Prokaryotic Metabolism

Energy and Nutrient Requirements for Prokaryotes

Prokaryotes need a source of energy, a source of carbon, macronutrients, and micronutrients to survive.

Learning Objectives

Summarize what prokaryotes need to remain alive and functioning

Key Takeaways

Key Points

  • The main components of the organic compounds in a prokaryotic cell are macronutrients (such as carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur) that make up important biomolecules, including proteins and nucleic acids.
  • Nutrients that are needed by prokaryotes in small quantities to perform cellular functions, including certain electron transport chain reactions, are known as micronutrients (e.g., iron, boron, chromium, and manganese).
  • Autotrophic prokaryotes are able to fix inorganic compounds, such as carbon dioxide, to obtain carbon, while heterotrophic prokaryotes use organic compounds as their source of carbon.

Key Terms

  • macronutrient: any of the elements required in large amounts by all living things
  • chemotroph: an organism that obtains energy by the oxidation of electron-donating molecules in the environment
  • micronutrient: a mineral, vitamin, or other substance that is essential, even in very small quantities, for growth or metabolism

Needs of Prokaryotes

The diverse environments and ecosystems on Earth have a wide range of conditions in terms of temperature, available nutrients, acidity, salinity, and energy sources. Prokaryotes are very well equipped to make their living out of a vast array of nutrients and conditions. To live, prokaryotes need a source of energy, a source of carbon, and some additional nutrients.

Macronutrients

Cells are essentially a well-organized assemblage of macromolecules and water. Recall that macromolecules are produced by the polymerization of smaller units called monomers. For cells to build all of the molecules required to sustain life, they need certain substances, collectively called nutrients. When prokaryotes grow in nature, they obtain their nutrients from the environment. Nutrients that are required in large amounts are called macronutrients, whereas those required in smaller or trace amounts are called micronutrients. Just a handful of elements are considered macronutrients: carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur. (A mnemonic for remembering these elements is the acronym CHONPS. )

Why are these macronutrients needed in large amounts? They are the components of organic compounds in cells. Carbon is the major element in all macromolecules: carbohydrates, proteins, nucleic acids, lipids, and many other compounds. Carbon accounts for about 50 percent of the composition of the cell. Nitrogen represents 12 percent of the total dry weight of a typical cell and is a component of proteins, nucleic acids, and other cell constituents. Most of the nitrogen available in nature is either atmospheric nitrogen (N2) or another inorganic form. Diatomic (N2) nitrogen, however, can be converted into an organic form only by certain organisms, called nitrogen-fixing organisms. Both hydrogen and oxygen are part of many organic compounds and of water. Phosphorus is required by all organisms for the synthesis of nucleotides and phospholipids. Sulfur is part of the structure of some amino acids such as cysteine and methionine. It is also present in several vitamins and coenzymes. Other important macronutrients are potassium (K), magnesium (Mg), calcium (Ca), and sodium (Na). Although these elements are required in smaller amounts, they are very important for the structure and function of the prokaryotic cell.

Micronutrients

image

Filaments of photosynthetic cyanobacteria: Cyanobacteria are an example of phototrophic prokaryotes.

In addition to these macronutrients, prokaryotes require various metallic elements in small amounts. These are referred to as micronutrients or trace elements. For example, iron is necessary for the function of the cytochromes involved in electron-transport reactions. Some prokaryotes require other elements (such as boron (B), chromium (Cr), and manganese (Mn)) primarily as enzyme cofactors.

The Ways in Which Prokaryotes Obtain Energy

Prokaryotes can use different sources of energy to assemble macromolecules from smaller molecules. Phototrophs (or phototrophic organisms) obtain their energy from sunlight. Chemotrophs (or chemosynthetic organisms) obtain their energy from chemical compounds. Chemotrophs that can use organic compounds as energy sources are called chemoorganotrophs. Those that can also use inorganic compounds as energy sources are called chemolithotrophs.

The Ways in Which Prokaryotes Obtain Carbon

Just as prokaryotes can use different sources of energy, they can also utilize different sources of carbon compounds. Recall that organisms that are able to fix inorganic carbon are called autotrophs. Autotrophic prokaryotes synthesize organic molecules from carbon dioxide. In contrast, heterotrophic prokaryotes obtain carbon from organic compounds. To make the picture more complex, the terms that describe how prokaryotes obtain energy and carbon can be combined. Thus, photoautotrophs use energy from sunlight and carbon from carbon dioxide and water, whereas chemoheterotrophs obtain energy and carbon from an organic chemical source. Chemolithoautotrophs obtain their energy from inorganic compounds, while building their complex molecules from carbon dioxide. Table 1 summarizes carbon and energy sources in prokaryotes.

image

Table 1. Carbon and energy sources in prokaryotes: This table summarizes the types of energy and carbon sources for different types of prokaryotes.

The Role of Prokaryotes in Ecosystems

Prokaryotes play vital roles in the movement of carbon dioxide and nitrogen in the carbon and nitrogen cycles.

Learning Objectives

Give examples of the beneficial roles played by prokaryotes in different ecosystems

Key Takeaways

Key Points

  • Carbon and nitrogen are both macronutrients that are necessary for life on earth; prokaryotes play vital roles in their cycles.
  • The carbon cycle is maintained by prokaryotes that remove carbon dioxide and return it to the atmosphere.
  • Prokaryotes play a major role in the nitrogen cycle by fixing atomspheric nitrogen into ammonia that plants can use and by converting ammonia into other forms of nitrogen sources.

Key Terms

  • carbon cycle: the physical cycle of carbon through the earth’s biosphere, geosphere, hydrosphere, and atmosphere that includes such processes as photosynthesis, decomposition, respiration and carbonification
  • nitrogen cycle: the natural circulation of nitrogen, in which atmospheric nitrogen is converted to nitrogen oxides and deposited in the soil, where it is used by organisms or decomposed back to elemental nitrogen
  • nitrogen fixation: the conversion of atmospheric nitrogen into ammonia and organic derivatives, by natural means, especially by microorganisms in the soil, into a form that can be assimilated by plants

Role of Prokaryotes in Ecosystems

Prokaryotes are ubiquitous: There is no niche or ecosystem in which they are not present. Prokaryotes play many roles in the environments they occupy, but 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. Prokaryotes play an important role in the carbon cycle. 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.

image

Carbon cycle: Prokaryotes play a significant role in continuously moving carbon through the biosphere.

A large amount of available carbon is found in land plants, which are producers that 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, releasing 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).

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.

Prokaryotes and the Nitrogen Cycle

Nitrogen is a very important element for life because it is part of proteins and nucleic acids. As a macronutrient 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 solely by prokaryotes; they 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 by natural means, especially by microorganisms (prokayotes) in the soil. Ammonia can then be used by plants or converted to other forms.

image

Nitrogen cycle: Prokaryotes play a key role in the nitrogen cycle.

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 perform the reverse process, the reduction of nitrate from the soils to gaseous compounds such as N2O, NO, and N2, a process called denitrification.