Environmental Microbiology of Aquatic and Waste Systems

62 4 Taxonomy, Physiology, and Ecology of Aquatic Microorganisms

even hollow balls. Some filamentous colonies

show the ability to differentiate into several differ­

ent cell types: vegetative cells, the normal, photo­

synthetic cells that are formed under favorable

growing conditions; akinetes, the climate­resistant

spores that may form when environmental condi­

tions become harsh; and thick­walled heterocysts,

which contain the enzyme nitrogenase, vital for

nitrogen fixation. Heterocysts may also form under

the appropriate environmental conditions (anoxic)

wherever nitrogen is necessary. Heterocyst­

forming species are specialized for nitrogen fixa­

tion and are able to fix nitrogen gas, which cannot

be used by plants, into ammonia (NH 3 ), nitrites

(NO 2 −), or nitrates (NO 3 −), which can be absorbed

by plants and converted to protein and nucleic

acids. The rice paddies of Asia, which produce

about 75% of the world’s rice, do so because of the

high populations of nitrogen­fixing cyanobacteria

in the rice paddy fields.

Photosynthesis in cyanobacteria generally uses

water as an electron donor and produces oxygen as

a by­product, though some may also use hydrogen

sulfide as is the case among other photosynthetic

bacteria. Carbon dioxide is reduced to form carbo­

hydrates via the Calvin cycle. In most forms, the

photosynthetic machinery is embedded into folds

of the cell membrane, similar to thylakoids found

in the chloroplasts of higher plants.

The cyanobacteria are traditionally classified

by morphology into five sections, I–V: Chroo coc­

cales, Pleurocapsales, Oscillatoriales, Nostocales,

and Stigonematales. The latter two contain

hetrocysts. The members of Chroococcales are

unicellular and usually aggregated in colonies. In

Pleurocapsales, the cells have the ability to form

internal spores (baeocytes). In Oscillatorialles, the

cells are singly arranged and do not form special­

ized cells, (akinets and heterocysts). In Nostocalles

and Stigonematalles, the cells have the ability to

develop heterocysts under certain conditions.

15. Gram positive bacteria (including Mycoplasmas
    and Actinobacteria)
    Like the Proteobacteria, the Gram positive bacteria
    are very diverse; they contain many bacteria encoun­
    tered in everyday life as agents of disease and
    inputs of production in industry or as important
    organisms in food microbiology. Some of them
    (the Mycoplasma) lack cell walls.

Gram­positive bacteria fall into two major phylo­

genetic divisions, “low-G + C” and “high-G + C.”:

(a) Low G + C group: G + C below 50%;

(b) High G + C group: G + C higher than 50%

(a) Low G + C Group: G + C Below 50%

Non­sporulating Low G + C Group

Staphylococcus: The staphylococci have

spherical cells often found in groups resem­

bling clusters of grapes. Bacteria of this

genus were originally grouped with other sphe­

rical microorganisms, especially of the genus

Micrococcus, since these two genera often

shared similar habitats. However, physiological

studies and phylogenetic analysis have shown

that these two genera are very different from

one another. The differences between staphy­

lococci and micrococci are discussed below.

Lactic Acid Bacteria: The lactic acid bacteria

are Gram­positive rods and cocci that produce

lactic acid as their primary end product. An

important group characteristic is the absence

of cytochromes, porphyrins and respiratory

enzymes. They are therefore incapable of oxi­

dative phosphorylation or any type of respira­

tion and are totally dependent on fermentation.

Lactic acid bacteria do, however, contain

mechanisms to deal with the toxic byproducts

of oxygen, which categorizes them as aeroto­

lerant anaerobes. They include Streptococcus,

Leuconostoc, Pediococcus, Lactococcus,

Enterococcus, and Lactobacillus. Lactic acid

bacteria are primarily differentiated based on

the types of end products they form. Homo­

fermentative lactic acid bacteria produce only

lactic acid as an end product, while heterofer­

mentative lactic acid bacteria produce lactate,

ethanol, and CO 2 as well (Axelssson and Ahrne

2000 ; Narayanan et al. 2004 ).

Sporulating Low G + C Group

Bacillus: These are spore­forming aerobic

rods. Bacillus is a genus of rod­shaped, beta­

hemolytic Gram­positive bacteria. Bacillus

species are catalase­positive obligate or facul­

tative aerobes. Ubiquitous in nature, Bacillus

includes both free­living in soil, water and air,

as well as some pathogenic species. Under