Chapter 6
Aquatic Environments
Todd R. Sandrin, Scot E. Dowd, David C. Herman, and Raina M. Maier
1.
Define the four types of microbial habitats found in aquatic environments.
The planktonic environment refers to microbial communities suspended in the water
column.
The benthic environment refers to the habitat that is the transition zone between the water
column and the mineral surface
A microbial mat is a compressed aerobic-anaerobic habitat at the mineral-planktonic
interface in aquatic environments. Photosynthetic microbes predominate at the
interface while anaerobic heterotrophs predominate in the interior of the mat.
A biofilm is a surface association of microorganisms that are strongly attached via
production of extracellular polymers. A biofilm community efficiently harvests
nutrients from water flowing over it. Biofilm communities are resistant to
stresses such as antibiotics and disinfectants.
2.
What is meant by the term microbial loop?
The microbial loop refers to secondary production by bacteria in the aquatic environment.
During secondary production the bacteria utilize dissolved organic matter released from
primary producers (phytoplankton).
3.
Describe the process of biofilm formation.
Biofilm production is initiated by the reversible attachment of bacteria to a solid surface
that has been conditioned by organic compounds flowing over it. Reversible attachment
is governed by transitory physicochemical attractions such as hydrophobic, electrostatic,
and van der Waals forces. This is followed by irreversible attachment which is initiated
by the excretion of extracellular polymers by the bacteria. These polymer create a matrix
that surrounds the cell and forms a strong chemical bridge to the solid surface.
4.
A recently trained environmental engineer is hired to investigate solutions to
clogging of water distribution lines by a persistent, thick and gelatinous
material. The astute engineer quickly recognizes that this recurring problem
may be caused by microorganisms and rushes to isolate and characterize the
microorganisms clogging the pipelines. The engineer is successful in
culturing several microorganisms in broth cultures (i.e., flasks containing
liquid microbiological media) from the material found in the pipelines. In
these broth cultures, the engineer determines the amount of an antimicrobial
compound necessary to kill these microorganisms. To be certain an adequate
amount of this antimicrobial compound is delivered, the engineer adds twice
as much as the broth culture-based tests suggested would be necessary.
Much to the engineer’s surprise and dissatisfaction, the treatment is
ineffective in killing the microorganisms found within the pipelines. Why did
this treatment fail? What additional measures might the engineer need to
take to solve the company’s problem with the clogged pipes? Can you devise
any novel strategies based on material presented in this chapter?
The treatment likely failed because too little antimicrobial compound was added. The
microorganisms clogging the lines are growing in biofilms, which are much more
resistant to antimicrobial compounds than the same microorganisms growing
planktonically (in broth culture). Thus, the engineer’s tests underestimated the amount of
antimicrobial compound necessary to kill the problem microorganisms. To solve the
company’s problem, the engineer would likely need to increase the amount of
antimicrobial compound. This, too, may fail, though as biofilms have been shown to be
capable of tolerating incredibly high concentrations of antimicrobial compounds.
Physical removal of the biofilms may be necessary. Several novel strategies might be
devised to deal with this problem. An effective antimicrobial compound might be
incorporated into new pipes to limit microbial growth. Alternatively, a strategy that
interferes with quorum sensing in the biofilm community might prove useful.
5.
Describe the types and numbers of microbes that predominate in the
different freshwater environments described (e.g., springs, rivers, lakes).
Springs: Photosynthetic bacteria and algal dominate spring environments with
communities ranging from 102 to 108 organisms/ml. These primary producers are present
in highest concentrations along the shallower edges of the spring and in association with
rock surfaces, where light is available and inorganic nutrients are in highest
concentrations. Although heterotrophs are also present, numbers are usually low (101 to
106 organisms/ml) because dissolved organic matter is low.
Rivers: Near their source, streams contain mostly primary producer communities,
especially when light can penetrate to the bottom of the stream. Photosynthetic
populations range from 100 to 108 organisms/ml and tend to be present as attached
communities associated with biofilms because of the flowing nature of the water column.
Phytoplankton also exist in streams, but because of the constant water movement, they do
not form spatially stable populations. As the stream develops (progresses away from the
source) and becomes larger, it tends to accumulate dissolved organic matter from surface
runoff and sediments. The increase in dissolved organic matter limits the penetration of
light and consequently begins to limit photoautotrophic populations. In turn,
heterotrophic populations begin to increase in response to increased dissolved organic
matter. In general, the concentration of heterotrophs in streams and rivers ranges from
104 to 109 organisms/ml, with microbial numbers increasing as dissolved organic matter
increases. Because of their flow patterns, stream and river waters are for the most part
well aerated. Therefore, heterotrophic populations are predominantly aerobic or
facultatively aerobic.
Lakes: Lakes contain extensive primary and secondary producer populations that interact
dynamically. The littoral zone (if there is one) has high primary activity where the
planktonic community is composed of algae (major) and cyanobacteria (minor). Attached
communities are dominated by the filamentous and epiphytic algae. The limnetic habitat
is also dominated by phytoplankton, which form distinct gradients based upon the
wavelength and the amount of light that penetrates to a given depth. Another type of
gradient is the oxygen gradient where aerobic primary producers are found nearer to the
surface and anaerobic primary produces which do not need oxygen and can utilize longer
wavelengths of light dominate at lower depths and even on sediment surfaces if light
penetrates that far. Lakes also have extensive heterotrophic communities. The amount of
secondary production is directly related to primary production. In general, secondary
production in the photic zone is 20 to 30% of primary production. Heterotrophic
concentrations vary with depth, but there are three areas that generally have elevated
numbers of heterotrophs. The first is the neuston layer, the second is the thermocline, and
the third is the upper layer of the sediment or benthos.
6.
What is a thermocline?
The thermocline is the region in an aquatic environment that is characterized by a rapid
change in temperature. This induces a density stratification (which is the
thermocline) because water is most dense at 4°C.
7.
Describe how marine environments differ from freshwater environments
physically, chemically, and microbially.
Chemically: Marine environments are characterized by salinity of approximately 3.5‰
while freshwater environments have an average of 0.5‰.
Physically: Oceans are large, deep (up to 11,000 m), and very active bodies of water with
considerable mixing occurring especially in surface water layers. Freshwater
environments are either fairly static (lakes) or running (rivers and streams). Lakes vary
in depth from a few to >1000 m and similarly vary considerably in size.
Microbially: Both freshwater and marine environments have a wide diversity of
microorganisms. The physical and chemical differences in these environments dictate
differences in the number and distribution of microorganisms. In general, lakes contain
extensive primary and secondary producers that interact dynamically. Primary producers
(photosynthetic microorganisms) are found at the surface to the depth of light
penetration. The amount of secondary production by heterotrophs is usually 20 to 30%
of primary production. Heterotroph concentrations vary with depth, but there are three
areas that generally have elevated numbers of heterotrophs, the neuston, the thermocline,
and the upper layer of the benthos where the populations are primarily anaerobic.
In ocean environments, especially in deep waters, microbial concentrations are highest at
the neuston layer. Total bacterial numbers are on average one order of magnitude higher
in coastal water than in the open ocean. As in lake environments, the vertical distribution
of the heterotrophs shows an increase at the thermocline. At greater depths, the numbers
of heterotrophs quickly diminish until, at a depth of 200 m, concentrations are very low.
Heterotrophs increase again immediately above the ocean floor. Thermocline-induced
stratification is less important in coastal waters because of the mixing of water by winds,
currents, and temperature. For this reason, bacterial numbers are uniform at all depths
except when the weather is very calm for long periods of time. In addition, seasonal
fluctuations occur in coastal bacterial numbers, which are not observed in the open ocean.
In general, there are two times of the year when there is an increase in bacterial
populations in coastal waters, late spring–early summer and late summer–early fall, times
when the phytoplankton are most active.