Placopsis Imshaugii

''Placopsis'' (bullseye lichen)USDA Natural Resources Conservation Service Name Search
/ref>Bullseye Lichen (Placopsis), Encyclopedia of Life
/ref> is a genus of lichenized fungi in the family Trapeliaceae.

''Placopsis'' Morphology

Most species of the ''Placopsis'' genus are characterized by a voluminous thallus.

Also distinctive to ''Palcopsis'' species are the characteristic radial patterns formed by distinct lobes in the center of the thallus. These radial patterns range in size but can span from the center to the very margin of the thallus.

The morphology of the lichen reveals its symbiotic relationship with its photobionts. The center of the lichenscephalodia structure houses colonies of cyanobacteria and the outer margins of the structure houses algal photobionts, the Placopsis antarctica showing a clear example of this type of morphological structure.

''Placopsis'' and ''Trapelia'' Comparative Phylogeny

The 60 or more species within the ''Placopsis'' genus are typically competitive strategists - they inhabit oligotrophic environments to ensure space for growth and little disturbance from competing species. ''Placopsis'' species are early successional pioneers which primarily reside on durable rock surfaces, though newer research finds them contributing to crust layers in soil. Researchers have mapped the thallus measurements of ''Placopsis'' against that other genera from the same lineage and have suggested that the symbiotic relationship between ''Placopsis'' and their cyanobacteria are what produce these distinctive thalli.

''Placopsis'' species’ symbiotic relationships with their cyanobacteria are an important evolutionary feature of this genus, distinguishing it from other precyanobacterial lineages such as its close relative, ''Trapelia''. ''Trapelia'' species, by comparison are often discovered in high-stress environments of more disturbed rock (such as gravel). They are characterized by a short lifecycle, and a small crustose thallus.

''Trapelia'' survives only off of the nutrients derived from symbiosis with a green algal photobion. Due to this inhibition, ''Trapelia'' thalli cannot grow larger than just a few millimeters in diameter. From ''Tapelia'' to ''Placopsis'', average thallus thickness differs by 150%. The low of thallic volume of ''Trapelia'' classifies it as a microlichen, as opposed to ''Placopsis'' which is classified as a macrolichen.

Other studies, however, suggest that the Genuses of ''Trapelia'' and ''Placopsis'' only differ in their symbiotic behaviors. Otherwise they both colonize rock because of the lack of ecological competition on this surface. Specimens of both ''Trapelia'' and ''Placopsis'' have been observed to grow larger on average when colonizing nutrient-rich surfaces, proving correlation between nutrient intake and thallus size

''Placopsis'' Symbionts

''Placopsiss remarkably thick thallic body is thanks to the advantageous microbial symbiosis between ''Placopsis'' and cyanobacteria. Cyanobacteria are photoautotrophs which can undergo photosynthetic processes to provide nutrients for their host organism.

The cyanobacteria which symbiotically reside in ''Placopsis'' are especially proficient at providing nitrogen that their acidic habitat lacks. It is the cyanobacteria’s supplemental Nitrogen that allows ''Placopsis'' to overcome size constraints and grow a notably thick and voluminous thallus.

In ''Placopsis'', green algae work in conjunction with the cyanobacteria as a photosynthetic aid or a photobion. Data collected from the estimated 15,000–20,000 species of lichen suggests that 10% of these species rely solely on the cyanobacterium as a photobiont. This same study assumes that 3% of the 15,000–20,000 species are "tripartite lichens" - meaning lichens which contain both green alga as a major photobiont and a cyanobacterium as the minor photobiont.

In addition to providing a nitrogen fixation for its host (converting nitrogen gas into ammonia or ammonium), cyanobacteria can also help protect its host from overexposure to the sun. The higher biomass that cyanolichens are able to accumulate allows them to better compete for sunlight.

''Placopsis'' Ecological Niche

Species of ''Placopsis'' are noted as constituents of biological soil crusts - crusts which are the first to form the uppermost millimeters of soil after it has been disturbed and aid in the process of compressing it. This compressing of soil is known as soil or ground consolidation, and it is an important process in dry climates because it protects the soil from undue erosion by wind and water.

Because ''Placopsis'' seek low-nutrient environments, ''Placopsis'' are often the main contributors of nitrogen to their habitats. ''Placopsis'' diversity is somewhat concentrated in the Southern hemisphere of the globe. In the arid, barren environments of the Southern hemisphere ''Placopsis'' finds most opportunity to serve their ecological niche. In the past few decades, research in the regions of Antarctica, New Zealand, and South Georgia has discovered new ''Placopsis'' species and revealed new ecological roles of these species.

In the nutrient-poor lands of Antarctica, species Placopsis Antarcitca was discovered to inhabit recently deglaciated rock faces which experience high rainfall. Placopsis Antarctica It is characterized by a more globular thallus with deep cracks, among other characteristics.

In New Zealand, ''Placopsis'' serve many different ecological niches in many different areas which range from rock to hard soil. Species ''Placopsis campbelliana'', and ''Placopsis erosa'' are recognized as ecologically significant in their contributions of nitrogen to the soil. However, ''Placopsis'' species ''Placopsis perrugosa'' and ''Placopsis Neofuscelia'' are also found on New Zealand's rock faces such as boulders, outcrops, and scree-slopes. Research conducted in the braided rivers of New Zealand attribute a great importance to species ''Placopsis illita'' in forming crusts on the soils of the river-beds. The braided river-region experiences great disturbances in the form of intense winds, sandstorms, and flooding. Along with mosses, vascular plants, and other lichens, The ''Placopsis illita'' thalli form crusts that help the soil resist these disturbances or else re-consolidate the soil after its been scattered. These crusts are visually striking, presenting usually as large brown swaths of color against the grey shingle that lines inactive river beds. However, the crusts are subject to parasitic fungal infections which make themsleves present by black spots within the swaths of brown. ''Placopsis''’s crust-communities are also under human threat, as their disappearance from the lower reaches of the braided may be linked to the anthropogenic activities of farming and recreation.

Species

, Species Fungorum (via the Catalog of Life) accepts 29 species of ''Placopsis''.
*''Placopsis ampliata''
*'' Placopsis antarctica''
*''Placopsis aspicilioides''
*''Placopsis bicolor''
*''Placopsis brachyloba''
*''Placopsis campbelliana''
*''Placopsis centrifuga''
*'' Placopsis clavifera''
*''Placopsis craterifera'' – Tanzania
*'' Placopsis cribellans''
*'' Placopsis dennanensis''
*'' Placopsis durietziorum''
*'' Placopsis elixii''
*'' Placopsis erosa''
*'' Placopsis fusciduloides''
*'' Placopsis gelida''
*'' Placopsis hertelii''
*'' Placopsis illita''
*'' Placopsis imshaugii''
*''Placopsis lambii''
*'' Placopsis macrospora''
*'' Placopsis murrayi''
*'' Placopsis parellina''
*'' Placopsis perrugosa''
*'' Placopsis polycarpa''
*'' Placopsis pruinosa''
*'' Placopsis rhodocarpa''
*'' Placopsis rhodophthalma''
*'' Placopsis stellata''
*'' Placopsis subcribellans''
*'' Placopsis venosa''

References

{{Taxonbar, from=Q7200416

Baeomycetales
Lichen genera
Baeomycetales genera
Taxa named by William Nylander (botanist)
Taxa described in 1861