The main physiological processes that lichens have are similar to those for green plants: photosynthesis, respiration, adaptation to water and temperature conditions, nitrogen acquisition, mineral absorption and growth behaviour. However their physiology is ‘odd’ to say the least; in 2005 the European Space Agency exposed Xanthoria elegans and some Peltigera species to 14.6 days in space (on the outside of the space craft). When returned to Earth they reverted to photosynthesis and respiration apparently unaffected by their trip to outer space!
Here is a brief outline of some of these processes:
Photosynthesis takes place in the lichen’s photobiont (algae or cyanobacteria) and requires carbon dioxide, water and sunlight to make carbohydrates. The process is one where electromagnetic radiation (sunlight) is converted to chemical energy (sugar). Oxygen is produced as a waste product (due to splitting water molecules). Carbohydrate is used by the photobiont and some is passed out to the fungal partner. Lichens that have a cyanobacterial photobiont (Genus Peltigera, for example) require less sunlight than those with an algal photobiont (Usnea and Ramalina species for example).
Respiration is essentially the reverse of photosynthesis in that it breaks down carbohydrate molecules (lichen’s food) to release chemical energy and returns carbon dioxide to the atmosphere. Growth will only occur in a lichen when the rate of photosynthesis is greater then the rate of respiration.
Water is important for the transporting of material around inside the lichen and as a source of hydrogen for building carbohydrates during photosynthesis. Lichens photosynthesise best when they are 50% to 70% saturated with water. Respiration functions best when water saturation is about 95%. Lichens are not good at holding on to the water they have in the thallus; they seem happy enough to depend on external water as they dry out and saturate with water rather rapidly if it rains. They can resume water-dependent physiological reactions (such as respiration and photosynthesis) very rapidly after it rains. Most lichens can absorb water directly from the atmosphere when the humidity is high enough; a few lichens, such as our Lepraria incana can only take water from the atmosphere and are unable to absorb water that falls on them directly.
Temperature: Desert lichens can tolerate temperatures as high as 90C (when dry, wet them and they cook!) and -195C (in liquid nitrogen). Lichens are a dominant part of Antarctica ecosystems.
Nitrogen is fixed (converted to a form plants can use) by cyanobacteria and supplied to the fungal partner for protein synthesis. Lichens whose photobiont is Nostac, Scytonema and/or Calothrix can fix nitrogen to produce nitrates or nitrites. Many lichens whose photobiont is a green algae also have some cyanobacteria in cephalodia that fix nitrogen. Studies have shown that the fixing process in the cephalodia is extremely efficient compared to that of cyanobacteria which act as the main photobiont in a lichen. Algal-dependent lichens that have no method to fix nitrogen must obtain it as a nitrate or ammonia from the air or substrate. This is a major limiting factor in the growth rate of non-cyanobacterial lichens.
Minerals are absorbed by lichens from the moisture in the air and from ground water. They cannot control the amount they absorb and this can lead to the photobiont being poisoned. Because of their inability to control mineral uptake, lichens are bioaccumulators and are used as biomonitors. See Lichens as Biomonitors.
Growth is a slow process in lichens. Even the fastest growing ones are very slow compared to flowering plants. Many discussions on growth are focused on radial growth, particularly in the case of crustose lichens. The typical radial annual growth rates for lichens in unpolluted habitats are as follows:
Crustose: 0.5 mm to 2.0 mm
Foliose: 0.5 mm to 4.0 mm
Fruticose: 1.5 mm to 10.0 mm
Some of the oldest growing lichens are in the Rhizocarpon group. A specimen of Rhizocarpon geographicum has been calculated to be over 4000 years old.