Diploschistes diacapsis
Diploschistes diacapsis is a ground-dwelling crustose lichen in the family Graphidaceae. It forms a thick, pale grey to greyish-white crust that cracks into small patches. Its fruiting bodies are sunken and often dusted with a powdery coating, and it produces brown, ascospores. The lichen body contains lecanoric and diploschistesic acids. The species typically grows in arid and semi-arid regions on calcium-rich or gypsum soils, where it can form extensive pale carpets within biological soil crusts. It is distinguished from similar soil-dwelling species by its thick, heavily thallus, eight-spored asci, and distinctive chemistry.
The species is most often reported from open, exposed habitats on calcareous or gypsiferous soils, where it can be a conspicuous component of biological soil crusts. It has been recorded across arid and semi-arid regions of North America and around the Mediterranean basin, with additional reports from Macaronesia, Eurasia, and northern Africa. Studies in south-east Spain and other dryland regions have examined its ecophysiology, its effects on runoff and erosion in soil crusts, and its interactions with lichenicolous lichens. In these environments, the lichen can influence water infiltration and erosion, and its thick crusts may affect the germination and establishment of certain plant species.
Taxonomy
The name Diploschistes diacapsis is based on Urceolaria diacapsis, described by Erik Acharius in 1810 from material collected on rocks and walls in England and Spain. In the original description, Acharius characterised it as a chalky white, finely cracked crust with immersed, urn-shaped apothecia whose black is dusted with a white, powdery coating and surrounded by a thick, inrolled rim ; he compared it with Urceolaria gypsacea and remarked that the almost free, inrolled margin gives it some resemblance to Gyalecta. Lumbsch transferred Acharius's species to Diploschistes in 1988, publishing the new combination Diploschistes diacapsis. To clarify which specimen the name refers to, he selected Lagasca's Spanish collection as the lectotype. Acharius had cited two original specimens, but Harriman's material belongs to an Aspicilia species rather than to Diploschistes. With that typification, he treated D. diacapsis as the correct name for the soil-dwelling taxon that had often been called D. steppicus.Molecular phylogenetic studies place Diploschistes within Graphidaceae and suggest the genus is most diverse in arid and semi-arid regions, with species growing on rock, soil, or over mosses and other lichens. The genus is traditionally characterised by a blackened rim of tissue around the fruiting body, sterile filaments alongside the spore-bearing structures, a Trebouxia algal partner, and the absence of a central column. Three main fruiting-body forms have been described:,, and . In that study, Fernández-Brime et al. placed D. diacapsis in Diploschistes subgenus Diploschistes. In their analyses, material identified as D. diacapsis fell into two distantly related clades. One clade was associated with thick thalli that become convex and loosely attached on gypsiferous and strongly calcareous inland soils. The other was associated with thinner, flatter thalli tightly attached to coastal soils, corresponding to material previously treated as D. diacapsis subsp. neutrophilus; on that basis, they supported recognising the coastal taxon at species rank as Diploschistes neutrophilus.
In a later six-locus study with broader sampling, specimens matching the two morphologies did not form separate clades. Thallus thickness and shape were treated as inconsistent characters in this group and the authors treated D. neutrophilus as a synonym of D. diacapsis; species-delimitation analyses also supported collapsing the two names. In an ITS-based phylogeny assembled for selected Diploschistes taxa, sequences identified as D. diacapsis grouped with D. neutrophilus, D. scruposus, and D. gypsaceus in a clade reported as having strong support in that analysis.
The common name "crater lichen" applies to the genus Diploschistes; the name "desert crater lichen" is used for this species.
Description
Diploschistes diacapsis is a soil-dwelling crustose lichen that forms a greyish to greyish-white crust broken by fine cracks into small, irregular . In the Greater Sonoran Desert flora, the thallus is described as cracked and sometimes warty, forming areoles 0.5–2.5 mm wide and a rough, dull, whitish to whitish-grey upper surface that may be scarcely to abundantly pruinose. The apothecia are urceolate, sessile, and up to 2.5 mm in diameter, with a concave, blackish disc. The thallus is often very thick and heavily pruinose, giving it a pale grey, sometimes slightly undulating appearance, and it may form patches more than 5 cm across; the areoles may reach about 2.5 mm in diameter, and rhizines are absent. The is a green alga of the Trebouxia type. The apothecia are usually urn-shaped, sometimes becoming more open, about 2–5 mm across, and may also be pruinose; the proper rim is brown and built of tightly packed tissue.Under the microscope, the hymenium is about 110–180 μm tall, with simple paraphyses 1–2 μm thick. The asci are subclavate to cylindrical and contain 4–8 spores. Ascospores are brown and muriform, typically 20–40 × 9–17 μm, with 3–6 transverse and 1–2 longitudinal septa. Asexual spores are produced in immersed pycnidia; the conidia are bacilliform, about 4–6 × 1.0–1.5 μm. D. diacapsis contains lecanoric acid and diploschistesic acid. In standard spot tests the thallus is K+ and C+, and PD−. D. diacapsis contains diploschistesic and lecanoric acids as major components and orsellinic acid as a minor constituent.
In gypsum soil-crust systems in central Spain, D. diacapsis has been reported to contain more than one Trebouxia photobiont lineage within a single thallus and to provide photobiont cells to some lichenicolous lichens, whose thalli either retain the host's microalgae or switch to different algal partners. On calcarenite in south-east Spain, Souza-Egipsy and colleagues reported that much of the thallus consists of fungal tissue mixed with fine mineral fragments incorporated during growth, and that calcium oxalate minerals occur within the thallus and were characterised using Raman spectroscopy.
Similar species
Diploschistes diacapsis can resemble Diploschistes muscorum, but that species is described as typically lacking pruina and being smaller and flatter with a thinner thallus, and it is reported mainly from non-calcareous soils. The lookalike, which begins life as a parasite on Cladonia species, also differs from D. diacapsis in having four-spored asci rather than eight-spored asci. The Sonoran Desert flora treats North American reports of Diploschistes ocellatus as misidentifications of D. diacapsis, stating that D. ocellatus does not occur in the Western Hemisphere; it is distinguished by lecanoroid ascomata and the presence of norstictic acid.Habitat and distribution
Diploschistes diacapsis is a soil-dwelling crustose lichen of open, exposed habitats, reported chiefly from calcareous soils and also from gypsum-rich ground. Lumbsch characterised its overall range as "northern xeric" and listed collections from the United States, Mexico, Macaronesia, Eurasia, and northern Africa; in Europe, he reported its northern limit as southern Switzerland. In western North America it is reported as common on gypsiferous soils in cool-desert regions, including coastal Sonora and southern California grasslands and chaparral, and it has also been reported from a wider range of soils farther south and east. In the Greater Sonoran Desert Region, D. diacapsis is described as common in semi-arid exposed habitats and coastal areas at 5–250 m and up to about 800 m in Baja California and southern California. It is also reported from higher elevations, occurring in open bushlands or open pine forests at about 1,600–1,800 m in Arizona and Chihuahua; the same flora describes its world distribution as nearly cosmopolitan in open habitats with Mediterranean to arid climates.Across multiple site reports, the species appears regularly in biological soil crusts in dry Mediterranean and steppe settings. In southern Tunisia it is reported as a dominant lichen in soil crusts of Stipa tenacissima steppe on alkaline sandy-loam soils with caliche layers and gypsum outcrops. In central Spain, it is among the characteristic crustose lichens of gypsum soil crusts and has been described there as a preferential gypsophyte, though also reported from other substrates. Spanish locality records include Triassic gypsum at Gobantes-Meliones in Málaga Province, where mean annual precipitation was reported as 427.9 mm, with about 64 precipitation days per year, and calcarenite at Cerro de Enmedio.
Ecology
In the Tabernas badlands of south-east Spain, slope aspect creates strong contrasts in microclimate and vegetation, and D. diacapsis is reported mainly from north-facing slopes and adjacent flatter pediments. There it may form extensive whitish carpets that have been associated with reduced erosion and with the development of plant patches, whereas the sunnier south-facing slopes are more eroded and commonly support little vegetation. Laboratory measurements were made by comparing thalli from north-facing and south-facing slopes and measuring exchange under controlled light, temperature, and thallus water content. In these experiments, the sun-exposed population generally showed higher net photosynthesis, while measures of photosynthetic efficiency and the light level where photosynthesis balances respiration were broadly similar between the two populations. After adjusting the results for dry mass and chlorophyll content, sun thalli still showed higher net photosynthesis. Shade thalli, however, had more lichen biomass per unit area and held more water per unit area, a pattern interpreted as supporting longer hydrated periods in shadier microhabitats; they summarised this as a trade-off between higher photosynthetic rates during shorter active periods on south-facing slopes and longer active periods with lower rates on north-facing slopes. Studies of continuous lichen-dominated soil crusts describe rapid ponding and runoff, reducing water infiltration into the soil, while still producing lower sediment loss than bare ground. Souza-Egipsy and colleagues investigated this pattern in D. diacapsis using scanning electron microscopy, and suggested that infiltration depends largely on breaks in the crust rather than on the crust alone.In a growth-chamber experiment using the steppe grass Macrochloa tenacissima, soil covered by a D. diacapsis crust delayed germination and reduced mean germination compared with bare soil: first germination occurred after 22 days and mean germination was 9%. Seedlings that germinated were less likely to emerge above the surface and their roots were less likely to penetrate into the soil, while seed viability was similar across treatments. Possible contributors included reduced water availability where the crust seals the surface, physical resistance from a thick, continuous thallus, and species-specific exudates.
On gypsum biological soil crusts of the Iberian Peninsula, D. diacapsis can act as a substrate for other lichen-forming fungi: Acarospora species may begin as parasites on D. diacapsis and later develop independent thalli, and Rhizocarpon malenconianum has been described as an obligate lichenicolous lichen on it ; Diplotomma rivas-martinezii is also reported to occur close to D. diacapsis in the same crust communities. Where it grows on exposed gypsum bedrock, thalli can shield the substrate beneath them from direct rainfall, while adjacent unprotected gypsum continues to dissolve and lower in surface level; this contrast can produce small conical or rounded gypsum mounds associated with the lichens, up to about 15 mm high, which Mottershead and Lucas described as an "umbrella effect". On calcarenite in south-east Spain, D. diacapsis has been reported to break down the rock by weathering the carbonate cement that binds mineral fragments, with hyphae growing between carbonate fragments, quartz grains, and mica within the substratum; the same study noted that little or no growth of other microorganisms occurred immediately adjacent to the crustose thallus, in contrast to the more microbially diverse communities observed beneath foliose lichens at the same site.
In the Colorado Plateau region it has been described as a good field indicator of gypsum-rich soils and may form large populations that locally dominate soil-crust communities, often reported with Psora decipiens, Gyalolechia bracteata, and Squamarina lentigera.