Draft:Lichens and air pollution

Lichens are composite organisms that form when algae or cyanobacteria live in the filaments of fungi in a mutualistic relationship. Lichen, which lack roots, obtain most of their necessary elements from the air; the elemental levels in lichen can reflect the composition of ambient air. As a result, many studies of lichens have emphasized their role as bioindicators of air quality, although not all lichens are equally sensitive to different air pollutants.

Overview
(is this necessary?)

Effects of air pollution
If lichens are exposed to air pollutants at all times, without any deciduous parts, they are unable to avoid the accumulation of pollutants. Also lacking stomata and a cuticle, lichens may absorb aerosols and gases over the entire thallus surface from which they may readily diffuse to the photobiont layer. Because lichens do not possess roots, their primary source of most elements is the air, and therefore elemental levels in lichens often reflect the accumulated composition of ambient air. The processes by which atmospheric deposition occurs include fog and dew, gaseous absorption, and dry deposition. Consequently, many environmental studies with lichens emphasize their feasibility as effective biomonitors of atmospheric quality. Not all lichens are equally sensitive to air pollutants, so different lichen species show different levels of sensitivity to specific atmospheric pollutants. The sensitivity of a lichen to air pollution is directly related to the energy needs of the mycobiont, so that the stronger the dependency of the mycobiont on the photobiont, the more sensitive the lichen is to air pollution. Upon exposure to air pollution, the photobiont may use metabolic energy for repair of its cellular structures that would otherwise be used for maintenance of its photosynthetic activity, therefore leaving less metabolic energy available for the mycobiont. The alteration of the balance between the photobiont and mycobiont can lead to the breakdown of the symbiotic association. Therefore, lichen decline may result not only from the accumulation of toxic substances, but also from altered nutrient supplies that favor one symbiont over the other.

History
Erasmus Darwin, grandfather of Charles Darwin, is sometimes claimed to be the first person to suggest that lichens were bioindicators of air pollution from a 1791 poem, believed to refer to the copper mines at Parys Mountain in Anglesey, Wales.

WIP notes

 * Discuss with about how to structure article
 * Brief overview of lichens, how they are affected by air pollutants broadly, how certain species are affected by certain pollutants specifically (a few key examples?), bioindicators, consequences of the loss of lichens
 * Figures of increases, photos of dying lichens would be helpful visuals (we'll need to make sure they're dying because of air pollution)
 * has a picture of a Punctelia dying from pollution, and it's open access (CC BY-SA 4.0 Deed)
 * Consideration of 'lichen as indicators of air pollution' and 'lichen affected by air pollution in their own right'
 * History of use as bioindicators
 * Impact of industrial revolution: original high mortality and subsequent (at least partial) recovery
 * Impact of modern farming practices
 * Why are some species more impacted than others? How do some species "fight off" effects?