Aquatic Habitats

Oceans, covering more than 70% of the earth's surface, are the most abundant aquatic habitat, representing about 95% of the global water. They are the marine environment. Although lakes and rivers, the fresh water environments, represent only a small fraction of the total water, they are important sources of fresh water.

Deep lakes and oceans have characteristic zones that influence the distribution of microbial populations. The uppermost layer, where sufficient light penetrates, supports the growth of photosynthetic microorganisms, including algae and cyanobac-teria. The organic material synthesized by these primary producers gradually descends and is then metabolized by heterotrophs. In oligotrophic waters, meaning nutrient poor, the growth of photosynthetic organisms and other autotrophs is limited by the lack of certain inorganic nutrients, particularly phosphate, nitrate, and iron. When waters are eutrophic, or nutrient rich, photosynthetic organisms flourish, sometimes forming a visible layer on the surface (figure 30.6). In turn, the photosynthetic activities of these organisms produce organic compounds that permit luxuriant growth of heterotrophs in lower layers. The heterotrophs consume dissolved O2 as they metabolize the organic material. Because O2 consumption can outpace the slow rate of diffusion of atmospheric O2 into the waters, the environment may become very low in dissolved oxygen, or hypoxic. Lack of sufficient O2 leads to the death of resident fish and other aquatic animals.

Marine Environments

Marine environments range from the deep sea, where nutrients are scarce, to the shallower coastal regions, where nutrients may be abundant due to runoff from the land. Seawater contains about 3.5% salt, compared with about 0.05% for fresh water. Consequently, it supports the growth of halophilic organisms, which prefer or require high salt concentrations, and halotoler-ant ones. Temperatures may vary widely at the surface, depending upon locale and other factors, but decrease with depth until

Figure 30.6 Eutrophication in a Polluted River

Nester-Anderson-Roberts: I V. Applied Microbiology I 30. Microbial Ecology I © The McGraw-Hill

Microbiology, A Human Companies, 2003

Perspective, Fourth Edition

30.2 Aquatic Habitats 771

reaching about 2°C in the deeper waters; an exception is the areas around hydrothermal vents, which will be described later.

■ hydrothermal vents, p. 778

Ocean waters are typically oligotrophic, limiting the growth of microorganisms. The little organic material produced by pho-tosynthetic organisms is consumed as it descends, so that only scant amounts reach the sediments below. Even in the deep sea, marine water is O2-saturated due to mixing associated with tides, currents, and wind action.

The ecology of inshore areas is not as stable as the deep sea, and can be dramatically affected by nutrient-rich runoff. An unfortunate example is a region referred to as the "dead zone" that forms every spring in the Gulf of Mexico. The Mississippi River, carrying nutrients accumulated as it runs through agricultural, industrial, and urbanized regions, feeds into the Gulf. As a consequence of this nutrient-enrichment, populations of algae and cyanobacteria flourish in the Gulf waters in the spring and summer when sunlight is also plentiful. Consumption of the organic compounds synthesized by these primary producers causes a large region in the Gulf, sometimes in excess of 7,000 square miles, to become hypoxic and essentially devoid of marine life. Animals in the area either flee or die. Enrichment of coastal waters also contributes to blooms of toxic algae.

Freshwater Environments

Lakes and streams are among the most obvious freshwater environments. As with marine environments, particularly inshore regions, the types and relative numbers of microbes inhabiting fresh waters depend on multiple factors including light, concentration of dissolved O2 and nutrients, and temperature.

Oligotrophic lakes in temperate climates may have anaerobic layers due to thermal stratification resulting from seasonal temperature changes (figure 30.7). During the summer months, the surface water warms. This decreases the density of the water, causing it to form a distinct layer that does not mix with the cooler, denser water below. The upper layer, called the epilimnion, is generally oxygen-rich due to the activities of photosynthetic organisms. In contrast, the lower layer, the hypolimnion, may be anaerobic due the consumption of O2 by heterotrophs. Separating these two layers is the thermocline, a zone of rapid temperature change. As the weather cools, the waters mix, providing O2 to the deep water.

Rapidly moving waters, such as rivers and streams, are very different from lakes. They are usually shallow and turbulent, facilitating O2 circulation, so they are generally aerobic. Light may penetrate to their bottom, making photosynthesis possible. Sheathed bacteria such as Sphaerotilus and Leptothrix species may adhere to rocks and other solid structures, enabling the microbes to remain stationary, utilizing nutrients that flow by.

Specialized Aquatic Environments

Specialized aquatic environments include salt lakes, such as the Great Salt Lake in Utah, which have no outlets. Water in these lakes evaporates, leaving concentrations of salt much higher than in seawater. Extreme halophiles thrive in this environment. Other

Thermal Stratification (Summer)

Prevailing wind ■

Prevailing wind ■

Epilimnion

25°—22°C

Thermocline

20°—10°C ^Ê

Hypolimnion

5°-4°C

Seasonal Upwelling

(Fall and Spring)

Nutrients

(Fall and Spring)

Nutrients

Figure 30.7 Thermal Stratification of a Lake (a) During the summer months, the surface water warms, creating a less-dense layer called the epilimnion.The thermocline separates this layer from the cooler, denser, hypolimnion. (b) As the weather cools, the layers mix, providing oxygen to the deep water.

Figure 30.7 Thermal Stratification of a Lake (a) During the summer months, the surface water warms, creating a less-dense layer called the epilimnion.The thermocline separates this layer from the cooler, denser, hypolimnion. (b) As the weather cools, the layers mix, providing oxygen to the deep water.

specialized habitats include iron springs that contain large quantities of ferrous ions; these springs are habitats for species of Gallionella and Sphaerotilus. Sulfur springs support the growth of both photosynthetic and non-photosynthetic sulfur bacteria. There are many other aquatic environments, both natural and human-made, ranging from groundwater to stagnant ponds and swimming pools to drainage ditches, each offering its own opportunity for bacterial growth. ■ extreme halophiles, p. 90

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