Chemical Cycles in the Biosphere
1956. Minamata Bay, Japan. Children stopped their play near a seashore to watch a strange sight. A cat contorted with grotesque jerking and leaping movements ran across a field and in a sudden violent lurch threw himself into the ocean bay where he was quickly swallowed up and drowned.
2004 Kunming, China. Students in this inland city are on the average healthier and more intelligent than their parents and grandparents. One of the reasons for this better health and higher intelligence is the more adequate amounts of the elements iodine and iron that they now get in their diet, chemicals that were lacking in their parents and grandparents diet.
2024. Black Earth, Wisconsin. The local farmer's cooperative had a close-out sale on all remaining nitrogen fertilizer, the final three tons going for ten cents on the dollar. The coop's new hybrid corn seed, on the other hand, was in short supply this spring because of the high demand. Farmers throughout the corn belt are finding that the nitrogen-fixing ability of the new corn hybrid has almost eliminated the need for artificial fertilizer on their corn fields.
This is the story of chemicals in the living world, the biosphere. The story of chemicals, good and bad. First the bad.
It was in 1956 that the citizens of Minamata, Japan first began noticing the strange behavior and the diminishing number of cats in town. Then people, too, began coming down with a strange new disease that seemed to affect the nervous system leading to convulsions and sometimes to death.
Over the next twenty years a grim story unfolded as more people were affected in and around Minamata, including children born defective to mothers whose diet included substantial amounts of locally caught fish. Eventually the truth was apparent. A local chemical factory had for years been discharging waste chemicals that included small quantities of mercury salts into Minamata Bay.
Over the years the mercury had been taken up in the food chain, concentrated and changed into a deadly form of methyl mercury that lodged in the flesh of fish. When cats or people ate the fish, it was the methyl mercury that acted so destructively on the nervous system and led to so many birth defects in the children.
Minamata was one of the worst examples, but this story has been repeated in other parts of the world with other chemical wastes, so that it is no wonder some people today are extremely wary of any chemical.
And yet we know that chemicals can be useful as well as harmful. Chemicals like soap and gasoline, paint and film, aspirin and penicillin.
And when you get down to it, all natural products like paper and wood, cotton and wool, leather and honey are just as much chemicals as plastic and ink, fertilizer and drugs- that is, they are all made of carbon, hydrogen, oxygen, nitrogen, iron and other common elements.
When you really get down to it, oranges and apples, rabbits and horses, you and I are made of chemicals. We are chemicals.
And just as more knowledge of chemicals in the biosphere can lead to ways of preventing any more Minamatas, so too it can lead to a multiplication of Kunmings and of Black Earths.
We didn't explain the good chemicals examples yet. Good news takes longer to make interesting. The details of both the Kunming and the Black Earth story--and many thousand others--get ahead of our story- the role of chemicals as they cycle in the biosphere.
To understand chemicals in the biosphere let's start with the air.
We can survive a few weeks without food, a few days without water, but only a few minutes without that mixture of chemicals we call air.
Air is a mixture of four main chemicals, oxygen (21%), nitrogen (79%), carbon dioxide and water vapor (less than 1%). Air also has very small quantities of other chemicals, some put there through natural processes and some put there through human initiated processes.
Each of the four major gases in the air is absolutely essential to the life of the biosphere and each one is in continual movement into and out of living creatures.
Oxygen, for instance. As you listen and watch these words and pictures, oxygen molecules from the air are coming into your lungs, getting into your blood, carried by your blood to your heart or brain or big toe, there to be combined with food to give you energy.
That same oxygen molecule was only very recently fixed in a green leaf outside your window (or who knows, maybe in the algae of Minamata Bay fifty years ago). Here it was combined with hydrogen in a water molecule of that leaf.
Then in a process known as photosynthesis, the oxygen suddenly found itself dumped out of the leaf, into the air, a waste product so far as the leaf was concerned.
Note that in this diagram of the oxygen-carbon dioxide cycle, the oxygen atoms keep changing partners as they move into and out of plants and animals, into and out of water, the soil and the air. If you were to follow a single oxygen atom it might start coming out of the tree leaf, then move into the cow's lungs, be used by the cow's cells in respiration, then breathed out, having changed partners, hooking up with carbon in carbon dioxide, C02, into the air, back to the leaf, for photosynthesis.
When the sun is shining, plants are continually using carbon dioxide and water to produce sugar and oxygen. All of the time, sun shining or not, both plants and animals are continually using sugar and oxygen to produce carbon dioxide and water. Photosynthesis and respiration are thus two complimentary processes.
Besides respiration, other natural processes on the globe also fix oxygen into carbon dioxide and water. Fires, whether in furnaces, in an automobile engine or in a campfire all make oxygen from the air join with carbon and hydrogen and nitrogen to produce carbon dioxide, water, and nitrogen oxides.
So too the natural corrosion of metals take oxygen from the air, forcing it to combine with metal atoms to form rust (iron oxide), aluminum oxide, copper oxide, etc.
And finally, in the special case of decay, certain bacteria need oxygen to live and carry out their essential function in breaking down the complex chemicals in dead plant and animal remains.
The oxygen-carbon dioxide cycle has been going on with little change for many millions of years on earth. In the last hundred years this cycle may be changing. Slightly. But significantly.
The rapidly increasing number of fires from furnaces, power plants and especially from automobiles, trucks and airplanes have been putting carbon dioxide into the atmosphere in greater quantities than the plants, corrosion, bacteria and ocean water take it out. Many experts think that this may lead to a gradual warming of the atmosphere. This global warming could have dramatic effects on human activities everywhere on earth.
Let's turn now to another important chemical cycle, the nitrogen cycle. About 79% of the air is pure nitrogen gas, N2, two atoms to the molecule. This nitrogen does not stay pure N2 for long but also participates in the life dance.
Nitrogen gas is relatively inert chemically. That means it does not easily combine with other atoms to make compounds. It is difficult to fix we sometimes say.
Now all living creatures are constructed, that is, are literally built out of nitrogen rich chemicals called proteins, and operated with nitrogen rich chemicals called enzymes. Both proteins and enzymes have nitrogen atoms as key parts of their structures. So a general problem of life is, how do you get nitrogen atoms from the air fixed into the nitrogen proteins and enzymes of life?
A first guess might be, why not just breathe nitrogen in, like oxygen, and let it do its trick in our blood and cells. Unfortunately, this won't work. The nitrogen you breathe in just gets breathed right back out. Unchanged. Unfixed. How do you get it into your cells then?
The answer is, you can't, but certain special kinds of bacteria can and do. Fortunately there exist in the biosphere large populations of nitrogen-fixing bacteria that, almost alone among living things, are able to take nitrogen in its pure air form and through their living metabolism make that nitrogen combine with hydrogen. To fix it, in other words, into ammonia, a molecule of three atoms of hydrogen to one atom of nitrogen. NH3. Other microorganisms are able then to add oxygen to the ammonia and change the ammonia into nitrites and nitrates
These nitrogen rich compounds can be then used by plants to make the very large life-forming molecules called proteins and enzymes. These plant proteins and enzymes, in turn, can be eaten by animals like us to make animal or human proteins and enzymes.
Unfortunately, so far these bacteria will only thrive in certain special niches on earth. The roots of leguminous plants like beans, peas, clover, alfalfa make good homes for the nitrogen-fixing bacteria. Also some varieties live in blue green algae and even in some soils. The big news recently is that they have been coaxed into living on special hybrid corn roots too.
That is the center of the Black Earth story. This latest breakthrough has only recently happened at the University of Wisconsin and a number of other agricultural research laboratories around the world. Not only corn, but also wheat, rice and other important grain crops are being studied and experimented with in the hopes of coming up with varieties that will harbor nitrogen-fixing bacteria on their roots.
This may someday lessen, or even eliminate, the need for nitrogen rich chemical fertilizers. Which in turn could lead to less air and water pollution, more efficient energy use and increased agricultural production. A caution however. The Black Earth story takes place in 2024. The breakthrough looks promising today, but a great deal of research and development needs to be done before such nitrogen fixing corn hybrids will actually be available for farm production.
Because this nitrogen cycle is so important and so often misunderstood let's summarize it once more with the help of a diagram. Gaseous nitrogen can be fixed into the soil by nitrogen fixing bacteria, by lightning or by other natural decay processes. In the form then of ammonium, nitrate or nitrite ions, it is taken up by green plants, eaten by animals that build these chemicals into proteins and enzymes, into their own flesh and blood.
The animals and plants in their turn die and decay, and have their proteins and enzymes broken back down into ammonium, nitrate and nitrite ions. This is why so-called "organic" fertilizer-cow manure, compost, garbage wastes are useful as fertilizers.
Ammonium or nitrate ions can also get into the soil by way of commercial chemical fertilizers. That is, they can be made in a chemical factory which is also able to fix nitrogen from the air into ammonia by way of an ingenious human invented process called the Haber process.
Finally, there are de-nitrifying bacteria which complete the cycle by breaking down the ammonium and nitrate and nitrite ions into gaseous oxygen, nitrogen and water. And so our nitrogen cycle is complete.
Nitrates are not the only nutrient plants need. Chlorophyll, for instance, the key molecule in photosynthesis, needs small amounts of magnesium. Other plant molecules need phosphorus, calcium, sulfur, iron, copper, potassium, sodium, iodine and about fifteen more common elements in small traces. All of these must also be absorbed into plant roots from the soil. And all of these have their own simple or complex cycles in the biosphere.
Sometimes, for instance, the soil in a given area may be deficient in certain elements. Iodine, for instance, is common in fish and sea food and in soils near the ocean. It is often, however, in short supply in inland soils. Vegetables and animal products grown in many of these inland areas do not get enough iodine. When humans have to rely solely on these local food supplies humans too do not get enough iodine. This deficiency can result in diseases of the thyroid gland and more important in a lowering of intellectual capacity by as much as 10 to 15 percentage points.
Similarly, about 40% of the developing world's people suffer from a deficiency of another trace element, iron. This lack of iron in the diet can result in similar drops in children 's IQs.
Right now the United Nation's Children's Fund is sponsoring projects to artificially fortify foods with iodine, iron, vitamin A and other trace elements and compounds needed by the human body but often absent or severely restricted in the diets of poor countries of the world.
Deficiencies in the soil are not the only problem with trace elements. Plant cells can be fooled into accepting dangerous substitutes for the real thing. Instead of selecting and concentrating needed calcium, for instance, they will accept a very similar element, strontium 90, which is radioactive and capable of doing great damage to living cells.
As the plant cells concentrate the strontium 90 they in turn are eaten by animals, who further concentrate it in their bones. If one animal is eaten by another there is still more concentration.
And so it happened in Minamata Bay with mercury. It became more and more concentrated as it went up the food chain through algae to small fish to large fish to cats or humans.
Today, in the 21st century, ecologists warn that mercury is still and again a dangerous chemical when it cycles through the ecosystem and becomes selectively concentrated in fish.
Coal contains mercury. Coal-fired power plants around the world are prominent mercury polluters. New federal regulations in the United States require new kinds of filtering equipment on power plants to cut back on the emitted mercury. This country, Canada, Japan and Western Europe have made rapid strides in controlling this chemical but there is still more work to be done. Unfortunately developing counties like China and India are building new coal power plants that often do not include this filtering equipment.
Mercury is one example. Many other chemicals in small quantities cycle through the biosphere. Most of them are helpful, indeed essential to life on earth. Some of them can be harmful.
Pesticides like DDT, for instance, cycle through the soil, mosquitoes, birds, into bird's eggs and leading to near extinction of some bird species. PCBs, (polychlorinated biphenols), created in factories, cycle into your salmon or your milk or your breakfast cereal. Sulfur and nitrogen oxides emitted from coal fired power plants dissolve in rain water and lead to an acid rain that may harm some forested areas. Lead, asbestos, herbicide residues and radioactive chemicals from natural rocks, from nuclear plants, from coal-fired power plants and from outer space all continually cycle through the biosphere.
In most cases the quantities of these dangerous chemicals are so small that it has been difficult to prove much harmful effect on living things. In other cases higher concentrations of chemicals like lead, mercury and asbestos have caused serious health problems. Research and control efforts are continuing. And there are problems and honest disputes within the scientific community as well as the political community about the possible effects of these chemicals.
Farmed salmon, for instance, because of the way they are fed in confined pens, have PCB levels higher than wild salmon. On the other hand these levels are still far below the limits set by the federal government as being safe to eat. In addition, most of the PCB contaminants are in the skin of the salmon and the fat just beneath the skin, parts of the salmon that most people do not eat.
Surprisingly enough, in some cases of both chemical and radiation exposure new research has shown that very small amounts may not be harmful at all. In fact they may actually be beneficial to living things.
In most cases of both the good and the bad, the chemical is carried in the last of our key natural cycling chemicals, water. Water moves into and out of all living systems. So much so that the ancient philosophers thought all living things were made of water.
We are made of over seventy percent water. And inside our bodies water is not only used directly in chemical reactions, it is also an essential carrier of all the other chemicals.
Here you can see how the water cycle works on this spaceship. Notice that if you were to follow a single molecule of water it would make a fascinating world journey. Up out of the ocean into a cloud, floating across the continent to fall on a mountain peak as snow, melting down the rushing stream or perhaps sinking into the ground to become a part of the water table, surfacing in springs, ponds, marshes, and slowly finding its way back to the ocean.
The most subtle and interesting part of water's cyclic journey is its path into and out of living systems. Here it gets intertwined with the oxygen-carbon dioxide cycle. From the trees roots into the leaf, built into sugar, eaten by an animal, burned in the mitochondria of animal cells, released again as waste product water, into the blood, out the lungs, or excreted by the kidneys, back to the air, the earth, the ocean. And so the eternal cycle of water goes on and on and on.
Chemical cycles in the biosphere. An old story. But one always surprising us with new chapters.
Oxygen, hydrogen, nitrogen, carbon, phosphorus, iron -- all these and fifteen to twenty more of the most chemical elements have been built--for millions of years-- into the delightful architectural combinations of chlorophyll, glucose, adenosine triphosphate, cellulose, Iysine, and the millions of other enzymes, salts, acids and diverse chemicals we all a leaf.
And the same chemical elements have been built--for millions of years--into other architectural combinations of hemoglobin, glycogen, lysine, thymine, guanine, adenosine triphosphate, sugar, salt, and the millions of other enzymes, proteins, acids and nutrients that we call a hand. Or a brain.
Operating rules for small spaceships:
#1. Don't be unduly afraid of chemicals. You are made of chemicals. Learn to tell the good from the bad. And learn to enhance the good and diminish the bad.
Operating rule #2. On a spaceship like earth you cannot throw anything away. There is no away. At best you can shove it into an unused closet, where it could be dangerous (or useful) in years to come. Learn and plan for today and tomorrow.