Rough, Hydrophobic Surface Allows Gas-exchange

Biological Strategy

Lecanora conizaeoides lichen

AskNature Team

Protect From Excess Liquids

While water is essential to life, too much water or other liquids can overwhelm living systems. Excess liquids can, for example, decrease a living system’s access to oxygen, promote excessive bacterial or fungal growth, or strip away soil and nutrients. To prevent the accumulation of excess liquids, living systems must control the movement of liquids across their boundaries or surfaces. They do so using waterproofing materials or structures, slowing flow, and/or facilitating flow to move the liquid away. Plant leaves, for example, commonly have waxy surfaces comprised of water-repelling chemicals to keep water from engorging the leaves or facilitating bacterial and fungal growth.

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Lichens

Lichen (“to lick”): Iceland moss, reindeer lichen, rock tripe

Lichen are composed of two organisms: algae (or cyanobacteria) and fungi. In this symbiotic, or beneficial relationship, algae produce simple sugars through photosynthesis which are consumed by fungi, and fungi provide algae with water and shade. Sometimes referred to as “moss”, lichen superficially resemble, but are not true moss. It is estimated that lichen cover 8% of the Earth’s land surface. From the subzero temperatures of the Arctic to sweltering deserts, lichen can be found on flat surfaces; gravestones, city buildings, rocks, trees, soil, and other flat surfaces. While some lichen have a cracked, crusty appearance, others are more foliose, or leaf-like in appearance. Lichen are currently being used as an inexpensive way to assess air quality, as some are sensitive to pollution and metal contamination. In the Arctic, caribou depend on reindeer lichen (Cladonia spp.) as their main source of food during migration.

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Pores on lichen surface allow water to roll off it and gas to be exchanged under saturated conditions due to rough surface and hydrophobic compounds.

This particular species of lichen utilizes hydrophobic interactions at its surface to avoid absorbing contaminated water (such as acid rain). Most lichen are susceptible to airborne pollution because their “open structure and tendency to dry and re-hydrate in response to drought mean that alga and fungus are exposed to high quantities of unbuffered water” (Shirtcliffe et al. 2005: 1193). Lecanora conizaeoides have adapted to counter this through their rough structure and hydrophobic pores. Hydrophobic means “water-fearing,” in other words, these pores reject interaction with water molecules. The rough surface of the lichen creates unevenness. When rain falls, the water molecules sit upon the topmost part of the lichen’s rough, multi-layered surface. Below these top layers sit micropores that are coated in hydrophobic compounds. This design creates openings for gasses to come through while preventing clogging by water. So even when a large amount of water is present, the lichen is still exchanging gasses. Such gas exchange is important for photosynthetic processes, meaning that while other plants become waterlogged, these lichen are still photosynthesizing by allowing water to roll off their surface.