Microbes and Plants - Freshwater Ecology: Concepts and Environmental Applications - page 131

Coccus (sphere)

Bacillus ( rod)

Spirilla (spiral shaped)

Streptobacillus Streptococcus

Filamentous

Sarcina

Vibrio (curved rod)

Staphylococci

Spirochete
FIGURE 8.3

Possible bacterial morphologies.

(Ward et al., 1990). In a study of Octopus Spring in Yellowstone National
Park, rRNA sequences were obtained from natural samples. Few matched
known sequences even though hot springs likely contain simple microbial
communities, and numerous microbes have been cultured from the study
site. More benign but variable habitats likely will have a much higher
diversity.

Cyanobacteria (Blue-Green Algae or Cyanophytes)
Aquatic ecologists are very concerned with the cyanobacteria because
of their tremendous impact on water quality; they form blooms or ex-
tremely high cell densities in eutrophic waters. Many researchers have
adopted the more modern terminology “cyanobacteria” rather than “blue-
green algae” to clearly delineate their bacterial origins. The taxonomy of
these organisms has been studied more completely than that of other bac-
teria because most are large and morphologically distinct under the light
microscope.
Most cyanobacteria are O 2 -producing photosynthetic bacteria. Fossils
similar to extant cyanobacteria are the oldest known records of life (Schopf,
1993). Cyanobacteria have been successful for billions of years and are
currently able to exploit some of the most extreme habitats on Earth, in-
cluding very cold, very hot, and extremely saline environments.
Cyanobacteria are found in most habitats and can range from 1 m

in diameter to several 100 m (Fig. 8.4). Shape ranges from simple spheres

to complex branching structures. Specialized cells include akenites (resting
cells) and heterocysts (the site of most nitrogen fixation). Nitrogen fixation
is the acquisition of gaseous N 2 into cellular nitrogen and will be discussed
more thoroughly in the context of nitrogen cycling (see Chapter 13).
Proteinaceous vacuoles called gas vesicles lend buoyancy to the cyanobac-
teria and lead to formation of surface scums under calm conditions. These
surface scums can be up to 1 m thick and have very objectionable odors and