Discussion of Lichen Biology (Nash 2008)
Chapter 4: Morphology (Budel and Scheidegger)
The morphology (growth form) of the lichen plays a large role in how the lichen functions in relation to its environment, and you can determine a lot about the environment in which you find a lichen by looking at its morphological features.
But before diving into how the lichen morphology and environment interact, Budel and Scheidegger first outline the primary goals of the lichen, afterwards they address the different morphological strategies to meet those goals in various environmental contexts.
Primary Goals of the Lichen Symbiosis:
Must be able to perform photosynthesis in a manner that affords a net gain and sufficient growth (think of a business, it can acquire income, but until it starts creating a profit the business will not grow)
To meet the #1 goal, the lichen must achieve the following goals:
light – not too much (or photosynthesis will cease) and not too little.
gas exchange – CO2 needs to reach the photobiont to create sugars, the supply must be ample
water – not too much (or gas exchange is slower) and not too little.
Along with the photosynthesizing functions of the photobiont, the structural form of the lichen is the primary way in which the lichen meets these goal, and the mycobiont is the primary structural agent of the symbiosis. The authors of this chapter, B. Budel and C. Scheidegger state that “the appearance of the lichen thallus is primarily determined by the mycobiont” (40). They list a few exceptions to this rule, including the Coenogonium, Ephebe, Cystocoleu, and Racodium. They also note that the photobiont does exert unknown influences on the morphology of the lichen, stating: “knowledge of the influence of the photobiont on the lichen morphogenesis is important, because only after the establishment of the symbiosis is the characteristic thallus of a lichen developed” (41).
Now lets get into some of the morphological features and how these are adapted to certain environments.
The thallus of crustose species are firmly attached to the substrate. This allows them to loose water only from the upper cortex. Budel and Scheidegger mention that inclined rock faces allow surface water to pass over them giving them moisture (41), and we certainly see this in our area.
One interesting thing I’ve noticed in our area is that rocks with a flat table like surface tend to have a great diversity of soil crust lichen and bryophytes, while the sloping side of rocks tend to have your more typical rock-crustose lichen that are pretty thin and so totally firmly attached that even a knife struggles to scrape them off – these are endolithic lichen, and that thin strongly attached upper cortex is the lithocortex – a dense conglutinated hyphal layer. The thicker, more easily detached crustose lichen growing on the top surface of rocks are probably not endolithic, or if they are, they tend to be more areolate, and seem to indicate that the environment is more moist; an increase in areolae especially peltate areolae may indicate that the surface area is increasing, which would allow for an increased amount of water loss – hence there must be alot of water absorbed by the lichen if the structure of the lichen is allowing for alot of water loss. It may also say something about the development of soil on top of these rocks by the action of cryptogams. In sum, the thick crustose lichen growing on the flat tops of rocks may give some indication about the amount of water available during temperatures and daylight hours that allow for net gain photosynthesis.
I also wonder about the morphology of the lichen that are growing under a rock underhang, since they don’t get surface water flow, and I speculate that the presence of dew would also be limited in those nooks – the lichen growing in these areas may help us read a bit about the air moisture available to plants and insects in those nooks, and it would be helpful from a climate change adaptation perspective to read whether the amount of humidty is increasing or decreasing over the next decade. The thick crustose lichen growing on the flat tops of rocks may give some indication about the amount of water available during temperatures and daylight hours that allow for net gain photosynthesis.
But I really wonder about how exactly the photobiont gets established on rocks, especially in areas where the UV rays have a large chance of causing damage to the photobionts DNA. Perhaps the soredia and isidia help to move them around, and the hyphal strands knotted around the photobiont help reflect the sunlight does the outer cortex of many foliose lichen during dessication periods. Well be diving more into this in our discussion of Chapter 5, Morphogenesis.
Types of crustose:
Endolithic – growing in rock. examples include i) Verrucaria rubrocincta, forms a micrite layer (in contrast to the lithocortex, this layer has only a few hyphae – and micrite is short for microcrystalline calcite, formed by the recrystallization of lime mud – so if I am understanding micrite correctly, it seems as though perhaps the lichen is recrystallizing lime mud or into a layer that reflects light, which is not unusual since a lot of literature on micrite concerns microorganisms creating it; and ii) Lecidea sarcogynoides, the photobiont layer of the medulla is located within the rock, research shows that hyphae extend up to 2mm deep into the matrix of the sandstone! A study by Wessels and Schoeman 1988 shows that in semi-humid climate of South Africa L. sarcogynoides weathers sandstone at rate of 9.6mm per 100 years (how fast are the non-lichen caused erosion rates of sandstone in this area?) – Note: see upcoming page on the abstract for “Life in extreme environments: survival strategy of the endolithic desert lichen Verrucaria rubrocincta” by Garvie et al. 2008.
- Soil crusts — grows on the surface of soil.
Endophloeodic – growing underneath cuticle of stem or leaves.
Shell eaters: these crustose etching on the side of molluscs and Balanus shells (e.g. Arthopyrenia halodytes).
Monument eaters: these crustose deteriorate historical monuments (e.g. Leproplaca chrysodeta, Dirina massiliensis).
Parts of a crustose thallus:
- Prothallus – the margin around the lichen that is just composed of fungi, also found between the areolae of the lichen – the fungi in this section may be hydrophilic, and pull water into the lichen thallus as in the case of Cryptothecia rubrocincta (42).
- Lithocortex – dense layer on endolithic lichen that consists of densely conglutinated hyphae (e.g. Acrocordia conoidea, Petractis clausa, Rinodina immersa, Verrucaria baldensis, V. marmorea).
- Areolae – polygonal sections containing both mycobiont and photobiont, often under wet conditions the areolae will inflate and fuse together. Areolae are formed in one of two ways: from a primary thallus that develops cracks from the upper cortex to deep in the medulla, kind of like the cracks in desert soil, or from a “single thallus primordia developing on a prothallus” (42).
- Effigurate thallus – on the margins there are lobes that are “prolonged” and “radially arranged” (e.g. Caloplaca, Dimelaena, Acarospora, Pleopsidium).
- Squamulose thallus – considered by Budel and Scheidegger to be the most complex. The squamules are thought to be formed from areolae that have become enlarged and partially free from the substrate. They often look like overlapping scales (e.g. Catapyrenium, Peltula, Psora, Toninia).
- Squamulose peltate, meaning they are flat scales with a central attachment on the lower surface: Peltula euploca, Anema nummularium, Peltate forms of crustose lichen are often found in lichens that are colonizing soil or rocks in hot arid deserts. Peltula radicata is completely immersed in soil with flat structures above the soil to take in light for the photobiont – also in Eremastrella and Toninia.
- Squamulose bultate, they squamules are inflated and look like bucket worms.
- Squamulose suffructose, which are sqaumules shaped like “corralloid tufted cushions” (43).
- Or the squamules are described as lobate, for example in the Caloplaca and Lecanora. “radially striate with marginal partially raised lobes” (43).
These are our leaf like lichen that are flat, usually have distinctly different sides, and their attachment to substrate is partial. They usually have lobes that have different branching patterns which help to distinguish between species and may also indicate environmental conditions. In our region we usually only find foliose lichen in the forested areas, however there are a few exceptions: Umbillicaria spp. may be found growing on rocks in the prairie, near riparian areas and wetland margins, and Peltigera may be found growing on top of mosses in the wetland margins and possibly moist seeps in the prairie and vernal pools.
Distinguishing characteristics of foliose lichen:
Laciniate lichen: typical foliose, having lobes (they are “lobate”), they range in size and may be stratified (heteromerous) or non-stratified (homoiomerous) e.g. Collema, Leptogium, Physma.
radially arranged: Parmelia.
overlapping: e.g. Peltigera, Hypocoenomyce.
inflated and hollow lobes: e.g. Menegazzia.
rhizines (e.g. Peltigera), cilia (e.g. Physcia tenella), or tomentum (e.g. Lobaria) which may act as attachment surfaces.
Umbilicate lichen: circular thallus, with a central holdfast made of conglutinated hyphae; these have evolved separately in unrelated groups including Dermatocarpaceae, Parmeliaceae, Physciaceae and Umbilicariaceae.
Lobes can be unbranched or numerous lobes that have limited branching patterns.
Central holdfast may leave navel-like depression on outer cortex’s topography.
Vagrant lichen: these are found in desert like areas, and in the absence of lichen their thallus rolls up enabling them to protect the photobiont layer while to move easily with the wind; when water is absorbed (e.g. from dew) the thallus unfolds and the photobiont layer is exposed and can begin photosynthesizing.
And now our Fruticose Lichen:
Fruticose lichens look like hair, or straps, or shrubby, and their lobes may be flat (with both sides looking the same) or cylindrical). Fruticose lichen are never closely attached to the substrate, they are like little trees, growing away from the substrate. Because fruticose lichen are the most sensitive to water loss, we will usually find fruticose lichen in our forested areas, but there will be some growing on the shrubs near riparian areas, I haven’t found any fruticose growing in prairie habitat yet, although the wooden fences in agricultural areas host a lot of different fruticose lichen particularly Letharia.
Fruticose lichen are the morphological form that is most sensitive to water loss because they have a much higher surface to volume ratio than the other two morphological forms. Their presence can indicate moist air because water can be rapidly absorbed as well as rapidly lost, so we’ll find these in rainforests as well as foggy deserts like those in Namibia, and arid areas that have regular dew, like our semi-arid region in the eastern portion of the inland northwest.
I’ve found that an interesting possible environmental indicators may be the hydrophobic medulla of the Usnea genus – the more loose and large the hydrophobic medullary layer (in contrast to the cortex and cord thickness) the more that Usnea must be responding to being inundated with water during photosynthesizing periods; concurrently I would assume that an Usnea that has a medullary layer that is thinner and more dense (less hydrophobic air pockets) then that would indicate air that is more arid throughout the growing season. This hypothesis has made looking at those percentages of “Medulla to Cortex to Cord” (see McCune and Geiser’s Macrolichens of the Pacific Northwest) less of a nit-picky process, and more of a mystery to be uncovered. It also would be interesting to look at the differences in these proportions between two Usnea species collected in the same location at the same time of year back 50 years ago compared to one collected today – that may tell us a bit more about our local climate change experienced at the biological level. Of course the amount of variation of these percentages within a species would have to be found as well to determine if there has been a significant change in the proportions over time, or if that is just a normal variation between two different individuals.
Thallus: Some have lobes that look like they have distinct sides, these are called dorsiventral lobes, e.g. Evernia prunastri. But most fruticose lichen lobes are more radially arranged, kinda like spaghetti.
Size: this is major distinguisher for many species such as those in the Usnea and Bryoria genus, some Usnea are only 1 to 2 mm high. Differentiating these mature tiny species from immature larger species can be tricky.
Branching patterns of lobes are critical: i.e. when a lobe branches, is each subsequent lobe equal in diameter, or is one larger than the other?
Two growth forms: Some fruticose lichen have a thallus that consists of two growth forms – one that is fruticose and one that is squamulose. Budel and Scheidegger call these thallus verticalis and thallus horizontalis (respectively). Others, such as McCune and Geiser in their guide Macrolichens of the Pacific Northwest call these secondary thallus and primary thallus.
Thallus verticalis can be:
Podetia: originating from the primordia of the fruit body, bears apothecia, and are hollow. e.g. Cladonia.
Pseudopodetium: develops from primordia of thallus horizontalis. e.g. Stereocaulon.
Reticulate podetia: medulla and algal layer are partially exposed allowing for rapid uptake and loss of water, thus indicating very moist environments. e.g. Cladia.
Central Cord: this is found in the genus Usnea, the strand is made of “periclinally arranged, conglutinated hyphae that provide mechanical strength along the longitudinal axis” (46). Although Budel and Scheidegger do not elaborate on the function of this mechanical strength, perhaps this cord helps the lichen to endure the changing weight of the lichen as it absorbs and looses water from the surrounding air, and perhaps usnic acid plays some kind of role in either the ability to quickly absorb or release water, or in the pendant growth form of the Usnea lichen, or enhances the fragility of the the lichen’s structure and thus a large concentration of usnic acid requires the central cord.
The way the lobes fold together or hang loosely may also be other indicators, such as our air polluted Evernia prunastri (see blog post about trial run at Cheney wetlands) – these Evernia respond to eutrophic air pollution by curling up, almost creating a surface area that is less exposed to the air than the non-affected Evernia which let their lobes hang loose.
Also see future pages discussing the following articles:
“Response of desert biological soil crusts to alterations in precipitation frequency” by Belnap J. et al. Oecologia 2004.
“Life in extreme environments: survival strategy of the endolithic desert lichen Verrucaria rubrocincta” by Garvie et al. 2008.
Definitions of some critical terms :
- Exhabitant: the mycobiont
- Inhabitant: the photobiont
- Homoiomerous: non-stratified, no distinct parts or layers to the lichen – photobiont is found throughout the medulla.
- Heteromerous: stratified, photobiont is found in discrete areas of the thallus.
- Conglutinated hyphae: found in the foliose umbilicate lichen, attaches thallus to substrate; also found in the central cord of Usnea, a fruticose lichen.