Lichens

Lichens are interesting organisms that are actually a symbiosis between different organisms - a mycobiont (which is a fungus) and the photobiont (which is one or more of either a green alga or a cyanobacterium). The fungi provides the structure and the algae/cyanobacteria the green  ‘chlorophyll’ part.  Lichens are photosynthetic due to the photosynthetic properties of the algae or cyanobacteria that they contain.
Lichens are are classified by the fungal partner in this symbiotic relationship. 

The fungus partner in this symbiosis (the mycobiont) protects the alga/cyanobacterium (photobiont) in a tissue known as the thallus, it anchors it to a substrate, and it absorbs water and nutrients.   
The non-fungal partner in this symbiosis (the photobiont - either alga or cyanobacterium) is able to manufacture carbohydrates through photosynthesis. The fungus meets its need for carbohydrates by ‘harvesting’ it from the algae.

 This symbiotic relationship allows lichens to thrive in inhospitable environments, such as directly on rock.

There are now about 20,000 species of lichens. In Australia there are 3,500 known species of lichenised fungi spanning all habitats, including the arid interior.

Identification of lichens may be difficult, and can require fruiting bodies that are not always present.
Colours range from grey to orange, and they occur on many different substrates including dead branches, rotting logs, on the soil and on rocks.

[Definition of Substrate (in biology): the natural environment in which an organism lives, or the surface or medium on which an organism grows or is attached]

Lichens help to provide soil surface protection against erosion in dry habitats.
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Lichens form fascinating shapes and structures and a number of broad groups/forms are recognised:

1. Foliose 

Foliose lichens have leafy, lobed growth forms.
They are a sandwich consisting of fungal layer with an algal mat in the middle, folded with distinct edges that are not adhered to the substrate. They have circular, flat sheets of tissue, often leaf-like in shape
They have small rootlets called rhizines for attachment to the substrate.

2. Fruticose 

Fructiose lichens are markedly three dimensional and are attached to the substrate  by an anchor at their base. The rest of the body is free and either erect or pendulous, and slow growing.
The fungal layer is on the outside, and the algal layer within.
They are beard-like or tangled. 

Fruticose lichen growing in Victoria, Australia

3. Crustose 
Crustose lichens are crust-like and tightly attached over their entire underside to the substrate.
In some crustose lichens the body of the lichen is immersed within the substrate.

​Crustose lichens have three main structures:

1. Thallus – body of the lichen where photosynthesis and growth take place, always fused to the substrate.
2. Margin – the edge of the lichen where the fungus grows ahead of the photosymbiont, often a different colour, unlobed and flatter than the thallus
3. Apothecia – reproductive structures where spores are formed and disseminated, typically cup or wart shaped, and a different colour from the thallus

4. Squamulose
Squamulose lichens look like thin flakes or crumbs scattered over the surface

References:

https://www.anbg.gov.au/lichen/what-is-lichen.html

https://www.rbgsyd.nsw.gov.au/Science/Our-work-discoveries/Natural-Areas-Management/Ecology-of-Cumberland-Plain-Woodland/Plant-species-in-the-woodland/Lichens​

https://fungimap.org.au/about-fungi/lichens/​

https://en.wikipedia.org/wiki/Substrate​

Lichen, Pixie Cups, Victoria, Australia

Lichen, Pixie Cups, Victoria, Australia

Lichen's growing in Victoria, Australia

Lichen's growing in Victoria, Australia

Lichen growing on a tree trunk. The Blue Mountains, Australia.

Lichen's growing in Victoria, Australia

Fruticose lichen growing on a tree trunk in The Blue Mountains, Australia.