50 4 Taxonomy, Physiology, and Ecology of Aquatic Microorganisms
Several mnemonics exist to help with remembering
the correct order of the listing of taxonomic groups.
Two are given below:
Domain Kingdom Phylum Class Order Family Genus Species
David Kindly Pay Cash Or FurnishGood Security
Dignified Kings Play Chess On Fine Green Silk
The names of the ascending taxonomic groups to
which the fruit fly, humans, peas, and the bacterium,
E coli belong are given in Table 4.2.
4.1.3.1 Definition of Species
In the Domain Eukarya, a species is defined as a group
of organisms which can mate and produce fertile
offspring. Even though goats are of different kinds, they
will mate and produce fertile offspring; similarly with
dogs. The horse and the donkey are of different species,
because although they can mate and produce offspring,
the offspring are not fertile. This definition is occasion
ally complicated by the lateral transfer of genes.
In Bacteria and Archae, the definition is a little
different. A species in these Domains is a collection of
strains that share many stable properties and differ
significantly from other groups.
4.1.3.2 Nomenclature of Biological Objects
Biological objects, including microorganisms are
named in the binomial system devised by Carolus
Linnaeus. The genus name is written first and begins
with an uppercase (capital) letter; the other half of the
name is written in lowercase (small) letters and is the
species name. The two are written in italics or
underlined if written in long hand. When written for
mally, the name of the author who first described the
organism is included and the year of the publication is
given; the names are usually written in Latin or lati
nized. The name of a hypothetical Bacillus discovered
in water by John Smith and published in 2007 could be
Bacillus aquanensis Smith 2007. Usually only the
genus and species names are given; the author and year
of publication are omitted (van Regenmortel 1999 ).4.1.3.3 Criteria and Methods for the
Identification and Classification
of Bacteria and Archae: Morphological,
Physiological, Nucleic Acid,
and Chemical Properties
Whereas members of the Domain Eukarya are classified
largely on their morphological characteristics which are
adequately diverse, morphological types are very lim
ited in the Domains Bacteria and Archae. Therefore,
while morphological properties are used, other charac
teristics are employed in addition to morphology. The
properties used for classifying and identifying unknowns
among organisms in the Domains Bacteria and Archae
are given in Table 4. 3. The principles of methods used
are described briefly below.Morphological and Physiological Methods- Nutritional types of Bacteria
Living things are classified into major nutritional
types on the basis of the following attributes:
(a) Carbon source utilized
A carbon skeleton is required for the compounds
used for growth and development such as carbo
hydrates, amino acids, fats, etc. The organism is
autotrophic if it manufactures its food and
obtains its carbon through fixing CO 2 such as is
the case with plants, algae, and some bacteria.
When the organism cannot manufacture its own
food from CO 2 but must utilize food already
manufactured from CO 2 , in the form of carbo
hydrates, proteins etc., it is heterotrophic. This
is the case with animals and most bacteria.
Table 4.2 Taxonomic groups of some organisms (From Anonymous 2008 a)
Rank Human Fruit fly Pea E. coli
Domain Eukarya Eukarya Eukarya Bacteria
Kingdom Animalia Animalia Plantae Bacteria
Phylum (animals) or division (plants) Chordata Arthropoda Magnoliophyta Proteobacteria
Class Mammalia Insecta Magnoliopsida Gammaproteobacteria
Order Primates Diptera Fabales Enterobacteriales
Family Hominidae Drosophilidae Fabaceae Enterobacteriaceae
Genus Homo Drosophila Pisum Escherichia
Species H. sapiens D. melanogaster P. sativum E. coli