Lichens Overview

This phylum encompasses approximately 25,000 lichen species that associate with fungi and photobionts.
Lichens exhibit a symbiotic relationship, primarily with fungi known as Mycobionts, of which there are around 18,000 species identified.
Approximately 80% of all fungal components within lichens belong to Sac fungi, specifically the Ascomycota group.
The photobionts involved in lichens include algae and cyanobacteria.
Certain lichen species are sensitive to pollutants, particularly acid rain, which makes them useful as bioindicators for environmental changes.
Lichens are characterized by their slow growth rates and long lifespan, capable of thriving in environments where minerals and nutrients are scarce.
They have a unique capability to absorb and accumulate nutrients that are deposited in their environment through the atmosphere.

Lichens are often found in extreme habitats, particularly as primary colonizers on bare rock. They are exposed to severe conditions such as temperature fluctuations, high radiation levels, and desiccation.
They are described as dual organisms due to the combination of a fungal component (Mycobiont) and an algal component (Photobiont).
The fungal component of the lichen plays a key role in obtaining water and minerals, creating a thallus structure, and producing reproductive structures.
The algal component functions by living within the thallus and performing photosynthesis within it.
Fungi primarily belong to the Ascomycota or Basidiomycota groups, while the algae can be either prokaryotic cyanobacteria or eukaryotic green algae (Chlorophyta).

Lichens and moss initiate the process of primary succession.
After initial colonization, other species such as bluebell, yarrow, blueberry, juniper, jack pine, black spruce, aspen, balsam fir, and paper birch may follow.
A climax forest could develop over an extended period, potentially taking up to 1000 years for significant succession stages to occur.

A comprehensive categorization of photobionts includes:
- Trebouxia: Unicellular green algae commonly found in most fruticose lichens.
- Trentepohlia: Filamentous green algae frequently found in crustose lichens.
- Nostoc: A type of cyanobacteria (blue-green algae) that appears gelatinous, found in all gelatinous crustose lichens.

The thallus of lichens is primarily composed of fungal hyphae, which have distinct layers:
- Upper Cortex: Protective layer comprising tightly packed hyphae.
- Medulla: Middle layer that provides structural integrity and space for gas exchange.
- Lower Cortex: Protects the underside of the thallus.
The mycobiont typically reproduces sexually, as seen with the Ascomycota fungi, resulting in the production of cup-shaped apothecia.

Approximately one-third of lichen species reproduce asexually through specialized structures such as soredia or isidia:
- Soredia: Small clumps of hyphae associated with a few algal cells that can disperse and generate clones of the lichen.
- Isidia: Similar to soredia, these structures can also form clones and are involved in asexual reproduction.

Soredia: Found in structures referred to as soralia, each soredium consists of the surrounding hyphal clumps and algal cells.
Detailed structures include different layers within the thallus:
- Algal cells contribute to the photosynthetic capability.
- Fungal hyphae provide structural support and nutrient absorption.
Microscopic dimensions are indicated, with some lichen cross-sections displaying the upper cortex, medulla, lower cortex, and rhizines effectively.

Based on their morphology, lichens can be classified into three primary forms:
- Crustose Lichens: Characterized by a crust-like appearance, tightly adhered to their substrate.
- Foliose Lichens: Leaf-like structure with a defined upper and lower surface, often with marginal cilia.
- Fruticose Lichens: Hair-like or branched appearance, often resembling miniature shrubs.

The crustose structure is composed of the algal layer, medulla, and substrate, showcasing adherence to a solid base.
Example: Candelina submexicana.

The structure is layered, with cortex, algal layer, and medulla present, exhibiting a more developed system of nutrient uptake and photosynthesis.
Example: Peltigera canina, which includes rhizines and apothecia in its morphology.
Collema furfuraceum: Notable for its gelatinous foliose structure and associated colonies of Nostoc.

Theophrastus is credited with naming lichens during 370-285 B.C., deriving from the Greek word for 'wort' or 'eruption'.

The classification and ordering of lichens depend on the type of ascoma produced by the mycobiont:
- Ascomas include: Apothecium, perithecium, pseudoperithecium, etc.
Additional classification methods involve:
- Growth form analysis: Categorization based on overall morphology.
- Anatomical structure examination: Features such as rhizines and marginal cilia.
- Chemical tests: Using reagents like potassium hydroxide (KOH) or acids, leading to color changes or crystal formations for species identification.

Lichens serve critical ecological roles, including nesting material for certain bird species.
Certain lichen species are significant food sources, particularly for herbivores like caribou, deer, and mountain goats, especially during winter months when food availability is scarce.
Example: Cladina stellaris, known as the star-tipped reindeer lichen, is consumed by these animals.
Another example of an edible lichen is Umbilicaria esculenta, which, when correctly prepared, is consumed as food and has medicinal applications in cultures such as Korea and Japan.