Earth Scale Lichen
Collema tenax
Earth Scale Lichen (Collema tenax) is a gelatinous lichen species belonging to the family Collemataceae. Unlike leafy or crustose lichens, Collema tenax has a distinctive jelly-like, blackish-green thallus that swells dramatically when wet and shrinks to a thin, inconspicuous crust when dry. This remarkable ability to absorb and retain water has made it a subject of scientific interest in studies of desiccation tolerance and biological soil crust formation.
• Lichens are symbiotic organisms composed of a fungal partner (mycobiont) and one or more photosynthetic partners (photobiont — either algae or cyanobacteria)
• In Collema tenax, the photobiont is the cyanobacterium Nostoc, which enables the lichen to fix atmospheric nitrogen
• The species name "tenax" is Latin for "tenacious" or "tough," reflecting its resilience in harsh, dry environments
Taxonomy
• Frequently encountered on calcareous soils, limestone outcrops, and base-rich substrates
• Often one of the pioneer species colonizing bare, disturbed, or nutrient-poor ground
• The genus Collema belongs to the order Peltigerales, a group of lichens that predominantly associate with cyanobacterial photobionts
• Fossil and molecular evidence suggests that the Collemataceae family diverged during the Mesozoic era, though direct fossil records of Collema tenax itself are not well documented
Thallus:
• Dark olive-green to blackish when moist; dark brown to black when dry
• Gelatinous and swollen when wet, becoming thin, shrunken, and crust-like upon desiccation
• Lobes are rounded to irregular, typically 2–5 mm wide, often overlapping
• Upper surface smooth to slightly wrinkled; lower surface paler with scattered rhizines (root-like anchoring structures)
• Thallus thickness when hydrated: approximately 0.5–2 mm
Reproductive Structures:
• Produces apothecia (fruiting bodies) that are sessile to slightly immersed in the thallus
• Apothecia are reddish-brown to dark brown, flat to convex, 0.5–2 mm in diameter
• Asci are 8-spored; ascospores are ellipsoid, simple (non-septate), hyaline, measuring approximately 15–25 × 6–10 µm
• Also reproduces vegetatively through fragmentation of the thallus
Internal Structure:
• Lacks a distinct cortex — the photobiont (Nostoc) cells are distributed throughout the medulla in chains
• This homoiomerous internal structure (photobiont evenly dispersed) is characteristic of the Collemataceae family
Habitat:
• Calcareous and base-rich soils, including chalk, limestone, and marl
• Open, sun-exposed ground with minimal vascular plant competition
• Disturbed soils, track edges, and eroded slopes
• Often found in grasslands, steppes, and Mediterranean scrubland
Ecological Roles:
• Nitrogen fixation: the cyanobacterial partner (Nostoc) converts atmospheric nitrogen (N₂) into bioavailable ammonium (NH₄⁺), enriching nutrient-poor soils
• Soil stabilization: the gelatinous thallus binds soil particles, reducing wind and water erosion
• Moisture retention: the hydrated thallus can absorb several times its dry weight in water, creating microhabitats for microorganisms
• Pioneer colonization: among the first organisms to establish on bare or degraded ground, facilitating succession
Environmental Tolerance:
• Extremely desiccation-tolerant — can survive months of complete drying and rapidly resume metabolic activity upon rehydration
• Tolerates high light intensity and UV radiation
• Sensitive to heavy metal pollution and nitrogen deposition, making it a potential bioindicator species
Substrate:
• Requires calcareous or base-rich (alkaline) soils with a pH typically above 7.0
• Does not grow on acidic substrates
Light:
• Prefers full sun to light shade
• Requires open, unshaded conditions for optimal growth
Water:
• Highly drought-tolerant once established
• Relies on atmospheric moisture, dew, and rainfall rather than soil moisture
• The gelatinous thallus absorbs water directly from rain, fog, and humidity
Propagation:
• Primarily through thallus fragmentation — small pieces of thallus can be scattered over suitable substrate
• Spore propagation is possible but slow and less reliable in field conditions
• In restoration projects, soil crust slurries containing Collema tenax fragments have been experimentally applied to degraded land
Common Challenges:
• Extremely slow-growing — recovery from disturbance can take years to decades
• Vulnerable to trampling, off-road vehicle traffic, and overgrazing
• Competition from vascular plants and faster-growing mosses can suppress establishment
Fun Fact
Collema tenax is a master of resurrection — it can lose over 95% of its cellular water and enter a state of suspended animation for months, only to spring back to full metabolic life within minutes of rehydration. • When dry, the thallus becomes so thin and dark that it is nearly invisible against the soil surface — earning it the common name "Earth Scale Lichen" • Upon wetting, it swells dramatically, transforming from an inconspicuous dark film into a conspicuous jelly-like green-black mat • The cyanobacterium Nostoc, its photosynthetic partner, is one of the oldest known lineages of photosynthetic organisms on Earth, with fossils dating back over 2 billion years • Biological soil crusts containing Collema tenax are estimated to cover approximately 12% of the Earth's land surface and contribute significantly to global nitrogen cycling • In some arid ecosystems, lichen-dominated biocrusts fix more nitrogen per unit area than many legume crops • The extreme desiccation tolerance of Collema tenax has made it a model organism for studying anhydrobiosis — the ability to survive almost complete dehydration — with potential applications in preserving biological materials and developing drought-resistant crops
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