Which one of the following is the well known example of bioindicator of air pollution ?

Question

UnlockIAS · Accepted Answer

Correct — A, Lichens. A lichen is a symbiotic association of a fungus with an alga or a cyanobacterium. It has no roots, no protective cuticle and no stomata, so it absorbs water, minerals — and whatever pollutants are dissolved in them — directly from rain and air over its entire surface, and it has no way of excreting what it takes up. That makes lichens extraordinarily sensitive to sulphur dioxide (SO2) and other gaseous pollutants, which destroy the chlorophyll of the algal partner. Lichens flourish on tree bark, walls and rock wherever the air is clean and disappear wherever it is polluted, leaving the bare 'lichen desert' typical of industrial and heavily built-up areas. Their presence, absence and species composition are therefore the classic biological indicator (bioindicator) of air quality.

Why the other options are wrong — (b) Wrong — methyl mercury is not an organism at all but an organic compound of mercury. It is itself a pollutant, notorious for biomagnifying up aquatic food chains (the agent of Minamata disease). A bioindicator, by definition, has to be a living organism.  (c) Wrong — the rose is a cultivated ornamental shrub with no standing as an air-pollution indicator. It has neither the fully exposed absorbing surface nor the documented sensitivity to SO2 that makes the lichen the textbook choice.  (d) Wrong — the sunflower is associated with phytoremediation, where it is grown to draw heavy metals and radionuclides out of contaminated soil and water. That is the clean-up of soil and water pollution, not the indication of air pollution.

Concept: A bioindicator is a living organism whose presence, absence, abundance or condition reveals the state of the environment around it. Lichens are the standard bioindicator for air. The fungal partner (mycobiont) supplies structure and absorption while the algal or cyanobacterial partner (photobiont) photosynthesises, so the whole thallus lives directly off the atmosphere — no roots drawing on soil, no waxy cuticle keeping pollutants out. Sulphur dioxide destroys the photobiont's chlorophyll, which is why lichens are among the first organisms to vanish as SO2 levels climb. Sensitivity varies with growth form: the bushy fruticose lichens die back first, the leafy foliose next, and the crust-like crustose lichens hold on longest — so the lichen community itself grades the air quality of a place.

State PCS 2019 Q24 — Environment & Ecology PYQ with Answer

State PCS 2019 Q24 — Environment & Ecology PYQ with Answer