Diversity of Living Organisms - Viruses, Viroids, and Lichens

Viruses are non-cellular organisms characterized by having an inert crystalline structure outside the living cell. They are considered a link between living and non-living entities.

The name virus was given by Pasteur. D.J. Ivanovsky ($1892$) recognized certain microbes as the causal organism of the mosaic disease of tobacco. M.W. Beijerinck ($1898$) demonstrated that the extract of the infected plants of tobacco could cause infection in healthy plants and called the fluid $Contagium \ vivum \ fluidum$ (infectious living fluid).

Viruses contain either $DNA$ or $RNA$. No virus contains both $RNA$ and $DNA$. Generally, viruses that infect plants have single-stranded $RNA$ ($ssRNA$) and viruses that infect animals have either single or double-stranded $RNA$ or double-stranded $DNA$ ($dsDNA$).

The protein coat of a virus is called the capsid, which is made of small subunits called capsomeres. This capsid protects the nucleic acid. These capsomeres are arranged in helical or polyhedral geometric forms.

Viroids were discovered by T.O. Diener in $1971$. They are smaller than viruses and cause potato spindle tuber disease. They consist of a free $RNA$ of low molecular weight and lack the protein coat (capsid) found in viruses.

Lichens are symbiotic associations, i.e., mutually useful associations, between algae and fungi. The algal component is known as the phycobiont (autotrophic) and the fungal component as mycobiont (heterotrophic).

Lichens are very good pollution indicators; they do not grow in areas polluted with sulfur dioxide ($SO_2$).