Lichens

EPE Triassic 250-200mya

Paleosols Plants Animals Insects

Lichens

Lichens appear to many people to be a plant form that goes back in deep time. A ‘lichen’ is when a fungus hugs an alga and doesn’t let go. In this sweet arrangement the fungus offers shelter, and algae feed the fungus. They’re separate species, but typically the fungi cannot survive on their own. So, lichens have long been studied as a single organism.

Lichens dominate about 7% of the planet’s surface. You’ll find more than 13,500 species embellishing soil, rocks, tree bark and even dead animals, even in Earth’s harshest climates.

In some ecosystems, like forests, drylands, and tundras, lichens can make up a large portion of the ground layer biomass, even dominating the landscape

Many people I ask 'How did soil form?' reply 'Lichens'. The BBC Earth ’ TV series hosted by Chris Packham, cite ‘lichens’ as the first possible soil. This programme set out to show "how plant life turned the Earth from a barren rock into a vibrant green world" But there is no further mention of soils.

Triassic Lichens

Thanks to Arizona's Petrified Forest Park

Lichens are indicators of air quality.

Different types of lichen on a petrified log

Colourful lichen on petrified wood

Fungi

The great majority (>95%) of endophytic (EF) and endolichenic fungi (ELF) are filamentous Ascomycota, concentrated in six classes of the subphylum Pezizomycotina (Arnold et al 2009). All known lichen-forming ascomycetes (ascolichens, AL; ≈20,000 species) comprise most of the remaining classes of this subphylum (James et al 2006)

Lichens weather rocks, but they do not make soils. They were not siginifcant in the early soil development, but they do come into their own in this period.

Rock weathering

Certainly lichens help weather rocks. “Numerous investigations of the interface between lichens and their rock substrates strongly suggest that the weathering of minerals can be accelerated by the growth of at least some lichen species. The effects of lichens on their mineral substrates can be attributed to both physical and chemical processes. The physical effects are reflected by the mechanical disruption of rocks caused by hyphal penetration, expansion and contraction of lichen thallus, swelling action of the organic and inorganic salts originating from lichen activity. Lichens also have significant impact in the chemical weathering of rocks by the excretion of various organic acids, particularly oxalic acid, which can effectively dissolve minerals and chelate metallic cations.” (Chen, Blume, and Beyer, 2000) Would they have been particaulry effective in the methane atmosphere and acidic conditions?

Were lichens the pioneer colonisers of terrestrial habitats, as suggested by many including Heckman et al 2001 ?

My hypothesis is that lichens diversified in this period to colonise rocks, helping weather them turning them into sediment that lead to soil formation. Many believe this happened 400 mya, but the evidence is that it did not happen till this Triassic period. Later, 100mya, similar developments occurred that may help us unravel what went on in this period

Many other plants benefit from the presence of lichens. The green algae component of lichens can transform nitrogen in the air, which is unusable to most organisms, into a form which is essential for life. This is especially important in arid climates where lack of nitrogen is known to limit productivity.

A new minimum age for the origin of the lichen symbiosis is now put at 415 Mya (Honegger et al 2012).

Nelson et al., 2019 determined that lichen-forming fungi (LFF) first evolved about 250 million years ago,

Gaya et al 2015 "found that anthraquinones evolved in these lichens (Teloschistaceae), allowed them to colonise swathes of unexploited habitats worldwide" about 100mya

Family Tree

Despite what many people think, lichens did not appear to be widespread till this period. One recent study, checking the family trees of relevant fungi and algae, overturned the idea that lichens are ancient. “Contrary to conventional wisdom that lichens were some of the earliest arrivals on land (eg. Heckman et al 2001), a study (Nelson et al., 2019) adds to the case that lichens most likely made their way to land some 100 million years after vascular plants, such as ferns. The study’s findings upturn our understanding of deep time, the tree of life and how lichens, plants and fungi transformed Earth’s evolving climate"

This fits with a previous study, where a lichenologist at Duke University, also found that lichens evolved with or after, but not before, vascular plants (Lutzoni et al., 2018). This points to plants not competing with lichens, and lichens not being the big climate changers that readied the world for vascular plants. But they do seem to play a vital role in the Triassic recovery.

New minimum age for origin of lichens

"Recently discovered fossils that represent the oldest known lichens, with photobionts and anatomy typical of extant lichens, suggest a new minimum age for the origin of the lichen symbiosis at 415 Ma (Honegger et al 2012). The earliest possible origin of extant lichens must have followed the origin of Pezizomycotina hyphae (590–467 Ma), contradicting a proposed Neoproterozoic or earlier origin of extant lichens and their status as pioneer colonizers of terrestrial habitats (Heckman et al 2001) Because EF are concentrated in the Pezizomycetes and Leotiomyceta, endophytism in the Ascomycota most likely originated in the stem lineage of the Pezizomycotina (≈590–467 Ma). It is not yet clear whether these early filamentous ascomycetes occupied embryophytes as endophytes, existed as saprotrophs, or were associated with terrestrial microalgae and cyanobacteria, perhaps as a prelude to endolichenic symbioses, followed by transitions to endophytism as new lineages of land plants originated and diversified" (Arnold et al 2009)

While the origins of lichens was around 400mya, their radiation is about 250mya (see above)

Triassic diversification

Nelson et al., 2019 determined that lichen-forming fungi (LFF) first evolved about 250 million years ago, in this Triassic period.— long after plants were originally rooted on land. The timing varies though, suggesting that different fungi developed their algae-hugging habits independently, and didn’t inherit it from one main ancestor.

Why & how?

The lichens diversified, but why and how?

They may well have been important recolonisers of rocks and crusts, following the great extinction, and so helped the vascular plants grow back. This is what they were supposed to have done 400mya, but there is no evidence from then.

Whereas, we can see that a particular group of fungal lichens did exactly this - about 100mya "We found that anthraquinones evolved in these lichens (Teloschistaceae), in conjunction with an ecological switch to exposed, rocky environments, allowed them to colonise swathes of unexploited habitats worldwide and sparking an acceleration in diversification. (Gaya et al 2015)

Lichens likely played a crucial and distinctive role in the recovery of Earth's ecosystems and soils after the EPE. While often overlooked due to their simple structure, lichens are resilient organisms that can thrive in harsh conditions, and they would have had a significant impact on stabilising and enriching soils during a time of extreme environmental stress.

The emergence and proliferation of lichens during the Triassic period were significant. Their development as key organisms during this time reflects the complex interplay between environmental conditions and biological responses following the EPE. The lichens were likely key players in the post-Permian recovery of terrestrial ecosystems, particularly in stabilizing degraded landscapes and contributing to soil formation and nutrient cycling. their resilience helping to rebuild ecosystems from the ground up, quite literally. Without lichens and their role in forming biological soil crusts, the recovery of soils—and by extension, more complex plant and animal life—would have been much slower and more difficult.

How Lichens Contributed to Post-Extinction Recovery:

1. Early Colonizers of Barren Landscapes:

Lichens are among the first organisms to colonize bare, rocky, or degraded surfaces, making them pioneers in ecosystem recovery. After the End-Permian extinction, much of the land was likely barren, stripped of vegetation, and left vulnerable to erosion due to the collapse of ecosystems.
Lichens are capable of surviving in extreme conditions (such as intense UV radiation, desiccation, and nutrient-poor environments), which would have been typical following the extinction event. Their ability to photosynthesize (algae or cyanobacteria component) and to resist drought and nutrient scarcity allowed them to establish themselves on rock surfaces, crusts, and soils that were otherwise uninhabitable by most plants.

2. Soil Stabilization and Protection:

Lichens form a protective layer on exposed surfaces, reducing wind and water erosion. By binding soil particles and adhering to rock surfaces, lichens would have helped prevent further degradation of already fragile soils.
This ability to stabilize the surface would have been particularly important in regions where biological soil crusts were forming, as lichens were a critical component of these crusts. Their presence would have slowed the loss of precious topsoil and helped preserve what little organic material was left after the extinction event.

3. Contribution to Soil Formation (Pedogenesis):

Lichens actively contribute to soil formation, a process known as pedogenesis. They do this through a combination of mechanical and chemical weathering of rock surfaces. Lichens secrete organic acids that can dissolve minerals from rocks, slowly breaking them down into finer particles that contribute to the development of soil.
Over time, as lichens decay and are replaced by new generations, they add organic material to the soil. This organic material is critical in building humus, which improves soil structure, moisture retention, and nutrient content.
Lichens also act as "biogeochemical agents" by accelerating the release of important nutrients (such as nitrogen, phosphorus, and potassium) from the mineral matrix, making these elements available for plant uptake as ecosystems began to recover.

4. Facilitating Plant Recolonization:

As lichens helped to stabilize the soil and enrich it with organic material, they made the environment more suitable for the recolonization of plants. Pioneering plants like ferns, mosses, and lycophytes would have been among the first to benefit from the nutrient-enriched soil left behind by lichens.
The presence of lichen-rich biological soil crusts would have also moderated surface temperatures and retained moisture, creating microhabitats where seeds and spores of early plants could establish themselves.

5. Symbiotic Partnerships:

Lichens are symbiotic organisms, typically consisting of a fungus and either an alga or cyanobacterium. This symbiosis enables lichens to photosynthesize (through the algal or cyanobacterial partner) and fix nitrogen (through cyanobacteria in some species), which is critical for soil fertility. By fixing nitrogen, lichens helped to replenish nutrients in nutrient-poor soils, accelerating the recovery of terrestrial ecosystems.
The nitrogen-fixing ability of cyanobacterial lichens would have been particularly valuable during the post-extinction period, when soil nitrogen levels were likely low due to widespread die-offs of plants and soil biota.

6. Surviving Harsh Environmental Conditions:

The End-Permian extinction event triggered extreme environmental conditions, including increased UV radiation (due to ozone depletion), acid rain, and global warming. Lichens are known for their ability to survive extreme climates, including high radiation, desiccation, and fluctuating temperatures.
Lichens' ability to enter a state of dormancy during dry periods and rapidly revive when conditions become favorable allowed them to persist in regions where other life forms would have struggled.

7. Albedo and Microclimate Regulation:

By covering the surface with light-colored crusts, lichens may have contributed to local albedo effects (reflecting sunlight) and regulated the microclimate. This could have reduced evaporation and mitigated some of the extreme temperature fluctuations on the surface, creating more favorable conditions for other organisms to colonize the land.

By stabilising soils, contributing to nutrient cycling, and facilitating the colonisation of plants, lichens helped kickstart the long process of ecosystem recovery. They are often viewed as pioneers in modern disturbed ecosystems, and they probably played a similarly vital role in this deep-time event.

Cold

They do appear to be more associated with colder biomes, if today’s distribution is anything to go by. The cold temperate ‘biome’ is characterized nowadays by boreal conifer forests and widely spaced trees with a ground cover of mosses and lichens and a short growing season The arctic biome represents a tree-less tundra with a diminutive flora of herbs, like grasses, mosses, lichens, and some dwarf shrubs, again because of short growing season. This may indicate the conditions from which the rebirth of the soil and plants had to emerge 250 mya.

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