Biological Classification: Applications in Biotechnology, Medicine & Agriculture

Agricultural Applications

Cyanobacteria as biofertilisers: Nostoc and Anabaena fix atmospheric nitrogen in their heterocysts. Used as biofertilisers in paddy fields, reducing chemical nitrogen fertiliser requirements. Anabaena is also symbiotic with the water fern Azolla.
Rhizopus (Phycomycetes): Produces enzymes used in the production of lactic acid and fermented foods.
Trichoderma (Deuteromycetes): Used as a biological fungicide in agriculture — it is mycoparasitic and produces antifungal compounds that protect crops from pathogenic fungi.
Puccinia and Ustilago (Basidiomycetes): Major agricultural pathogens — Puccinia causes wheat rust and Ustilago causes smut disease, leading to billions in crop losses globally.
Methanobacterium (Archaebacteria): Used in biogas plants; converts organic waste in cattle dung to methane ( $CH_{4}$ ) as fuel.

Biotechnology Applications

Saccharomyces cerevisiae (Ascomycetes): Used in bread making ( $CO_{2}$ leavening), alcohol fermentation (beer, wine, ethanol), and as a recombinant protein production platform (produces insulin, hepatitis B vaccine).
Neurospora crassa (Ascomycetes): Classical genetics model organism; used by Beadle and Tatum to formulate the "one gene–one enzyme hypothesis."
Aspergillus niger (Ascomycetes): Industrial production of citric acid; also used for amylase and glucoamylase production.
Diatoms (Chrysophytes): Diatomaceous earth used in filtration systems (water treatment, swimming pools), polishing compounds, and as a natural insecticide (disrupts insect cuticle).

Medical/Clinical Applications

Plasmodium (protozoan — Sporozoa): Causes malaria; four species infect humans (P. vivax, P. falciparum, P. malariae, P. ovale). P. falciparum causes the most severe (cerebral) malaria.
Trypanosoma (protozoan — Flagellata): Causes African sleeping sickness (T. brucei, transmitted by tsetse fly) and Chagas disease (T. cruzi).
Mycoplasma: Causes atypical ("walking") pneumonia and various plant diseases; resistant to penicillin because it has no cell wall.
Claviceps purpurea (Ascomycetes): Causes ergotism — a disease in humans who consume contaminated grain; produces alkaloids causing vasoconstriction, gangrene, and hallucinations. Historically important.
Prions: Cause BSE (bovine spongiform encephalopathy) in cattle and Creutzfeldt-Jakob disease (CJD) in humans. No effective treatment — proteins cannot be inactivated by standard sterilisation.
Lichens in pollution monitoring: Their extreme $SO_{2}$ sensitivity makes them natural air quality monitors used by environmental scientists. Lichen diversity = air quality index in many countries.