Prescott’s Microbiology, 9th Edition
30
Microorganisms in Marine and Freshwater
Ecosystems
CHAPTER OVERVIEW
This chapter discusses the general characteristics of marine and freshwater environments and describes some
of the microorganisms that live there. Life in aquatic environments is most impacted by the levels of light,
oxygen, and nutrients present. These are determined by physical properties and by the activities of
microorganisms. The chapter discusses life in estuaries and salt marshes, the photic zone of the open ocean,
and the benthic zone at the seafloor. Freshwater environments discussed include ices, rivers, and lakes.
LEARNING OUTCOMES
After reading this chapter you should be able to:
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explain the carbonate equilibrium system and why it is important in the study of marine microbial biology
explain the ecological and economic importance of estuaries and harmful algal blooms
diagram carbon and sulfur flux in a Winogradsky column
assess the importance of marine microbial primary productivity in the global carbon budget
evaluate the interplay between carbon, nitrogen, and iron in the open ocean and how the relative
concentrations of these elements influence the productivity of these waters
appreciate the scarcity of freshwater on Earth
contrast and compare the microbial communities found in lotic systems with those in lentic systems
describe the differences between the microbial communities found in a deep, stratified lake with those of
a shallow lake that lacks stratification
describe how seasonal changes influence lake microbial communities and how these changes can lead to
blooms
CHAPTER OUTLINE
I.
Water as a Microbial Habitat
A. Gases in aquatic environments
1. Oxygen
a. Aquatic environments are low-oxygen diffusion environments; this can lead to the
formation of hypoxic or anoxic zones, which are inhabited by anaerobic microbes
b. At higher temperatures and with lower pressure the solubility of oxygen in water is
further decreased
2. Carbon dioxide
a. Important in many chemical and biological processes
b. The carbonate equilibrium system buffers the pH of water, with seawater highly buffered
by the interplay of carbon dioxide, bicarbonate, and carbonate
c. This equilibrium is impacted by the activity of aquatic microorganisms
d. Oceans serve as a major carbon sink
3. Other gases such as nitrogen, hydrogen, and methane are important; they vary in terms of their
solubility in water, with methane being the least soluble
B. Light is critical for primary production by autotrophs in marine and freshwater ecosystems; the zone
where light penetrates is the photoic zone; solar radiation can warm surface waters and create a
thermocline where warm waters float on cooler waters
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Prescott’s Microbiology, 9th Edition
C.
II.
Nutrient cycling in marine and freshwater environments
1. Major source of organic matter in illuminated water is phytoplankton (including tiny
picoplankton), consisting of photoautotrophic organisms that acquire needed nitrogen and
phosphorous from surrounding water; microplankton include diatoms and dinoflagellates
2. Redfield ratio—ratio of carbon-nitrogen-phosphorus (C:N:P) in phytoplankton; is important
for following nutrient dynamics and for studying factors that limit microbial growth
3. The microbial loop recycles much of the organic matter produced by phytoplankton
(photosynthate in the form of dissolved organic matter); the chemoheterotrophic bacteria that
function in the microbial loop (thought of as particulate organic matter) are consumed by a
series of larger predators (protists)
Microorganisms in Marine Ecosystems
A. Marine environments represent the major portion of biosphere; contain 96% of the Earth’s water;
vital to global biogeochemical cycles
B. Microorganisms in coastal ecosystems
1. In estuaries, tidal mixing of freshwater and saltwater creates a salinity profile characterized by
salt wedges, where heavier saltwater forms a layer below freshwater
2. The salinity varies in the estuary in time and space; microbial inhabitants need to be
halotolerant (withstands salinity changes)
3. These nutrient-rich waters are often polluted and this can create dead zones or greatly increase
microbial growth; harmful algal blooms (HABs), including red tides caused by dinoflagellates,
occur when algae grow to high numbers and then release toxic compounds
C. Winogradsky columns can be used to model salt marshes and illustrate the interactions and
gradients that occur in aquatic environments
1. Made by mixing together mud, water, and sources of nutrients (e.g., cellulose and other
materials), then incubating the column in light
2. In the bottom, anaerobic conditions foster the activity of fermentative microorganisms, which
leads to the accumulation of fermentation products
3. Other anaerobic microorganisms use the fermentation products to carry out anaerobic
respiration using sulfate as the electron acceptor and producing sulfide
4. Sulfide diffuses upward, creating an anaerobic, sulfide-rich zone where anoxygenic
photosynthetic bacteria reside
5. Further up in the column, chemolithotrophic and mixotrophic organisms may use hydrogen
sulfide as an energy source and oxygen as the electron acceptor
6. At the top are oxygenic photosynthetic organisms such as diatoms and cyanobacteria
D. Microorganisms in the open ocean
1. The open ocean regions (pelagic) account for half of the world's photosynthesis; it is an
oligotrophic environment (very low nutrient levels) that supports a diverse microbial
community
2. Organic matter found in the photic zone (upper 200–300 meters) where light penetrates falls as
marine snow into lower depths; much of the marine snow is not easily degraded; it is colonized
by microbes that consume it, such that only about 1% reaches the seafloor; the collection of
organic matter on the seafloor is important in moderating the effects of global warming; the
addition of iron may increase draw down of carbon dioxide in high-nutrient low-chlorophyll
(HNLC) ocean areas
3. Cyanobacteria (including Trichodesmium) are important in fixing nitrogen in the pelagic zone;
some bacteria can perform the anammox reaction wherein ammonia is oxidized to nitrogen gas
anaerobically below the photic zone, and lost from the seas
4. The most abundant monophyletic group of organisms is an -proteobacterial lineage called
SAR11; these small bacteria have recently been isolated and have small, efficient genomes;
they are responsible for about 50% of bacterial biomass production and DOM flux in some
marine environments
5. Proteorhodopsin may supplement ATP production for a variety of microorganisms in nutrientpoor waters; aerobic anoxygenic phototrophs do not fix carbon, but use light energy to
generate ATP; lithoheterotrophs also do not fix carbon, but use inorganic chemicals as a
source of energy
E. Aquatic viruses
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in any manner. This document may not be copied, scanned, duplicated, forwarded, distributed, or posted on a website, in whole or part.
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Prescott’s Microbiology, 9th Edition
1.
Virioplankton are the most numerous members of marine ecosystems; counting viruslike
particles (VLPs) can be challenging
2. Virioplankton are major agents of mortality in the sea, influence microbial loop, are involved
in horizontal gene transfer, and influence microbial community diversity
F. Microorganisms in benthic marine environments
1. Benthos—oceanic sediments and the seafloor including hydrothermal vents; mainly high
pressure, low light, and cold (1 to 4 °C) environments
2. Microbes found not only on seafloor but to a depth of at least 0.6 km where pressures are high
and the organisms must be piezophilic (barophilic)
3. Hydrocarbon metabolism fuels microbial communities on continental margins; massive
deposits of methane hydrates are found at the ocean floor below 500 meters
III. Microorganisms in Freshwater Ecosystems
A. Microorganisms in glaciers and permanently frozen lakes
1. These ancient deposits hold microorganisms that may provide information about the
biogeographic distribution of microorganisms and life on icy, extraterrestrial worlds (e.g.,
Europa)
2. Metabolically active environments that support a variety of microbes; frozen lakes may have
thick ice caps that block solar radiation, leading to chemosynthetic communities
B. Microorganisms in streams and rivers
1. Differ from lakes in that horizontal movement minimizes vertical stratification and most of the
functional biomass is attached to surfaces (not planktonic); can be lotic (free running) or lentic
(free standing)
2. Support benthic communities including biofilms of diverse microorganisms including
photosynthetic primary producers; phytoplankton also are supported by dissolved and
particulate organic carbon
3. Nutrients available in streams and rivers can be from in-stream production (autochthonous) or
from out-stream sources (allochthonous) such as leaves, and runoff from riparian areas; under
most conditions, such added organic material does not exceed the oxidative capacity of the
stream and it remains productive and aesthetically pleasing
4. Ability to process organic matter is limited
a. If the amount of organic material is excessive (eutrophic), oxygen is used faster than it
can be replenished, which causes the water to become anaerobic
1) Point sources of pollution include inadequately treated municipal wastes and other
materials from specific locations
2) Nonpoint sources of pollution include field and feedlot runoffs
b. If the amount of organic material is not excessive, algae will grow, which leads to oxygen
production in the daytime and respiration at night (diurnal oxygen shifts)
C. Microorganisms in lakes
1. Lakes vary in nutrient status
a. Oligotrophic lakes are nutrient-poor; typically oxic throughout
b. Eutrophic lakes are nutrient-rich; typically anoxic at the bottom
2. Deep lakes have a littoral zone where light penetrates and a lower pelagic zone, each with
distinct nutrient cycles
3. Lakes can be thermally stratified; stratified waters undergo seasonal turnovers because of
temperature and specific gravity changes
a. Epilimnion—warm, aerobic, upper layer
b. Thermocline—region of rapid temperature decrease that spans the metalimnion; acts as a
barrier to mixing
c. Hypolimnion—cold, often anaerobic (particularly in nutrient-rich lakes) lower layer
d. Oligotrophic lakes remain oxic and do not exhibit oxygen stratification; eutrophic lakes
usually have bottoms rich in organic matter; cyanobacterial and heterotrophic bacterial
blooms can degrade eutrophic waters
CRITICAL THINKING
3
© 2014 by McGraw-Hill Education. This is proprietary material solely for authorized instructor use. Not authorized for sale or distribution
in any manner. This document may not be copied, scanned, duplicated, forwarded, distributed, or posted on a website, in whole or part.
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Prescott’s Microbiology, 9th Edition
1.
Ocean microbiota are critical for global nutrient cycling. Given that the oceans are closely connected to
the atmosphere, how might global climate change be expected to affect microbial communities in the
oceans? How might ocean chemistry be affected? Will the influx of nutrients cause changes to ocean
communities, and how might this affect global nutrient cycles?
2.
There are enormous accumulations of methane hydrates on the ocean floor. What kind of microbial
communities do you think would form around these deposits? What types of metabolisms would you
expect to find? What do you think would be the consequences of harvesting these deposits for our energy
needs?
3.
Phosphorus is typically a limiting factor in eutrophic aquatic systems. How does the reformulation of
detergents from phosphate anion to sulfate anion effect the impact of runoff into lakes and streams?
4.
How does a Winogradsky column act as a model system for an aquatic ecosystem and in what ways does
it enrich for particular bacterial types.
CONCEPT MAPPING CHALLENGE
Construct a concept map that features aspects of marine microbes that are specific to growth in the open ocean.
You will need to provide your own linking terms.
Oligotrophic adaptations Picoplankton Bacteria Archaea
Proteorhodopsin
Protists
AAnP Lithoheterotrophy Viruses
Microbial loop
N 2 fixation
Stratification
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© 2014 by McGraw-Hill Education. This is proprietary material solely for authorized instructor use. Not authorized for sale or distribution
in any manner. This document may not be copied, scanned, duplicated, forwarded, distributed, or posted on a website, in whole or part.
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