State two economically important uses of archaebacteria.

(i) The biological classification of plants and animals was first proposed by Aristotle on the basis of simple morphological characters.

(ii) Carolus Linnaeus later classified all living organisms into two kingdoms- Plantae and Animalia.

(iii) The Linnaean system of classification of plants on the basis of number and arrangement of stamens is artificial.

(iv) The classification based on evolutionary sequence and the genetic relationships among the organisms is termed phylogenetic system.

(v) $\mathrm{R}.\mathrm{H}.$ Whittaker ($1969$) proposed five kingdom classification- Monera, Protista, Fungi, Plantae and Animalia.

(vi) The main criteria of the five kingdom classification were cell structure, body organisation, mode of nutrition and reproduction, and phylogenetic relationships.

(vii) The most recent classification of the living world is the six kingdom system proposed by Carl Woese, $\mathrm{O}$. Kandler and $\mathrm{M}. \mathrm{L}.$ Wheelis in $1990$.

(viii) Woese, Kandler and Wheelis divided living organisms into three major groups called domains above kingdom level: Eukarya, Bacteria and Archaea.

2. Kingdom Monera:

(i) In the five kingdom classification, bacteria are included in Kingdom Monera.

(ii) Bacteria are cosmopolitan in distribution and show the most extensive metabolic diversity.

(iii) Bacteria may be autotrophic or heterotrophic in their mode of nutrition.

(iv) Kingdom Monera includes those organisms which do not possess true nuclei.

(v) Two major groups of Monerans are represented by Eubacteria and Cyanobacteria. In these prokaryotes, the central region of a cell is occupied by genetic material consisting of aggregated DNA called nucleoid.

(vi) Bacteria are differentiated into Gram-positive and Gram-negative on the basis of Gram staining.

(vii) Endospores are asexual and highly resistant structures found in Gram-positive bacteria.

3. Kingdom Protista:

(i) Kingdom Protista includes all single-celled eukaryotes such as Chrysophytes, Dinoflagellates, Euglenoids, Slime moulds and Protozoans.

(ii) Protists have defined nucleus and other membrane-bound organelles.

(iii) Euglena forms a connecting link between animals and plants.

(iv) The Protozoa are animal-like protists and the term 'protozoa' was coined by Georg August Goldfuss.

4. Kingdom Fungi:

(i) Members of Kingdom Fungi show a great diversity in structures and habitat with saprophytic in their mode of nutrition.

(ii) The four classes under kingdom fungi are Phycomycetes, Ascomycetes, Basidiomycetes and Deuteromycetes.

(iii) Mycorrhiza is an association between fungus and the roots of higher plants exhibiting symbiosis.

(iv) Fungi are useful for the production of drugs, organic acids, enzymes, Single Cell Protein (SCP), alcoholic fermentation, etc.

5. Kingdom Plantae:

(i) The Plantae includes all eukaryotic chlorophyll-containing organisms like algae, bryophytes, pteridophytes, gymnosperms and angiosperms.

(ii) The life cycle of plants exhibit alternation of generations.

6. Viruses, Viroids and Lichens:

Some acellular organisms like viruses are not included in the five kingdom system of classification.