A typical woodland, freshwater pond teems with microbial life. © Stephen Durr, author. Licensed for use, ASM MicrobeLibrary.

It’s hard to say enough about the importance of microbes to the rest of us living things on Earth. In short, microbes are responsible for the make-up of the biosphere, the portion of the planet that supports life as we know it. Without microbes, much of the carbon, nitrogen, oxygen, sulfur, phosphorus, and other chemical elements that are necessary for life would be locked up in minerals and gases that other organisms can’t access. Fortunately for us, microbes are like little biochemical factories that transform these elements into forms the rest of us can use. The very oxygen we breathe, for example, was first released into the atmosphere by ancient microbes that inhabited the young planet Earth. We have microbes to thank for making Earth a more hospitable place.

Earth’s supply of these vital elements is limited. They are constantly being recycled -- stored, transformed, used and reused, and stored again in a balanced manner that has developed and stabilized over the eons. We must learn exactly how these cycles work so that we don’t upset the balance. Our population size and technological capabilities have reached a point where we humans are capable of altering the various inputs into these natural cycles in ways that have never occurred before. The big question is, can microbes adapt to these changing inputs and still be able to maintain the balance of the biosphere? The answer is, we simply don’t know. Understanding the complex interactions between microbes and the environment has thus become a research priority for scientists in Delaware, around the nation, and throughout the world.

The essential elements are stored in large quantities referred to as pools or reservoirs, either in the atmosphere or in the Earth’s crust. For the most part, it’s the activity of microbes that converts these elements into forms that can be used or stored by plants and animals. Microbes are also largely responsible for breaking down organic matter and returning the elements to their storage pools in the atmosphere or soil, completing the cycle. Let’s explore a few of these cycles in more detail. We can illustrate the microbial roles in these cycles through diagrams without going too deeply into the specific chemical reactions involved. In the following diagrams for each cycle, yellow arrows represent processes carried out by microbes, white arrows are inorganic (physical or chemical) processes or processes that involve other organisms, and red arrows represent human inputs. Click on the words and animals in each diagram for a brief explanation of each step in the cycle. Links in the text below will help you learn more about some of the processes involved.

The Carbon Cycle

Carbon isn’t the most abundant element in our bodies (that would be oxygen), but it’s considered the most basic building block of life. That’s because the organic molecules that make up most of our tissues and those of other organisms all have a “backbone” of carbon atoms linked together in chains or rings. The length of the chain of carbon atoms and the other kinds of atoms that are attached give organic molecules their unique properties. Sugars are relatively short, simple chains. Proteins are comparatively more complex chains that are folded in precise ways.

In the carbon cycle, green plants and algae remove carbon dioxide (CO₂) from the air (or its dissolved form from the water) and convert it to sugars and starches through the process of photosynthesis. Sugars and starches are basically energy-storing molecules. The original source of that energy is, of course, the sun. Other organisms consume this organic carbon, often employing microbes to help them digest it, and use it for growth and for energy. Some CO₂ returns to the atmosphere through the respiration of plants and animals. More carbon is returned to the atmosphere, either as CO₂ or methane (CH₄), through the decomposition of organic wastes and dead organisms, a process carried out primarily by bacteria and fungi. Fossil fuels such as coal and oil -- which are carbon from the decomposed bodies of microbes, plants, and animals that were buried in sediment millions of years ago -- are an additional storage pool of carbon.

Humans are changing the carbon cycle through agriculture and carbon burning. The burning of petroleum products and coal in our vehicles and power plants as well as the burning of forests and agricultural fields are transferring carbon from its various terrestrial storage pools to the atmospheric pool at a much greater rate than has previously occurred. The average level of carbon dioxide in the atmosphere today is 25 percent higher than at any time in the last 420,000 years. Most of this increase has occurred in the last 50 years. In addition, cattle raised for food each produce about 40 liters of methane gas per day. The concern is that both CO₂ and CH₄ are “greenhouse gases” -- that is, they trap heat in the atmosphere. In fact, methane is seven times more potent as a greenhouse gas than carbon dioxide. While specific effects are still uncertain, an overwhelming majority of scientists now agree that this tipping of the carbon cycle toward atmospheric carbon will likely alter Earth’s climate over the coming decades.

The Nitrogen Cycle

Nitrogen is a major constituent of all living cells. It is an ingredient of proteins and nucleic acids -- the DNA and RNA that contain the genetic code for cellular function and reproduction. The atmospheric pool of nitrogen is very large; nitrogen gas (N₂) makes up 79 percent of the atmosphere. However, plants and animals can’t use this gaseous form.

Complex organisms rely upon certain bacteria in the soil and oceans to “fix” nitrogen gas by transforming it into ammonia (NH₃), a form that can be incorporated into living cells and enter the food chain. This dependence is so strong that some plants, primarily in the legume family, have developed special relationships with nitrogen-fixing bacteria, providing a home and nutrients for them in specialized root nodules. In return, the bacteria supply the plant with abundant nitrogen. Legumes include some of our most important food and forage crops such as beans and peas, soybeans, peanuts, alfalfa, and clover. Thus, nitrogen fixation is an important process, not only for the plants, but also for those of us who eat them.

Some nitrogen-fixing bacteria are free-living in the environment. The ammonia they produce is available to other bacteria that transform it into nitrite (NO₂^-) or nitrate (N0₃^-), a process called nitrification. (These forms may also be used by other organisms.) More bacteria work to decompose plant and animal wastes and dead organisms, releasing organic nitrogen compounds into the environment. Many bacteria then convert this organic nitrogen into ammonium. Still other microbes turn nitrogen in the soil or water back to nitrogen gas.

	A boat channel in the Delaware Inland Bays.

Because the bacterial transformation of nitrogen gas to ammonia is the main way new nitrogen is naturally made available to plants (a tiny amount is fixed by lightning), the lack of nitrogen often limits plant growth. The discovery of this limitation by humans led to the agricultural use of chemical fertilizers, which are largely composed of ammonium (NH₄⁺) and nitrate (NO₃^-). We have more than doubled the amount of nitrogen naturally available to Earth’s ecosystems over the past century. However, only an estimated 20 to 30 percent of the applied nitrogen is used by crop plants. The rest either runs off the field with rain or irrigation and enters ground or surface waters or disappears into the atmosphere. One consequence is that surface waters enriched with nitrogen can support larger populations of algae. Algal blooms may become problematic if the algae remove too much oxygen from the water or produce toxins resulting in fish kills and human health hazards. Additionally, groundwater supplies may become tainted with high levels of nitrate, a human health concern.

The Sulfur Cycle

Sulfur is a small but important component of proteins and enzymes in plants and animals. Several amino acids, which are the building blocks of proteins, contain sulfur. Plants can absorb sulfur that has been dissolved in water and incorporate it into organic molecules such as amino acids. Humans, like most animals, must consume sulfur in their diet.

An important difference between the sulfur cycle and the carbon and nitrogen cycles is that sulfur doesn’t need to be “fixed.” In other words, microbes aren’t needed to take sulfur from the atmosphere and make it available to living organisms. Large pools of sulfate ions (SO₄^2-), the most useful form, exist in the Earth’s crust, aquatic sediments, and oceans. In addition to sulfate, sulfur is available in the environment in a number of different forms, including elemental sulfur, sulfite, thiosulfate, hydrogen sulfide, and a vast array of organic sulfur compounds, the most significant of which is dimethyl sulfide. Microbes carry out a number of processes that convert these forms back and forth. Some of the reactions that occur in the sulfur cycle serve as a source of energy and thus can support life in such unlikely places as deep-sea hydrothermal vents.

Sulfur also enters the atmosphere, from both natural and human sources. Natural inputs include volcanic eruptions, evaporation, and bacterial processes including decomposition (that’s the source of that rotten egg smell in swampy areas, for example). However, human industrial processes dump as much sulfur dioxide (SO₂) into the atmosphere as all the natural inputs combined. In the atmosphere, sulfur dioxide reacts with oxygen and water vapor to produce sulfuric acid (H₂SO₄), which falls to the Earth as acid rain.

Other human activities can affect the sulfur cycle on a smaller, more localized scale. For example, hydrogen sulfide (H₂S) is toxic to most multicellular organisms. Certain bacteria produce H₂S under anaerobic conditions in places like aquatic sediments. When human activities such as dredging disturb these sediments, the H₂S is released into the water, harming other aquatic organisms. In Delaware, recent fish kills in the Torquay Canal and Eagle Creek were linked to the deepening of boat channels in those waterways. Which sulfur reactions dominate depends on the environmental conditions and types of microbes present, so understanding these combinations is very important to predicting the environmental outcomes of our actions.

Additional Resources

Microbial Ecology

With sections such as DirtLand, WaterWorld, and even a Snack Bar, the Microbe Zoo web site presents important microorganisms according to their roles in the environment. The Microbe Zoo is also available on CD-ROM.

Photosynthesis, Respiration, Nitrogen Fixation

Flying Turtle.org, an award-winning science education site, has a good web page that explains photosynthesis in humorous and easily understood terms.

The Center for the Study of Early Events in Photosynthesis at Arizona State University maintains a web site with good information and lots of links to educational resources about photosynthesis at levels ranging from middle school to college.

In conjunction with its science program Newton’s Apple, Twin Cities Public Television has created a web site with video links, teacher guides, and related activities. Check out the episode on photosynthesis.

Respiration is more than just breathing. This site goes into the complex cellular process in some detail for high school students and above.

Listen to National Public Radio’s Science Friday program about photosynthesis and nitrogen fixation in cyanobacteria, also known as blue-green algae, and what scientists are learning about circadian rhythms (our internal clocks) from studying this lowly “pond scum.” The Science Friday Kids Connection site also has additional resources, discussion questions, and activites.

From biogeochemical cycles to photosynthesis, Real Trees 4 Kids, a site developed by the National Christmas Tree Association, offers lots of appealing, grade-appropriate information.

An on-line biology textbook explains in more detail how the symbiosis between rhizobia and legumes works.

Dr. Janine Sherrier studies the relationship between plants in the legume family and soil microbes in her lab at the University of Delaware.

Fossil Fuels

The U.S. Department of Energy and its Energy Information Administration have lots of information about fossil fuels on its web site, including pages for kids.

Greenhouse Gases

Faculty at the University of Delaware’s College of Earth, Ocean, and Environment have put together a web site summarizing current information about atmospheric carbon dioxide and it’s effects.

Nitrogen Pollution and Harmful Algal Blooms

Read what the World Resources Institute has to say about the consequences of human inputs into the global nitrogen cycle.

This article from National Geographic News tells about the effects of nitrogen pollution on aquatic ecosystems.

This web site devoted to harmful algal blooms was created by the Woods Hole Oceanographic Institute with support from NOAA.

Deep-Sea Hydrothermal Vents

Explore the Venture Deep Ocean site put together by the Education and Outreach team of Ridge 2000, a long-term program of scientific research into mid-ocean ridges and tectonics.

Learn more about the geology and life forms associated with hydrothermal vents at Extreme 2004, a web site created by the University of Delaware College of Marine and Earth Science in association with a scientific expedition to study the vents.

Experience this Online Adventure Into the Abyss, courtesy of the PBS television series NOVA.

Acid Rain

View simple information and teaching resources on acid rain from the Atmosphere, Climate, and Environment Information Programme in the United Kingdom.

The Environmental Protection Agency has put together an interactive site for kids about acid rain featuring games and activities.