21. The Nature of Microorganisms




Are We Killing the Oceans?

Fertilizer may be the cause.

Dead zones are areas in the world’s oceans where the oxygen level is so low that most organisms die. In 2006, the United Nations published a list of over 200 places in the world’s oceans where dead zones developed for at least part of each year. Similar dead zones occur in some lakes. Dead zones normally develop at the mouths of rivers as a result of human action and the activity of various kinds of microorganisms. One of the largest dead zones is in the Gulf of Mexico off the mouth of the Mississippi River. Fertilizer from agricultural runoff, waste from large industrial livestock operations, and the effluent from poorly controlled municipal and industrial sewage flow into rivers that ultimately empty into an ocean (see photo). These nutrients stimulate the growth of single-celled and multi-cellular algae during the warmer months of the year. Eventually, these organisms die and sink to the bottom, where decomposer bacteria use oxygen in the process of aerobic respiration to break down the dead organic matter. This lowers the amount of dissolved oxygen in the water, causing the death of bottomdwelling animals.

• Why are bacteria able to live in regions of low oxygen when animals cannot?

• Why do dead zones occur off the mouths of rivers?

• Should the amount of fertilizer farmers apply to their fields to increase yields be regulated?


ü  Background Check

Concepts you should already know to get the most out of this chapter:

• The processes of natural selection and evolution (chapter 13)

• Prokaryotic organisms have a simpler cellular structure than eukaryotic organisms (chapter 4)

• How structural and life history characteristics are used to classify organisms (chapter 20)


21.1. What Are Microorganisms?


A microorganism, or microbe, is a tiny organism that usually cannot be seen without the aid of a microscope. These are terms of convenience for a wide variety of organisms, including the domains Bacteria and Archaea and the kingdoms Protista and Fungi in the domain Eucarya. Often, tiny animals, particularly those that cause disease (mites, worms, etc.), are considered microorganisms as well. Viruses are also treated as microbes. However, we will not consider animals or viruses in this chapter. Viruses were discussed in chapter 20 and animals are discussed in chapter 23.

In a very general sense, microorganisms share several characteristics. These organisms generally consist of cells, which function independently. Many are single-celled organisms, although some single-celled microbes form loose aggregations, called colonies. Others are multicellular and have some specialization of cells for certain functions. Their primary method of reproduction is asexual reproduction, in which one cell divides to become two cells, although most kinds are also capable of sexual reproduction. Many have special structures involved in the production of gametes. Many microbes, particularly Bacteria, have the ability to exchange pieces of genetic material, which creates new combinations of genes. In some cases this even involves gene exchange between organisms that are considered to be of different species.

Microbes are extremely common organisms. It is estimated that the total biomass of microbes is larger than the biomass of all other kinds of organisms combined. They live in any aquatic or moist environment and occur in huge numbers in the oceans and other bodies of water and in soil. It has been suggested that if one were able to instantly remove all living things from Earth except microbes, everything on Earth would still be visible in outline form because microbes cover all surfaces, including living things. Because they are small, their moist habitat does not need to be large. Microbes can maintain huge populations in places such as human skin or intestine, temporary puddles, and soil. Your skin, mouth, and gut each contains trillions of microbes. Most die if they dry out, but some have the special ability to become dormant and survive long periods without water. When moistened, they become actively growing cells again.



1. What taxonomic groups are included in the category known as microorganisms?

2. List three general characteristics the various kinds of microbes share.