| Pulchrocladia retipora | |
|---|---|
 | |
Scientific classification | |
| Kingdom: | Fungi |
| Division: | Ascomycota |
| Class: | Lecanoromycetes |
| Order: | Lecanorales |
| Family: | Cladoniaceae |
| Genus: | Pulchrocladia |
| Species: | P. retipora |
| Binomial name | |
| Pulchrocladia retipora | |
| Synonyms[3] | |
List
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Pulchrocladia retipora, commonly known as thecoral lichen, is aspecies offruticose lichen in the familyCladoniaceae. Found predominantly inAustralasia, itshabitats range from theAustralian Capital Territory to New Zealand'sNorth andSouth Islands, and even the Pacific region ofNew Caledonia, where it grows in coastal and alpineheathlands. The lichen features coral-like branches and subbranches with numerous intricate, netlike perforations. It is known by multiple names, with some sources referring to it by itssynonymCladia retipora, or thecommon namelace lichen.
The lichen was firstscientifically collected by the French biologistJacques Labillardière on boardBruni d'Entrecasteaux's 1792 expedition. Labillardière erroneously classified it as analga rather than a lichen.Pulchrocladia retipora holds the distinction of being the first Australian lichen to be scientifically documented. It is now thetype species of the genusPulchrocladia. This genus was established in 2018 as a result of amolecular phylogenetics-led restructuring of the Cladoniaceae.
The species was firstformally described byJacques Labillardière in 1806, asBaeomyces reteporus. Thetype specimen was collected fromTasmania.[4] This sample was obtained as part of the botanicalcollections he made during Labillardière's 1791–1794 voyage to theSouth Seas with the French explorerAntoine Bruni d'Entrecasteaux in an unsuccessful search for the French naval officerJean-François de Galaup, comte de Lapérouse. Labillardière had first collected the lichen in 1792, and published the description inNovae Hollandiae Plantarum Specimen, erroneously describing the lichen as an alga.[5] According to the Australian lichenologistRex Filson, this lichen was the first to be described for Australia.[6]
In itstaxonomic history, the species has been shuffled to several genera, some of which are no longer used or have beensynonymized with other genera.[3] In 1814, the Swedish botanistErik Acharius changed the spelling of thespecific epithet toretipora (fromretepora) when he introduced thenew combinationCenomyce retipora.[7] It is not known for certain if Acharius's change was intentional, as he still referred to thebasionym asBaeomyces retiporus Labill. The termrete translates to "net", and the correctconnecting vowel in such compounds is "i", makingretiporus the appropriate spelling. The original spelling by Labillardière is considered anorthographical variant due to the lack of evidence indicating a deliberate choice of spelling. Sinceretipora is widely recognized, the current spelling has been maintained to avoid confusion.[8]
Other genera that have housed the species arePycnothelia (Fée, 1825),Cladonia (Fries, 1826),Cladina (Nylander, 1868),Cladia (Nylander, 1876), andClathrina (Johann Müller, 1883).[3] Nylander proposed the genusCladia in 1870 to contain threeCladonia species based on their shared trait of having a cortex made ofconglutinate (stuck together), longitudinally aligned filaments. In 1883,[9] the Swiss botanistJohannes Müller Argoviensis suggested that the nameCladia could be confused withCladium (a genus ofsedges), and proposed a replacement genusClathrina.[10] Nylander persisted with the genus nameCladia, a decision that was largely accepted by later taxonomists.Cladia retiporus was the most prevalent name for the lichen for more than a century.[10]
Pulchrocladia retipora is now thetype species of genusPulchrocladia, which wascircumscribed in 2018 by Soili Stenroos, Raquel Pino-Bodas,Helge Thorsten Lumbsch, andTeuvo Ahti following a large-scalemolecular phylogenetic analysis of the familyCladoniaceae.[11]
It iscommonly known as the coral lichen,[12] or the lace lichen.[13]
Like most species in the family Cladoniaceae,Pulchrocladia retipora has acladoniiform growth form, meaning it has both a primary (horizontal) and secondary (vertical)thallus. Theprimary thallus is nodular (i.e. with small raised areas or swellings), white, and only lasts for a short period.[11]Podetia are thesecondary thallus that originate from the primary thallus. The podetia are typically up to 5 cm (2 in) tall, white to pale grey in colour, sometimes tinged pinkish or yellowing or superficially blackening at tips. They are rigid when dry, but become spongy when wet. They are irregularly or dichotomously branched, forming cushiony clumps. The wall is highly perforated (about 5–11 perforations per centimetre), with large, round to ellipsoidal holes. The surface of the podetium is continuouslycorticate and lackssoredia. The innermedulla is made of twisted strands ofhyphae with a cobweb-like form.[11] The lichen, nonetheless, generates soredia-like clusters of algal cells and hyphae within the strands of the inner medulla.[14]
Apothecia occur infrequently. When present, they are small, black,peltate, and crowd together at the ends of small, terminal branches (branchlets). The colour of thehymenium ranges from dark reddish-brown to black.Ascospores have dimensions of 25–27 by 5 μm. Theconidiomata end on branchlets, and are covered with translucent slime; they produce curved or straightconidia measuring 6 by 1 μm.[11] Specimens collected in Australia tend to have a brownish colouring, while New Zealand specimens range from pure white to grey, to grayish-green, or slightly yellowish.[15] Thephotobiont partner isgreen algae from theTrebouxia. Occasionally, free-living algae become trapped in the irregularly intertwined hyphae of the medulla.[16]
Usnic acid andatranorin are the majorsecondary compounds present inPulchrocladia retipora. Other compounds present in smaller quantities, identifiable throughthin-layer chromatography, areprotolichesterinic andursolic acids,[16] as well asrangiformic acid andnorrangiformic acids in many cases.[11] Usnic acid is thought to be responsible for theantimicrobial,antiviral andcytotoxicbiological activity ofPulchrocladia retipora lichen extracts tested inin vitro experiments.[17] The hue ofP. retipora is determined by the concentration of usnic acid in the thallus, leading to a colour spectrum that ranges from an opaque greyish-white through yellowish-white to a distinct yellow.[14]
Pulchrocladia retipora andP. corallaizon are closely related species, and their similarities may lead to confusion in identifying them. However, one can distinguish them by observing the inner-medulla ofPulchrocladia retipora, which is characteristically tightly packed. In the more maturepseudopodetia ofC. corallaizon, there may be areas where the inner medulla is missing or appears less dense. Nonetheless, the medulla always remains compacted in the top branches of the pseudopodetia, ensuring it never appears stranded or corticated.[18]
Pulchrocladia retipora is widely distributed throughout Australasia. In Australia, it has been recorded from theAustralian Capital Territory,New South Wales,Queensland,Victoria, andTasmania. In New Zealand, it is known from both theNorth andSouth Islands as well as theAntipodes Islands,Auckland Islands,Campbell Island, andChatham Islands. In the Pacific, it occurs inNew Caledonia.[11]
The coral lichen is common in subalpine peatbogs; it is often found in association with the lichensCladonia confusa,Rexiella sullivani, andStereocaulon ramulosum. It is found on peaty soils amongtussocks or inheaths comprisingDracophyllum andLeptospermum, most often at the margins ofNothofagus forests, infellfield, or rarely on surfaces such as rocks, logs, and sand dunes.[12] In themoorlands of the Meredith Range area in Tasmania, it thrives in well-drained, elevated locations, especially close to decomposingbuttongrass hummocks.[19]
The lichen replicatesvegetatively when new podetia grow from fragments of old podetia.[15] It has highly variable growth rates, ranging from less than 1 mm per year to up to a few centimetres per year.[20] It has been noted to grow in clusters, sometimes up a metre in diameter. The unique morphology of the lichen helps it to survive the exposed heaths it inhabits, as the coral structure increasesgas exchange, moderates temperature extremes, and maximises light and water access.[16]
Pulchrocladia retipora grows in cushion-like growths ranging in diameter from about 10 centimetres (4 in) to 100 centimetres (40 in).[12] The New Zealand botanist William Martin remarked finding square metre-sized cushions in the area ofLewis Pass in Canterbury, New Zealand. Lichen cushions about the size of afootball have been observed growing on the mountain range in Australia'sGrampians National Park. According to Martin, the lichen forms large growths only occur insubalpine zones, with lowland forms being only 5–10 centimetres (2–4 in) in size.[21]
The complex net-like structures ofPulchrocladia retipora are known asfenestrations.[16]Rosmarie Honegger referred to the thallus of this species as "likely to be among the most complex vegetative structures ever produced in the fungal kingdom".[22] The appearance of the lichen has been described as "of considerable beauty resembling lace or coral". As a result, it has been used infloral decoration andarchitectural design.[12] In his review ofThe Lichenologist journal's cover designs, German lichenologistRobert Lücking highlighted the design of volume 37, issue 1, from 2005, as particularly striking. This issue featured an image ofPulchrocladia retipora set against a blue-themed background.[23]
Understanding the branching pattern ofPulchrocladia retipora is important for comprehending its uniquedevelopmental biology, and some studies have focused on this aspect. The first structure to emerge from the primary thallus is a meristem, which is a solid bundle of tissue comprising only fungal cells. Two adjacentmeristem bundles give rise to the erect secondary thallus, thepodetium. These bundles continue to splitdichotomously, resulting in groups of three meristem bundles. Because the development of same-age bundles is unequal, the developmental differences become more pronounced as the meristem bundles grow farther apart. Splitting of the meristem is not synchronized between approximately same-age bundles; consequently, one meristem bundle or one side of the podetium can grow beyond the others. During its early development, the meristem ofP. retipora undergoes slight bends and twists. These early changes become noticeable later in the form of varying angles between the meristem bundles. Perforations that are developmentally unrelated to the central perforation tend to occur early in the lichenised tissuedistal to the meristem, while perforations that occur between meristem bundles happen later in development.[24]
ThePulchrocladia retipora lichen thallus has been successfully resynthesized from isolatedmycobiont and photobiont under laboratory conditions. In these experiments, lichenprimordia, consisting of fungalmycelia and enclosed algae appear after about one month. After four months, the cultures form small scales that are the starting units for the development of the complexhyphal network that becomes the thallus. Later, the scales differentiate into columns that grow together vertically to form a thin network, and more hyphae join and fuse together to strengthen and stabilize the network. Eventually, algaecolonise the network prior to further development of the inner medulla and connecting of the fenestrations. In the laboratory, the entire process takes about two years.[16]
The Baseline Air Pollution Station in Tasmania (part of theWorld Meteorological Organization-Global Atmosphere Watch network) has usedPulchrocladia retipora as abioindicator to identify howatmospheric nitrogen andsulphur deposition in Tasmania is affected byhuman pollution.[20]