Download File

Notes Unit 22
Aquatic Microbiology
Introduction
Water, because it provides a unique physical environment, favors the existence of many types of
microorganisms that are not common in soils. They perform many roles in the water environment: base of the
food chain, produce oxygen, increase nutrient capture by other organisms.
The Ocean Ecology
In terms of sheer volume, the marine environment represents the largest portion of the biosphere, containing
97% of the earth’s water. The microbes in this environment can be divided into three main groups based on
where they reside in the ocean: the top layer is the barotolerant range (microbes that grow at normal pressure),
the middle layer is the moderate barophilic range (microbes that tolerate some pressure), and the very bottom
layer is the extreme barophilic range (microbes that thrive under very high pressure).
As with other ecosystems, the major source of organic matter in marine systems is photosynthetic activity,
primarily from phytoplankton (composed of algae and Cyanobacteria) in illuminated surface waters. There are
also a large number of viruses in surface waters, mostly which infect the Cyanobacteria.
Most nutrient cycling in the oceans occurs in the top 300 m where the light penetrates. Light allows marine
phytoplankton to grow and fall as a “marine snow” to the seabed, although this can take a month or longer to
happen. In fact, only 1% of photosynthetically derived material reaches the deep-sea floor unaltered.
In at least one way, oceans bear a resemblance to tropical rainforests with their enormous life-supporting
canopies. The upper layer of water is an active zone of primary food production. Like the rainforest canopy,
the upper layer of water is where light from the sun can penetrate and photosynthesis occurs. Just as on land,
this first step in transferring energy from the sun into the ocean’s web of life is a critical step for the majority of
the familiar, larger life forms that populate the upper reaches of the ocean.
There is a major difference, however, in who is responsible for primary food production. In the rainforest, as in
all other land-based ecosystems, it’s plants. In the ocean, primary food production is the exclusive province of
microbes. Bacteria and algae – together known as phytoplankton – are responsible for energy capture by
photosynthesis just like land-based plants. They live on or near the ocean surface, floating in the relative
warmth where they have ready access to the sunlight they need.
At the ocean surface, complex communities of microscopic and macroscopic life interact, transferring and
recycling materials necessary for growth and reproduction in the watery environment. It is a certainty that the
same sorts of partnerships between microbes and larger life forms that exist on land are present and operating to
keep life rolling along in the ocean. Microbes play as important a role in the cycling of materials among the
ocean’s residents as they do on our more familiar earth.
1
The Role of Microbes in the Ocean
Although water may seem like just water to us, in reality the aquatic terrain varies geophysically every bit as
much as the more familiar expanses of land do. The living arrangements within these different environments in
the ocean are as distinctive and different as those found on land. Studying these ecosystems has offered some
major surprises in terms of who is feeding whom!
a. Coral Reefs – Coral reefs are underwater islands of diverse species all depending on each other for
their existence. The entire reef is microbially powered. Even the physical characteristics of the reef
are the direct consequence of a microbe-animal partnership.
i. The coral itself is formed by a living animal with a hard, calcified exterior sometimes called
an exoskeleton. The coral depends on a partnership with a specialized kind of algae living in
its tissues. Neither can survive without the other. The growth of the coral depends on food
produced by the photosynthetic algae. The growth of the algae depends on a waste product,
ammonia, excreted by the coral. Even the formation of the coral’s hard skeleton is dependent
on the removal of carbon dioxide from the water by the algae.
ii. Clams, anemones, sea fans, and sponges all contain photosynthetic algae and cyanobacteria
in their tissues. These mutualistic relationships are so important that the health of the entire
reef community is threatened when their exchanges of carbon, nitrogen, phosphorus, and
hydrogen are interrupted.
iii. Many of our world’s coral reefs are now dying as a result of rising tropical sea temperatures.
This is called “coral bleaching”, and it occurs like this: coral gets its color from the algae
that live within it. When the coral gets stressed, like when temperatures rise, it spits out its
algal partner. The coral now turns white, and can no longer grow or reproduce, and will
eventually die. When the stress is removed, the algae return, and the coral begins to thrive
again. To date, almost 10% of the world’s coral has died, and if current trends continue,
scientists estimate that an additional 10-20% will be lost.
b. Ocean Microbes and Climate – scientists have learned that microbes play a direct role in the
warming and cooling of the earth. When the sun hits the ocean surface, it causes the algae to
“bloom”, or increase in number. The presence of more algae increases the amount of a chemical
called “DMS, or dimethyl sulfide”, released into the atmosphere. This in turn causes clouds to
reflect heat away from the ocean surface, cooling down surface waters and the earth’s surface. This
then slows the rate of growth of algae, which slows the rate of DMS release, which decreases the
amount of cloud cover, and causes the ocean’s temperatures to rise again!
c. Deep Sea Trenches – two miles below the ocean’s surface lies another whole world. At this level in
the ocean, there is no light, the pressure is enormous, and temperatures are near freezing. However,
there is a complete community of life forms, including microbes! At this level in the oceans, there
exist hydrothermal vents. Through these vents shoot water that has been heated by the earth’s core
to temperatures of 350 – 400 degrees Celsius. Minerals such as iron, copper, manganese, and zinc
sulfides are found in this hot water. It is bacteria that utilize these minerals that create energy for
many other life forms found at the bottom of the ocean. Some of these bacteria “breathe” sulfur, and
capture the energy from its chemical bonds, couple it with carbon dioxide dissolved in the water, and
create food that supports all other living creatures.
i. Giant Tube Worms – these are giant strange-looking worms that live at the bottom of the
ocean, and they have a symbiotic relationship with bacteria. Bacteria live inside these
worms, and produces the food the worm needs to live. In return, the worm protects the
bacteria, providing it with hydrogen sulfide, oxygen, and carbon dioxide it needs to live.
2
d. Phytoplankton – phytoplankton is a term for the plant-like plankton, and it includes single-celled
algae and cyanobacteria. It is known that green plants liberate oxygen and produce carbohydrates, a
basic link in the food chain of plants to animals to people. Collectively, this chemical process is
referred to as photosynthesis (photo = light, synthesis = to make). In these tiny food factories, there
is a chemical compound called chlorophyll that, in combination with sunlight, converts carbon
dioxide, water, and minerals into edible carbohydrates, proteins, and fats. Thus, these phytoplankton
are the basis for the oceanic food chain. Animals cannot perform this biological food-making
process. Two-thirds of all the photosynthesis that takes place on this earth occurs in the oceans that
yearly create 80 to 160 billion tons of carbohydrates. So numerous are these tiny plant forms that
they often turn the water green, brown, or reddish.
e. Bioluminescence – some ocean organisms have the ability to produce their own light, either to
attract prey or ward off predators. One example of this is the Anglerfish. This species dwells far
down in the ocean where the suns’ light can not penetrate. A normal esca (a fleshy growth found
hanging from its head) is therefore invisible. The deep living Angler fish species have solved this by
entering a symbiotic relationship with a certain type of bacteria than produce light. The bacteria
colonize the esca and make it glow in the dark, an example of so called bioluminescence.
Freshwater Ecology & Microbes
Microbes are natural and vital members of all aquatic communities, and are the foundation of lake and stream
ecology—without them the natural water worlds would not be possible. Microbes include bacteria, bacteria-like
organisms called archaea, viruses, protozoa, helminths, and protists. Certain microbes, however, when present
in excessive numbers, pose a threat to human health.
Like all ecosystems, fresh-water ecosystems require energy inputs to sustain the organisms within. In lakes and
streams, plants and also certain microbes conduct photosynthesis to harvest the Sun's energy. Microbial
photosynthesizers include protists (known as algae) and Cyanobacteria. Other protists and animals feed on these
organisms, forming the next link in the food chain. Plant material from the land also enters lakes and streams at
their edges, providing an important nutrient source for many bodies of water.
Decomposers form an especially important part of fresh-water ecosystems because they consume dead bodies of
plants, animals, and other microbes. These microbial agents of decay are an important part of the ecosystem
because they convert detritus (dead and decaying matter) and organic materials into needed nutrients, such as
nitrate, phosphate, and sulfate. Decomposers and other microbes are thus essential to the major biogeochemical
cycles by which nutrients are exchanged between the various parts of the ecosystem, both living and nonliving.
Without microbial decomposers, minerals and nutrients critical to plant and animal growth would not be made
available to support other levels of the fresh-water food chain.
Aerobes and Anaerobes
Aerobic decomposers in water need oxygen to survive and do their work. The lapping waves and babbling
brook help increase the level of dissolved oxygen that is crucial to so many creatures in lake and stream
ecosystems, none more so than the bacteria. If there is not enough oxygen in the water, many parts of the
system suffer: the aerobic decomposers cannot digest plant matter, insects cannot develop and mature, and the
fish cannot play their part, whether browsing for small food particles or eating other fish. Eventually, the stream
or pond will be changed, starting at the microbial level.
3
Human interaction can jeopardize parts of this system in a variety if ways. One principal way is through the
runoff of fertilizers or sewage into a body of water. Both contain nutrients that plants, algae, and Cyanobacteria
can use to grow; and excessive nutrient amounts can lead to very rapid growth. Interconnected sequences of
physical, biological, and chemical events may eventually deplete the water's dissolved oxygen supply, leading
to changes in the aquatic ecosystem. If the conditions become severe enough, only a few species (known as
anaerobes) tolerant of low-oxygen conditions will survive. This process, called cultural eutrophication, can have
profound and lasting consequences on the body of water
Eutrophication
Natural eutrophication is usually a fairly slow and gradual process, occurring over a period of many centuries.
It occurs naturally when for some reason, production and consumption within the lake do not cancel each other
out and the lake slowly becomes overfertilized. While not rare in nature, it does not happen frequently or
quickly. However, artificial or human-caused eutrophication has become so common that the word
eutrophication by itself has come to mean a very harmful increase and acceleration of nutrients. It is as if
something receives too much fertilizer or has too much of what is a good thing.
Human activities almost always result in the creation of waste, and many of these waste products often contain
nitrates and phosphates. Nitrates are a compound of nitrogen, and most are produced by bacteria. Phosphates
are phosphorous compounds. Both nitrates and phosphates are absorbed by plants and are needed for growth.
However, the human use of detergents and chemical fertilizers has greatly increased the amount of nitrates and
phosphates that are washed into our lakes and ponds. When this occurs in a sufficient quantity, they act like
fertilizer for plants and algae and speed up their rate of growth.
Algae are a group of plantlike organisms that live in water and can make their own food through photosynthesis
(using sunlight to make food from simple chemicals). When additional phosphates are added to a body of water,
the plants begin to grow explosively and algae takes off or "blooms." In the process, the plants and algae
consume greater amounts of oxygen in the water, robbing fish and other species of necessary oxygen.
All algae eventually die, and when they do, oxygen is required by bacteria in order for them to decompose or
break down the dead algae. A cycle then begins in which more bacteria decompose more dead algae, consuming
even more oxygen in the process. The bacteria then release more phosphates back into the water, which feed
more algae. As levels of oxygen in the body of water become lower, species such as fish and mollusks literally
suffocate to death.
Eventually, the lake or pond begins to fill in and starts to be choked with plant growth. As the plants die and
turn to sediment that sinks, the lake bottom starts to rise. The waters grow shallower and finally the body of
water is filled completely and disappears. This also can happen to wetlands, which are already shallow.
Eventually, there are shrubs growing where a body of water used to be.
In the 1960s and 1970s, Lake Erie was the most publicized example of eutrophication. Called a "dead lake," the
smallest and shallowest of the five Great Lakes was swamped for decades with nutrients from heavily
developed agricultural and urban lands. As a result, plant and algae growth choked out most other species living
in the lake, and left the beaches unusable due to the smell of decaying algae that washed up on the shores. New
pollution controls for sewage treatment plants and agricultural methods by the United States and Canada led to
drastic reductions in the amount of nutrients entering the lake. Forty years later, while still not totally free of
pollutants and nutrients, Lake Erie is again a biologically thriving lake.
4