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Tag words: carbon cycle, primary production, nitrogen cycle, sulfur cycle, biodgradation, methanogen, methanogenesis, nitrogen fixation, denitrification, nitrification, anoxygenic photosynthesis, anaerobic respiration, lithotroph, lithotrophy.

Kenneth Todar currently teaches Microbiology 100 at the University of Wisconsin-Madison.  His main teaching interest include general microbiology, bacterial diversity, microbial ecology and pathogenic bacteriology.

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Bacteria and Archaea and the Cycles of Elements in the Environment (page 1)

(This chapter has 4 pages)

© Kenneth Todar, PhD

The most significant effect that the procaryotes, bacteria and archaea, have on their environment is their underlying ability to recycle the essential elements that make up cells. The earth is a closed system with limited amounts of certain elements in forms that are utilized by cells. These element are generally acted upon first by microbes to assimilate them into living matter. The total biomass of microbial cells in the biosphere, their metabolic diversity, and their persistence in all habitats that support life, guarantee that microbes will play crucial roles in the transformations and recycling of these elements among all forms of life.

The table below lists the major elements that make up a typical procaryotic cell (in this case, E. coli). As expected, over 90 percent of the elemental analysis consists of carbon, hydrogen, oxygen, nitrogen, phosphorus and sulfur. These are the elements that become combined to form all the biochemicals and macromolecules that comprise living systems. C, H, O, N, P and  S are the constituents of organic material (An organic compound is a chemical that contains a carbon to hydrogen bond. Organic compounds on earth are evidence of life.  Organic compounds may be symbolized as CH2O, which is the empirical formula for a sugar such as glucose.) H and O are the constituents of water (H2O), that makes up over 95 percent of the cell composition. Calcium (Ca++), iron (Fe++), magnesium (Mg++) and potassium (K+) are present as inorganic salts in the cytoplasm of cells.

Table 1. Major elements, their sources and functions in cells.

Element % of dry weight Source Function
Carbon 50 organic compounds or CO2 Main constituent of cellular material
Oxygen 20 H2O, organic compounds, CO2, and O2 Constituent of cell material and cell water; O2 is electron acceptor in aerobic respiration
Nitrogen 14 NH3, NO3, organic compounds, N2 Constituent of amino acids, nucleic acids nucleotides, and coenzymes
Hydrogen 8 H2O, organic compounds, H2 Main constituent of organic compounds and cell water
Phosphorus 3 inorganic phosphates (PO4) Constituent of nucleic acids, nucleotides, phospholipids, LPS, teichoic acids
Sulfur 1 SO4, H2S, S, organic sulfur compounds Constituent of cysteine, methionine, glutathione, several coenzymes
Potassium 1 Potassium salts Main cellular inorganic cation and cofactor for certain enzymes
Magnesium 0.5 Magnesium salts Inorganic cellular cation, cofactor for certain enzymatic reactions
Calcium 0.5 Calcium salts Inorganic cellular cation, cofactor for certain enzymes and a component of endospores
Iron 0.2 Iron salts Component of cytochromes and certain nonheme iron-proteins and a cofactor for some enzymatic reactions

The table ignores the occurrence of "trace elements" in cells. Trace elements are metal ions required in cellular nutrition in such small amounts that it is difficult to determine or demonstrate their presence in cells. The usual metals that qualify as trace elements are Mn++, Co++, Zn++, Cu++ and Mo++. Trace elements are usually built into vitamins and enzymes. For example, vitamin B12 contains cobalt (Co++) and the bacterial nitrogenase enzyme contains molybdenum (Mo++). 

Microbes and the Cycles of Elements

Of course, all living organisms play a role in the cycles of elements, but for the most part, it is the procaryotes that play major and sometimes unique roles. Herein, we discuss total microbial contribution to the cycles of the major elements, but major emphasis is placed on procaryotes.

The fungi (molds and yeasts). The molds are aerobic organisms that utilize organic compounds for growth.  They play an important role in decomposition or biodegradation of organic matter, particularly in soil. Yeast can grow anaerobically (without oxygen) through the process of fermentation. They play a role in fermentations in environments high in sugar. The prominent role of fungi in the environment is in the carbon cycle, during the process of decomposition, especially in the soil.

The algae are also an important part of the carbon cycle. They are the predominant photosynthetic organisms in many aquatic environments. The algae are autotrophs, which means they use  carbon dioxide (CO2) as a source of carbon for growth. Hence they convert atmospheric CO2 into organic material (i.e., algal cells). Algae also play a role in the oxygen (O2)  cycle since their style of photosynthesis, similar to plants,  produces O2 in the atmosphere. The cyanobacteria are a group of procaryotic microbes, as prevalent as algae, that have this type of metabolism.  Photosynthetic algae and cyanobacteria can be found in most environments where there is moisture and light. They are a major component of marine plankton which the basis of the food chain in the oceans.

Protozoans are heterotrophic organisms that have to catch or trap their own food. Therefore, they have developed elaborate mechanisms for movement and acquiring organic food which they can digest. Their food usually turns out to be bacterial cells, so one might argue that they are ecological predators that keep bacterial populations under control in soil, aquatic environments, intestinal tracts of animals, and many other environments.

The procaryotic bacteria and archaea, as a result of their diversity and unique types of metabolism, are involved in the cycles of virtually all essential elements.  In two cases, methanogenesis (conversion of carbon dioxide into methane) and nitrogen fixation (conversion of nitrogen in the atmosphere into biological nitrogen) are unique to procaryotes and earns them their "essential role" in the carbon and nitrogen cycles.

There are  other metabolic processes that are unique, or nearly so, in the procaryotes that bear significantly on the cycles of elements. For example, procaryotes called lithotrophs use inorganic compounds like ammonia and hydrogen sulfide as a source of energy, and others called anaerobic respirers use nitrate (NO3) or sulfate (SO4) in the place of oxygen, so they can respire without air. Most of the archaea are lithotrophs that use hydrogen (H2) or  hydrogen sulfide (H2S) as a source of energy, while many soil bacteria are anaerobic respirers that can use their efficient respiratory metabolism in the absence of O2.

The basic processes of heterotrophy are spread throughout the bacteria. Most of the bacteria in the soil and water, and in associations with animals and plants, are heterotrophs. Heterotrophy means living off of dead organic matter, usually by some means of respiration (same as animals) or fermentation (same as yeast or lactic acid bacteria). Bacterial heterotrophs in the carbon chain are important in the processes of biodegradation and decomposition under aerobic and anaerobic conditions.

In bacteria, there is a unique type of photosynthesis that does not use H2O or produce O2 which impacts on the carbon and sulfur cycles.

Meanwhile, the cyanobacteria (mentioned above) fix CO2 and produce O2 during photosynthesis, and they make a very large contribution to the carbon and oxygen cycles.

The list of examples of microbial involvement in the cycles of elements that make up living systems is endless, and probably every microbe in the web is involved in an intimate and unique way.

The Oxygen Cycle

Basically, O2 is derived from the photolysis of H2O during plant (oxygenic) photosynthesis. Two major groups of microorganisms are involved in this process, the eucaryotic algae, and the procaryotic cyanobacteria (formerly known as "blue-green algae"). The cyanobacteria and algae are the source of much of the O2 in the earth's atmosphere.  Of course, plants account for some O2 production as well, but the microbes predominate in marine habitats which cover the majority of the planet.

Since most aerobic organisms need the O2 that results from plant photosynthesis, this establishes a  relationship between plant photosynthesis and aerobic respiration, two prominent types of metabolism on earth. Photosynthesis produces O2 needed for aerobic respiration. Respiration produces CO2 needed for autotrophic growth.

CO2 + H2O-----------------> CH2O (organic material) + O2  plant (oxygenic) photosynthesis

CH2O + O2-----------------> CO2 + H2O aerobic respiration

Since these photosynthetic microbes are also autotrophic (meaning they convert CO2 to organic material during growth) they have a similar impact on the carbon cycle (page 2).

chapter continued

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