0.30 Bis2a 08.0 metabolism from a microbes perspective  (Page 2/14)

Micronutrients

In addition to these macronutrients, bacteria and archaea 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 bacteria and archaea obtain energy

Bacteria and Archaea 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 chemolitotrophs.

The ways in which bacteria and archaea obtain carbon

Bacteria and Archaea not only can use different sources of energy but also different sources of carbon compounds. Recall that organisms that are able to fix inorganic carbon are called autotrophs . Autotrophic organisms synthesize organic molecules from carbon dioxide. In contrast, heterotrophic organisms obtain carbon from organic compounds. To make the picture more complex, the terms that describe how organisms 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. Chemolitoautotrophs obtain their energy from inorganic compounds, and they build their complex molecules from carbon dioxide. The table below ( [link] ) summarizes carbon and energy sources.

Role of microbes in ecosystems

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

Microbes and the carbon cycle

Carbon is one of the most important macronutrients, and prokaryotes play an important role in the carbon cycle ( [link] ). 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 bacteria, and is returned to the atmosphere via the respiration of chemoorganotrophic organisms, including bacteria, 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.