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6.3: Role of Prokaryotes in Ecosystems

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    Learning Objectives
    • Describe the roles of prokaryotes in the carbon cycle
    • Describe the roles of prokaryotes in the nitrogen cycle

    Prokaryotes are ubiquitous: There is no niche or ecosystem in which they are not present. Prokaryotes play many roles in the environments they occupy. The roles they play in the carbon and nitrogen cycles are vital to life on Earth.

    Prokaryotes and the Carbon Cycle

    Carbon is one of the most important macronutrients, and prokaryotes play an important role in the carbon cycle (Figure 1). Carbon is cycled through Earth’s major reservoirs: land, the atmosphere, aquatic environments, sediments and rocks, and biomass. The movement of carbon is via carbon dioxide, which is removed from the atmosphere by land plants and marine prokaryotes, and is returned to the atmosphere via the respiration of chemoorganotrophic organisms, including prokaryotes, fungi, and animals. Although the largest carbon reservoir in terrestrial ecosystems is in rocks and sediments, that carbon is not readily available.

    A large amount of available carbon is found in land plants. Plants, which are producers, use carbon dioxide from the air to synthesize carbon compounds. Related to this, one very significant source of carbon compounds is humus, which is a mixture of organic materials from dead plants and prokaryotes that have resisted decomposition. Consumers such as animals use organic compounds generated by producers and release carbon dioxide to the atmosphere. Then, bacteria and fungi, collectively called decomposers, carry out the breakdown (decomposition) of plants and animals and their organic compounds. The most important contributor of carbon dioxide to the atmosphere is microbial decomposition of dead material (dead animals, plants, and humus) that undergo respiration.

    In aqueous environments and their anoxic sediments, there is another carbon cycle taking place. In this case, the cycle is based on one-carbon compounds. In anoxic sediments, prokaryotes, mostly archaea, produce methane (CH4). This methane moves into the zone above the sediment, which is richer in oxygen and supports bacteria called methane oxidizers that oxidize methane to carbon dioxide, which then returns to the atmosphere.

    This illustration shows the role of bacteria in the carbon cycle. Bacteria break down organic carbon, which is released as carbon dioxide into the atmosphere.
    Figure 1. Prokaryotes play a significant role in continuously moving carbon through the biosphere. (credit: modification of work by John M. Evans and Howard Perlman, USGS)

    Prokaryotes and the Nitrogen Cycle

    Nitrogen is a very important element for life because it is part of proteins and nucleic acids. It is a macronutrient, and in nature, it is recycled from organic compounds to ammonia, ammonium ions, nitrate, nitrite, and nitrogen gas by myriad processes, many of which are carried out only by prokaryotes. As illustrated in Figure 2, prokaryotes are key to the nitrogen cycle. The largest pool of nitrogen available in the terrestrial ecosystem is gaseous nitrogen from the air, but this nitrogen is not usable by plants, which are primary producers. Gaseous nitrogen is transformed, or “fixed” into more readily available forms such as ammonia through the process of nitrogen fixation. Ammonia can be used by plants or converted to other forms.

    Another source of ammonia is ammonification, the process by which ammonia is released during the decomposition of nitrogen-containing organic compounds. Ammonia released to the atmosphere, however, represents only 15 percent of the total nitrogen released; the rest is as N2 and N2O. Ammonia is catabolized anaerobically by some prokaryotes, yielding N2 as the final product. Nitrification is the conversion of ammonium to nitrite and nitrate. Nitrification in soils is carried out by bacteria belonging to the genera Nitrosomas, Nitrobacter, and Nitrospira. The bacteria performs the reverse process, the reduction of nitrate from the soils to gaseous compounds such as N2O, NO, and N2, a process called denitrification.

    This illustration shows the role of bacteria in the nitrogen cycle. Nitrogen-fixing bacteria in root nodules of legumes convert nitrogen gas, or N2, into organic nitrogen found in plants. Nitrogen-fixing soil bacteria produce ammonium ion, or NH4+. Decomposers, including bacteria and fungi, decompose organic matter, also releasing NH4+. Nitrification is the process by which nitrifying bacteria produce nitrites (NO2-) and nitrates (NO3-). Nitrates are assimilated by plants, then animals, then decomposers. Denitrifying bacteria convert nitrates to nitrogen gas, completing the cycle.
    Figure 2. Prokaryotes play a key role in the nitrogen cycle. (credit: Environmental Protection Agency)
    Practice Questions

    Which of the following statements about the nitrogen cycle is false?

    1. Nitrogen fixing bacteria exist on the root nodules of legumes and in the soil.
    2. Denitrifying bacteria convert nitrates (NO3) into nitrogen gas (N2).
    3. Ammonification is the process by which ammonium ion (NH4+) is released from decomposing organic compounds.
    4. Nitrification is the process by which nitrites (NO2) are converted to ammonium ion (NH4+).

    [reveal-answer q=”123240″]Show Answer[/reveal-answer]
    [hidden-answer a=”123240″]Answer d is false.


    Think about the conditions (temperature, light, pressure, and organic and inorganic materials) that you may find in a deep-sea hydrothermal vent. What type of prokaryotes, in terms of their metabolic needs (autotrophs, phototrophs, chemotrophs, etc.), would you expect to find there?

    [practice-area rows=”4″][/practice-area]
    [reveal-answer q=”566708″]See Our Thoughts[/reveal-answer]
    [hidden-answer a=”566708″]Responses will vary. In a deep-sea hydrothermal vent, there is no light, so prokaryotes would be chemotrophs instead of phototrophs. The source of carbon would be carbon dioxide dissolved in the ocean, so they would be autotrophs. There is not a lot of organic material in the ocean, so prokaryotes would probably use inorganic sources, thus they would be chemolitotrophs. The temperatures are very high in the hydrothermal vent, so the prokaryotes would be thermophilic.[/hidden-answer]

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