Nephroma arcticum, the arctic kidney lichen, is a species of foliose (leafy), terricolous (ground-dwelling) lichen in the family Parmeliaceae. It has a yellowish-green thallus up to across made of large , with a dark, lower surface. It is a , consisting of a fungus and two photobiont partners: a species of nitrogen-fixing cyanobacteria (contained within dark, blistered cephalodia), and a species of green alga. First described by Carl Linnaeus in 1753, it is one of only two Nephroma species in North America that use green algae as their primary photobiont.
Nephroma arcticum is widely distributed throughout the circumpolar regions of Asia, Europe, and North America, where it grows in semi-open coniferous forests and mountainous birch forests on shaded and moist ground, on moss carpets or on mossy rocks. The species is ecologically significant for its role in nitrogen fixation and as a food source for various animals including reindeer and mountain goats. Alaska Natives have historically used it both as food and traditional medicine. While generally secure across its range, its abundance varies regionally, being common in many arctic and subarctic areas but rare or imperilled in some southern portions of its distribution.
The species has been the subject of extensive research into its photosynthetic characteristics, nitrogen metabolism, and bacterial associations. Studies have revealed several key adaptations of N. arcticum to arctic conditions, including its reflective thallus surface that regulates light exposure and physiological mechanisms that enable photosynthesis during short summer seasons. The species shows considerable variation in its symbiotic relationships, with high genetic diversity among its cyanobacterial partners and the ability to develop separate cyanobacterial growth forms under certain conditions. The lichen's southern distribution limits are determined primarily by biological factors, particularly gastropod grazing of nitrogen-rich cyanobacterial tissues, rather than by temperature tolerance.
Taxonomy
The species, originally named Lichen arcticus, was one of the first few dozen lichen species formally described by the Swedish botanist Carl Linnaeus in his 1753 work Species Plantarum. Linnaeus derived the phrase from his earlier Flora Svecica (1745), adding a single descriptive term, (smooth). He also referenced Flora Lapponica (1737). Linnaeus's original concept of the species was represented by five herbarium sheets in the Linnaean Herbarium (LINN), all annotated by Linnaeus himself. Two of these specimens were later selected as possible lectotypes: Reginald Heber Howe, Jr. (1912) designated specimen 1273.183, while Peter James and F. Joy White (1987) designated specimen 1273.180. Howe's typification has priority, and specimen 1273.183, which bears numbers from both Species Plantarum and Flora Svecica, is regarded as the lectotype, serving as the generitype of Nephroma.
Linnaeus's brief diagnosis included several of its key features:
the lichen has a creeping growth form, with flat, smooth, and lobed leaves (the thallus). are wide and obtuse, and the underside of the thallus is covered in hairs. the upper surface of the thallus is smooth, while the underside is snowy white. the lichen's reproductive structures (calyces, an early term for apothecia, the lichen's reproductive structures) were described as flat and oval, attached to small, lace-like extensions of the thallus.
Linnaeus noted that the lichen grows on the ground, particularly under junipers in northern Sweden. He further described the shield (apothecia) underside as "large, livid-flesh-coloured, about the size of a thumb", remarking that it is among the widest of all known lichens at the time. Linnaeus's description summarised what he considered the distinctive features of this species that allowed for its identification and differentiation from other lichens in the region.
Since its original description, the taxon has been proposed for inclusion in several genera throughout its taxonomic history, including Peltigera (Raeusch., 1797), Peltidea (Göran Wahlenberg, 1812), Parmelia (Linds., 1871), Nephromium (Johan Havaas, 1897), and Opisteria (Edvard Vainio, 1909). The Swedish lichenologist Gustav Torssell transferred it to the genus Nephroma in 1843, establishing the binomial name by which it is now known.
A 2002 molecular phylogenetics study confirmed that Nephroma forms a monophyletic group with Peltigera as its sister genus. Within Nephroma, N. arcticum was found to be relatively divergent from other species, suggesting an early evolutionary split. The study also demonstrated that tripartite species like N. arcticum, which contain both green algal and cyanobacterial photobionts, do not form a monophyletic group within the genus, indicating that the transition between bipartite and tripartite forms has occurred multiple times during the evolution of Nephroma.
In North America, it is commonly known as the "arctic kidney lichen". Other vernacular names include "green paw", and "greenlight lichen" ( in French).
Description
Nephroma arcticum has a foliose thallus that sometimes forms rosettes and sometimes is irregular, reaching up to about , or rarely up to across. Individual thalli can merge to form loose colonies up to wide. Individual of the thallus are up to wide; they are smooth or slightly pitted, tongue-like, and have ascending margins. The upper thallus surface is yellow-green, blue-green or bright green, and often glossy. In contrast, the undersurface is dull with paler margins, and is darker towards the centre. It has a thick . The absence of and rhizines helps distinguish this species from similar-looking Peltigera species.
Apothecia are common in Nephroma arcticum. They are red-orange to red-brown in colour, kidney-shaped, and typically measure in diameter. Spores are 23–30 by 4–5 μm, have a somewhat shape, and contain three septa (internal partitions). Conidiomata are rare; when they occur, they appear at the margins of the lobes. They produce conidia (asexual spores) 3–4 by 1–2 μm. The photobiont is green (from the genus Coccomyxa); cyanobacteria (from genus Nostoc) are present in large, bluish cephalodia that are readily apparent in moist thalli. Neither isidia nor soredia, vegetative propagules found in many lichens, occur in this species.
A blue-green (same fungal partner but different algae or cyanobacteria) of Nephroma arcticum was first reported from Norway in 1983. The chemistry of the blue-green phycotype is identical to that of the more common green phycotype, except for the absence of usnic acid in the blue-green.
The cephalodia of N. arcticum can occasionally grow into separate thalli of moderate size. These outgrowths represent a cyanobacterial form of the species that has been found in moist inland forests of British Columbia at lower elevations. While these forms resemble N. silvae-veteris, they can be distinguished by their chemistry (medulla K−, PD−) and their physical connection to typical N. arcticum lobes.
In North America, N. arcticum is one of only two members of the genus Nephroma that contain green algae as their primary photobiont, the other being N. expallidum (alpine kidney lichen). While both species possess cyanobacteria in cephalodia, N. expallidum can be distinguished by its narrower lobes with distinctly margins, and its cephalodia are not visible on the upper surface. N. expallidum also has a more restricted distribution, being confined primarily to arctic tundra and northern boreal woodlands. All other North American Nephroma species form symbioses exclusively with cyanobacteria, resulting in darker thalli that appear brown when moistened.
Chemistry
Lichen products that have been isolated from Nephroma arcticum include zeorin, usnic acid, hyphonephroarctin, nephroarctin, and phenarctin. Lichen products are confined to the green algal part of the thallus, not the cephalodia. The expected chemical spot test reactions for Nephroma arcticum are C–, K+ (yellowish), KC+ (yellow), Pd+ (yellow) in the thallus, and UV+ in the medulla.
The usnic acid in the cortex of N. arcticum serves an important photoprotective function. The compound occurs as tiny crystals outside fungal hyphae and helps screen harmful ultraviolet (UV) radiation while also reducing transmittance of photosynthetically active radiation to underlying cells. The concentration of usnic acid varies with habitat, with higher concentrations found in populations from well-lit alpine and subalpine sites compared to shaded lowland forest populations. This adaptation helps protect the photobiont from excess light exposure in high-light environments, while allowing maximum light penetration in shaded forest habitats. Other UV-absorbing compounds in the species, such as nephroarctin and phenarctin, show an opposite pattern with higher concentrations in shaded forest populations, suggesting these medullary compounds may serve different functions such as herbivore defense.
Aqueous extracts of the lichen have been shown in in vitro experiments to have antifungal effects against a wide range of fungi.
Habitat and distribution
The species has a primarily arctic range, occurring throughout northern Canada from Alaska to Newfoundland, with isolated populations extending into New England and Wyoming. In Canada, it is abundant (S5) in British Columbia, Labrador, Quebec, and Yukon Territory, while being rare (S1) in Saskatchewan and imperiled (S2) in Nova Scotia and New Brunswick. In coastal Alaska, the species can be found growing on tree trunks, particularly those of Sitka spruce (Picea sitchensis) and Western hemlock (Tsuga heterophylla) in high-elevation forests. In British Columbia, it occurs in the Spruce-Willow-Birch biogeoclimactic zone.
In Greenland, N. arcticum grows among mosses on soil in dwarf shrub heath. Nitrogen fixation by its cyanobacterial partner enables N. arcticum to outcompete neighbouring plants and colonise moss-covered rocks in nutrient-poor environments. The species reaches its largest sizes and produces abundant reproductive structures in arctic climates. It is classed as a "somewhat oceanic lichen", occurring most frequently in oceanic areas and more rarely in continental areas. One study of nearly 300 species of Western Carpathian flora (including vascular plants, bryophytes, and terrestrial lichens) suggests that Nephroma arcticum is one of the few species with a high probability of being a glacial relict. It is rare in the United Kingdom.
Ecology
Nephroma arcticum plays ecological roles in arctic and subarctic environments through its symbiotic relationships, nitrogen fixation in nutrient-poor soils, environmental adaptations, and interactions with diverse organisms.
Microbial associations and community structure
The bacterial communities associated with the thalli of Nephroma arcticum have been investigated. The composition of the identified endophytic and epiphytic bacterial groups changes depending on from where on the thallus they are sampled. Members of the Alphaproteobacteria, Betaproteobacteria, Gammaproteobacteria, Acidobacteria, Actinobacteria, Firmicutes, and Bacteroidetes are all components of the microbiota of Nephroma arcticum.
Nephroma arcticum hosts several lichenicolous fungi, including Everniicola flexispora, which creates bleached spots with dark rims. It has been recorded from the Murmansk Oblast, the Sayan Mountains, and from Finland and Norway. Another is Lichenopeltella lobariae, which infects the underside of the thallus, including the rhizines.