Bacteria are the sole members of Kingdom Monera, being the most abundant microorganisms found almost everywhere, even in extreme habitats. Many also live as parasites.

Bacterial Shapes (Figure 2.1)

• Coccus (pl.: cocci): Spherical
• Bacillus (pl.: bacilli): Rod-shaped
• Vibrium (pl.: vibrio): Comma-shaped
• Spirillum (pl.: spirilla): Spiral

Conceptual diagram representing different bacterial shapes: Cocci, Bacilli, Spirilla, Vibrio.

Bacteria show extensive metabolic diversity, with some being autotrophic (photosynthetic or chemosynthetic) and the vast majority being heterotrophic (decomposers, saprophytic, or parasitic).

2.1.1 Archaebacteria

• Live in harsh habitats: halophiles (extreme salty), thermoacidophiles (hot springs), methanogens (marshy areas).
• Differ from other bacteria by a unique cell wall structure, aiding survival in extreme conditions.
• Methanogens produce methane (biogas) from dung of ruminant animals.

2.1.2 Eubacteria (‘True Bacteria’)

• Characterised by a rigid cell wall and a flagellum (if motile).
• Cyanobacteria (Blue-green algae): Photosynthetic autotrophs with chlorophyll ‘a’. Unicellular, colonial, or filamentous; form blooms in polluted water. Some (Nostoc, Anabaena) fix atmospheric nitrogen in heterocysts (Figure 2.2).
• Chemosynthetic autotrophic bacteria: Oxidise inorganic substances (nitrates, nitrites, ammonia) for ATP production. Important in nutrient recycling (N, P, Fe, S).
• Heterotrophic bacteria: Most abundant in nature, important decomposers. Many are beneficial (curd production, antibiotics, nitrogen fixation in legumes) but some are pathogens (causing cholera, typhoid, tetanus, citrus canker).

Conceptual diagram of a filamentous blue-green alga – Nostoc (Figure 2.2).

Bacteria primarily reproduce by fission (Figure 2.3). Under unfavourable conditions, they form spores. Primitive DNA transfer (sexual reproduction) also occurs.

Conceptual diagram of a dividing bacterium (Figure 2.3).

Mycoplasma: Organisms completely lacking a cell wall. Smallest known living cells, can survive without oxygen. Many are pathogenic.

Fungi constitute a unique kingdom of heterotrophic organisms, showing great diversity in morphology and habitat. They are cosmopolitan, preferring warm and humid places.

Most fungi are filamentous, consisting of long, slender, thread-like structures called hyphae, which form a network called mycelium. Some hyphae are coenocytic (continuous tubes with multinucleated cytoplasm) while others are septate (with cross walls). Their cell walls are composed of chitin and polysaccharides.

Most fungi are saprophytes (absorb soluble organic matter from dead substrates). Some are parasites (depend on living plants/animals). They can also live as symbionts (e.g., lichens with algae, mycorrhiza with plant roots).

Reproduction in Fungi

• Vegetative means: Fragmentation, fission, and budding.
• Asexual reproduction: By spores called conidia, sporangiospores, or zoospores, produced in fruiting bodies.
• Sexual reproduction: By oospores, ascospores, and basidiospores, also produced in distinct fruiting bodies.

Sexual Cycle Steps:

1. Plasmogamy: Fusion of protoplasms between two motile or non-motile gametes.
2. Karyogamy: Fusion of two nuclei.
3. Meiosis: In zygote, resulting in haploid spores.

In some fungi, plasmogamy immediately leads to diploid cells. In Ascomycetes and Basidiomycetes, an intervening dikaryotic stage ($n+n$) occurs before karyogamy.

Classes of Kingdom Fungi

Classification into classes is based on morphology of mycelium, mode of spore formation, and fruiting bodies.

• 2.3.1 Phycomycetes:
  • Found in aquatic habitats, decaying wood, damp places, or as obligate parasites on plants.
  • Mycelium is aseptate and coenocytic.
  • Asexual reproduction by zoospores (motile) or aplanospores (non-motile), endogenously produced.
  • Zygospore formed by fusion of isogamous (similar) or anisogamous/oogamous (dissimilar) gametes.
  • Examples: *Mucor* (Figure 2.5a), *Rhizopus* (bread mould), *Albugo* (on mustard).
• 2.3.2 Ascomycetes (‘Sac-fungi’):
  • Mostly multicellular (e.g., *Penicillium*), rarely unicellular (e.g., yeast, *Saccharomyces*).
  • Saprophytic, decomposers, parasitic, or coprophilous (dung-growing).
  • Mycelium is branched and septate.
  • Asexual spores: Conidia, produced exogenously on conidiophores.
  • Sexual spores: Ascospores, produced endogenously in sac-like asci. Asci arranged in ascocarps.
  • Examples: *Aspergillus* (Figure 2.5b), *Claviceps*, *Neurospora* (used in biochemical/genetic work). Some are edible (morels, truffles).
• 2.3.3 Basidiomycetes (Mushrooms, Bracket fungi, Puffballs):
  • Grow in soil, on logs/tree stumps, or as parasites (rusts, smuts).
  • Mycelium is branched and septate.
  • Asexual spores generally absent; vegetative reproduction by fragmentation is common.
  • Sex organs absent; plasmogamy by fusion of vegetative/somatic cells. Leads to dikaryotic structure, ultimately forming basidium.
  • Karyogamy and meiosis in basidium produce four basidiospores, produced exogenously on basidia, arranged in basidiocarps.
  • Examples: *Agaricus* (mushroom) (Figure 2.5c), *Ustilago* (smut), *Puccinia* (rust fungus).
• 2.3.4 Deuteromycetes (‘Imperfect Fungi’):
  • Known only by asexual or vegetative phases. Sexual forms, if discovered, lead to reclassification into Ascomycetes or Basidiomycetes.
  • Reproduce only by asexual spores (conidia). Mycelium is septate and branched.
  • Saprophytes or parasites; many are decomposers of litter, helping mineral cycling.
  • Examples: *Alternaria*, *Colletotrichum*, *Trichoderma*.

Conceptual diagrams illustrating diverse fungi: Mucor, Aspergillus, Agaricus (Figure 2.5).