Vaccari D.A., Strom P.F., Alleman J.E. Environmental Biology for Engineers and Scientists

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the visc osity of the water. A microscopic zooplankter, on the other hand, stops within

milliseconds if it stops propelling itself.

Salt water is more viscous than fresh water. Seawater is about 25% more viscous at 0



than at 20



C. Figure 15.28 shows how two species of one genus react to this difference.

The warm-water form has more elaborate appendages to slow its sedimentation rate in the

less viscous water.

Many marine animals control their position not only by swim ming but by buoyancy.

Several mollusks have rigid air containers. These include the genus Nautilus, the cuttleﬁsh

Sepia, and the squid Spirula. Their air chambers just balance the buoyant weight of the

rest of their bodies, making them neutrally buoyant. Some of the boney ﬁsh have a swim

bladder to store air. The bladder is ﬁlled either by extracting gases from the blood or by

direct ingestion through the esophagus. Active swimmers, such as the tuna, or ﬁsh that

live on the bottom do not have swim bladders.

15.4.3 Marine Communities

Several marine communities are important or interesting enough to merit special

attention.

Estuaries. Estuaries have high productivity because they receive a constant nutrient input

from rivers, and tidal motions provide an energy subsidy that improves oxygen transfer

Figure 15.28 Adaptation of two species of the copepod genus Oithona to viscosity differences due

to temperature: ðaÞ warm-water species; ðbÞ cold-water species. (From Garrison, 1993. #

Wadsworth Publishing Co. Used with permission.)

558

ECOSYSTEMS AND APPLICATI ONS

and mass transfer of nutrients from the sediment. They receive detritus inputs from adja-

cent salt marshes. The organisms that populate estuaries are similar to those already

described for salt marshes.

Rocky Intertidal Communities. This group constitute one type of littoral zone; others

include the sand beach and the salt marsh. The wave energy that strikes these ecosystems

would seem to make it an inhospitable place. Besides the crushing wave energy, they

are subjected to sharp swings in moisture and temperature. In fact, they are highly pro-

ductive ecosystems and have one of the highest species richness and diversities of any

marine ecosystem. For this reason, and also because they are among the most accessible

of the high-richness ecosystems, they are among the most interesting. There are few other

en vironments where the casual observer could ﬁnd so many different animals in one place.

Several reasons explain why. The wave energy provides mixing, gas transfer, and ﬂush-

ing of wastes. It also eliminates competition from species not specially adapted for this

environment. The tides add to the energy subsidy. Detritus is brought in by the tides. Tides

naturally create a range of habitats, classiﬁed by the fraction of the time that each is

exposed. Areas near the low-tide mark are submerged almost all the time, high-tide

areas only brieﬂy. Since the intertidal zone spends time both above and below the

water, organisms there are subjected to attack by predators from the land and the sea.

Heavy predation is one of the factors that contributes directly to species diversity. Finally,

the rocky intertidal environment itself provides many diverse habitats and niches: tide

pools, crevices, exposed rock, and gravel beds.

Algae encrust rocks or grow in the form of large, tough, elastic, and slippery plants,

such as kelp, to resist wave energy. Sessile ﬁlter-feeding animals such as barnacles and

mussels attach themselves to rocks. Snails scrape algae from the rocks. Clams, octopi,

starﬁsh, crabs, sea urchins, anemones, sponges, shrimp, ﬁsh, and shorebirds are all

present.

Coral Reefs. Coral reefs are unique and important communities formed from colonial

types of coral. Corals are cnidarians with a polyp body plan that sits in a cuplike exoske-

leton made of calcium carbonate. The opening at the top of the polyp has tentacles with

stinging cells for capturing prey. The outer layer of the coral tissue has embedded within it

thousands of dinoﬂagellates called zooxanthellae. There may be as many as 30,000 cells

per milliliter of coral tissue, up to 75% by mass, and they provide the bright and varied

colors for which coral reefs are famous. The coral and the zooxanthellae exist in a sym-

biotic relationship in which the coral provide shelter and nutrients and the algae provide

food and removal of the coral’s waste products (which are nutrients to them). The zoox-

anthellae also seem to give the coral its ability to secrete large amounts of calcium car-

bonate. The coral–algae association seems a most intimate form of nutrient recycling,

which must be essential since coral tends to grow in nutrient-depleted waters.

Coral grows on a solid substrate. When the coral dies, the exoskeleton remains, and

new organisms can grow on top of this. Over many hundreds and thousands of years

the skeletons accumulate, forming reef structures. Coral can only grow in warm, shallow

waters. If the land subsides or sea level increases (as has been happening over the last

dozen millennia or so), the reef grows to maintain its position relative to the surface, form-

ing offshore barrier reefs or, where a central island has become submerged, a ring-shaped

atoll with a lagoon. Reef-forming coral grows almost exclusively in waters with a

seasonal temperature minimum above 20



MARINE AND ESTUARINE ECOSYSTEMS 559

The mas sive reef structure creates numerous ecological niches. The zooxanthellae,

plus ﬁlamentous green algae and other algae, form the base of the coral reef food pyramid

(Figure 15.29). The corals themselves are primary consumers, as well as ﬁsh, clams, sea

urchins, crustaceans, and other invertebrates. Secondary consumers include sea urchins,

sea anemones, sea stars, and ﬁsh. The top carnivores are eels, octopi, and barracuda. The

animal diversity of coral reefs is the highest of all marine communities. Despite their high

productivity, coral reefs do not support large ﬁsheries. It is thought that high predation

rates limit production at each level of the food chain. The overall effect is a low transfer

efﬁciency.

Coral reefs will grow at depths no greater than 150 m, where light levels in the clear

water they require falls below 4% of the surface levels. Too high a nutrient level hurts

coral because other benthic plants and suspension feeders such as clams will outcompete

them. Higher nutrient levels also brings increased plankton concentrations and resulting

turbidity, which also inhibits coral growth.

Abyssalpelagic and Abyssal Benthos. With the exception of hot vents described below,

the deep-sea bottom is sparsely populated. While the nearshore sediments may have

5000 g of biomass per square meter and the continental shelf could have 200, the abyssal

Seabirds

B = 15

P = 81

Benthic heterotrophs

B = 1.7e5

P = 5.1e5

Zooplankton

B = 899

P = 3.6e6

Small pelagics

B = 1836

P = 2020

Lobsters and Crabs

B = 1348

P = 701

Bottom fish

B = 94

P = 30

Monk seals

B = 63

P = 189

Reef fish

B = 13966

P = 20949

Sharks, jacks, scombrids

B = 536 P = 192

Phytoplankton

B = 3.3e3

P = 2.3e5

Benthic algae

B = 2.0e5

P = 2.5e6

Green turtles

B = 15

P = 12

2.3e5 2.3e4 2.4e6 3.3e4

1.3e4

2.3e4

1.1e4 6.1e4 87

1.6e4

826

191432

243

847

2500

219

150

4.3e5

Seabirds

B = 15

P = 81

Benthic heterotrophs

B = 1.7e5

P = 5.1e5

Zooplankton

B = 899

P = 3.6e6

Small pelagics

B = 1836

P = 2020

Lobsters and Crabs

B = 1348

P = 701

Bottom fish

B = 94

P = 30

Monk seals

B = 63

P = 189

Reef fish

B = 13966

P = 20949

Sharks, jacks, scombrids

B = 536 P = 192

Phytoplankton

B = 3.3e3

P = 2.3e5

Benthic algae

B = 2.0e5

P = 2.5e6

Green turtles

B = 15

P = 12

2.3e5 2.3e4 2.4e6 3.3e4

1.3e4

2.3e4

1.1e4 6.1e4 87

1.6e4

826

191432

243

847

2500

219

150

4.3e5

Figure 15.29 Food web of a coral reef with a biomass budget. B is average annual biomass in kg/

; P is the production in kg/m

 yr. (Based on Barnes and Mann, 1991; original source is R. W.,

Grigg, J. J. Polovina, and M. J. Atkinson, 1984, Coral Reefs, Vol. 3, pp. 32–37).

560

ECOSYSTEMS AND APPLICATI ONS

benthos typically has less than 1 mg. Furthermore, the deeper it is, the less biomass there

will be. This is because it is entirely a detritus-based food web, the energy input comes

from sedimentation from the euphotic zone, and this input has increasing chances of being

intercepted by pelagic organisms on its way down.

The environment is permanently dark, cold, and currents are weak. Adaptations

include low rates of metabolism and growth, and enzymes that are specially optimized

to function at the high pressures found on the o cean ﬂoor. Most organisms are blind,

but some, such as the lanternﬁsh, use bioluminescence (the biological production of

light) to ﬁnd and lure prey. All the animals are predators or scavengers. There are no

plants, so there are no herbivores.

Mesopelagic ﬁsh have swim bladders, which makes them difﬁcult to harvest without

rupturing them and killing the ﬁsh. Fish from below 1000 m depth typically lack swim

bladders. They and other organisms from this depth can survive in aquaria at the surface if

the temperature and other physical factors are favorable. Some ﬁsh, such as the lantern-

ﬁsh, migrate diurnally to the euphotic zone at night to feed, then descend to between

700 and 900 m during the day. More than 2000 species of animals live in the aphotic

zone, including copepods, ostracods, jellyﬁsh, prawns, mysids, amphipods, swimming

worms, and a group of strange-looking ﬁsh. The ﬁsh are typically small but have huge

mouths. Since prey are scarce, they need not to have to reject larger victims.

Feeding strategies of deep-sea creatures are often bizarre. Some bury themselves in the

sediment with their mouths open at the surface. Other creatur es mistake them for caves

and crawl in for shelter, only to be forced to crawl right into the stomach by downward-

projecting spines. Others can smell dead organisms from kilometers away, then spend

weeks crawling to the source of the scent. Some organisms feed less than once per

year and live for hundre ds of years.

Surprisingly, species diversity at the bottom of the sea is very high. This may be

explained by the principle of competitive exclusion, which states that high predation

pressure prevents any single species from dominating.

Hot Vent Communities. In 1977, scientists in the submersible Alvin disc overed a new

ecosystem northeast of the Gala

pagos Islands while searching for a source of heated

water at a depth of 3000 m. There, at the rift where two of the tectonic plates that

form Earth’s crust are spreading apart, hydrothermal vents were spewing water at

350



C, laden with dissolved minerals, including H

But the astonishing thing about the vents were the abundance of benthic invertebrates

surrounding them, many of them huge in size and previously unknown to science. Besides

large crabs, clams, shrimp, and anemones, there were huge ‘‘tube worms,’’ contained

in parchmentlike tubes the diameter of a human arm and about 3 to 4 m. Three species

have been found so far, forming a new genus called Riftia, in the phylum Pogonophora.

Altogether, about 100 species live in vent communities.

The vent community had no plants and was too dense to subsist on detritus from above.

What was the source of the energy to maintain it? Furthermore, the worms had no mouth,

digestive tract, or anus. Instead, the worms and some clams contained ‘‘feeding bodies’’

packed wi th chemoautotrophic bacteria. The worms would absorb hydrogen sulﬁde from

the water and transport it to the feeding bodies. These would then produce the carbohy-

drates to sustain the ecosystem. Thus, this unique ecosystem is based not on the energy

from sunlight, as is every other ecosystem on Earth, but on geochemical energy from deep

within the Earth itself.

MARINE AND ESTUARINE ECOSYSTEMS 561

Hot vent communities have been discovered in seas throughout the world, including

sites on the mid-Atlantic ridge, the Gulf of California, offshore from the state of Washing-

ton, and near Okinawa. Similar communities have been found where hydrogen sulﬁde–

rich water seeps from the base of the continental shelf off Florida, Oregon, and Japan.

These communities also have tube worms. In a third type of deep ocean chemoautotrophic

ecosystem, sites were found in the Gulf of Mexico and Bafﬁn Bay, where ecosystems

subsist on hydrogen sulﬁde and methane seeping from natural hydrocarbon deposits.

15.4.4 Adverse Impacts on Marine Ecosystems

Many years ago, the vastness of the oceans made it easy to assume that nothing that

humans could do to it could have any long-term or widespread effect. Today, however,

our species has become so successful that nothing in the biosphere can be held to be

insensitive to our activities.

Overﬁshing. Improvement of ﬁshing technology led to overﬁshing in many marine ﬁsh-

eries. There are many instances of this. Peru led the world in its catch with 4 million

metric tons in 1960. In 10 years it increased the harvest to 13 million metric tons. But

this high level, combined with a series of El Nin

o events, devastated the industry until

it was reduced to 100,000 tons in 1985. Cod and herring are seriously depleted in the

North Atlantic. The Paciﬁc sardine has been reduced to commercial extinction,in

which it is no longer proﬁtable to harvest it. Of 11 species of large whales that have

been hunted commercially, eight are now commercially extinct.

When a ﬁshery becomes overﬁshed, instead of prudently reducing its take, ﬁshermen

instead increase their efforts by adding more boats and more efﬁcient methods. One of

these methods is called drift net ﬁshing, in which a 7-m-wide net up to 80 km long is

deployed vertically overnight, then hauled in the next morning. This method ensnares tur-

tles, birds, and marine mammals that are not intended for harvesting. Furthermore, about

1600 km of these nets are lost each season, becoming ‘‘ghost nets’’ that drift and kill for

decades.

Oil Pollution. Oil pollution in the ocean is sometimes very dramatic. However, spills due

to accidents are a small part of the total discharge: only 13% in 1985. A signiﬁcant frac-

tion, about 8%, is of natural origin. A major source (about one-third) is due to routine

ﬂushing of oil tankers when loading and unloading. Another third comes from use and

disposal on land via rivers and streams. Once in the water, fractions of crude oil may dis-

perse by evaporating, dissolving, forming emulsions, or settling to the bottom. Floating oil

slicks may directly harm wildli fe at the surface, especially birds, and can devastate

benthic organisms of the littoral zone. These effects are most severe and may persist

for many years. The dispersed fractions are biodegraded or assimilated by zooplankton

and higher organisms, thus entering the food chain. Compounds absorbed by organisms

have toxic effects at all trophic levels. Also, they may affect the commercial use of the

resource by imparting an unpleasant taste to ﬁsh, shellﬁsh, and so on. Spills of reﬁned

hydrocarbons can be more damaging than crude oil, since it contains lighter fractions

and additives that are more easily taken up by organisms and are often more toxi c.

Water Pollution. Coastal areas are affected by discharge of water pollutants from land

sources via rivers, direct pipeline disposal, or dumping by barges. Barge dumping was

562 ECOSYSTEMS AND APPLICATIONS

practiced by communities in the New York Metropolitan Area up to the early 1990s. Ori-

ginally, sewage sludge dumping was conﬁned to a 10-km-square site some 20 km from the

coast of New Jersey and Long Island. When it became known that sludge was accumulat-

ing at that site, an interim site was chosen in deeper waters at a distance of 196 km from

New York Harbor. The new site provided enhanced dilution. Ultimately, ocean dumping

was banned altogether in favor of land-based alternatives.

When these discharges are used as an alternative to treatment, they discharge toxic

materials and nutr ients to the marine environment. The nutrients can cause algal blooms

in excess of what might occur naturally. Sometimes, the algae themselves are toxic to

marine life. Other times, the algae have been responsible for ﬁsh or benthos kills due

to deoxygenation, when the algae decompose. Discharge of sewage and sludge have

also been blamed for diseases of ﬁnﬁsh, including ‘‘black gill’’ and ‘‘ﬁn rot.’’

Severe cases of hypoxia (low-dissolved-oxygen condi tions) have been observed in the

New York Bight and in the Gulf of Mexico. The New York Bight is the continental shelf

area into which the Hudson River discharges. In one inciden t in the late 1980s, dissolved

oxygen levels dropped to low levels over a 300  100 km area in the Bight, falling all the

way to zero in a portion of the region off Atlantic City, New Jersey. A similar incident

occurred in a 420-km band along Louisiana in the Gulf of Mexico. Even in the absence of

such catastrophes, continuing sanitary pollution causes public health risks to swimmers in

the ocean and eliminates many shellﬁsh beds from use as a resource due to contamination

by pathogens.

Coral Diseases. In 1983, scientists discovered a disturbing disease of coral called coral

bleaching. For unknown reasons, the coral will expel its zooxanthellae, leaving the coral

a pale color. Without their symbionts, coral cannot deposit calcium carbonate and are

subject to erosion. If it does not regain its zooxanthellae, the coral dies.

This was ﬁrst noticed in the Paciﬁc but has since been found in Caribbean coral reefs as

well. The cause of this disease is still unknown and is being sought with some urgency by

marine biologists. Some blame a combination of pollution and high temperatures. The

original observation occur red during a particularly severe El Nin

o event in the eastern

Paciﬁc, when sea surface temperatures averaged 30 to 31



C for 5 to 6 months.

Coral are vulnerable to other problems as well. The crown-of-thorns sea sta r Acantha-

ster planci is destroying coral reefs in the western Paciﬁc. A number of other diseases are

found on coral, including white-band disease. Anecdotal evidence points to many of

these problems as having proliferated recently. Thus, human activities are being blamed,

although there is no strong evidence for it.

Thermal Pollution. Industries, especially steam-powered electrical generating plants,

often discharge large amounts of heated water to adjacent surface waters. At low levels

these may provide an energy subsidy to the ecosystem, encouraging growth. The beneﬁts

may vary by organism, resulting in population shifts. Detrimental effects include stimula-

tion of fungal organisms, which may cause disease for aquatic organisms. Cyanobacter

have been found to dominate at temperatures above 35



C, green algae between 28 and



C, and diatoms at lower temperatures. Oxygen consumption rates increase with tem-

perature, but oxygen solubility decreases, both effects increasing oxygen depletion. Stress

may occur if the heated discharge is suddenly halted due to operating considerations of

the source.

MARINE AND ESTUARINE ECOSYSTEMS 563

The sensitivity of animals to temperature may be measured by the upper lethal tem-

perature (ULT), the temperature at which 50% mortality occurs over long-term exposure.

The ULT for the ﬁshes Salmo trutta and Salmo gairdneri was measured at 25 and 24



respectively. Others, including the ﬁsh Carassius auratus, Cyprinus carpio, and Lepomis

macrochirus and the invertebrates Gammarus fasciatus, Procladius, and Cryptochirono-

mus had ULT between 30 and 35



C. At lower temperatures, growth and reproduction will

be inhibited.

Use of large volumes of surface water for cooling can have a direct effect as the water

is passed through pumps and heat exchangers. Shear and pressure forces can kill or

damage organisms. Discharge of water from below the thermocline in reservoirs can

produce a low-termperature anomaly. These usually have nonlethal effects, such as

population shifts or inhibition of spawning of ﬁsh.

15.5 MICROBIAL ECOLOGY

The study of microorganisms too often treats each species as a pure strain. However, in

the environment this is rarely the case. Many of the important behaviors depend on the

interactions among microbial populations and between them and other organisms. Here

we describe some of these interactions.

Cooperation is the positive interaction among organisms within a single population.

Bacteria cooperate in nume rous ways. Bacteria form ﬂocs, bioﬁlms, and other kinds of

aggregates for the purpose of degrading solid substrates, whether lignin, cellulose, or

rocks. The aggregates can beneﬁt more efﬁciently from the investment of extracelllular

products than a single individual can. In laboratory-batch microorganism growth experi-

ments, a lag phase of growth is often missing when a large initial concentration is used.

It may be that at a high initial density , the cells share gro wth factors with each other . A certain

minimum number of microorganisms, called the infective dose,isrequiredtotransmitadisease

to a host. The bacteria in this case cooperate to overwhelm the host’ s defenses.

Individuals also compete within a population. They certainly use the same reso urces. In

the case of photoautotrophic microbes, there is competition for light.

Microorganisms exhibit most of the types of two-population interactions shown in

Table 14.5. Neutralism (0/0) is uncommon among microorganisms if there is any overlap

in their niches. Commensalism (þ/0) is common. One microbial population may solubi-

lize rock minerals, and other microbes beneﬁt from the nutrients. One microbe may man-

ufacture growth factors that others require. Biotin is a growth factor that is very important

in marine habitats. Methanogenesis may result from commensal relationships. Desulfovi-

brio produces acetate and hydrogen by fermentation, which is then used by Methanobac-

terium to reduce CO

to methane. Several of the micro bially mediated pathways in

biogeochemical cycling involve one population using the products of another (e.g., Nitro-

somonas and Nitrobacter).

Cometabolism is, by some deﬁnitions, commensal. Mycobacterium vaccae can come-

tabolize cyclohexane if it has propane available for its own beneﬁt. The cyclohexane is

converted to cyclohexanone, which can be used by other bacteria. The Mycobacterium

does not assimilate the cyclohexanone itself.

One population may utilize a substance that is toxic to another population. An example

is the oxidation of H

S. Some marine bacteria produce organics that chelate heavy metals,

reducing their bioavailability.

564 ECOSYSTEMS AND APPLICATIONS

Synergism (þ/þ) refers to a loose two-way beneﬁt, in which both populations can exist

separately, but beneﬁt when together. Sometimes two populations together can produce

metabolic products that neither can produce by themselves. For example, neither Sterpto-

coccus faecalis nor Escherichia coli can convert arginine to putrescine, but together, they

can. Cyclohexane can be degraded by Nocardia and Pseudomonas together, but not sepa-

rately. The reason is that Nocardia needs biotin and other growth factors produced by

Pseudomonas to do the job. Together, Arthrobacter and Streptomyces can degrade and

grow on the organophosphate pesticide diazinon, but not separately. Pseudomonas stutzeri

combines with Pseudomonas aeruginosa to mineralize parathion. One form of synergism

is called syntrophy or cross-feeding. In this situation one species provides a nutritional

requirement for anot her, and vice-versa. For example, Enterococcus faec alis requires folic

acid for growth, and Lactobacillus arabinosus requires phenylalanine. Together, they can

grow in a medium that contains neither, because E. faecalis produces pheynylalanine and

L arabinosus produces folic acid.

Mutualism,orsymbiosis (þ/þ), is an obligatory relationship between two organisms

that enables them to occupy a habitat that they otherwise could not. Lichens, an associa-

tion between a fungus and a photoautotroph, are the best known example of symbiosis.

Some protozoans form mutualistic associations with algae. For example, Paramecium can

contain many cells of the alga Chlorella within its protoplasm. Under conditions of stress,

the protozoan may digest its algae. Paramecium aurelia can also harbo r a bacterium of the

Rickettsia group. These paramecia have an advantage over a different strain from the same

species that does not contain the bacteria. Apparently, the Rickettsia manufacture a toxin

used to inhibit neighbors.

Some methanogenic cultures, once thought to be pure, have been found to consist of

mutualistic associations. Methanobacterium omeliansk ii is associated with another strain,

called ‘‘S.’’ Methane formation involves an electron transfer reaction that actually starts in

the S organism and ends in the Methanobacterium. It is thought that some other metha-

nogenic mechanism may involve interactions among three organisms. Methanogens also

participate in a mutualistic relationship with eubacteria. The methanogens require simple

substrates such as acetate, CO

, formate, and methanol. These are waste products for the

eubacterial degradation of more complex organics. Thus eubacter beneﬁts by having their

wastes removed, and the archaean methanogens beneﬁt by having substrates provided.

Bacteria can also enter into symbiotic relationships with viruses. Corynebacte rium

diphtheriae harbors a virus that enables it to produce a toxin and to infect host organisms,

causing the disease diptheria. Without the virus, it cannot do either.

Competition ( /) is also uncommon because of the principle of competitive exclu-

sion. Unless competition is weak, two species cannot occupy the same niche. However, if

each species has some distinguishing feature, they can compete in overlapping portions of

their niches. The classic example of these behaviors is laboratory work by Gause

(Figure 15.30). This shows two species of Paramecium that althoug h they do not attack

each other or secrete toxins, compete to the degree that one species is eliminated after

16 days. In a separate experiment, P. caudatum and P. bursaria were found to be able

to coexist because they tended to occupy different areas of the laboratory ﬂask.

Nutrient limitations can also produce coexistence. When the diatoms Asterionella

formosa and Cyclotella meneghiniana were cultured together, the former would be limited

by silica and the latter by phosphate.

Some organisms are relatively efﬁcient at low substrate concentrations but are outcom-

peted at high nutrient levels. This causes the microb ial populations in raw sewage to shift

MICROBIAL ECOLOGY 565

to the typical populations found in rivers and streams as substrate is consumed. This also

explains why some organisms that cause ﬁlamentous bulking in the activated sludge pro-

cess, such as Sphaerotilus natans, are favored by low-loading conditions. Figure 15.31

shows how this could occur by comparing the growth rate, r

, as a function of substrate

concentration, S, for two hypothetical microorganisms. Species A has a higher maximum

growth rate, m

; species B has a lower Monod coefﬁcient, K

. As a result, species B has

the higher growth rate at low substrate concentration (to the left of the dashed line).

Among bacteria, amensalism (antagonism) (0/), usually comes in the form of allelo-

pathy, the production of chemical inhibitors. In some cases it takes the form of acid

production leading to pH changes, many produce toxic short-chain fatty acids, and

some produce complex inhibitors that we exploit as antibiotics. Inhibitors allow the

ﬁrst population to gain a foothold in an environment to exclude newcomers.

Thiobacillus oxidans oxidizes sulfur, producing sulfuric acid, which lowers the pH of

mine drainage to between 1 and 2, inhibiting most other microbes. Microorganisms on the

skin produce fatty acids that are believed to prevent colonization by yeasts and other

microbes. The fatty acids produced during methanogenesis are not just intermediates in

the reaction but inhibit microbes that otherwise would disrupt the electron transfer

between Methanobacterium and S.

Antibiotics are substances that kill or inhibit at very low concentration. Their role in

natural habitats is unclear. They do not seem to accumulate to effective levels under

natural conditions.

P. aurelia

100

150

0.0 5.0 10.0 15.0 20.0

Days

Volume

P. caudatum

0.0 5.0 10.0 15.0 20.0

Volume

Figure 15.30 Competition between P. aurelia and P. caudatum. Solid diamonds show growth of

each paramecium species in the absence of the other. Open circles show growth in mixed culture.

The curves are for equation (14.28) ﬁtted to the data by eye. For P.aurelia, r ¼ 1:5 day

1

, K ¼ 100

volume units, and b ¼ 1:5. For P.caudatum, r ¼ 1:0 day

1

, K ¼ 60, and b ¼ 0:8. (Original data

from Gause, 1934.)

566

ECOSYSTEMS AND APPLICATI ONS

Bacteria, fungi, algae, and protozoans all have their parasites. Many of the parasites

are viruses, which are obligate parasites. Vibrio cholerae is thought to be eliminated from

river water following fecal contamination by the action of viral parasites. Bdellovibrio is a

bacterium that is parasitic on bacteria. It attaches to larger gram-negative bacteria, causing

cell lysis. E. coli has been found to be partially protected from Bdellovibrio by the

presence of clay particles. Other bacteria produce extracellular enzymes that can lyse

bacteria, algae, or protozoans, making their cell contents available for upta ke.

It can sometimes be difﬁcult to distinguish predation from parasitism among microbes.

The distinction can be made if predation is limited to cases where the prey is actually

ingested by the predator. Some protozoans prey on other protozoans. Didinium preys

on Paramecium. Many protozoans prey on bacteria. When bacteria are under strong pre-

dation pressure, they may change their growth habits: for example, growing in an attached

form instead of dispersed. Again, clay particles have been observed to reduce the vulner-

ability of some bacteria.

Even algae and fungi can prey on other organisms. Some dinoﬂagellates consume bac-

teria, other algae, and other dinoﬂagellates. Slime molds such as Dictyostelium prey on

bacteria. The nematode-trapping fungi capture nematodes by adhesives or by ‘‘lasso-

ing’’ them with constricting rings. When it attempts to swim through a ring, the ring

swells, trapping the worm. Despite thrashing motions, the nematode cannot free itself.

Eventually, the fungus hyphae grow into the body of the prey. Interestingly, the fungi

do not produce the trapping structures in laboratory culture unless nematodes are also

present in the culture.

15.6 BIOLOGICAL EFFECTS OF GREENHOUSE GASES

AND CLIMATE CHANGE

Our industrial economy is almost entirely heterotrophic, relying on energy stored in coal

and petroleum deposits. These were formed from atmospheric CO

removed over the eons

0.000

0.020

0.040

0.060

0.080

0.100

0.120

0.140

0.160

0 20 40 60 80 100 120 140 160 180

S (mg/L)

(day

−1

)

Species A

Species B

Figure 15.31 Effect of substrate concentration on competition. Maximum growth rate, m

, for

species A and B are 0.2 and 0:12 day

1

, respectively, and K

values are 50 and 5.0 mg/L,

respectively.

BIOLOGICAL EFFECTS OF GREENHOUSE GASES AND CLIMATE CHANGE 567