Environmental Microbiology of Aquatic and Waste Systems

48 4 Taxonomy, Physiology, and Ecology of Aquatic Microorganisms

Taxonomy is important because it:

1. Allows the orderly organization of huge amounts
   of information regarding organisms


2. Enables predictions about their properties and
   formation of hypothesis about them


3. Facilitates the accurate characterization and
   identification of “unknown” organisms


4. Places organisms in meaningful manageable groups
   and thus facilitates scientific communication

4.1.2 Evolution of the Classification of Living Things

Landmarks in the evolution and development of bio­
logical classification may be ascribed to the contribu­
tions of the following:

1. Linnaeus (1707–1778)
   The Swedish naturalist, Carolus Linnaeus, is credited
   with introducing the earliest organized classification
   of living things in his Systema Naturae or natural
   system. He divided living things into plants and ani­
   mals. Based on morphology and motility, the dis­
   tinction between the two groups of organisms was
   clear: plants were green and did not move; on the
   other hand, animals were not green, but moved.


2. Ernst Haeckel (1834–1919)
   Soon after the discovery of the microscope, previ­
   ously invisible microscopic organisms were observed,
   some of which had properties common to both plants
   and animals. Some such as Euglena were green like,
   but they also moved about like animals. Because the
   clear­cut criteria which separated plants from animals
   were absent in these “new” organisms, the German
   biologist who was a contemporary of Charles Darwin,
   in 1866 coined the name Protista for a third kingdom,
   in addition to the Plant and Animal Kingdoms.


3. Robert Harding Whittaker (1920–1980)
   Whittaker was an American. Born in Wichita, Kansas,
   he worked in various places including the University
   of California, Irvine, and Cornell University. In 1968,
   he proposed the five­kingdom taxonomic classifica­
   tion of living things into the Animalia, Plantae, Fungi,
   Protista (Algae and Protozoa), and Monera (Bacteria).
   His categorization of living things was based on three
   criteria: cell­type (whether prokaryotic or eukaryotic);
   organizational level (unicellular or multi­cellular);
   nutritional type (autotrophy or heterotrophy).


4. Carl R. Woese (1928–)
   The current classification of living things is based on
   the work of Carl Robert Woese of the University of

Illinois. While earlier classifications were based

mainly on morphological characteristics and cell­

type, following our greater understanding of living

things at the molecular level, Woese’s classification

is based on the sequence of the gene of the ribosomal

RNA (rRNA) in the 16S of the small subunit of the

prokaryotic ribosome, or the 18S of the small subunit

of the eukaryotic ribosome (Petti et al. 2006 ).

The sequence of the rRNA in the 16S or 18S of the

small subunit of the ribosome is used for the following

reasons:

(a) The ribosome is an important organelle in all

living things where it is used for a basic function

for the support of life, namely, protein synthesis.

(b) The 16S (prokaryote) or 18 S (eukaryote) rRNA is

an essential component of the ribosome.

(c) The function of 16S or 18 S rRNA is identical in

all ribosomes.

(d) The sequences of the 16S or 18 S rRNA are ancient

(or highly conserved) and change only slowly with

evolutionary time.

(e) Organisms can generally inherit genes in two ways:

From parent to offspring (vertical gene transfer), or

by horizontal or lateral gene transfer, in which genes

jump between unrelated organisms, a common

phenomenon in prokaryotes. There is little or no

lateral gene transfer in the sequences in the 16S or

18 S RNA of the ribosomal small units.

All the above properties make the sequence of the

rRNA in the 16S or 18S of the small subunit of the

ribosome useful as molecular chronometers for mea­

suring evolutionary changes among organisms. Using

this method, living things are now divided into three

domains: Archae, Bacteria, and Eukarya. A diagram­

matic representation of the three domains is given in

Fig. 4. 1 , and their distinguishing properties are given

in Table 4. 1 (Woese 1987 , 2000, 2002 ).

4.1.3 Determining Taxonomic Groups Within Domains

The smallest unit of biological classification is the

Species. Species sharing similar properties are put in a

Genus. Genera (plural of genus) sharing similar charac­

teristics are put in a Family. Families with similar pro­

perties are arranged in an Order. Orders with similar

properties are classified as into a Class. Classes which

share similar properties are grouped into a Phylum.

Phyla (plural of phylum) with similar properties are put

in a Kingdom and similar kingdoms are in a Domain.