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Sulfur Cycle
The sulfur cycle is also an important biogeochemical cycle. Sulfur, like carbon and nitrogen, is needed
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by living organisms, where it is a constituent of protein. However, it is not required in such large
amounts as carbon and nitrogen and has not been reported as a limiting nutrient in environments.
Higher plants usually obtain sulfur in the form of sulfate, whereas animals
obtain it through amino acids (cysteine, cystine, and methionine) in their diet, either from plant protein
or from the protein of other
animals or microorganisms
(as
in
the
ruminant
animals).
In
contrast,
microorganisms use sulfur
compounds in other ways.
For example, some
use
sulfur containing substances
as energy sources, others
use it as electron acceptors in anaerobic respiration, and
some use it as hydrogen donors for photosynthesis.
Because of these diverse activities, the sulfur cycle is
dominated by microbial processes and is one of the
most fascinating of the elemental cycles. The oxidation
state of inorganic sulfur ranges in its cycling from –2 in
sulfide to +6 in sulfate.
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Sulfur Oxidation Reduced inorganic forms of sulfur, including not only sulfide and elemental sulfur, but
thiosulfate and other ions as well, can be oxidized by several different groups of organisms . This
process is called sulfur oxidation. Photosynthetic bacteria use hydrogen sulfide produced by sulfate
reducers in anaerobic environments as electron acceptors for the ultimate reduction of carbon dioxide
for the synthesis of organic materials. Included in this group of organisms are the purple sulfur and
green sulfur bacteria. These organisms oxidize the sulfide to elemental sulfur and ultimately to sulfate.
In addition, certain nonphotosynthetic bacteria also oxidize reduced sulfur compounds. Some of these
are chemosynthetic and therefore use the reduced sulfur compounds as energy sources and inorganic
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carbon as a carbon source for growth. Others are heterotrophic. For the most part, these are obligately
aerobic organisms, although a few can use nitrate as an electron acceptor in nitrate respiration. Both
filamentous and unicellular eubacteria are involved in this process as well as some acidophilic,
thermophilic archaeobacteria..
Sulfur Reduction The groups of sulfur-reducing bacteria are less well studied. The best-studied group
is sulfate- reducing Bacteria, which obtain carbon from organic compounds and use sulfate as an
electron acceptor in sulfate respiration. Some of these also use hydrogen gas as an energy source and
grow autotrophically by carbon dioxide fixation. This process of sulfate reduction is referred to as
dissimilatory sulfate reduction to distinguish it from assimilatory sulfate reduction, the process by
which plants, algae, and many aerobic bacteria obtain sulfur for synthesis of amino acids. The
dissimilatory sulfate reduction process requires large amounts of sulfate and occurs in anaerobic muds.
It is the sulfate-reducing bacteria that produce the sulfide smell typical of black muds from lake and
marine sediments. A number of thermophilic Archaea such as Pyrodictium spp. respire with sulfur.
Elemental sulfur is used by most of them as an electron acceptor, although a few use sulfate.
Acid rain is rain or any other form of precipitation that is unusually acidic. It has harmful effects on the
environment and on structures. Acid rain is mostly caused by emissions due to human activity of sulfur
and nitrogen compounds which react in the atmosphere to produce acids. In recent years, many
governments have introduced laws to reduce these emissions.
Effects of Acid Rain
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Acid rain causes acidification of lakes and streams and contributes to the damage of trees at high
elevations (for example, red spruce trees above 2,000 feet) and many sensitive forest soils. In
addition, acid rain accelerates the decay of building materials and paints, including irreplaceable
buildings, statues, and sculptures that are part of our nation's cultural heritage. Prior to falling to the
earth, sulfur dioxide (SO2) and nitrogen oxide (NOx) gases and their particulate matter derivatives—
sulfates and nitrates—contribute to visibility degradation and harm public health.
Surface Waters and Aquatic Animals
The ecological effects of acid rain are most clearly seen in the aquatic, or water, environments, such as
streams, lakes, and marshes. Acid rain flows into streams, lakes, and marshes after falling on forests,
fields, buildings, and roads. Acid rain also falls directly on aquatic habitats. Most lakes and streams
have a pH between 6 and 8, although some lakes are naturally acidic even without the effects of acid
rain. Acid rain primarily affects sensitive bodies of water, which are located in watersheds whose soils
have a limited ability to neutralize acidic compounds (called “buffering capacity”). Lakes and streams
become acidic (i.e., the pH value goes down) when the water itself and its surrounding soil cannot
buffer the acid rain enough to neutralize it. In areas where buffering capacity is low, acid rain releases
aluminum from soils into lakes and streams; aluminum is highly toxic to many species of aquatic
organisms.
Forests
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Over the years, scientists, foresters, and others have noted a slowed growth of some forests. Leaves
and needles turn brown and fall off when they should be green and healthy. In extreme cases, individual
trees or entire areas of the forest simply die off without an obvious reason.
After much analysis, researchers now know that acid rain causes slower growth, injury, or death of
forests. Acid rain has been implicated in forest and soil degradation in many areas of the eastern U.S.,
particularly high elevation forests of the Appalachian Mountains from Maine to Georgia that include
areas such as the Shenandoah and Great Smoky Mountain National Parks. Of course, acid rain is not
the only cause of such conditions. Other factors contribute to the overall stress of these areas, including
air pollutants, insects, disease, drought, or very cold weather. In most cases, in fact, the impacts of acid
rain on trees are due to the combined effects of acid rain and these other environmental stressors. After
many years of collecting information on the chemistry and biology of forests, researchers are beginning
to understand how acid rain works on the forest soil, trees, and other plants.
Automotive Coatings
Over the past two decades, there have been numerous reports of damage to automotive paints and
other coatings. The reported damage typically occurs on horizontal surfaces and appears as irregularly
shaped, permanently etched areas. The damage can best be detected under fluorescent lamps, can be
most easily observed on dark colored vehicles, and appears to occur after evaporation of a moisture
droplet. In addition, some evidence suggests damage occurs most frequently on freshly painted
vehicles. Usually the damage is permanent; once it has occurred, the only solution is to repaint.
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Materials
Acid rain and the dry deposition of acidic particles contribute to the corrosion of metals (such as
bronze) and the deterioration of paint and stone (such as marble and limestone). These effects
significantly reduce the societal value of buildings, bridges, cultural objects (such as statues,
monuments, and tombstones), and cars.
Dry deposition of acidic compounds can also dirty buildings and other structures, leading to increased
maintenance costs.
Visibility
Sulfates and nitrates that form in the atmosphere from sulfur dioxide (SO2) and nitrogen oxides (NOx)
emissions contribute to visibility impairment, meaning we cannot see as far or as clearly through the
air.
Human Health
Acid rain looks, feels, and tastes just like clean rain. The harm to people from acid rain is not direct.
Walking in acid rain, or even swimming in an acid lake, is no more dangerous than walking or
swimming in clean water. However, the pollutants that cause acid rain—sulfur dioxide (SO2) and
nitrogen oxides (NOx)—do damage human health. These gases interact in the atmosphere to form fine
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sulfate and nitrate particles that can be transported long distances by winds and inhaled deep into
people's lungs. Fine particles can also penetrate indoors. Many scientific studies have identified a
relationship between elevated levels of fine particles and increased illness and premature death from
heart and lung disorders, such as asthma and bronchitis.
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