4.1 Taxonomy of Microorganisms in Aquatic Environments 61
tral pH, decaying plant material, and dung of ani
mals. In freshwater environments, they are found
on riverbanks and lake shores, in estuaries, bottom
sediments, and algal mats. They are also common
in sewage treatment plants, especially at the latter
stages where only recalcitrant molecules remain.
Cytophaga tend to degrade polymers such as cel
lulose and have been shown to be the major cellu
lose degraders in some lakes. A few species have
been isolated from the oral cavity of humans where
they appear to be part of the normal flora, but can
occasionally cause septicemias. Some Cytophaga
strains are pathogens of fish.- Planctomyces/Pirella
Planctomyces, Pirella, Gemmata, and Isosphaera
form a phylogenetically related group of microor
ganisms that have many unusual properties. They
are the only bacteria, other than the confusing case
of the Chlamydia, whose cells lack peptidoglycan.
Cells of this group divide by budding. Some mem
bers of the group produce long appendages, called
stalks, and new cells are motile, developing stalks
as they mature. Some members of this group have
structures resembling nuclear membranes (Bauld
and Staley 1976 ): others have fimbrin.
Cells of this group can be pigmented (light rose,
bright red, or yellow to ochre) or nonpigmented.
An example of the species is Planctomyces limno
philus, which is ovoid, has a diameter of 1.5 mm,
and forms red pigmented colonies. It grows slowly
at temperatures between 17°C and 39°C and takes
at least a week to form colonies. Stalks of the
organism are very thin and cannot be seen by light
microscopy. These stalks appear to be made of
thin fibers twisted into a bundle that emanates
from one pole of the ovoid cell. Cells multiply by
budding and new cells are motile and stalkless,
but eventually grow stalks as part of a maturation
process similar to that seen for Caulobacter.
These microbes are common inhabitants of
freshwater lakes, marine habitats, and salt ponds,
but most have been difficult to isolate in pure
culture. For example, three of the four species in
Planctomyces have only been observed in lake
water and never isolated. - Verrucomicrobia
Verrucomicrobia, with the best example as
Verrucomicrobia spinosum, has been isolated from
freshwater, soil environments, and human feces. It
produces cytoplasmic appendages called prosth
eca. Prostheca are like warts and the name of the
group comes from the Greek word for warts. Both
mother and daughter contain prostheca at the time
of the cell division.- Chlamydia
Chlamydia are obligate intracellular pathogens
with poor metabolic capabilities. They cannot syn
thesize biomolecules such as amino acids which
they obtain from their hosts. Many Chlamydiae
coexist in an asymptomatic state within specific
hosts, and it is widely believed that these hosts pro
vide a natural reservoir for these species.
Chlamydiae exist in two states: a metabolically
inert elementary body (EB) and a metabolically
active reticulate body (RB) found only inside cells.
EB is similar to the virions of viruses and enters the
body by phagocytosis. Once ingested and inside the
cell, EB divides and becomes RB. After it has killed
the cell, it becomes EB again and is ready to be
transmitted. Chlamydiae are spread by aerosol or
by contact and require no alternate vector.
Diseases caused by Chlymidia include sexually
transmitted infections (STIs) (Chlamydia tracho
matosis), pneumonia (Chlamydia pneumoniae),
and bird pneumonia (Chlamydia psittaci). - Cyanobacteria
Cyanobacteria (Greek: kuanόs (kyanós) = blue +
bacterium) obtain their energy through photosyn
thesis. They are often referred to as bluegreen
algae, because they were once thought to be algae.
They are a significant component of the marine
nitrogen cycle system and an important primary
producer in many areas of the ocean. Their ability
to perform oxygenic (plantlike) photosynthesis is
thought to have converted the reducing atmosphere
of the early earth into an oxidizing one, which dra
matically changed the life forms on Earth and
provoked an explosion of biodiversity.
Cyanobacteria are found in almost every
conceivable habitat, from oceans to freshwater to
bare rock to soil. Most are found in freshwater, while
others are marine, occur in damp soil, or even tempo
rarily moistened rocks in deserts. A few are endo
symbionts in lichens, plants, various protists, or
sponges and provide energy for the host. Some live
in the fur of sloths, providing a form of camouflage.
Cyanobacteria include unicellular and colonial
species. Colonies may form filaments, sheets, or