Environmental Encyclopedia

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Environmental Encyclopedia 3

Methyl tertiary butyl ether

of plants, and the belching of cattle. Methane is the major

component of

natural gas

, making up about 85 percent of

that fuel. Environmental scientists are increasingly con-

cerned about methane as a possible greenhouse gas. Like

carbon dioxide

, methane traps heat reflected from the earth

and, therefore, may contribute to global warming. Increases

in agricultural and dairying activities have resulted in an

increase in methane production, possibly contributing to

climate

change. See also Greenhouse effect; Greenhouse

gases

Methane digester

Methane

digesters are systems that use anaerobes to produce

methane through fermentation. Methane is a main constit-

uent of

natural gas

and can be readily substituted for that

nonrenewable resource.

The anaerobes used in methane digesters are methano-

genes, bacteria belonging to the genera Methanobacterium;

Methanosarcina; and Methanoccus. They can be found in the

gastrointestinal tracts of animals such as cows and other

ruminants, as well as in

soil

, water, and sewage. In septic

tanks, bacteria liquefy some of the organic matter; which

releases energy for the bacteria and by products such as

methane and

carbon dioxide

Methane digesters are also known as biogass digesters

and organic digesters. The central portion is an airtight

drum, called a digester unit, which contains the methano-

genes. Raw material is place into the drum, and the unit is

kept at a constant temperature of about 95°F (35°C). Some

of the methane produced by the digester heats the water.

Outlets on the digester unit take away the various products

of the system. Liquid and solid fertilizers are collected to

be used for crops and other plants. Methane is stored in a

tank, from which it can be drawn off for fuel for a variety

of purposes.

Carbon

dioxide and

hydrogen

sulfide can be

filtered out of the methane and put under pressure for use

in turning turbines.

Common household organic wastes can be put into a

digester, and the methane produced can be used to make

electricity. It can also be used for cooking, illumination,

heating, and

automobile

fuel. One system at the University

of Maine produced over $8,000 worth of power a year in

the early 1990s; the digester also produced a

sludge

that

could be used as a nutritious and relatively odorless plant

fertilizer

. Some sludges, however can contain high level of

metals if the original material is unsorted municipal waste.

Methane is a clean and nontoxic automobile fuel, and

it produces no pollutants when burned. It has an octane

number of 130. Italy has used it as a motor fuel for over 40

years, and Modesto, California, has a small fleet of methane-

897

powered cars. Because of its cleanliness, it extends the life

of engines as well as making starting easier.

The organic matter this system converts into other

uses would otherwise have decomposed in a

landfill

leach-

ing

into the surrounding

environment

and contaminating

groundwater

supplies. In a digester, it becomes a useful

resource. See also Alternative energy sources; Alternative

fuels; Anaerobic digestion; Bioremediation; Groundwater

pollution

[Nikola Vrtis]

ESOURCES

ERIODICALS

Trans, W. B. “Just Plug It Into That Cherry Tomato Over There.” Sierra

75 (May-June 1990): 20–21.

Methanol

Methanol is an organic compound with the chemical form

OH. It is also known as methyl alcohol or wood alcohol.

Like most alcohols, methanol is very toxic. Its ingestion can

cause severe nerve damage leading to blindness, insanity,

and death. Methanol can be prepared through destructive

distillation of

coal

, wood and wood products,

garbage

sewage

sludge

, and other forms of

biomass

. It is an excel-

lent automotive fuel and has long been used to power racing

cars. Existing methods of production are still too expensive,

however, to make it an economically viable alternative to

gasoline

in the general market. See also Alternative fuels;

Fuel-switching

Methyl tertiary butyl ether

Methyl tertiary butyl ether (MTBE) is a flammable, volatile,

and colorless liquid fuel additive that is manufactured by

the chemical reaction of

methanol

and isobutylene. It is

one of a group of

chemicals

referred to as oxygenates, be-

cause they raise the level of oxygen in

gasoline

. Oxygen

helps gasoline burn more completely, thus reducing emis-

sions.

MTBE has a strong odor similar to a general anesthe-

tic. Humans can detect it by smell at very low concentrations:

parts per billion

(ppb) in air and 20 to 40 ppb in water.

MTBE is very soluble in water and more soluble than other

gasoline constituents. It is also persistent and resistant to

degradation.

As a gasoline additive, it is used to increase the octane

level (replacing the use of

lead

additives) and to reduce

vehicular emissions of

carbon monoxide

(CO) and ozone-

forming pollutants. It was first used in the United States in

Environmental Encyclopedia 3

Methyl tertiary butyl ether

the 1970s, with its use increasing during the 1990s, when the

Clean Air Act

Amendments of 1990 created the Oxygenated

Fuel Program. Areas of the country with severe

air pollution

problems are required to add oxygenates to gasoline during

winters to reduce vehicular emissions of CO. Although regu-

latory requirements do not specify the types of oxygenates

required, MTBE and

ethanol

are the main oxygenates used.

Ethanol is the primary oxygenate used in the Winter Oxyfuel

Program. Areas with problems meeting

ozone

standards

require the use of reformulated gasoline (RFG) year round

to reduce ozone-forming pollutants. RFG is oxygenated gas-

oline (with a minimum of 2% oxygen by weight) that is

specially blended to have fewer polluting compounds than

conventional gasoline. As the addition of ethanol increases

the vapor pressure of gasoline such that it is difficult for

ethanol-containing gasoline to comply with summertime

volatility requirements, MTBE has been the preferred oxy-

genate for use during the summer. In 1970, MTBE was

the 39th-highest produced organic chemical in the United

States; by 1998, it was the 4th-highest produced organic

chemical. In 1999 over 200,000 barrels of MTBE per day

were produced.

The use of RFG has improved the quality of air in

the United States. The U.S.

Environmental Protection

Agency

estimates that smog-forming pollutants are being

reduced annually bey at least 105 million tons and toxic

chemicals by at least 24 million tons.

However, the success of the

air quality

program that

utilizes MTBE to reduce emissions has been offset by con-

tamination of ground and surface waters with MTBE.

MTBE has been detected in

soil

, surface water, and ground

water throughout the United States. Point sources of MTBE

include releases from underground and above-ground stor-

age tanks and pipelines due to leaks, overfilling, and faulty

construction. Non-point sources of MTBE include

urban

runoff

, precipitation, vehicle accidents, and motorized water

craft. MTBE may also be released to the

environment

through gasoline fumes during vehicle refueling. A study by

the U.S.

Geological Survey

in 1993–1994 showed that

MTBE was the second most frequently detected volatile

organic chemical of 60 chemicals in samples collected from

shallow ground water in eight urban areas. Another report

in 1999 by the U.S. Environmental Protection Agency’s

Blue Ribbon Panel on Oxygenates in Gasoline indicated

that between five and ten percent of drinking water

wells

in areas with high use of MTBE had detectable levels of

MTBE, while one percent had levels higher than 20 micro-

grams per liter. During the summer of 1996, the City of

Santa Monica in California stopped pumping ground water

from two of its well fields because of persistent and increasing

levels of MTBE. These two wells provided about 50% of

the city’s drinking water supply.

898

MTBE has been shown to have the potential to pro-

duce adverse effects associated with central nervous system

depression, including headaches, dizziness, nausea, and dis-

orientation. These effects are reversible if exposure is discon-

tinued. Evidence from animal studies suggests that MTBE

is a potential human

carcinogen

. When it enters the human

body through inhalation or

absorption

through the skin,

it may metabolize into two compounds, tertiary butyl alcohol

and formaldehyde, that are carcinogenic in animals and are

also classified as potential human carcinogens.

In 1997 the U.S. Environmental Protection Agency

issued a Drinking Water Advisory for MTBE. The Advisory

recommended that the levels of MTBE in drinking water

by limited to 20 to 40 micrograms per liter. This limit was

set to assure consumer acceptance with regards to taste and

odor and to provide an adequate margin of safety from toxic

effects. Based on this Advisory, several states have set their

own drinking water standards for MTBE. A nation-wide

standard for MTBE under the

Safe Drinking Water Act

is not expected until after February 2005, due to lack of

information regarding health effects and occurrence data. In

March of 2000, the U.S. Environmental Protection Agency

formally began regulatory action to eliminate or phase down

MTBE, issuing an Advance Notice of Proposed Rulemaking

under Section 6 of the Toxic Substances Control Act, which

gives the U.S. Environmental Protection Agency authority

to ban, phase out, limit or control the manufacture of any

chemical substance deemed to pose an unreasonable risk to

the public or the environment. The State of California is

considering a ban on the use of MTBE by the end of 2003.

As of 2002, bans on MTBE are also under consideration

in at least 16 other states. Research is being conducted to

develop alternative oxygenates so if MTBE is banned, air

quality benefits of the use of oxygenated fuels will be pre-

served.

Because of its physical properties, during a release

MTBE migrates rapidly through the soil column. Upon

reaching the ground water, MTBE moves at the same veloc-

ity as water and faster and farther than other gasoline constit-

uents. As the gasoline contaminant

plume

degrades over

time,

benzene

toluene

, ethyl benzene, and

xylene

(BTEX) decrease in concentration more than MTBE, which

is more resistant to degradation. Eventually MTBE may be

the only contaminant remaining from the release.

The extent of MTBE contamination in the subsurface

has led to the investigation of treatment technologies. If

MTBE in soil is not dissolved in water, it can be removed

by such treatment methods as soil vapor extraction. How-

ever, the high solubility of MTBE in water and its

resis-

tance

to degradation makes it difficult and time consuming

to remove from ground and surface waters using many com-

mon remedial technologies. Promising technologies for re-

Environmental Encyclopedia 3

Methylmercury seed dressings

moval of MTBE that are being studied include chemical

oxidation using ultraviolet light and

hydrogen

peroxide,

phytoremediation

using deep-rooted trees, and cometabo-

lic degradation processes.

[Judith L. Sims]

ESOURCES

OOKS

Jacobs, James J., Jacques Guertin, and Christy Herron. MTBE: Effects on

Soil and Groundwater Resources. Boca Raton: Lewis Publishers, 2000.

THER

Report on Methyl Tertiary Butyl Ether (MTBE). Arizona Department of

Environmental Quality, October 1, 1999.

U.S. Environmental Protection Agency. Methyl Tertiary Butyl Ether

(MTBE). June 22, 2001 [cited June 23, 2002]. <http://www.epa.gov/

mtbe>.

Methylation

A chemical reaction in which the methyl radical (-CH

)is

added to some substance. The most common mechanism by

which methylation occurs in the

environment

biological

methylation

, which involves the action of living organisms.

Bacteria in oxygen-poor soils, for example, can convert me-

tallic

mercury

to an organic compound, methyl mercury.

Similar reactions occur with other metals, including

arsenic

selenium, tin, and

lead

. These reactions are significant be-

cause they convert non-soluble metals of low toxicity into

soluble forms with high toxicity. Light can also induce meth-

ylation. Photomethylation has occurred in the laboratory,

but its relative importance in environmental systems is not

yet well understood.

Methylmercury seed dressings

Seed dressings were devised to prevent diseases caused by a

wide variety of seed-borne plant-pathogenic

fungi

, to pro-

tect the germinating seeds against secondary infections, and

to increase crop yields. Various

chemicals

, including several

heavy metals

, have been used as fungicides to treat seeds

since the end of the nineteenth century. The effectiveness

of these fungicides was greatly increased when aryl organo-

mercurials were introduced around 1914. They had a wider

spectrum of fungicidal activity than nonmercurial formula-

tions and were used extensively until the mid-1980s to con-

trol fungus diseases through their application as seed dress-

ings on many grains, vegetables, and nuts, as well as to

protect fruit trees, rice, turf grasses, and

golf courses

. How-

ever, with the introduction of the more effective alkyl

mer-

cury

compounds in the 1930s, especially methyl mercury

899

and ethyl mercury, severe

poisoning

incidents followed. In

developing countries hundreds of people died or became

incapacitated due to either the consumption of grains treated

with alkylmercury compounds or meat from animals that

had eaten such treated seeds.

Poisonings from eating alkylmercury-treated grains

occurred on several occasions in various parts of Iraq. Desti-

tute, illiterate rural families either did not understand the

words or poison symbols on the bags of grain or did not

believe government warnings that it was unsafe to eat. In

some instances the families fed some of the grain to chickens

and swine first. When they did not observe poisoning symp-

toms in the livestock and poultry after a few days, the farmers

became convinced that the warnings were false and the grain

was safe to eat. However, depending on the amount of

methyl mercury consumed, there is a

latency

period of

weeks or months between exposure and the development of

poisoning symptoms.

When the seed grain was ground into flour, baked

into bread, and consumed by the rural victims, both sexes

and all ages were affected. The ingested quantities of methyl-

mercury ranged from small amounts that produced no overt

effects to lethal doses. Fetuses suffered the most damage.

Among the rest of the population, the severity of the neuro-

logical and psychiatric symptoms was almost directly propor-

tional to the amount of bread consumed. In cities where

bread was produced from government-inspected flour mills,

not a single case of poisoning was reported.

The first documented incident occurred in Iraq in

1956. Of the two hundred persons afflicted, seventy died.

In 1960 an estimated 1000 persons were affected in a similar

incident and over 200 died. During the 1960s other smaller

but similar episodes of alkylmercury poisoning occurred on

a more limited scale in West Pakistan, Guatemala, Ghana,

and in other countries such as Mexico and the United States.

The total death toll in these countries was 42 with approxi-

mately 197 individuals less seriously affected.

But the most serious outbreak of poisoning from eating

methyl mercury-poisoned bread occurred in Iraq early in

1972. In October and November of 1971 a total of about

73,000 tons of high-yield Mexipac wheat seed grain and

22,000 tons of barley seed grain, all treated with alkyl mer-

cury

fungicide

, had been distributed by cooperatives to

farmers for planting throughout the country. Some of this

grain, as before, was used to prepare homemade bread. The

Iraqi government estimated that the treated grain was dis-

tributed to no more than five percent of the rural population,

about 200,000 people. Most of the fatalities occurred within

three months after the end of the exposure, although a few

long-term illnesses resulted in fatalities as well.

By March 1973, up to 40,000 persons, residents of

every province, were unofficially estimated to have been poi-

Environmental Encyclopedia 3

Mexico City, Mexico

soned. The total number of casualties will never be known

precisely because hospitals were quickly overloaded, and

many victims did not have access to them. In addition, most

of the poisonings occurred in rural areas, and many were

not reported to authorities. The government officially ac-

knowledged that 6,530 persons were hospitalized and 459

died. These figures were not confirmed because news report-

ers were denied entry to the country and the movements of

foreigners were restricted. However, tourists reported that

large numbers of Iraqis suffered brain damage, blindness,

and paralysis. Since then, with the exception of a few follow-

up scientific reports published between 1985 and 1989,

hardly any new information relative to the long term health

effects on the thousands of victims has been published or

released by the Iraqi government although this was the larg-

est such tragedy of this kind. Because of the highly toxic

nature of the alkyl mercurials, as well as the severity of the

accidents caused by misuse of the treated seeds, they were

banned in 1970 in the United States and many other coun-

tries. See also Agricultural chemicals; Birth defects; Heavy

metals and heavy metal poisoning; Minamata disease; Plant

pathology; Teratogen; Xenobiotic

[Frank M. D’Itri]

ESOURCES

OOKS

D’Itri, P. A., and F. M. D’Itri. Mercury Contamination: A Human Tragedy.

New York: Wiley-Interscience, 1977.

ERIODICALS

Bakir, F., et al. “Methylmercury Poisoning in Iraq.” Science 181 (1973):

230–241.

“Conference on Intoxication Due to Alkyl Mercury-Treated Seed.” World

Health Organization 53 (Suppl.) (1976): 138.

Greenwood, M. R. “Methylmercury Poisoning in Iraq: An Epidemiological

Study of the 1971-72 Outbreak.” Journal of Applied Toxicology 5 (1985):

148–159.

Mexico City, Mexico

Founded in the fourteenth century, Mexico City has been

a center for three great civilizations: the Aztecs, the Spanish,

and the modern-day Mexicans. But in addition to an impos-

ing political background, its geographical location has as-

sured the city a fascinating ecological and

environmental

history

. Mexico City lies in a basin 7,350 feet (2,240 m)

high. It is surrounded by mountain ranges on all sides, and

the presence of the extinct volcanoes Ixtacihuatl and Popoca-

tepetl to the east are a reminder that the city lies on an

active

earthquake

fault.

Most of Mexico City’s environmental problems are

caused by a combination of its geographical location and

900

growing population. In 1900, the population was estimated

at 350,000; the rest of the century has seen nothing but

rapid growth. Population has leapt from 1,029,000 in 1930

to 4,871,000 by 1960, and then to 12,000,000 in the mid-

1970s, and then 15,000,000 by 1981. According to some

estimates, Mexico City will have more than 32 million in-

habitants by the year 2000, making it the most populous

urban area in the world.

The amount of

pollution

produced by a city of this

size would be difficult to control in even the most favorable

geographic circumstances. But the basin in which Mexico

City is built traps the

ozone

nitrogen oxides

, sulfur, and

particulates that are released each day. Soft

coal

, wood, and

low-grade

gasoline

and oil are burned widely throughout

the city, contributing greatly to this problem. In addition,

prevailing winds from the northeast carry dust particles into

the city from rural areas, further degrading

air quality

. The

city has gone from having one of the most perfect natural

settings and ideal climates in the world to being the most

heavily polluted. Pollution levels can rise so high that schools

are occasionally forced to close so students can remain in-

doors and not breathe the polluted air. Entrepreneurs have

even set up booths on city streets where people can pay to

breath oxygen from tanks.

Lying near the boundary of the Pacific and North

American geological plates, Mexico City has long been at

risk for major earthquakes, a risk which has been greatly

increased by the city’s history. When the Aztecs first settled

the area, it was largely covered by an enormous lake, Lake

Tenochititla

n, which either was filled in or dried out as the

city began to grow. Today, Mexico City sits on a soft

subsoil

that is highly unstable. Some parts it are actually sinking

into the old lake bed, while the whole area rides out each

earthquake like a boat on an unsettled ocean.

The geographical instability of the area has been

worsened by the fact that residents traditionally obtain their

water from

wells

. As the population grows, more water is

removed and

subsidence

increases. In some parts of the

old city, buildings have actually sunk more than 6 ft (2 m)

below street level. Since measurements were first made in

1891, subsidence in some areas has exceeded 26 ft (8 m),

and it measures at least 13 ft (4 m) in nearly all parts of the

city. See also Air pollution control; Air quality criteria; Los

Angeles Basin; Nitrogen cycle; Population growth; Sulfur

cycle; Sulfur dioxide

[David E. Newton]

ESOURCES

OOKS

Poland, J. F., and G. H. Davis, “Land Subsidence Due to Withdrawal of

Fluids.” In Man’s Impact on the Environment, edited by T. R. Detwyler.

New York: McGraw-Hill, 1971.

Environmental Encyclopedia 3

Microbes (microorganisms)

Polluted air over Mexico City, Mexico. (Photograph by A. J. Copley. Visuals Unlimited. Reproduced by permission.)

ERIODICALS

“Line Up to Breathe.” BioScience (September 1991): 591.

“School Days and Lethal Haze.” Environment 30 (March 1988): 23.

Microbes (microorganisms)

Microbes, also known as microorganisms, are defined solely

on the basis of their size—they are too small to be seen by

the naked eye. Microorganisms can only be viewed if they

are magnified in size, using an optical or electron microscope.

Apart from their size, the major groups of microorgan-

isms have little affinity in terms of their evolutionary history

and systematics. Included among the microorganisms are

viruses, bacteria, blue-green bacteria, some algae, some

fungi

, yeasts, and protozoans.

Viruses

Although viruses are commonly considered to be mi-

croorganisms, they are actually “pseudo-organisms” because

they do not display all of the characteristics of life. Viruses

consist only of bits of

nucleic acid

(either deoxyribonucleic

acid

(DNA), or

ribonucleic acid

(RNA)) surrounded by a

901

protein capsule (called a capsid). Viruses are not capable of

independent reproduction, and they cannot perform other

important metabolic functions. To reproduce and grow vi-

ruses must invade and parasitize the living cells of other

organisms, and appropriate the metabolic capabilities of

their host.

It is probable that all living cells are infected by viruses

of various sorts, and in some cases serious diseases are caused.

Viral diseases of humans include colds, flu, and more deadly

ailments such as smallpox, yellow fever, rabies, herpes, polio,

and human immunodeficiency syndrome (HIV, also known

AIDS

). Some viral diseases can be controlled by vaccina-

tion, a practice that involves infecting hosts with killed but

intact

virus

bodies. These are non-virulent but nevertheless

cause the host to develop

resistance

to pathogenic forms

of the strain. Vaccination made it possible to achieve the

eradication of smallpox, a disease that had long been a

scourge of humans.

Bacteria and Blue-green Bacteria

Bacteria and blue-green bacteria are in the kingdom

of life known as Monera. Most

species

in this group are

Environmental Encyclopedia 3

Microbes (microorganisms)

bacteria; these have rigid or semi-rigid cell walls, propagate

by binary division of the cell, and do not display mitosis

during cell division. Blue-green bacteria or cyanobacteria

(sometimes incorrectly referred to as blue-green algae) utilize

chlorophyll dispersed within their cytoplasm as a pigment for

capturing light energy during

photosynthesis

. The genetic

material of monerans is organized as a single strand of DNA,

and it is not contained within a membrane-bounded organ-

elle called a nucleus, so these microorganisms are referred

to as being prokaryotic. (All of the other kingdoms have a

nucleus within their cells, and are known as eukaryotic.) In

addition, prokaryotes do not display meiosis or mitosis, their

reproduction is by asexual cellular division, and they do

not have organelles such as chloroplasts, mitochondria, or

flagella. Prokaryotes were the first organisms to evolve, about

3.5 million years ago. It was not until 2 million years later

that the first eukaryotes evolved.

About 4,800 species of bacteria have been named, but

microbiologists believe that there are many additional species

that have not yet been discovered. Bacteria are capable of

exploiting an astonishing range of ecological and metabolic

opportunities. Some species can only function in the presence

of oxygen, and others only under

anaerobic

conditions,

although some species are able to opportunistically switch

between these metabolic types. Some bacteria can tolerate

very extreme environments, surviving in hot springs at tem-

peratures as hot as 172°F (78°C), while other species have

been found active in the frigid conditions that occur as deep

as 436 yd (400 m) in glaciers.

Most free-living bacteria are heterotrophic, and among

the diversity of bacterial species are some that are capable

of metabolizing virtually any organic substances as a source

of nutrition. Other species of bacteria are photosynthetic

(including blue-green bacteria), capable of capturing sunlight

and using it to reduce

carbon dioxide

and water into simple

sugars, which are used as a source of energy in more complex

biochemical syntheses. Other bacteria are chemosynthetic,

coupling their biosynthetic abilities to energy released during

the oxidation of certain inorganic compounds, as when pyri-

tic sulfur or sulfide are oxidized to sulfate.

Many species of bacteria are not free-living, and in-

stead live in a mutualistic

symbiosis

with more complex

organisms, such as plants or invertebrate or vertebrate ani-

mals. For example, numerous species of bacteria live as a

microbial community within the rumen of cows and sheep,

while others live in the gut of humans and other primates.

These gut bacteria help with the digestion of complex or-

ganic foods, and they also synthesize vitamins and micronu-

trients that are useful to their host. Other bacteria in the

genus Rhizobium live in a

mutualism

with the roots of peas,

clovers, and other leguminous plants, fixing atmospheric

dinitrogen gas (N

) into ammonia (NH

), which after con-

902

version to ammonium (NH

), it becomes a source of

nitro-

gen

that plants can utilize as a

nutrient

Many bacteria are

parasites

of other organisms, and

some cause important diseases. Significant diseases of hu-

mans caused by bacteria include various kinds of infections,

bacterial pneumonia,

cholera

, diphtheria, gastric ulcers,

gonorrhea, Legionnaire’s disease, leprosy, scarlet fever, syph-

ilis, tetanus, tooth decay, tuberculosis, whooping cough,

most types of food

poisoning

, and the “flesh-eating disease,”

which is caused by a virulent strain of Streptococcus. Bacteria

also cause diseases of other species, and this is sometimes

used to the advantage of humans. For example,

Bacillus

thuringiensis

is a pathogen of many species of moths, but-

terflies, and blackflies, and strains of this bacterium have

been used as a biological insecticide against certain insect

pests in agriculture and forestry.

Microscopic Protists

The kingdom Protista consists of a wide range of

simple eukaryotic organisms, including numerous unicellular

and multicellular species. Microbial protists include protozo-

ans, foraminifera, slime molds, single-celled algae, and

multicellular algae. (Some other multicellular algae are not

microscopic—the largest seaweeds, known as kelps, can grow

fronds longer than 11 yd [10 m]!) Eukaryotic organisms

have their genetic material organized within a membrane-

bounded nucleus, containing paired chromosomes of DNA.

Protists have flagellated spores, and mitochondria and plas-

tids are often, but not always, present.

Recent systematic treatments have divided the king-

dom Protista into about 14 phyla, consisting of about 40,000

named species. Many of these phyla, however, differ enor-

mously from the others in basic elements of their biology. It

is likely that additional systematic research of this extremely

diverse group will result in the Protista being divided into

several kingdoms.

Several phyla of protists are photosynthetic, and these

are collectively known as algae. Most species in the following

groups are microscopic: diatoms (phylum Bacillariophyta),

green algae (Chlorophyta), dinoflagellates (Dinoflagellata),

euglenoids (Euglenophyta), and red algae (Rhodophyta).

The brown algae and kelps (Phaeophyta) are macroscopic,

as are some colonial species in several of the other groups.

Algae are important primary producers in marine and fresh-

water ecosystems. Uncommon phenomena known as “red

tides” are natural blooms of certain species of marine dino-

flagellates that produce toxic biochemicals.

Other phyla of microbial protists have a heterotrophic

nutrition. Protozoans are single-celled microorganisms, gen-

erally considered to be microscopic animals. Protozoans re-

produce by binary fission, and are often motile, usually using

cilia or flagella for propulsion. Some protozoans are colonial.

Protozoans are abundant in most aquatic environments.

Environmental Encyclopedia 3

Microclimate

Other heterotrophic protists include the ciliates (Cilio-

phora), forams (Foraminifera), amoebae (Rhizopoda), and

unicellular flagellates (Zoomastigina). Forams are unicellular

microorganisms that form a “shell” of calcium carbonate.

Chalk is a mineral that is formed from the remains of forams

that have accumulated over geologically long periods of time.

(The white cliffs of Dover, England, consist of the fossil

remains of innumerable forams).

Some species of protists cause diseases of humans,

such as Plasmodium, a unicellular blood parasite that causes

malaria

and is spread to people and other vertebrate animals

by mosquitoes. Some species of amoebae are parasites of

animals, causing amoebic dysentery in humans. Hiker’s diar-

rhea (or beaver fever) is a water-borne disease caused by the

ciliate Giardia, which is the reason why even the cleanest-

looking natural waters should be boiled or otherwise disin-

fected before drinking.

Microscopic Fungi

The kingdom Fungi consists of single-celled microor-

ganisms known as yeasts, plus the fungi, which are multi-

celled, filamentous in their growth form, and reproduce by

budding or by cellular fission. All yeasts and fungi are hetero-

trophic, excreting enzymes that digest complex organic ma-

terials, allowing simple organic compounds to be ingested.

A few species of yeasts are economically important

because they have the ability to ferment sugars under anaero-

bic (O

-deficient) conditions, yielding gaseous CO

and

ethyl alcohol (or

ethanol

). The CO

produced during fer-

mentation can be utilized to raise bread dough prior to

baking, while the alcohol can be used by brewers to produce

beer, wine, and other intoxicating beverages.

Microorganisms are characterized only by their micro-

scopic size. Otherwise they comprise a wide range of diverse

but not necessarily related groups of organisms. As a larger

group, microorganisms are extremely important as primary

producers in ecosystems, as agents of decay of dead organ-

isms and

recycling

of the nutrients contained in their

bio-

mass

, and as parasites and diseases of humans and other

species.

[Bill Freedman Ph.D.]

ESOURCES

OOKS

Atlas, R. M., and R. Bartha. Microbial Ecology. Menlo Park, CA: Benjamin/

Cummings, 1987

Raven, P. H,. and G.B. Johnson. Biology. 3rd ed, St. Louis: Mosby Year

Book, 1992.

Starr, C., and R. Taggart. Biology. The Unity and Diversity of Life. Belmont,

CA: Wadsworth Pub. Co., 1992.

903

Microbial pathogens

Microbial pathogens are

microorganisms

that are capable

of producing disease. Virtually all groups of bacteria have

some members that are pathogens. One notable exception is

the Kingdom Archaea, where there are no known pathogenic

members. Other disease-causing microbial agents are viruses

and parasitic protozoa. Earlier methods of detecting and

identifying microbial pathogens involved culturing and iso-

lating bacterial colonies in growth media in the lab. With

the advent of polymerase

chain reaction

(PCR) assays, iden-

tification of microorganisms that were difficult or impossible

to culture became possible. Many microbial pathogens can

be controlled with antimicrobial drugs called antibiotics.

However, these drugs are not effective against viruses or

parasites

, and indiscriminant use may cause resistant strains

of pathogens to evolve.

Antibiotic resistance

has resulted

in the reemergence of several disease, and as of 2002, most of

the major bacterial diseases that infect humans are becoming

resistant to antibiotics.

[Marie H. Bundy]

Microclimate

In general,

climate

conditions near the ground are called

microclimates. More specifically, microclimate refers to the

climate characteristics of highly localized areas, ranging from

the area around an individual plant to a field of crops or a

small forested area. The horizontal area considered may be

less than one square meter or up to several thousands of

square meters. The vertical extent may range from a few

centimeters involving the still layers of air within a plant

canopy, for instance, to 100 meters or more, when the

atmo-

sphere

surrounding a forested area is studied.

Microclimates are governed to a large extent by the

interactions of surface features with the overlying atmo-

sphere, and their characteristics may differ markedly from

those of the surrounding large-scale climate. Microclimates

exhibit great ranges in environmental conditions depending

on the moisture and radiation properties of the surface.

They typically show large diurnal temperature ranges and

are highly influenced by slope, aspect, and elevation. Most

plants and animals are adapted to highly specific microcli-

matic conditions.

Micronutrient

see

Trace element/micronutrient

Environmental Encyclopedia 3

Migration

Although some scientists define animal migration as any

animal movement, this definition becomes cumbersome be-

cause it does not distinguish between small-scale daily move-

ments, annual migrations, and irrupting dispersions. Mobile

animals tend to move frequently, and migration should be

distinct from emigration (directional one-way movement)

and dispersal (non-directional one-way movement), and it

should refer primarily to regular round trip movement that

happens at least once in the life span of the organism. Migra-

tion is a spatial behavior pattern that allows animals to locate

themselves in the most favorable portions of their

habitat

for as long as necessary. Such favorable conditions may vary

according to season or life history, but in both cases it is

related to adaptive fitness. This allows the organism to take

in nutrients in excess of energy expenditures and to success-

fully reproduce.

Generally animal migration can be divided into two

areas of study: the behavioral aspects, which concentrates

on “how” migration happens; and the ecological aspects,

which addresses “why” migrations takes place. The ecological

questions also concern

evolution

, for spatial behavior is an

evolved compromise between differing requirements of an

organism’s life. Most migratory behavior depends on food

abundance. In most habitats productivity varies with the

seasons, and thus energy availability also varies at all upper

levels of food chains. Migration must often accommodate

several energy and reproductive requirements. As a result

migration patterns tend to be complicated with subsections

of migrants taking slightly differing paths at various times

to serve different needs.

Many

species

of North American waterfowl use several

breeding areas, from the northern prairies to the Arctic coast,

and travel along several major flyways to wintering areas that

range from the southern prairies to the estuaries of northern

South America. Within that framework, males

desert

the

hens during nesting season and make shorter migrations to

molting areas, afterward meeting with the females and newly

fledged young during the fall migration. In either of those

sites, requirements for habitat differ in terms of water-cover

ratios, water permanence, and food preferences.

The altitudinal migration of mountain sheep (Ovis

canadensis) in the Rocky Mountains brings them into more

productive habitats during the summer in the alpine mead-

ows and into the mountain forest during the winter. Thus,

food intake requirements are accommodated and, simultane-

ously, the sheep take advantage of the

microclimate

of the

forested area during the winter to supplement energy losses

from body temperature maintenance.

Migration can also be a response to particular breeding

site requirements, mate location, and a combination of sev-

904

eral forces acting together. The longest mammal migration,

that of the California gray whale (Eschrichtius robustus),

places the breeding-ready adults and newly impregnated

females in the productive shallows of the Bering and Chuk-

chi Seas, the non-breeding animals in the more patchy feed-

ing grounds of the northeast Pacific Coast, and the calving

females in the warm shallow lagoons of the Baja Peninsula.

[David A. Duffus]

ESOURCES

ERIODICALS

Clark, C. W. “Moving With the Heard (Hydrophonic Monitoring of

Migratory Bowhead Whales).” Natural History (March 1991): 38–42.

Dybas, C. L. “Secret Creatures of the Nigh; When the Moon is New and

Darkness Falls, American Eels Begin Their Eerie Autumn Migration.”

National Wildlife 28 (October-November 1990): 18–23.

Hansson, L. “The Lemming Phenomenon: Or Why the Legendary Mass

Migrations of Rodents Are Restricted to the Extreme North.” Natural

History (December 1989): 38–43.

Pallace, D. R. “Avian Nations: The Patterns and Problems of Migrating

Birds.” Wilderness 54 (Fall 1990): 42–9+.

“Recent Developments in the Study of Animal Migration.” [Symposium

on Recent Developments in the Study of Animal Migration.] American

Zoologist 31 (1991): 151–276.

Milankovitch weather cycles

According to the theory of the Milankovitch weather cycles,

ice ages are cyclical, caused by changes in the earth’s orbit.

The theory was developed by Serbian geophysicist Milutin

Milankovitch (1879–1958) in the 1930s, and it postulates

that the amount of available sunlight in the northern hemi-

sphere is affected by the earth’s orientation in space. Because

the earth is a globe in motion, sunlight strikes the earth

differently depending on the following factors: The eccen-

tricity of its orbit, which returns to the same point every

100,000 years; the tilt of the axis of its rotation, a 41,000-

year cycle; and the precession of the equinoxes, a 23,000-

year cycle. Milankovitch proposed that decreased sunlight

prevents ice and snow from melting in the summer in the

northern latitudes. This, in turn, cools the

atmosphere

because ice reflects 90 percent of solar radiation back into

space, and over a long period of time, the ice accumulates

and moves south.

The theory has gone through its own cycles of accept-

ance and rejection. The first problem for proponents of the

theory was to prove the occurrence of glacial-interglacial

patterns during the Pleistocene era, the epoch preceding

ours. It was also necessary to date the onset of each

ice age

in order to calculate the date of the next one. During the

last fifty years, data collection improvements in astronomy

and geology, especially in satellites and deep-sea

sediment

Environmental Encyclopedia 3

Minamata disease

coring, have challenged and refined the theory. Supported

first by astronomical and geological data, the theory encoun-

tered opposition in the 1950s when scientists used the newly-

developed Carbon-14 method to date warm-era fossils in

supposed ice-age deposits. But examination of fossilized mi-

croscopic sea creatures on the ocean bed, known as foramini-

fera, revealed two types, the larger of which flourished when

the ocean was warm. In 1955, geologist Cesare Emiliani

refined dating techniques of the foraminifera retrieved by

deep-sea coring. He isolated oxygen isotopes. Oxygen 16,

a lighter

isotope

, evaporates more quickly and becomes

trapped in ice, while oxygen 18, which thrives in warm

waters, was absorbed by the shells of the foraminifera. From

the evidence of theses oxygen isotopes, Emiliani was able

to reconstruct glacial-interglacial periods for 300,000 years.

His date agreed with Milankovitch’s orbital dates.

In 1971 John Imbrie of Brown University created Proj-

ect CLIMAP, which coordinated all of the data relevant to

Milankovitch’s cycles. From this data, scientists were able to

develop a “rosetta stone” for Pleistocene glacial-interglacial

dates. In the 1980s, Project SPECMAP brought together

all of the deep-sea core data. It was from this data that

scientists predicted the next ice age was imminent.

But critics have argued that the Milankovitch orbital

theory does not take into account the concept of chaos as

well as the climatic influence of the heat transfer between

the atmosphere and the ocean. In 1992, United States

Geo-

logical Survey

hydrogeologist Isaac Winograd challenged

the Milankovitch theory when a scuba team with a submers-

ible drill retrieved a 14-in (36-cm) core of calcite in a water-

filled fault called Devils Hole in Nevada. The ice age dates

for this core of calcite differed from the Milankovitch dates,

and this suggested to Winograd that ice ages were tied less

to orbital cycles than to an interaction of heat and moisture

between the atmosphere, the ocean, and ice sheets. Imbrie

and many others defended Milankovitch against these find-

ings. They stood on their mountain of ocean-sediment core

data, and they argued that the calcite core from Devils Hole

reflected local rather than global changes.

Another problem for the Milankovitch theory is iden-

tifying the effects of human activity on the

environment

The current interglacial period began 10,000 years ago, and

according to Imbrie, it might become a “super interglacial”

period due to the burning of

fossil fuels

and the subsequent

greenhouse effect

. Imbrie has argued that the natural cool-

ing cycle which began 7,000 years ago will be postponed

until the excess

carbon dioxide

is exhausted in 2,000 years.

Then, after another 1,000 years, serious cooling will set in,

expected to last for 23,000 years. See also Climate; Glaciation;

Ice age refugia; Ozone layer depletion; Radiocarbon dating

[Stephanie Ocko]

905

ESOURCES

OOKS

Imbrie, J., and K. P. Imbrie. Ice Ages: Solving the Mystery. Short Hills, NJ:

Enslow, 1979.

ERIODICALS

Kerr, R. A. “Milankovitch Climate Cycles Through the Ages.” Science 235

(February 27, 1987): 973–4.

Winograd, I. J., et al. “Continuous 500,000-Year Climate Record from

Vein Calcite in Devils Hole, Nevada.” Science 258 (October 9, 1992):

255–260.

THER

Berger, A., et al. Milankovitch and Climate. Proceedings of the NATO

Advanced Research Workshop on Milankovitch and Climate. Dordrecht:

D. Reidel Publishing, 1982.

Milfoil

see

Eurasian milfoil

Minamata disease

The town of Minamata, near the southern tip of Japan’s

Kyushu Island, gave its name to one of the most notorious

examples of environmental contamination known. Mina-

mata disease is really alkylmercury

poisoning

, caused by

eating food such as fish or grain contaminated with

mercury

or its derivatives. At Minamata people were poisoned when

they ate large quantities of methylmercury-contaminated

fish. Often the victims were the poorest members of society,

who could not afford to stop eating the cheap fish known

to be affected by

effluent

discharged from the local chemical

company. The first victims were reported in 1956; before

that, cats were seen moving strangely, sometimes flinging

themselves into the sea, and birds were observed flying awk-

wardly or even falling out of the sky.

The Chisso Company, a leading chemical manufac-

turer, produced acetaldehyde by passing acetylene gas across

an inorganic mercury catalyst, leaving methylmercury as a

by-product. The company was a major local employer and

the only significant major source of industrial waste dis-

charged into Minamata Bay. There, methylmercury in the

water biomagnified to high levels in fish and shellfish con-

sumed by the local inhabitants. Methylmercury concentrates

in specific regions of the central nervous system and readily

crosses the blood/brain barrier as well as the placental barrier.

It is a human

teratogen

which causes brain damage during

prenatal exposures, resulting in congenital or fetal Minamata

disease. The compound has a

half-life

of about 70 days in

the human body, and the damage is generally irreversible.

Although company officials knew that similar symptoms

could be induced in cats by feeding them fish taken from

the bay, Chisso was not blamed by the Japanese government

Environmental Encyclopedia 3

Minamata disease

A mother from Minamata, Japan, bathes her daughter who suffers from Minamata disease. The daughter’s

brain damage and birth defects were caused by alkylmercury poisoning from mercury-contaminated seafood

that her mother ate while pregnant. (Photograph by Eugene Smith. Black Star Publishing Company Inc. Reproduced by permission.)

for the disaster until 1968. Despite the evidence, it was

difficult to be certified as a Minamata disease victim. Those

affected were not compensated until after another epidemic

occurred at Niigata, Japan, in 1965. That one was caused

by methylmercury waste discharged from the Showa Denko

Corporation Kanose factory into the Agano River. The com-

pany also manufactured acetaldehyde using the same process

as Chisso.

Nearly four decades after Minamata disease was iden-

tified, the full range of its neurological symptoms is still

not known, nor have the total number of sufferers been

determined. While some children were born with contorted

bodies and severe retardation, in milder cases the victims may

be moderately retarded or exhibit only sensory disturbances.

Even as the symptoms advance in adult patients, it is often

difficult to determine whether they are the result of long-

term poisoning, delayed effects of residual methylmercury,

aging, or other complications. Therefore, even thirty years

later, as of December 31, 1992, only 2,945 individuals were

officially certified as Minamata Disease victims and 1,343 of

906

them had died. Another 13,761 had been denied certification

and the fate of 2,430 was still pending.

Lawsuits dragged on for more than twenty-five years,

and victims seeking compensation tried confrontation tactics

like an encampment that lasted a year and a half in front

of Chisso’s main headquarters in Tokyo.

Reclamation

of the

environment

has been even slower. The Chisso Company

installed waste treatment equipment in 1966 and stopped

making acetaldehyde in 1968. By then 400–600 tons of

mercury and 60,000 tons of

sludge

had been dumped into

the shallow Minamata Bay. Mercury levels in fish remained

elevated.

Dredging

scheduled to begin in 1975 did not

get underway until 1982. Since then, mercury levels in ten

species

of fish have dropped; but as of March 1993, they

were still two or three times higher than what the govern-

ment deems acceptable. Consequently, fishermen have lost

their livelihood and health and gained a social stigma. The

Chisso company’s fortunes also declined and were further

strained by lawsuits that mandated restitution. See also Ag-

ricultural chemicals; Biomagnification; Birth defects; Envi-