Prescott's Microbiology,4/e

		Microbiology, 4/e Prescott, Harley, Klein
		Instructor Resources

40 Microorganisms as Components of the Environment

CHAPTER OVERVIEW

This chapter discusses the roles of microorganisms in ecosystems. The organisms= physiological state, nutrient cycling, decomposition processes, and successional interactions are discussed. The fate and impact of genetically engineered organisms are also considered. The chapter includes a discussion of extreme environments and concludes with a discussion of the methods used in environmental studies.

CHAPTER OBJECTIVES

After reading this chapter you should be able to:

! discuss the populations and communities of microorganisms that are associated with various ecosystems environments and describe the roles they play in succession

! discuss the interactions of microorganisms with one another and with other nonmicrobial members of various ecosystems

! describe the ways in which microorganisms obtain their required nutrients from environmental sources

! discuss the fate and potential impact of the releasing genetically engineered microorganisms

! describe the microorganisms found in extreme environments and the adaptations necessary for their survival under these conditions

! discuss the various methods used to study microorganisms in the environment

CHAPTER OUTLINE

I. Microorganisms and the Structure of Ecosystems

A. Roles of organisms in an ecosystem

1. Primary producerCsynthesizes and accumulates organic matter

2. ConsumerCuses the accumulated organic matter as a food supply

3. DecomposerCcycles various nutrients from dead organisms into forms that can be used by the other members of the ecosystem

B. Roles of microorganisms in an ecosystem

1. ChemoautotrophsCprimary producers in limited environmental niches only, such as deep oceanic hydrothermal vents

2. PhotoautotrophsCprimary producers in many aquatic ecosystems

3. ChemoheterotrophsCdecomposers whose primary role is the mineralization of organic matter to simpler inorganic forms of nutrients

4. Other roles of microorganisms

a. Serve as food source for other organisms

b. Produce inhibitory compounds that decrease microbial activity, and thereby limit the survival and functioning of plants and animals dependent on those microorganisms

C. Interactions of microorganisms with their environments

1. PopulationsCmicroorganisms of the same type growing at a localized site

2. CommunitiesCdifferent types of interactive populations living together within an ecosystem

3. Microbial environments are complex and constantly changing; they are characterized by overlapping gradients of resources, toxic materials, and other limiting factors.

4. MicroenvironmentsCregions where resources interact (e.g., at the interface between aerobic and anaerobic environments); this sometimes gives rise to specialized groups of microorganisms suited to those unique microenvironments.

D. Microbial ecologyCthe term used for the study of microbes and their relationship with their particular environments

II. The Physiological State of Microorganisms in Ecosystems

A. Many environments contain low levels of nutrients, which spurs intense competition among organisms

B. Microorganisms may increase surface area to allow for more efficient uptake of limited nutrients

C. Microorganisms may attach to surfaces that may have a higher local concentration of necessary nutrients thereby creating biofilms

D. Microorganisms may sequester critical limiting nutrients, thereby making them less available to competing microorganisms

E. Natural chemicals can inhibit microbial growth in low-nutrient environments

III. Nutrient Cycling ProcessesCbiogeochemical cycling

A. Carbon cycleCcarbon can be interconverted between methane, complex organic matter, and carbon dioxide; carbon fixation can occur by the activities of cyanobacteria, the green algae, photosynthetic bacteria, and aerobic chemolithoautotrophs

B. Sulfur cycleCsulfur can be interconverted between elemental sulfur, sulfide, and sulfate forms by the actions of various microorganisms

1. Dissimilatory sulfate reduction produces sulfide which accumulates in the environment

2. Assimilatory sulfate reduction results in the reductin of sulfate for use in amino acid biosynthesis

C. Nitrogen cycleCa variety of microorganisms participate in this process

1. NitrificationCaerobic oxidation of ammonium ion to nitrite, and ultimately to nitrate

2. DenitrificationCreduction of nitrate to nitrite, nitrous oxide, and gaseous molecular nitrogen

3. Nitrogen assimilationCutilization of inorganic nitrogen and its incorporation into new microbial biomass

4. Nitrogen fixation

a. A series of sequential reduction steps to convert gaseous nitrogen to nitrate

b. Requires an expenditure of energy

c. Can be carried out by aerobes or anaerobes; the actual reduction process must be done anaerobically, even by aerobic microorganisms

d. Anaerobic conditions can be maintained by using physical barriers, O₂-scavenging molecules, and high rates of metabolic activity

D. Other cycling processes include iron cycling from ferrous to ferric ion, phosphorus cycling between inorganic and organic forms, and manganese cycling

IV. Interactions Between Microorganisms and MetalsCdiffering sensitivities of more complex organisms (usually less sensitive) and microorganisms (usually more sensitive) form the basis for many antiseptic procedures developed over the last 150 years; metals can be broadly characterized into three groups

A. Noble metals (silver, gold, platinum, etc.)Ccannot cross the blood-brain barrier of vertebrates, but have distinct effects on microorganisms

B. Metals and metalloids that can be methylated (mercury, arsenic, lead, selenium, and tin are important examples)Cmethylation enables them to cross the blood-brain barrier and affect the central nervous system of higher organisms; also affect microorganisms; methylated mercury can be concentrated in the food chain (a process known as biomagnification)

C. Metals that occur in ionic forms (copper, zinc, cobalt, etc.) can be directly toxic to microorganisms; often required as trace elements, but excess is toxic

V. Interactions in Substrate Utilization

A. CommensalismCwaste products accumulated by some microorganisms are used as nutrients by other microorganisms; microbial succession

B. Microbial succession also occurs when mixtures of electron acceptors are present and are used in a defined order

VI. Organic Substrate Use by MicroorganismsCdifferent organisms have different abilities to utilize various types of organic molecules as nutrient sources; most organic compounds can be degraded with or without oxygen, except some hydrocarbons and lignin

A. Factors influencing degradation include:

1. Elemental composition

2. Structure of basic repeating units

3. Linkages between repeating units

4. Nutrients present in the environment

5. Abiotic conditions (pH, oxidation-reduction potential, O₂, osmotic conditions)

6. Microbial community present

B. Hydrocarbon degradation usually requires oxygen: the first step involves addition of molecular oxygen; recently however, slow anaerobic digestion in the presence of sulfate or nitrate has been observed

C. LigninCfilamentous fungi that are major lignin degraders require oxygen; important in construction because wood pilings can be used below the water table where anaerobic conditions are maintained; however, if the water table drops, degradation can take place, thereby weakening the structure

D. Other organics, such as starch, cellulose, hemicellulose, chitin, protein, lipids, and biomass, can be used to provide most of the nutrients required by microorganisms; only previously grown microbial biomass contains all of the nutrients required for microbial growth

VII. Disease Causing MicrobesCSurvival and Fate

A. Allochthonous (introduced) microorganisms are constantly being added to natural environments, in human and animal wastes and in sewage materials

B. They generally do not grow well in natural environments, but may survive for long periods of time, particularly at lower temperatures

C. Detection of these organisms (particularly if pathogenic) is a major concern

D. Many occur naturally in the environment, especially in waters.

E. Many microorganisms recovered and cultured lose their ability to survive when re-introduced to their original environments

VIII. Genetically Engineered MicroorganismsCFate and Effects

A. Effects depend on survival, reproduction, and/or gene transfer to other microorganisms

B. Each of these will occur only at low probability, but the uncertainties are many

C. Microorganisms can be made less competitive for low nutrient levels and can be constructed with copy blocks that limit replication

D. Gene transfers to natural populations are a concern but are unlikely because of the low population density, nutrient levels, and growth rates

E. However, natural competent microorganisms coupled with stabilization of DNA released from dead microorganisms makes gene transfer a distinct possibility

IX. Extreme EnvironmentsCphysical and chemical conditions restrict the diversity of microbial types that can maintain themselves

A. SalinityCfavors extreme halophiles

B. High barometric pressure (e.g. deep sea environments) favor barotolerant, moderately barophilic, and extremely barophilic bacteria

C. High temperature (up to 113EC) favors thermophiles and extreme thermophiles

D. AcidityCacidophiles maintain a high internal pH relative to the environment

E. AlkalinityCalkalophiles maintain a low internal pH relative to the environment

X. Methods Used in Environmental Studies

A. Microscopic examination

B. Viable cell counting

C. Measurement of nutrient cycling

D. Measurement of organic carbon by biochemical oxygen demand (BOD), chemical oxygen demand (COD), or total organic carbon (TOC)

E. Nucleic acid probe technology can be used to look for specific organisms

F. Gel array microchips (genosensors) containing a mixture of probes can detect single subunit (ssu) ribosomal RNA in mixed populations

G. A summary of methods and their uses in various environments is given in Table 40.6 of the textbook

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