habitats

Characteristics of Soil

1. Soil represents an environment that can fluctuate abruptly and dramatically.

Microorganisms in Soil

1. The environmental conditions affect the density and composition of the flora of the soil.

The Rhizosphere

1. The concentration of microbes in the rhizosphere is generally much higher than that of the surrounding soil.

30.4 Biogeochemical Cycling and Energy Flow

1. Organisms use elements to produce biomass as sources of energy and as terminal electron acceptors.

Carbon Cycle (Figure 30.9)

1. One of the fundamental aspects of the carbon cycle is carbon fixation.

2. As consumers and decomposers degrade organic material, respiration and some fermentations release CO2.

Nitrogen Cycle (Figure 30.11)

1. The steps of the nitrogen cycle include nitrogen fixation, ammonification, nitrification, denitrification, and anammox. All the steps other than ammonification rely on prokaryotes.

Sulfur Cycle (Figure 30.12)

1. Like the nitrogen cycle, certain steps of the sulfur cycle— sulfur reduction and sulfate oxidation—depend on the activities of prokaryotes.

Phosphorus Cycle and Other Cycles

1. Most plants and microorganisms take up orthophosphate, assimilating it into biomass.

2. Iron, calcium, zinc, manganese, cobalt, and mercury are recycled by microorganisms.

Energy Sources for Ecosystems

1. Energy must be continually added to ecosystems.

2. Photosynthetic organisms convert solar energy to chemical bond energy in the form of organic compounds.

3. Chemolithoautotrophs harvest energy from reduced inorganic chemicals. (Figure 30.13)

30.5 Mutualistic Relationships Between Microorganisms and Eukaryotes


1. Mycorrhizae help plants take up phosphorus and other substances from soil; in turn the fungal partners gain nutrients for their own growth

2. Endomycorrhizal fungi penetrate root cells; ectomycorrhizal fungi grow around root cells.

Symbiotic Nitrogen-Fixers and Plants

1. Rhizobia fix nitrogen in nodules of leguminous plants. (Figure 30.14)

2. Frankia species fix nitrogen in nodules of alder and gingko.

3. A species of cyanobacteria fixes nitrogen in specialized sacs in the leaves of the Azolla fern.

Microorganisms and Herbivores

1. In order to subsist on grass and other plant material, herbivores rely on a community of microbes that inhabit a specialized digestive compartment, either a rumen or a cecum.

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