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Images of British Lichens



FAQ
Frequently Asked Questions (if anyone actually asked any)

These pages will be used to provide various notes and explanations.
I intend that the FAQ will expand into an illustrated glossary, but this will take time.


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What is a lichen?

Lichens are fungi. However, they are fungi with a difference – they contain an alga, which may be in the Green Algae (phylum Chlorophyta) or Blue-Green Algae (phylum Cyanobacteria, 'domain' Bacteria). Consequently, the fungus and the alga constitute a stable, dual organism, with the fungus (the ‘mycobiont’) and the alga (‘photobiont’ or ‘phycobiont’) in some kind of partnership. The algae are photosynthetic, i.e. they fix the sun’s energy, so one can think of a lichen as a fungus with solar energy cells.

Lichens are regularly cited as examples of "symbiosis", which is entirely true, but it should be appreciated that understanding of this term varies. Common British interpretation and common lay use imply mutual benefit, but the term is used here in the original sense of de Bary in 1879, in which any stable and continuous, close association of two species may be considered as symbiosis, including parasitism and commensalism (Hawksworth, 1988), a view also seen in American biology textbooks. In fact at least the majority of lichens can be viewed as mutualistic symbioses, implying evolutionary development at least on the part of the fungus but likely a degree of co-evolution. However, the balance of benefits varies and some lichen associations may well be regarded as parasitism. This is discussed further below.


 
 
Parmelia sulcata is a fungus containing algal cells in a layer (the 'photobiontic layer') in the medulla tissue below the upper cortex.
See below for explanation of these terms.

 
In this case the alga is a Green Alga (Chlorophyta), a species of Trebouxia.

The relationship is regarded as a mutualistic symbiosis but it is not necessary that this ‘partnership’ be viewed as an equal one.

Opinions vary, but an analogy may be the ‘partnership’ between humans and cattle.
We can argue that cattle benefit from the partnership in that their world range has been extended, they are provided with protected living space and nutritional needs, but the partnership could equally be described as exploitation.

The fungal use of algae in lichens could be viewed in the same way.


The fungus provides protected living space for the alga, and it is efficient at absorbing water and dissolved inorganic nutrients from its surroundings and undoubtedly these are passed on to the algal cells, at least as a result of concentration gradients. The alga is photosynthetic (converting solar energy to chemical energy) and releases soluble carbohydrate back to the fungus. However, continuing the cattle analogy, free-living algae may be captured by the lichen, they can escape, one lichen may steal algae from another, and this may involve armed robbery and murder!


 
 
Typical arrangement of tissues in a foliose lichen.

As a foliose lichen, it has both an upper and a lower cortex. The photobiontic layer is in the upper part of the medulla, where the algal cells will receive enough light, yet be protected from the full strength of the sun's rays by the upper cortex.

[Physcia parietina is an old synonym for Xanthoria parietina.]

The morphologically dominant partner is, perhaps with few exceptions, the fungus, and the Latin name of the lichen is indeed that of the fungal partner. Sometimes a fungus may contain two different algae at the same time, or certain lichens may exist in different forms as they can form lichenised associations with different algae, e.g. Sticta canariensis (page pending), which can form associations with both green algae and cyanobacteria. If the fungus species remains the same, then so does the name of the lichen, even if the lichen appearance varies.

 
The description of the lichen as the morphologically dominant partner is somewhat uncertain in the case of some of the filamentous lichens. Here, in the lichen Cystocoleus ebeneus, the fungal hyphae form a tight sheath around the filaments of the alga Trentepohlia and although the fungal association macroscopically differs in colour from free-living Trentepohlia and has longer filaments, it retains the filamentous growth form of its photobiont.

In the case of the filamentous lichen Ephebe lanata, in which the fungus wraps its hyphae around the cyanobacterium Stigonema, it is in fact difficult in the field to distinguish the lichen from free-living Stigonema colonies. The same might be said of the lichen Belonia nidarosiensis and free-living Trentepohlia umbrina.

In these cases, it is difficult to consider the lichen symbiosis to be any other than parasitism of the alga by the fungus.


 
What is NOT a lichen?

It will be noted that a lichen has been defined as a fungus that contains an alga in a reasonably stable relationship (a usually mutualistic symbiosis).

However, what if an alga contains a non-parasitic fungus in what may be a symbiotic partnership? Is it a lichen too?

 
Pelvetia canaliculata (Channelled Wrack) is a brown seaweed (phylum Phaeophyta), that typically forms extensive colonies on rocks high on the shore, at the very top of the brown seaweed zone, extending up to the beginnings of land vegetation. Shown here in fresh, hydrated state, it is often seen dried, and it appears to be more desiccation resistant than any other brown seaweed.

It is also known to contain a fungus that appears to be constantly present in its tissues. The mycelium of Stigmidium ascophylli (= Mycosphaerella ascophylli, also = M. pelvetiae) pervades the tissues of the host, but remains intercellular and appears to cause no injury (Sutherland, 1915, cited in Smith, 1921; Hawksworth, 1988). The fruiting bodies (pseudothecia) of the fungus are commonly seen as tiny black dots in the fertile portions (receptacles) of the seaweed.

It appears that P. canaliculata and S. ascophylli form a stable relationship, a symbiosis. It is not clear if S. ascophylli should be regarded as a parasite, though there is apparently no evidence for this, or if it is commensal, the seaweed merely providing a microhabitat for the fungus, or if in fact there is mutual benefit.

With regard to the last of these possibilities, is it a coincidence that this ecologically very distinct, unusually desiccation-resistant seaweed is also one that constantly contains a fungus of uncertain function? If the fungus, in some way, enhances the resilience of the seaweed, this would be a mutualistic symbiosis involving a fungus and an alga, i.e. a relationship that could well be considered to be a lichen.

As implied by its Latin name, Stigmidium ascophylli also occurs in the seaweed Ascophyllum nodosum, which also occurs in the upper part of the brown seaweed zone on a rocky shore, though generally at around the same level as Fucus species such as F. spiralis and submerged for somewhat longer periods than P. canaliculata. This association again seems constant, found even in the youngest Ascophyllum plants, and experimental work suggests that presence of the fungus may be required for survival of sporelings on the shore (Webber 1967).

It seems likely, therefore, that the fungus does confer unusual physiological properties on its hosts and that this is a mutualistic symbiosis, supporting the lichen concept. However it could still be that the fungus has adapted to these two hosts as they already spend less time submerged than other brown seaweeds.

In practice, a distinction is made between a fungus containing algal cells, i.e. a lichen, and a multicellular alga containing a fungus. The latter case may well be analogous to the many flowering plants that contain endophytic fungi in mutualistic symbioses, the fungi in these cases often reducing the palatability of their hosts to grazing animals. Consequently, neither the Stigmidium-Pelvetia nor Stigmidium-Ascophyllum associations are considered to be lichens. A term that has been used for such marine algal-fungal associations is 'mycophycobiosis' (Hawksworth & Hill, 1984; Hawksworth, 1988). Other cases of mycophycobioses are now known, again involving seaweeds high on the shore (Hawksworth, 1988).


References
•   Hawksworth, D.L., (1988). The variety of fungal-algal symbioses, their evolutionary significance and the nature of lichens. Botanical Journal of the Linnean Society 96: 3–20.
•   Hawksworth, D.L., & Hill, D.J., (1984). The lichen-forming fungi, Blackie, Glasgow.
•   Smith, A.L., (1921). Lichens, Cambridge University Press, Cambridge.
•   Webber, F.C., (1967). Observations on the structure, life history and biology of Mycosphaerella ascophylli. Transactions of the British Mycological Society 50: 583–601.


 

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© A.J. Silverside
October 2012, last modified December 2012
Original line illustration from A.L. Smith (1921) Lichens (out of copyright).