Encyclopedia of Environmental Science and Engineering, Volume I and II

MICROBIOLOGY 687

The prokaryotic blue-green algae, Cyanophyta, are

usually divided into 5 groups:

Chooccocales are unicellular. They sometimes occur in

irregular packets or colonies. Cells multiply by binary fi ssion.

Chamaesiphonales are unicellular, fi lamentous, or colo-

nial epiphytes or lithophytes. Cells show distinct polarity

from apex to base. The base usually has a holdfast which

permits attachment to the substrate. Cells multiply by inter-

nal septation or by formation of spherical cells ( gonidia ) at

the ends of fi laments.

Pleurocapsales are fi lamentous with differentiation into

aerial and nonaerial elements. Cells multiply by crosswall

formation or by internal septation.

Nostocales are fi lamentous without differentiation into

aerial and nonaerial elements. They are unbranched or falsely

branched and frequently have pale, empty-looking cells called

heterocysts and resting spores ( akinetes ). Reproduction is by

liberation of a short fi lament only a few cells long, called a

hormogonium, which then elongates.

a) Nostacaceae are unbranched and produce hetero-

cysts. They frequently produce akinetes.

b) Rivulariaceae are unbranched or falsely branched.

Filaments taper from base to tip. Heterocysts are

usually present at the base. There is some akinete

formation.

c) Scytonemataceae are false branched. Heterocysts

are frequently found at branch points.

d) Stigonematalis are filamentous with aerial and

nonaerial differentiation. Hormogonia and hetero-

cysts are present. They often show true branching

and have pit connections between cells. Akinetes

are rare.

Eukaryotes

Eukaryotic microorganisms include all the algae (except

the Cyanophyta ), all the protozoa, and most fungi. All are

microscopic in size.

The eukaryotic algae are separated into nine divi-

sions based upon their pigment and carbohydrate reserves

(Table 3). They are all photosynthetic and, like higher plants,

evolve oxygen during photosynthesis. Many algae are obli-

gate phototrophs. That is, they are completely dependent

upon photosynthesis: they can not use exogenously supplied

organic compounds for growth in either the dark or light.

Some algae are facultative phototrophs; they are able to uti-

lize organic compounds for growth in the dark but fi x carbon

dioxide photosynthetically in the light.

Occasionally algae, especially unicellular forms, per-

manently lose their chloroplasts by exposure to any one

of several adverse conditions, e.g. heat or chemicals. If the

organism had been a facultative phototroph, before the loss of

the chloroplasts, it has the enzymatic machinery necessary to

survive except that now, in its chloroplastless state, it is indis-

tinguishable from certain other unicellular organisms more

commonly called protozoa. The ease with which an organ-

ism at this primitive level of evolution may be interchanged

between groups containing a preponderance of plant-like or

animal-like attributes underlines the need for the term protist

rather than plant or animal to describe them. Indeed both

botanists and zoologists claim the protists. Some algae e.g.

Euglena spp., normally only form chloroplasts when they

grow in the light while others e.g. Chlorella spp. form chlo-

roplasts regardless of the presence of absence of light. There

is great diversity in size, shape, presence or absence of life

cycles, type of multiplication, motility, cell wall chemistry,

and chloroplast structure. Although these parameters are of

great assistance in defi ning affi nities among algae, there are

still groups whose proper place is debated.

Many algae are important as sources of food, chemi-

cal intermediates of industrial and medical importance, and

research tools. Others are nuisances which clog waterways

or poison other aquatic life with their potent toxins.

Eukaryotic Algal Groups

The eight groups are:

1) Chlorophyta (green algae) are either marine or fresh-

water forms. This large and diverse group includes

forms which are either unicellular, colonial, filamen-

tous, tetrasporal (cells separated but held together

in groups of four in a mucilaginous material), coe-

nobial (cells more or less attached to each other in

an aggregate), or siphonaceous (simple, nonseptate

filaments). They frequently have life cycles which

RELATIVE SIZE OF BACTERIA

Clostridium 1x3.10m

Salmonella 0.6x2.3m

Hemophilus 0.3x0.6–1.5m

Pseudomonas 0.5x1.3m

Fusibacterium 0.75–1.5x8.80m Neisseria 0.6x0.8m

Streptococcus 0.5–0.75m

Staphylococcus 0.8–1m

Erythocyte 7m diameter

FIGURE 2 Relative sizes of bacteria.

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