Beauty of some historical monuments is greatly affected by the growth of certain living organisms. These living organisms belong to which one of the following groups?

Correct — B, (b) Lichens. The organisms that disfigure old stone buildings — the grey, green, yellow and orange crusts and rosettes that spread across temple walls, forts and monuments — are lichens, and their damage is both mechanical and chemical.

A lichen is not a single organism but a symbiotic association of a fungus with an alga or a cyanobacterium. The algal partner, the phycobiont, photosynthesises and feeds the association; the fungal partner, the mycobiont, provides shelter and absorbs water and mineral nutrients. That partnership is what allows a lichen to grow where nothing else can, including on bare rock, which is why lichens are the classic pioneer species in a primary succession and why they are equally at home on a stone monument.

The damage follows from how they live. The fungal hyphae penetrate into the pores and micro-cracks of the stone, and repeated wetting and drying of the thallus loosens grains at the surface — that is the mechanical part, and it is why a lichen crust often takes a film of stone with it when it is removed. The chemical part is more serious. Lichens secrete acidic metabolites — carbon dioxide, lichen acids, and above all oxalic acid from the fungal partner — which attack the minerals of the stone. On the limestone and marble that so many monuments are built of, the attack dissolves the calcium carbonate and leaves crystals of calcium oxalate at the interface between rock and lichen; oxalates of magnesium, manganese and copper have been reported in the same position. The result is weathering of the surface and loss of the basic cations that hold the stone together. Conservators consequently treat lichen growth on monuments as biodeterioration and remove it.

One further property, which this question does not ask about but every examiner eventually does, is that lichens are highly sensitive to air pollution, especially to sulphur dioxide, and are used as bio-indicators: they simply do not grow in badly polluted areas.

Why the others are wrong

• (a)Amphibious plants — Amphibious plants are plants adapted to live partly in water and partly on land, at the margins of ponds, marshes and streams. They need soil or sediment to root in and standing or saturated water to live in, and they can do neither on the dry vertical face of a stone monument. Higher plants do sometimes damage historic structures — a peepal seedling germinating in a joint and prising the masonry apart with its roots is a familiar sight on old Indian buildings — but that is a rooted tree exploiting a crack, not an amphibious plant, and the question asks for the group whose growth spoils the beauty of the surface.
• (c)Bacteria — Bacteria are genuinely part of the community that colonises stone, and the microbiology of monument decay includes sulphur-oxidising and nitrifying bacteria whose acidic products attack masonry. So this is not a foolish option. But bacteria are microscopic and form thin films rather than the visible coloured crusts and rosettes that spoil the appearance of a monument, and the question is framed around beauty being ’greatly affected by the growth of certain living organisms’ — that is, around something you can see. What you see on an old wall is lichen. This is a good illustration of reading the stem for what it emphasises rather than for the first plausible cause.
• (d)Viruses — Viruses cannot grow on stone at all. A virus is not a free-living organism: it has no metabolism of its own and can replicate only inside a living host cell, so it cannot colonise an inert mineral surface, cannot secrete acids and cannot produce a visible growth. This option is eliminated by the definition of a virus rather than by any fact about monuments, which makes it the easiest of the three to discard and a useful reminder that a general biology principle often disposes of an option faster than specialised knowledge would.

A lichen is a mutually useful association between an alga or a cyanobacterium and a fungus. The algal component, called the phycobiont, is autotrophic and prepares food; the fungal component, called the mycobiont, is heterotrophic and provides shelter while absorbing mineral nutrients and water for its partner. The association is so intimate that a lichen looks like a single organism, and it survives conditions neither partner could face alone — bare rock, tree bark, desert soil, arctic tundra. That toughness makes lichens the standard pioneer species in a primary succession on bare rock: they colonise the rock, weather it chemically, trap dust and organic debris, and so begin the soil-forming process that later allows mosses and then higher plants to establish. Applied to a monument, the same sequence is destruction rather than construction. Lichens grow in three broad forms — crustose, which are tightly attached crusts, foliose, which are leaf-like, and fruticose, which are shrubby — and the more exposed forms are the more pollution-sensitive, which is the basis of their use as bio-indicators of air quality, particularly for sulphur dioxide. Lichens have other uses too: litmus, the acid-base indicator, is a dye extracted from lichens, and several species are used in dyeing and in perfumery.

Key facts

• Lichens are symbiotic associations of an alga or cyanobacterium, the phycobiont, with a fungus, the mycobiont.
• The alga prepares food by photosynthesis; the fungus provides shelter and absorbs mineral nutrients and water.
• Lichens damage stone mechanically, by hyphal penetration into pores and cracks, and chemically, by secreting acidic metabolites.
• Oxalic acid secreted by the fungal partner is among the most active agents of chemical attack, leaving calcium oxalate crystals at the rock-lichen interface.
• Monuments of limestone and marble are especially vulnerable, since the attack dissolves calcium carbonate and depletes the stone of basic cations.
• Lichens are pioneer species in a primary succession on bare rock, initiating the process of soil formation.
• Lichens are highly sensitive to air pollution, particularly sulphur dioxide, and are used as bio-indicators of air quality.
• Litmus, the acid-base indicator, is a dye obtained from lichens.
• Lichen growth forms are crustose, foliose and fruticose.

Common traps

• Choosing bacteria because microorganisms are known to attack stone. The stem is about visible growth spoiling the appearance.
• Treating a lichen as a single organism or as a kind of plant. It is an association of a fungus with an alga or cyanobacterium.
• Forgetting that the same organism is both a pioneer of soil formation and an agent of monument decay; the mechanism is identical, only the substrate differs.
• Overlooking that a virus cannot grow outside a living host cell, which rules it out of any question about growth on an inert surface.

• practice — not a real PYQ

  In a lichen, the fungal partner is known as the

  • (a)phycobiont
  • (b)mycobiont
  • (c)symbiont
  • (d)mycorrhiza

  Answer(b) mycobiont

• practice — not a real PYQ

  Lichens are widely used as bio-indicators of

  • (a)soil salinity
  • (b)water hardness
  • (c)air pollution, especially by sulphur dioxide
  • (d)radioactive contamination

  Answer(c) air pollution, especially by sulphur dioxide