The Lichen Symbiosis:
Lichen "Extremophiles" and Survival on Mars
The Lichen Symbiosis: Lichen "Extremophiles" and Survival on Mars
Journal of Astrobiology and Space Science Reviews, 1, 378-397, 2019

The Lichen Symbiosis: Lichen "Extremophiles" and Survival on Mars
Richard A. Armstrong
Dept. of Vision Sciences, Aston University, Birmingham B4 7ET, United Kingdom


Abstract

There continues to be speculation regarding whether lichens could survive on Mars. Lichens have been described as ‘extremophiles’ and could potentially withstand some aspects of the hostile environment of the planet especially if they are ‘endolithic’ and live within the rocks as in some deserts and dry valleys in Antarctica. Lichens, however, are composite organisms and their existence assumes the successful establishment of a variety of microorganisms on Mars including eukaryotic green algae, blue-green algae (cyanobacteria), and various types of fungi. To date, although ‘lichen-like’ and ‘fungal-like’ structures have been described from the surface of Mars, there is no conclusive evidence of the presence of lichens or their symbionts on the surface. If lichens are present on Mars, they would be subjected to a considerably more hostile environment than the most extreme on Earth, which is regarded as at the limit of tolerance of present day lichens. The probable lack of sufficient liquid water over most of the surface and the problem of obtaining sufficient nitrogen and other nutrients are particular problems that Martian lichens would have to overcome. This review introduces various aspects of the lichen symbiosis and discusses the adaptation of lichens to extreme conditions on Earth with special reference to those living in very dry and/or cold environments such as deserts and the polar regions. Given their considerable adaptability, the pros and cons of whether lichens and their symbionts could survive on Mars, and the theoretical attributes of a ‘putative’ Martian lichen, are discussed.


Keywords: Lichen, Physiology, Adaptation, Arctic/Alpine environments, Antarctica, Deserts, Extremophile, Mars.


Introduction

Lichens can be found in every major ecosystem on Earth from the poles to the tropics but they only become a significant component of the flora in more extreme environments such as hot arid and semi-arid deserts and the cold polar regions (Mattick 1954). Lichens exhibit many of the characteristics of ‘stress-tolerant’ organisms, i.e., slow growth rates, low demands for nutrients, longevity, and adaptations to stressful conditions (Grime, 1979). In these stressful environments, lichens are subjected to a variety of extreme conditions including hot or cold temperatures, high light intensity, and extreme aridity. The ability of lichens to tolerate these conditions involves structural and functional adaptation and changes in ecological behavior, the lichens often adapting to relatively protected niches within a more extreme environment. As a consequence of the ability of lichens to survive in particularly extreme environments, they have been collectively termed ‘extremophiles’ (Rothschild & Mancinelli 2001).

There continues to be speculation regarding whether lichens could survive successfully on Mars. Lichens could withstand some aspects of the hostile environment especially if they live within relatively protected environments such as within the rocks in some deserts and the dry valleys of Antarctica (‘endolithic lichens’). Lichens, however, are composite organisms and their existence presupposes the successful establishment and survival of their symbionts on Mars likely to include eukaryotic green algae, various types of fungi, and blue-green algae (cyanobacteria). To date, although evocative ‘lichen-like’ (Dass 2017, Joseph 2014, Rabb 2018, Small 2018) and ‘fungal-like’ (Joseph et al. 2019) structures have been described from the surface of Mars from images captured by various Martian missions, there remains no conclusive evidence of present life on the surface.

If lichens have either evolved on Mars or have colonized Mars from earth, they would be subjected to a considerably more hostile environment than the most extreme on Earth. The probable lack of sufficient liquid water over most of the surface and the difficulties of obtaining sufficient nitrogen resources and other nutrients such as phosphate are particular problems that Martian lichens would have to overcome. This chapter introduces various aspects of the lichen symbiosis and discusses the adaptation of lichens to various ‘extreme’ environments on Earth including deserts, the poles, and at high altitudes. Given this extreme adaptability, the pros and cons of whether lichens and their symbionts could survive on Mars, and the theoretical attributes of ‘putative’ Martian lichens, are discussed.

The Lichen Symbiosis

Structure of lichens: Lichens are composite organism and comprise two or more different symbionts, the most frequent on Earth being eukaryotic green algae and a fungus (Farrar 1976). Most commonly, the green alga is a member of the genera Trebouxia, Myrmecia, Coccomyxa, or Trentepohlia, while the fungus is a member of the Ascomycotina. More recently, other types of fungi have been found in association with lichens including Cyphobasidiales ‘yeasts’ and members of the basidiomycota such as Tremella, thus extending the frequency and diversity of microbes present in association with lichens (Aschenbrenner et al. 2014, Tuovinen et al. 2019). The organisms are so intimately associated that the term ‘mutualism’ or ‘symbiosis’ has been applied to them.

A lichen is composed mainly of fungal tissue with eukaryotic algal cells embedded in the upper cortical layers (Fig 1a). Cyanobacteria (blue-green algae), especially members of the genera Nostoc, Scytonema, Stigonema, and Gloeocapsa may also be present in the thallus (Fig 1b), and may be confined to special structures called ‘cephalodia’. The algal partner carries out photosynthesis and supplies the fungus with carbohydrate but there is little experimental evidence to suggest that the fungus supplies nutrients directly to the alga (Smith & Douglas 1987).


Figure 1. Vertical section through lichen thalli showing the various layers: (A) a typical foliose lichen Xanthoria parietina (L.) Th. Fr. UC = Upper cortex, AL = Algal layer, M = Medulla, LC = Lower cortex, (Bar = 100 um) and (B) a foliose species of Solorina (L.) Th. Fr. AP = Apothecium, AL = Algal layer, M = Medulla, CB = Layer of cyanobacteria (Nostoc), Bar = 60 um.


Figure 2. The main growth forms of lichens: (A) the crustose lichen Rhizocarpon geographicum (L.) DC, (B) the foliose lichen Xanthoparmelia conspersa (Ehrh. Ex Ach.) Hale, and (C) the fruticose lichen Ramalina subfarinacea (Nyl. ex Cromb.) Nyl. The proportions of these growth forms often varies with type of extreme environment.

The benefit to the alga may be the protection afforded by the fungal thallus enabling the alga to survive in significantly more hostile environments, thus extending its range. There are three main types of lichen growth form (Fig 2), viz., the fruticose type in which the lichen thallus is attached to the substratum at a single point often forming a complex branched structure, the foliose type comprising a series of radially arranged leaf-like lobes, and the crustose type composed of a thin crust that is tightly attached to the substratum. Additional growth forms include the ‘placodioid’ growth form in which a largely crustose thallus has distinct marginal lobes, a gelatinous growth form in which cyanobacteria such as Nostoc are generally distributed throughout the thallus, e.g., as in members of the genera Collema and Leptogium, and the ‘squamulose’ growth from exemplified by species of Cladonia, many species comprising a scale-like primary thallus on which develop reproductive ‘podetia’ (Fig 3).

Most habitats on Earth have a mixture of growth forms. Many of the claims of ‘lichen-like’ structures on Mars resemble members of the Earth family Icmadophilaea such as Dibaeis baeomyces (Rambold & Hertel; Brodo 2001, Seminari 2018). Such specimens on Earth comprise a scale-like or crustose thallus, not particularly evident in the observed Martian specimens. Moreover, stalked fungal ‘podedia’ develop on the thallus which possess reproductive structures (‘asci’) at their tips containing ascospores. Many lichens only produce fungal spores and reproduction must involve ‘lichenization’ which results when a germinating spore contacts a suitable alga. However, many species have also evolved vegetative ‘diaspores’ which disperse the fungal and algal symbionts together (Bailey 1976). The most common methods of vegetative reproduction include isidia (Armstrong 1981), soredia (Armstrong 1991), and thallus fragments (Armstrong 1990). This is an important issue as lichenization on Mars would presuppose the continued survival in some form of the symbionts whereas if Martian lichens reproduce by exclusively vegetative means, survival of the symbionts would not necessary.


Figure 3. Additional growth forms of lichens: (A) the ‘placodioid’ lichen Caloplaca aurantia (Pers.) Hillb., (B) a ‘gelatinous’ lichen of the genus Collema, and (C) the ‘squamulose’ lichen Cladonia fimbriata (L.) Fr.

Lichen growth: Most foliose and crustose lichens grow radially over the substratum resulting in a flat rounded thallus, but growth rates can be extremely slow. Hence, many foliose species have radial growth rates (RaGR) in the range 2-5 mm year-1 (Armstrong & Bradwell 2011) while many crustose lichens grow significantly more slowly with RaGR less than 0.5 mm year -1 (Armstrong & Bradwell 2010). Some species in the Arctic grow so slowly that larger thalli may be over 5000 years old, thus making them potential candidates for the oldest living organisms on Earth (Beschel 1961).

The slow growth of lichens is not necessarily attributable to slower than normal physiological processes but to the fact that lichens may spend considerable times in a dehydrated state in which there is little physiological activity. When a lichen thallus is wetted, however, there is often a loss of carbon due to ‘re-saturation respiration’ (Smith & Molesworth 1973) (Fig 3). After wetting, photosynthesis begins to replace the carbon that was lost but the lichen has to remain wet for a sufficient period in the light to make good the carbon losses and then to make new carbon for growth (Armstrong 1976). Frequent rain showers combined with rapid rates of drying in the sun can continually deplete carbon within the thallus with little left over for growth.

As a result of slow growth, however, a lichen may make relatively little demand on the environment for nutrients thus enabling the organism to grow in potentially extremely nutrient-poor habitats, a strategy which may be especially valuable on Mars. Nevertheless, an important question is whether it would be possible for a lichen to make sufficient carbon for growth, survival, and reproduction under Martian conditions (Kidron 2019).

Polyols and stress protection Many Earth lichens contain the eukaryotic green alga Trebouxia, and photosynthesis results in the release of the carbohydrate as the polyol ribitol, which is then converted into arabitol and mannitol by the fungus (Richardson et al. 1968) (Fig 4). In some foliose and fruticose lichens, transfer of carbohydrate occurs within an extra-cellular envelope of hydrophobic proteinaceous material creating an ‘apoplastic continuum’ but this system is unlikely to be present in crustose lichens where the mechanism of transfer is less clear (Honegger, 1998, Clayden 1998). Recently, polyol transfer by ‘yeast’ polyol transporter proteins has been reported suggesting the presence of yeast-like fungi in the lichen symbiosis (Yoshino et al 2019).

In lichens which have Trebouxia as the algal partner, arabitol and mannitol are used as carbohydrate reserves, arabitol as a respiratory reserve while mannitol functions in stress resistance (Farrar 1973). More general stress-tolerant mechanisms are also likely to be present including those involving the anti-oxidant system (Li & Wei 2016). Hence, Kranner et al. (2005) investigated the effect of desiccation and irradiation on the level of reactive oxygen species (ROS). Anti-oxidant and photo-protective mechanisms in Cladonia vulcani Savicz, were more effective by an order of magnitude compared with the isolated symbionts and it was concluded that the alga and fungus may induce increased regulation of the protective systems in the other.


Figure 4. The consequences of wetting a dry lichen. Little carbon is made when the lichen is dry. When the thallus is wetted, there is a burst of re-saturation respiration and further carbon is lost from the thallus. Photosynthesis then begins to replace the carbon lost but the lichen has to be wet long enough in the light to overcome the carbon losses and to make new carbon for growth.

Extremophile Lichens

Desert Environments: Lack of water is clearly the main problem facing lichens living in deserts on Earth and likely to be a critical factor on Mars (Kappen, 1973, 1988). Several structural adaptations to the desert environment have been described. First, there is an increase in the frequency of crustose species which possess very thin thalli as conditions become increasingly dry, which reduces thallus area and the volume of tissue needing to be supported. Second, there are changes in lichen pigmentation, including species which are either light-colored (Galun 1963) or possess light-colored powdery ‘pruina’ on a darker thallus (Weber 1962).

In extremely dry conditions, dark colored lichens containing cyanobacteria (‘cyanolichens’) occur (Marton & Galun 1981) while on the gravel plains of Namaqualand, orange/yellow lichens are common, a pigmentation protective against high light intensities (Kappen 1983). Third, dry thalli regardless of pigmentation can resist high temperatures of up to 70°C without heat damage (Lange 1953). Fourth, increasing cortical thickness relative to the other layers occurs and may reach a maximum if thalli are frequently exposed to sand blast (Kappen 1988), a problem which may also be present on Mars as a result of dust storms. Fifth, some species of Buellia exhibit an ‘inversion’ of the typical thallus structure in which the more sensitive algal layer is located at the base of the lichen rather than at its surface, an adaptive response to high light intensity (Broady 1986).

Functional adaptations to desert conditions have also been observed. In Chondropsis semiviridis (F. Muell. Ex. Nyl.) Nyl., in south Australia, for example, the optimum temperature for net photosynthesis (nPS) is approximately 30°C, one of the highest recorded for lichens (Rogers 1971). In the Sonoran desert in north America, temperature optima for nPS were significantly lower indicating adaptation to the cooler, dimmer conditions characteristic of winter rain periods in this region (Nash et al. 1982a, 1982b). Cyanolichens in the Judean desert and Arava valley occur exclusively in rain tracks as they cannot utilize water vapor for photosynthesis (Llimona 1982). By contrast, Ramalina maciformis (Delise) Bory, can use water vapor in the Negev desert very efficiently, nPS being recorded at very low water potentials (Lang & Bertsch 1965).

Some desert lichens can utilize the occasional fogs which occur under less extreme conditions (Kappen, 1988). Such lichens often have a large surface area to trap moisture and which also minimizes wind damage in open coastal areas and hence, have a fruticose, pendulous, or tufted growth form. Such adaptations are also seen in some crustose species, e.g., Caloplaca corallinoides (Tuck.) Hulting,, which adopt a pulvinate, branched, or coralloid form. Although there is very little water in the atmosphere of Mars, this adaptation may be useful in resisting Martian dust storms.

Desert lichens can alter their ecological behavior in response to extreme conditions. In the Negev desert, for example, thalli may occur on northerly-exposed rocks or at more shaded sites while in the Namib, south-westerly exposed sites are more favored or sites located under translucent rocks (Vogel 1955). In addition, in the Negev, different taxa may be distributed either at the top or sides of the stones (Kidron, 2002). Hence, the tops of the pebbles receive twice the average daily dew compared with their margins but there is greater moisture at the margin after rain as a result of capillary action in soil adjacent to the pebble. In the driest parts of the Atacama desert in Chile, weakly-welded rhyolitic ignimbrite is frequently colonized by endolithic lichens, the porous interior of the rock which as well as encouraging hydration, also protects the thalli from damaging UV radiation and excessive levels of visible light (Wierzchos et al. 2013). In addition, the endolithic species Verrucaria rubrocinta Bruess, inhabits the ‘caliche plates’, resulting from subsurface precipitates dominated by calcite exposed on the surface of the Sonoran desert where temperatures may exceed 60°C in summer (Garvie et al. 2008). An upper micrite layer within the rock is highly reflective thereby reducing light intensity levels experienced by the alga. Hence, the endolithic growth form is an effective adaptation to the environmental extremes of exposed rock surfaces in hot deserts and also offers a possible model of lichen life on Mars.

Arctic environments: Arctic lichens face many extremes including high radiation levels, low temperatures, short periods when metabolic activity is possible, drought or wetness, long-lasting snow cover, and the erosive action of wind (Kappen 1988). Most species are crustose or fruticose and there is considerable morphological variation. Morphological variation in Cladonia uncialis occurs along a gradient from southern Finland into the Arctic, the ratio of length to thickness of internodes and area of algal cell layer being at their lowest but relative percentage of reproductive ‘pycnidia’ and perforate axils at their highest in the Arctic (Kärenlampi & Pelkonen 1971). Many species may find shelter against the cooling of the wind in small depressions (Link and Nash 1984). Some species are darkly pigmented, an adaptation to absorb heat and therefore to increase the availability of water by melting snow more rapidly (Nordhagen 1928).

By contrast, lichens in deeper depressions are light colored or ‘greenish’, which may be a response to lower light conditions under snow. The Arctic can also be strongly heated in summer but subject to slower rates of evaporation. Hence, widely distributed taxa such as Rhizocarpon geographicum (Armstrong 2013) may change their ecological behavior, being characteristic of well-lit southern facing surfaces in more temperate regions (Armstrong 2002) but north facing rocks in the Arctic (Pitman 1973). In addition, fruticose lichens often occur in dense light-colored mats which dissipate solar radiation and cause cooler soil surface temperatures thus conserving moisture (Kershaw 1978).

Functional adaptations of Arctic lichens frequently take advantage of the short periods when the environment is favorable (Kappen 1988). The growth of the fruticose species Cetrariella delisei (Bory ex Schaer.) Kärnefelt & A. Thell., was measured in the high Arctic (Uchida et al. 2006). Positive nPS was recorded when thallus water contents were high, photosynthetic rates being lower on clearer days due to lower thallus water content. Most Arctic species are tolerant of freezing conditions with many species exhibiting CO2 uptake (Kallio & Heinonen 1971) and recovery of nPS and respiration after prolonged storage at low temperatures (Lange 1966). Soluble carbohydrates in cryptograms are often highest in spring and summer, especially trehalose, a non-reducing disaccharide which accumulates as a result of heat, cold or osmotic stress (Avonce et al. 2006) and which may be involved in acclimation to low temperature and partial dehydration (Monteil 2000, Armstrong & Smith 2009).

Availability of nitrogen may be an important limiting factor for Arctic lichens and other regions characterized by cold temperatures (Bliss 1962, Russel 1940, Haag 1974). Lichens capable of fixing nitrogen are particularly common at the poles including species of Peltigera, Nephroma, Solorina (Fig 1b) and Stereocaulon. Availability of nitrogen is likely to be an important factor on Mars and therefore the possible existence of nitrogen-fixing cyanobacteria on the surface a critical factor (de Vera et al. 2014, Olsson-Francis et al. 2009).

Alpine environments: Alpine environments pose similar challenges to the Arctic (Billings and Mooney 1968, Billings 1973, Bliss 1962) but growing seasons are generally longer, i.e., up to 7 months below 3000 m at favorable sites and up to two months at higher latitudes. On clear days, alpine environments are characterized by alternating high temperatures and frost (Kershaw 1983). Diurnal temperature changes are generally more extreme under alpine conditions and differences compared with those in the Arctic increase significantly with decreasing latitude (Bliss 1956, Vareschi 1956).

Lichens exhibit several adaptations to the severe conditions posed by high altitude. Cetraria nivalis (L.) Ach., for example, accumulates the UV-absorbing phenolic compound usnic acid in the upper cortex at higher altitudes. Moreover, lichens at high altitudes experience alternating periods of desiccation and hydration and their pools of polyols may contribute to the protection of cellular constituents and the preservation of intracellular structures during desiccation (Aubert et al. 2007). Hence, high rates of nPS when wet at low temperatures help alpine lichens to take advantage of the brief periods of hydration associated with ice melt. In Chile, at high altitude the haplotype of the algal partner may change compared with lower altitudes where there is higher water availability in the form of fog, condensation, and precipitation. In Pseudovernia furfuracea (L.) Zopf., (Strobl et al 1994), differences in protein composition occur with altitude, with an increase in some proteins and a decrease in others. Furthermore, changes in fatty acid composition have been reported with decreasing levels of unsaturation with altitude and which may reflect changes in cellular membrane composition (Piervittori et al. 1994).

Antarctic environments: Lichens are often dominant on the Antarctic continent and its adjacent islands (Llano 1965, Ahmadjian 1970, Longton 1979, Lindsay 1978, Smith 1984). There is a marked moisture gradient from the coast to inland areas with the degree of salinity, nutrient availability, oxygenation, and ice cover as additional variables (Convey et al. 2014). Drifting snow can be an important factor in part of the region but snowfall is rare south of the Antarctic circle resulting in an essentially ‘polar desert’ and the ‘dry valleys’ (Smiley & Zumberge 1971) and which may represent the most useful analogue of the Martian environment. Consequently, there is a decline in species diversity, coverage, and growth rates from the maritime Antarctic to the continental dry valleys (Raggio et al. 2016) with water availability and length of the growing period as important variables.

Adaptive changes in Antarctic lichens are generally similar to those of Arctic and Alpine species (Ahmadjian 1970). The freezing conditions can cause several effects at the cellular level. Hence, Schroeter and Scheidegger (1995) examined the effects of extreme cooling in water saturated thalli of Umbilicaria aprina Nyl. from the continental Antarctic. When thalli were slowly cooled to subzero temperatures, ice nucleation occurred at -5.4°C followed by extracellular freezing affecting the algal cells and vacuolation in the fungi, changes which were reversible on warming. As a consequence, reproduction in these lichens is often via vegetative diaspores such as soredia rather than by lichenization, and early development can be very slow (Ott 2004). Cortical thickening occurs commonly with the addition of ‘dead’ layers of hyphae, and structures to ensure thalli are anchored to the ground to withstand wind (Dodge 1973). Crustose species are usually dominant but stunted fruticose species are also common (Dodge 1973). Some crustose species have a ‘stipitate’ or ‘pulvinate’ growth, which may be an adaptation to the higher humidity resulting from fog in the maritime Antarctic (Kappen 1988). Moreover, Antarctic Usnea species usually grow vertically on rocks but unattached prostrate forms also occur which may result from competition from more rapidly growing bryophytes (Kappen 1985). As in the Arctic, many of the continental species are darkly pigmented, yellow to green thalli often predominating in cracks and fissures (Hertel 1984).

The endolithic growth form found in desert environments is frequently present in Antarctica, Lecidea phillipsiana Filson, being a common species in the east of the region (Friedmann 1977). Three types of endolithic organisms have been described: (1) ‘chasmoendoliths’ which occupy fissures and cracks in rocks, the organism being partially exposed on the surface, (2) ‘cryptoendoliths’ which occupy pores and pre-existing structural cavities, and (3) ‘euendoliths’ that bore into relatively soluble rock substrates such as those rich in carbonate (Lawrey 1984). In addition, numerous microorganisms have been recorded in the Beacon sandstone rocks of the dry valleys. Cyanobacteria are also present but the dominant flora is chasmoendolithic and cryptoendolithic lichens. The lichens occupy a narrow zone of the subsurface of the rock 10 mm thick and form colonies from a few centimeters to a meter in diameter (Friedmann 1982). The lichens exhibit a similar structure to those living on the surface but a true fungal zone is absent, the fungal hyphae filling the available pore space. In cross section, a typical rock consists of a black zone just below the surface containing a green alga, most frequently Trebouxia, and below that a white zone of fungal tissue, then a eukaryotic green layer of non-lichenized green algae (Hemichloris antarctica) (Tschermak-Woess & Friedmann 1984), and finally, in some samples, a layer of cyanobacteria (Fig 6). Cryptoendoliths only occasionally develop sexual reproductive structures and these usually appear on the surface of the rock as alga containing areoles (Friedmann 1982). Various metals are mobilized in the rock and are carried upwards by capillary movement and downwards by snow melt resulting in the lichen layers being low in metals. The subsurface layers are often colonized by fungal hyphae resulting in the upper surface peeling away to expose the lichen tissue. Further penetration of the rock then occurs, more rock layers being lost and the consequence of this ‘biogenic weathering’ is a characteristic pockmarked surface which is also visible on some Martian rocks, e.g., those at the Viking landing site (Armstrong 2004).


Figure 5. Nutritional relationships between lichen symbionts. In lichens containing the eukaryotic green alga Trebouxia, photosynthesis results in the release of carbohydrate as the polyol ribitol, which is then converted into arabitol and mannitol by the fungus.


Figure 6. A possible model for a Martian lichen. Vertical structure of a cryptoendolithic lichen and associated microorganisms living in the rocks of the Beacon sandstone, Antarctica (based on Friedmann 1977);*Cyanobacteria present only in some samples.

The snow covered regions of the Antarctic retain the severe cold of winter for long periods preventing any early warming (Pannewitz et al. 2003). Hence, the lichens may be at subzero temperatures for prolonged periods and physiological activity occurs only when snow finally melts. However, nPS and dark respiration can occur at subzero temperatures in continental Antarctica (Schroeter & Scheidegger 1995). There is a substantial decline in lichen growth rates from the warmer, wetter peninsula to the colder dry valleys (Sancho et al. 2007).

Lichens in the maritime region have a higher photosynthetic capacity and a wider temperature range of response than their counterparts on the continent. In addition, an extremely low nPS rate is characteristic of the endolithic species. Hence, in studies of species of Umbilicaria and Caloplaca, there is a considerable difference in duration of activity in relation to microclimate, as measured by chlorophyll fluorescence, between the maritime regions and dry valleys, Caloplaca species in particular exhibit strategies to improve thallus hydration (Raggio et al. 2016). Successful adaptation of lichens to continental Antarctic conditions also depends on the algal partner as strong adverse effects of radiation on nPS may be expected (Cao et al. 2015). Nevertheless, no photo-inhibition was observed in thalli of Umbilicaris decussata (Vill.) Zahlbr., when the photo-protective pigment melanin was present (Sadowsky et al. 2016). Bartak et al. (2004) studied the effect of high light exposure on Umbilicaria antarctica E. Frey & M. Lamb, and which caused increased levels of oxidized glutathione and the conversion of violaxanthin to zeaxanthin, both of which are involved in antioxidant resistance.

Could lichens survive on Mars?

The Martian environment: Mars is an arid, barren, and rocky planet unprotected from UV light and cosmic rays. Both shortwave-UV and cosmic rays are damaging to life, the former being up to 1000 times more intense than on Earth, and both are much less likely to be attenuated by passage through the Martian atmosphere than on Earth (Cockell & Raven 2004). In addition, Mars is subjected to freezing temperatures and frequent sandstorms (Rummel et al. 2014). In the past, however, Mars may have been warmer, with oceans, and an atmosphere closer in composition to that of Earth. Over long periods of time, the surface water has been lost resulting in the arid conditions observed today.

Recent temperature measurements have confirmed that the daytime surface temperature may vary from 26.6°C during rare sunny days to -93°C at the poles in winter, air temperature rarely rising above zero and decreasing markedly with altitude above the surface.

In addition, the atmosphere of Mars is composed mainly of CO2Nevertheless, subsequent studies using theoretical climate models and experiments on Earth which simulate Martian environments (Kuznetz & Gan 2002) demonstrated that liquid water may be stable for extended periods of time on the Martian surface under present-day conditions. These studies culminated in the discovery of water ice near the edge of the southern polar cap by Mars Odyssey using the Thermal Emission Imaging System (THEMIS) (Titus et al. 2003). Hence, surface water ice may be widespread around and under the CO2 polar cap. In addition, air pressure is approximately 1% of that of Earth and wind speeds recorded at the Viking Lander site were 2 -7 m s-1 but under conditions of low gravity could rise to 17 -30 m s-1during dust storms. This combination of a very dry and largely cold desert-like environment for lichens is significantly more extreme than on Earth and potentially too harsh for significant photosynthetic activity (Kidron 2019).

Lichen symbionts on Mars: A range of microorganisms are involved in the lichen symbiosis on Earth including eukaryotic green alga, fungal members of the Ascomycotina and Basidiomycota, and cyanobacteria. Most Earth lichens contain eukaryotic green alga but there is little evidence for such alga on Mars. Green algae might be present adjacent to areas which occasionally fill with water and along ‘water pathways’ and some of these pathways have been observed to be covered by very thin ‘green mats’ suggestive of soil algae (Krupa 2017). In addition, green alga could be present within the surface layers of the rocks as in Antarctica. In addition, studies of rocks in the <3.7 Ga Gillespie Lake region of Mars by Noffke (2015) suggested sedimentary changes over time whose spatial and temporal characteristics were similar to microbiological mats on Earth. In addition, Ruffi & Farmer (2016) detected silica structures in sediments resembling micro-stromatolites in hot springs in Chile supporting the existence of microorganisms on Mars.

Algal photosynthesis has been investigated in a series of experiments carried out at the European Biopan facility (Raggio et al. 2011) and the International Space Station (ISS) in which lichens and their symbionts were exposed to space and simulated Martian conditions (Brandt et al. 2014, 2015). Photosynthetic activity was demonstrated post-exposure with high viability of algal cells and low rates of photosynthetic impairment (Meesen et al. 2014). However, a strong impairment of photosynthesis and photo-protective mechanisms was also observed when the isolated algae were irradiated emphasizing the importance of protection within the lichen to survive very dry conditions (Meesen et al. 2014). By contrast, in Aspicilia fruticulosa during a 10 day spaceflight, solar electromagnetic radiation exposure between 100 - 200 nm caused reductions in chlorophyll ‘a’ yield, a reduction which recovered after 72 hours reactivation, indicative of the lichens capability of repairing space damage (Raggio et al. 2011).

Putative fungi have been observed on Mars, many resembling members of the Basiodiomycota such as ‘puffballs’ with images apparently showing fruiting bodies emerging from the ground (Joseph 2016). Some images even purport to show spores both on the surface of the specimens and littering adjacent ground (Joseph et al. 2019). On Earth, however, puff-balls frequently ‘burst’ releasing their spores into the atmosphere but a different method of dispersal may be present on Mars where the atmosphere is very thin. Alternative interpretations of the ‘puff-balls’ include that they could represent geological structures which have been alternately exposed and then covered by dust and soil. In addition, the majority of lichen fungi on Earth are members of the Ascomycota and very few species incorporate members of the Basidiomycota (Hale 1967). Whether the puff-balls actually represent Basidiomycota, however, is an important question because it would dramatically increase the probability that microscopic Ascomycota also occur.

The presence of bacteria on Mars, either living today or as fossils of past life, has been highly controversial. On August 1996, NASA announced that there was evidence of life in a Martian meteorite (ALH84001) that had entered Earth’s atmosphere 13,000 years ago and landed in Antarctica. The existence of life in the meteorite was based on four lines of evidence. First, that the carbonate patterns had a unique life signature consistent with those expected of terrestrial bacteria. Second, polycyclic aromatic hydrocarbons (PAH), usually created by bacteria, were present in the meteorite. There has been particular controversy regarding the origin of the PAH which are found in meterorites rich in carbonates and which could be abiogenic or the product of cellular decay (Treiman 2003, Thomas-Keprta et al. 2009), the latter suggesting the involvement of ancient iron bacteria, algae, or fungi. A detailed discussion of this controversy can be found in Joseph et al. (2019). Third, magnetite globules present in the meteorite can be created by bacteria on Earth as well as by some chemical processes. However, only bacteria may have caused the distinctive tear-shaped globules present in the Martian rock. Fourth, worm-like structures were observed in the meteorite. These structures were much smaller than most bacteria but recently similar sized terrestrial fossils have been discovered. In addition, biogenic features have been found in three Martian meteorites, including eight of the amino acids that are constituents of terrestrial proteins, but there is still no conclusive proof that these represent ancient life (Gibson et al. 2001).

On the surface of Mars itself, experiments have also produced controversial results. Hence, Viking carried out experiments to detect the presence of organic materials in Martian soil. One such experiment (Viking Labelled Release VLR) detected possible biological activity, the amplitude and kinetics of which was similar to samples tested on Earth, especially from colder regions (Levin 1976a, 1976b, 2010). Results also contrasted with those obtained from control samples in being highly organized and more typical of biological activity (Biancianti et al. 2012). The results of a second sample were discounted by NASA while the positive result could have been attributable to superoxides or peroxides present in the Martian soil and which reacted with the test solution when it was mixed with these oxides.

There have also been attempts to detect atmospheric biomarkers of subsurface life on Mars. Bacterial life below the surface may depend on hydrogen and carbon monoxide as energy sources (Summers et al. 2002) and it may be possible to observe the metabolic ‘signature’ of these organisms as trace gases in the atmosphere. Organic trace gases in the atmosphere often have very short chemical lifetimes but methane (CH4) is more persistent and tends to be more uniformly distributed.

The existence of CH4 in the Martian atmosphere and it’s possible sources has been controversial and the subject of intense debate. Hence, a spike in CH4 concentration of 15.5 ppb was recorded on 16 June 2013 above the Gale crater by Mars express one day after a similar spike was observed by the Curiosity rover (Giuranna et al. 2019). In addition, a substantial spike of 50 ppb was observed in the northern hemisphere of Mars (Mumma et al. 2009), modelling studies suggesting a time-varying methane source close to the surface (Temell et al. 2019). The source of the CH4 may be gas trapped below or within ice which is being released episodically (Giuranna et al. 2019). In addition, adsorption onto and diffusion through rocks may reproduce the cycle of CH4 release if the rocks are assumed to be impregnated with CH4 from either a prior plume or from below via micro-seepage (Moores et al. 2019).

Lichen thalli on Mars: Being extremophiles, lichens fulfill some of the requirements for survival in a Martian environment. Hence, lichen thalli were exposed to space and simulated Martian conditions on the ISS (Brandt et al. 2014, 2015). Lichens in space are exposed to the combined effects of insolation, UV irradiation, cosmic radiation, extreme low temperatures, and a vacuum (Brandt et al. 2015). Hence, after long-term exposure of 559 days to these conditions, Xanthoria elegansTh. Fr., showed considerable resistance with post-exposure activity at 50-80% of normal in the alga and 60-90% in the fungus (Brandt et al. 2014). Nevertheless, a degree of desiccation-induced breakdown of cell integrity was also observed which was more severe under space than Martian conditions.

Detailed examination of images of the Martian surface has led to claims of the presence of ‘lichen-like’ structures (Dass 2017; Joseph 2014; Rabb 2018; Small 2018). In the paper of Joseph et al. (2019), these images resemble the reproductive podetia (stalks) of the Earth lichen D. baeomyces, a species also similar to Icmadophila ericetorum (Zahlbr. several species of the genus Baeomyces and some Cladonia species. The algal partner of D. baeomyces is the green alga Coccomyxa Schmidl., and on earth D. baeomyces colonizes unstable soil, loose sand, and dry clay in full sun, disturbed ground being preferred. The species is a lichenized member of the Ascomycota and the apothecia containing the asci and spores are 1-4 mm on podetia up to 6 mm long, similar to those revealed in Martian images (Joseph et al. 2019).

Observations in extreme environments on Earth have suggested that lichens could also live within Martian rocks. Hence, dehydrated Antarctic crypto-endolithic communities and colonies of rock inhabiting black fungi were exposed to simulated Martian conditions for 18 months at the ISS (Onofri et al. 2015), less than 10% of the colonies proliferated while 60% of cells and rock communities remained intact. Endolithic lichens on Mars may be able to better tolerate the low temperatures of the surface and as in the dry valleys of Antarctica, the rock subsurface is likely to be warmer and subjected to smaller fluctuations than the surface of the rocks. Nevertheless, the lack of a ready supply of surface water would be a significant problem for these lichens. It is a possible, however, that in certain areas, water ice on the surface may melt and penetrate the boulders, the lichens remaining in a dehydrated condition during the long intervening periods.

In Antarctica, CO2 exchange takes place very slowly through a relatively thick surface crust (Kappen & Friedmann 1983) and this could presumably also take place in Martian rocks. In addition, in regions of high light intensity, approximately 1% of the light reaches the lichen zone inside Antarctic rocks, the harmful UV being screened out by the dark-pigmented fungal layer and this process would be even more important on Mars. The main source of nitrogen for endolithic lichens is abiotically fixed nitrogen by atmospheric electric discharge, the fixed nitrogen then being conveyed to the rock by atmospheric precipitation. However, there are only trace amounts of nitrogen gas in the Martian atmosphere (Nier et al. 2003) and hence, it is unclear how Martian endolithic lichens would obtain their nitrogen supply. One possibility is that cyanobacteria in the rocks can fix sufficient nitrogen from the trace levels available to supply small populations of endolithic lichens. Joseph et al. (2019) also review the evidence for the possible presence of cyanobacteria on Mars.

Alternatively, if sufficient water to result in a cloud cover existed on Mars in the past and a greater concentration of atmospheric nitrogen, convection could have resulted in electrical discharge, formation of nitric oxides, and the presence of nitrates in rainfall These nitrates may ultimately have been incorporated into rocks and become a nitrogen source for present day endolithic lichens. There are likely to be sufficient sources of phosphate present as weathering profiles on Mars indicate calcium phosphate-rich minerals have been lost during past reactions with water. Hence, both a high phosphate content and potential release of phosphate may occur on Mars (Adcock and Hausrath 2015). Another potential concern is whether there is sufficient oxygen to support aerobic respiration. Nevertheless, thermodynamic considerations suggest that Mars may support a liquid environment with sufficient dissolved oxygen in the form of salt solutions to support respiration, which also may explain the highly oxidized phase in Martian rocks (Stamenkovic et al. 2018).

Putative Martian lichens: Given these adaptations and assuming Martian lichens resemble those on Earth in their adaptive properties, the following theoretical predictions may be made (Table 2). First, Marian lichens are likely to be very slow growing crustose species rather possessing a foliose or fruticose life form. Second, they are more likely to be endolithic, as in the Antarctic, rather than growing on the surface of the rocks or soil. Third, they are likely to propagate by vegetative reproduction using diaspores rather than by lichenization. Fourth, they are likely to spend much of their time in a dehydrated state and hence, to have extremely low nPS and very low growth rates. Fifth, if lichens do grow on the surfaces of substrata, they are likely to possess an ‘inverted’ thallus structure to protect the algae, to be darkly pigmented, or exist as scattered areolae on a black prothallus, the later to maximize the absorption of heat. Sixth, obtaining sufficient nitrogen is a significant problem for Martian lichens and association with nitrogen-fixing cyanobacteria may be essential.

Conclusions

Lichens are often described as ‘extremophiles’, a testament to their ability to adapt and thrive in some of the most extreme environments on Earth (Rothschild & Mancinelli 2001). A number of experiments have suggested that the lichen symbiosis is much more tolerant of extreme conditions than their symbionts. This resistance is likely to be attributable to morphological and physiological adaptations specific to the lichen such as the layered thallus, the presence of chemical compounds, and their ability to survive very dry conditions (Meesen et al. 2014). Hence, given this adaptability, lichens do fulfill some of the requirements for survival on Mars and appear to be able to survive conditions in space and simulated Martian environments for at least a period of time. Nevertheless, given our knowledge of the structure, function, and adaptability of Earth lichens and the Martian environment, is it conceivable that similar lichens could survive, grow, and reproduce where there is so little potential for physiological activity?

A particularly intriguing question is to what degree can extremophile lichens adapt to the even more extreme conditions on Mars? Possibly the slowest RaGR yet measured for a crustose lichen on Earth is by Benedict (2008) in a study over 16 years of Rhizocarpon superficiale (Schaer.) Vain. from the Front Range in Colorado where an overall average RaGR of 0.006 mm yr-1 being recorded. Hence, it is conceivable that given Martian conditions, lichens might exhibit a positive RaGR but only measurable over time-scales of hundreds or thousands of years.

Future Mars missions should increase our knowledge of the environmental conditions close to the ground, in the soil, and within the rocks, especially regarding water potential and the availability of oxygen and nitrogen, and will ultimately enable a more decisive conclusion to be reached regarding whether lichens could survive on Mars.

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