Figure 4.3 (a) Nitrogen-fixing bacteria such asRhizobiumlive in the root nodules of legumes such as clover. (b)
This micrograph of the root nodule shows bacteroids (bacterium-like cells or modified bacterial cells) within the plant
cells. The bacteroids are visible as darker ovals within the larger plant cell.
Another positive function of prokaryotes is in cleaning up the environment. Recently, some researchers focused on
the diversity and functions of prokaryotes in manmade environments. They found that some bacteria play a unique
role in degrading toxic chemicals that pollute water and soil.[3]
Despite all of the positive and helpful roles prokaryotes play, some are human pathogens that may cause illness or
infection when they enter the body. In addition, some bacteria can contaminate food, causing spoilage or foodborne
illness, which makes them subjects of concern in food preparation and safety. Less than 1% of prokaryotes (all of
them bacteria) are thought to be human pathogens, but collectively these species are responsible for a large number
of the diseases that afflict humans.
Besides pathogens, which have a direct impact on human health, prokaryotes also affect humans in many indirect
ways. For example, prokaryotes are now thought to be key players in the processes of climate change. In recent years,
as temperatures in the earth’s polar regions have risen, soil that was formerly frozen year-round (permafrost) has
begun to thaw. Carbon trapped in the permafrost is gradually released and metabolized by prokaryotes. This produces
massive amounts of carbon dioxide and methane, greenhouse gases that escape into the atmosphere and contribute to
the greenhouse effect.
- In what types of environments can prokaryotes be found?
- Name some ways that plants and animals rely on prokaryotes.
Symbiotic Relationships
As we have learned, prokaryotic microorganisms can associate with plants and animals. Often, this association results
in unique relationships between organisms. For example, bacteria living on the roots or leaves of a plant get nutrients
from the plant and, in return, produce substances that protect the plant from pathogens. On the other hand, some
bacteria are plant pathogens that use mechanisms of infection similar to bacterial pathogens of animals and humans.
Prokaryotes live in acommunity, or a group of interacting populations of organisms. A population is a group of
individual organisms belonging to the same biological species and limited to a certain geographic area. Populations
- A.M. Kravetz “Unique Bacteria Fights Man-Made Chemical Waste.” 2012. http://www.livescience.com/25181-bacteria-strain-cleans-up-
toxins-nsf-bts.html. Accessed March 9, 2015.
142 Chapter 4 | Prokaryotic Diversity
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