General characteristics of arctic species and their adaptations in the context of changes in climate and ultraviolet-B radiation levels

This is Section 7.3.2 of the Arctic Climate Impact Assessment Lead Author: Terry V. Callaghan; Contributing Authors: Lars Olof Björn, F. Stuart Chapin III,Yuri Chernov,Torben R. Christensen, Brian Huntley, Rolf Ims, Margareta Johansson, Dyanna Jolly Riedlinger, Sven Jonasson, Nadya Matveyeva,Walter Oechel, Nicolai Panikov, Gus Shaver; Consulting Authors: Josef Elster, Heikki Henttonen, Ingibjörg S. Jónsdóttir, Kari Laine, Sibyll Schaphoff, Stephen Sitch, Erja Taulavuori, Kari Taulavuori, Christoph Zöckler

 

Plants (7.3.2.1)

For the past 60 years, arctic plant ecologists have been concerned with the adaptations and traits of arctic plants that enable them to survive in harsh climates (e.g., [1]). It is now important to consider how plants that are adapted to harsh environments will respond to climatic warming, and particularly how former adaptations may constrain their survival in competition with more aggressive species that are projected to immigrate from the south. Only in the past 20 years have ecologists considered arctic plant adaptations to UV-B radiation (e.g., [2]).

Plant adaptations to the arctic climate are relatively few compared with adaptations of plants to more southerly environments[3] for the following reasons[4]:

  • arctic plants have inhabited the Arctic (except for ice-free refugia) for a relatively short period of time, particularly in Canada and the Yamal Peninsula;
  • life spans and generation times are long, with clonal reproduction predominating;
  • flowering and seed set are relatively low and insecure from year to year; and
  • the complexity of the plant canopy is relatively small and the canopy is low, so that climbing plants with tendrils, thorns, etc. are not present.

Annuals and ephemeral species are very few (e.g., cold eyebright – Euphrasia frigida; and Iceland purslane – Koenigia islandica). Many arctic plants are pre-adapted to arctic conditions[5] and have migrated to the Arctic along mountain chains[6] or along upland mires and bogs. Although specific adaptations to arctic climate and UV-B radiation levels are absent or rare, the climate and UV-B radiation regimes of the Arctic have selected for a range of plant characteristics (Table 7.5).

The first filter for plants that can grow in the Arctic is freezing tolerance, which excludes approximately 75% of the world’s vascular plants[7]. However, many temperature effects on plants, particularly those with roots and in the long term, are indirect[8]. Plant nutrients in arctic soils, particularly nitrogen, are available to higher plants (with roots) at low rates[9] because of slow microbial decomposition and mineralization rates of organic matter constrained by low temperatures[10]. Arctic plants use different strategies for nutrient uptake[11], and different sources of nitrogen, which reduces competition among plants and facilitates greater plant diversity[12].

 Table 7.5. Summary of major characteristics of current arctic plants related to climate and UV-B radiation.

 

Climatic factor

 General effects on plants

Adaptations/characteristics of arctic plants

References 

 Aboveground environment

 

 

Freezing temperatures

 Plant death

 Evergreen conifers tolerate temperatures between -40 and -90 °C; arctic herbaceous plants tolerate temperatures between -30 and -196 °C

 [13]

 Ice encapsulation

 Death through lack of oxygen

 Increased anoxia tolerance

 [14]

 Low summer temperatures

 Reduced growth

 Increased root growth, nutrient uptake, and respiration

 [15]

 

 

 Minimized coupling between the vegetation surface and the atmosphere: cushion plants can have temperature differentials of 25 °C

 [16]

 

 

 Occupation of sheltered microhabitats and south-facing slopes

 [17]

 Short, late growing seasons

 Constraint on available photosynthetically active radiation and time for developmental processes

 Long life cycles

 [18]

 

 

 Slow growth and productivity

 [19]

 

 

 Dependence on stored resources

 [20]

 

 

 Long flowering cycles with early flowering in some species

 [21]

 

 

 Increased importance of vegetative reproduction

 [22]

 

 

 Clonal growth; clones surviving for thousands of years

 [23]

 

 

 Long-lived leaves maximizing investment of carbon

 [24]

 Interannual variability

 Sporadic seed set and seedling recruitment

 Dependence on stored resources

 [25]

 

 

 Long development processes buffer effects of any one year

 [26]

 

 

 Clonal growth

 [27] see also [28]

 Snow depth and duration

 Negative: constrains length and timing of growing season

 Where snow accumulates, snow beds form in which specialized plant communities occur

 [29]

 

 

 Where snow is blown off exposed ridges (fellfields), plants are exposed to summer drought, winter herbivory, and extreme temperatures

 [30]

 

 Exerts mechanical pressure on plants

 Responses and adaptations not measured

 -

 

 Positive: Insulation in winter (it is seldom colder than -5 °C under a 0.5 m layer)

 Low plant stature

 [31]

 

 Reduction of plant temperature extremes and freeze–thaw cycles

 Low stature to remain below winter snow cover reduces the risk of premature dehardening

 [32]

 

 Protection from wind damage, abrasion by ice crystals, and some herbivory

 Low stature to remain below winter snow cover; growth in sheltered locations

 [33]

 

 Protection from winter desiccation when water loss exceeds water supply from frozen ground

 Low stature to remain below winter snow cover; deciduous growth

 [34]

 

 Protection from chlorophyll bleaching due to light damage in sunny habitats

 Low stature to remain below winter snow cover; deciduous growth

 [35]

 

 Source of water and nutrients late into the growing season

 Zonation of plant species related to snow depth and duration

[36]

 Increased UV-B radiation levels

 Damage to DNA that can be lethal or mutagenic

 Reflective/absorptive barriers such as thick cell walls and cuticles, waxes, and hairs on leaves; physiological responses such as the induction or presence of UV-B radiation absorbing pigments (e.g., flavonoids) and an ability to repair some UV-B radiation damage to DNA

 [37]

 

 

 Repair is mediated through the enzyme photolyase that is induced by UV-A radiation. There is so far no indication of adaptations to UV-B radiation that are specific to plants of the Arctic

 [38]

 Variable atmospheric CO2 concentrations

 Increased atmospheric CO2 concentrations usually stimulate photosynthesis and growth if other factors are nonlimiting; an increased ratio of carbon to nitrogen in plant tissues

 Photosynthesis in Alaskan graminoids acclimated to high CO2 concentrations within six weeks, with no long-term gain

 [39]

 

 

 The dwarf willow (Salix herbacea) has been able to alter its carbon metabolism and morphology in relation to changing CO2

concentrations throughout the last 9,000 years

 [40]

 

 

 Species such as the moss Hylocomium splendens are already adapted to high CO2 concentrations; frequently experiencing 400–450 parts per million (ppm), and sometimes over 1100 ppm, to compensate for low light intensities under mountain birch woodland. These high CO2 concentrations are caused by both soil and plant respiration close to the forest understory surface in still conditions and when light intensities are low

 [41]

 Soil environment

 

 

 Low rates of nutrient availability, particularly nitrogen

 Reduced growth and reproduction

 Conservation of nutrients in nutrient-poor tissues

 [42]

 

 

 Long nitrogen retention time resulting from considerable longevity of plant organs and resorption of nutrients from senescing tissues and retention of dead leaves within plant tufts and cushions

 [43]

 

 

 Substantial rates of nutrient uptake at low temperatures

 [44]

 

 

 Increased surface area for nutrient uptake by increased biomass of roots relative to shoots (up to 95% of plant biomass can be below ground)

 [45]

 

 

 Associations with mycorrhizal fungi

 [46]

 

 

 Nitrogen uptake by rhizomes

 [47]

 

 

 Some arctic plants can take up nutrients in organic forms, bypassing some of the slow decomposition and mineralization processes

 [48]

 

 

 Dependence on atmospheric nutrient deposition in mosses and lichens

 [49]

 

 

 Soil movement at various spatial scales resulting from freeze–thaw cycles, permafrost dynamics, and slope processes |Freeze–thaw cycles heave ill-adapted plants from the soil and cause seedling death |Areas of active movement select for species with elastic and shallow roots or cryptogams without roots

 [50]; Chapter 5

 

 

 Shallow active layer |Limits zone of soil biological activity and rooting depth; shallow rooting plates of trees can lead to falling |Shallow-rooting species, rhizome networks

 [51]

 Biotic environment

 

 

 Herbivory

 Removal of plant tissue sometimes leading to widespread defoliation and death

 Arctic plants do not have some morphological defenses (e.g., thorns) found elsewhere

 [52]

 

 

 Many plants have secondary metabolites that deter herbivores; some substances are induced by vertebrate and invertebrate herbivores

 [53]

 

 

 Protected growing points; continuous leaf growth in summer; rapid modular growth in some graminoids; regeneration from torn fragments of grass leaves, mosses, and lichens

 [54]

 Competition

 Suppression of some species and increased dominance of others leading to changes in community structure

 Secondary metabolites in some arctic species inhibit the germination and growth of neighboring species

 [55]

 Facilitation

 Mutual benefits to plant species that grow together

 Positive plant interactions are more important than plant competition in severe physical environments

 [56]

 

 

 Nitrogen-fixing species in expanding glacial forefields facilitate the colonization and growth of immigrant plant species

 [57]

 

 

 Plant aggregation can confer advantages of shelter from wind

 [58]

 

 

 Hemiparasites can stimulate nutrient cycling of potential benefit to the whole plant community

 [59]

Many of the adaptations of arctic species to their current environments, such as slow and low growth, are likely to limit their responses to climate warming and other environmental changes. If changes in climate and UV radiation levels adversely affect species such as mosses that play an important role in facilitation, normal community development and recovery after disturbance are likely to be constrained. Many arctic plant characteristics are likely to enable plants to cope with abiotic selective pressures (e.g., climate) more than biotic pressures (e.g., interspecific competition).This is likely to render arctic organisms more susceptible to biological invasions at their southern distributional limits, while populations at their northern range limit (e.g., boreal species in the tundra) are likely to respond more than species at their southern limit to warming per se. Thus, as during past environmental changes, arctic species are very likely to change their distributions rather than evolve significantly.

Summary

Plant adaptations to the arctic climate are absent or rare: many species are pre-adapted. The first filter for arctic plants is freezing tolerance, which excludes approximately 75% of the world’s vascular plants. Short growing seasons and low solar angles select for long life cycles in which slow growth often uses stored resources while development cycles are often extended over multiple growing seasons. Some plant species occupy microhabitats, or exhibit behavior or growth forms that maximize plant temperatures compared with ambient temperatures. Low soil temperatures reduce microbial activity and the rates and magnitude of nutrient availability to the roots of higher plants. Mechanisms to compensate for low nutrient availability include the conservation of nutrients in nutrient-poor tissues, resorption of nutrients from senescing tissues, enhanced rates of nutrient uptake at low temperatures, increased biomass of roots relative to shoots, associations with mycorrhizal fungi, uptake of nutrients in organic forms, and uptake of nitrogen by rhizomes. Temperature fluctuations around 0 °C cause frost-heave phenomena that can uproot ill-adapted plants.

Snow distribution determines the period over which plants can intercept solar radiation and grow. Snow cover insulates plants against low air temperatures in winter and extremes of temperature in spring, protects plants from physical damage from abrasion by ice crystals, and provides a source of water often late into the growing season. Where snow cover is thin (e.g., on exposed ridges), growing seasons are usually long but water can become limiting; where snow accumulates in sheltered depressions, snow beds form in which specialized plant communities occur.

Many arctic plants are pre-adapted to relatively high levels of UV-B radiation. They exhibit various mechanisms to protect DNA and sensitive tissues from UV-B radiation and an ability to repair some UV-B radiation damage to DNA. Thick cell walls and cuticles, waxes, and hairs on leaves, and the presence or induction of UV-B radiation absorbing chemical compounds in leaves, protect sensitive tissues. There appear to be no specific adaptations of arctic plant species to high atmospheric CO2 concentrations.

Arctic plant species do not show the often-complex interactions with other organisms prevalent in southern latitudes. Arctic plants are adapted to grazing and browsing mainly through chemical defenses rather than the possession of spines and thorns. Facilitation increases in importance relative to competition at high latitudes and altitudes.

Thus, many of the adaptations of arctic species to their current environments are likely to limit their responses to climate warming and other environmental changes. Many characteristics are likely to enable plants to cope with abiotic selective pressures (e.g., climate) more than biotic pressures (e.g., inter-specific competition). This is likely to render arctic organisms more susceptible to biological invasions and they are very likely to change their distributions rather than evolve significantly in response to warming.

Animals (7.3.2.1)

Classical arctic zoology typically focused on morphological and physiological adaptations to life under conditions of extremely low winter temperatures[60]. Physiological studies contribute to a mechanistic understanding of how arctic animals cope with extreme environmental conditions (especially low temperatures), and what makes them different from their temperate counterparts. Ecological and evolutionary studies focus on how life-history strategies of arctic animals have evolved to tolerate environmental variation in the Arctic, how flexible life histories (in terms of both phenotypic plasticity and genetic variation) are adapted to environmental variation, and how adjustments in life-history parameters such as survival and reproduction translate into population dynamics patterns.

Adaptations to low temperatures

Arctic animals have evolved a set of adaptations that enable them to conserve energy in low winter temperatures. Warm-blooded animals that persist throughout the arctic winter have thick coats of fur or feathers that often turn white[61]. The body shapes of high-arctic mammals such as reindeer/caribou, collared lemmings, Arctic hares (Lepus arcticus), and Arctic foxes (Alopex lagopus) are rounder and their extremities shorter than their temperate counterparts (Allen’s rule). Body size within some vertebrate taxa increases toward the north (Bergman’s rule), but there are several notable arctic exceptions to this (e.g., reindeer/caribou[62]; muskox[63]). There are few physiological adaptations in homeotherms (warm-blooded animals) that are unique to arctic animals. However, several adaptations may be considered to be typical of the Arctic, including fat storage (e.g., reindeer/caribou and Arctic fox[64]) and reduced body-core temperature and basal metabolism in the winter (e.g., Arctic fox[65]). While hibernation during the winter is found in a few arctic mammals such as the Arctic ground squirrel (Spermophilus parryii), most homeothermic animals are active throughout the year. Small mammals such as shrews (Sorex spp.), voles (Microtus, Clethrionomys spp.), and lemmings with relatively large heat losses due to a high surface-to-volume ratio stay in the subnivean space (a cavity below the snow) where they are protected from low temperatures during the winter. Even medium-sized birds and mammals such as ptarmigan (Lagopus mutus) and hares seek thermal refuges in snow caves when resting. In the high Arctic, the normal diurnal activity patterns observed at more southerly latitudes are replaced by activity patterns that are independent of the time of the day (e.g., Svalbard ptarmigan – Lagopus mutus hyperboreus[66]).

In heterothermic (cold-blooded) invertebrates, hairiness and melanism (dark pigmentation) enable them to warm up in the summer season. Invertebrates survive low winter temperatures in dormancy mainly due to two cold-hardiness strategies: freeze tolerance and freeze avoidance[67]. Typically, supercooling points are lower in arctic than in temperate invertebrates. Freeze tolerance, which appears to be an energetically less costly strategy than extended supercooling, is a common strategy in very cold regions. Wingless morphs occur frequently among arctic insects, probably because limited energy during the short growth season is allocated to development and reproduction, rather than in an energetically costly flight apparatus. A short growth season also constrains insect body size and number of generations per year. Life cycles are often extended in time and/or simplified because invertebrates may need several seasons to complete their life cycles. Small body size in arctic insects seems to be a strategy to shorten generation time[68]. Moreover, individuals from arctic populations are able to grow faster at a given temperature than southern conspecifics (e.g., [69]). Thus, arctic invertebrates may be particularly efficient in utilizing relatively short warm periods to complete life-cycle stages.

A short breeding season also underlies several life-history adaptations in birds and mammals, such as synchronized breeding, shortened breeding season, specific molting patterns, and mating systems[70]. Although adjustments to low temperatures and short growth seasons are widespread in arctic animals, successful species cannot be generalized with respect to particular life-history traits[71]. Both flexible and programmed life cycles are common in polar arthropods[72].

While there are many examples that show that winter temperatures lower than species-specific tolerance limits set the northern borders of the geographic distribution of animals, there are hardly any examples that demonstrate that high temperatures alone determine how far south terrestrial arctic animals are found. Southern range borders are typically set by a combination of abiotic factors (e.g., temperature and moisture in soil invertebrates) or, probably most often, by biotic factors such as food resources, competitors, and natural enemies.

Migration and habitat selection

Many vertebrates escape unfavorable conditions through movement (either long-distance migration or more short-range seasonal movement) between different habitats in the same landscape. Seasonal migration to southern overwintering areas is almost the rule in arctic birds. Climate may interfere in several ways with migrating birds, such as mismatched migration timing, habitat loss at stopover sites, and weather en route[73], and a mismatch in the timing of migration and the development of invertebrate food in arctic ponds (section 8.5.6). Many boreal forest insects invade the low-arctic tundra in quite large quantities every summer[74], but few of these are likely to return in the autumn. Birds residing in the tundra throughout the year are very few and include species such as Arctic redpoll (Carduelis hornemanni), willow grouse (Lagopus lagopus), ptarmigan, raven, gyrfalcon (Falco rusticolus), and snowy owl (Nyctea scandiaca). Like several other arctic predators that specialize in feeding on lemmings and Arctic voles, the snowy owl emigrates when cyclic lemming populations crash to seek high-density prey populations elsewhere in the Arctic and subarctic. A similar nomadic lifestyle is found in small passerine seed-eating birds such as redpolls and crossbills (Loxia spp.) in the forest tundra. These birds move between areas with asynchronous mast years (years with exceptionally abundant seed production) in birch and conifers. A substantial fraction of the Arctic fox population migrates after lemming peaks and sometimes these migrations may extend far into the taiga zone[75]. Most reindeer and caribou populations perform seasonal migrations from coastal tundra in summer to continental areas of forest tundra and taiga in the winter. Inuit ecological knowledge explains caribou migrations as triggered by seasonal "cues", such as day length, temperature, or ice thickness[76]. Reindeer on isolated arctic islands are more sedentary without pronounced seasonal migrations[77]. Lemmings and ptarmigans shift habitat seasonally within the same landscapes[78]. In peak population years, the seasonal habitat shifts of the Norway lemming (Lemmus lemmus) may become more long-distance mass movements[79]. For small mobile animals (e.g., wingless soil invertebrates such as Collembola and mites), habitat selection on a very small spatial scale (microhabitat selection) enables individuals to find spatial refuges with temperature and moisture regimes adequate for survival[80]. The variability in microclimatic conditions may be extremely large in the high Arctic[81].

Adaptations to the biotic environment

Generalists in terms of food and habitat selection seem to be more common among arctic animals than in communities further south (e.g., [82]). This may be either due to fewer competitors and a less tightly packed niche space in arctic animal communities and/or because food resource availability is less predictable and the appropriate strategy is to opt for more flexible diets. Notable exceptions to food resource generalism are lemming predators (e.g., least weasels – Mustela nivalis, and several owls – Asio, Nyctea spp., raptors – Buteo spp., and skuas – Stercorarius spp.) and a number of host-specific phytophagous insects (e.g., aphids –Acyrthosiphon spp. and sawflies – Symphyta). Many water birds, such as geese and sandpipers (Calidris spp.) with 75 and 90%, respectively, of species breeding in the Arctic, are habitat specialists. Some species exhibit a large flexibility in their reproductive strategy based on food resources. Coastal populations of Arctic foxes with a relatively predictable food supply from the marine ecosystem (e.g., seabird colonies) have smaller litter sizes than inland "lemming foxes" that rely on a highly variable food supply[83]. Specialists on highly fluctuating food resources such as seeds from birch and conifers, as well as lemmings and voles, respond to temporary superabundant food supplies by having extraordinarily high clutch or litter sizes.

High Arctic environments contain fewer natural enemy species (e.g., predators and parasites) and some animals seem to be less agile (e.g., Svalbard reindeer[84]) and are possibly less resistant to disease[85].

Ultraviolet-B radiation

Little is known about animal adaptations to UV-B radiation. Non-migrant species, such as reindeer/caribou, Arctic foxes, hares, and many birds, have white fur or feathers that presumably reflect some UV-B radiation. There is some evidence, however, that feathers can be affected by high UV-B radiation levels[86], although this early research needs to be repeated. There is also a possibility that fur absorbs UV-B radiation. Eyes of arctic vertebrates experience extremes of UV-B radiation, from dark winter conditions to high UV-B radiation environments in spring. However, mechanisms of tolerance are unknown. Invertebrates in general have DNA that is robust to UV-B radiation damage[87] and various adaptations to reduce the absorption of UV-B radiation. Some subarctic caterpillars possess pigmented cuticles that absorb in the UV-B wavelengths, while pre-exposure to UV-B radiation can induce pigmentation[88]. Collembolans and possibly other invertebrates have dark pigmentation that plays a role in both thermoregulation and UV-B radiation protection[89]

Patterns of population dynamics

In tundra habitats, population cycles in small- to medium-sized birds and mammals are the rule, with few exceptions. The period of the population cycle in lemmings and voles varies geographically, and is between three and five years. Cyclicity (e.g., spatial synchronicity and period between peak population years) seems to be associated with spatial climate gradients (coast to inland and south to north) in Fennoscandia[90], although the biotic mechanisms involved are still much debated[91]. Some lemming populations show geographic variation in the cycle period within arctic Siberia; and also (for example) exhibit a long period (5 years on Wrangel Island) and a relatively short period (3 years in Taymir) between peak population years[92]. Within regions (e.g., northern Fennoscandia), small rodent cycles may show distinct interspecific synchrony over large spatial scales[93]. However, recent spatially extensive surveys in northern Canada[94] and Siberia[95] have indicated that the spatial synchrony of lemming populations is not as large-scale as the snowshoe hare (Lepus americanus) cycles in boreal North America[96]. This is at least partly due to the geographically variable cycle period.

Populations of small- and medium-sized bird and mammal predators follow the dynamics of their lemming and vole prey species[97]. The signature of lemming and vole population dynamics can also be found in the reproductive success and demography of mammals and birds, for example, waders and geese (e.g., [98]), that serve as alternative prey for lemming predators. Among northern insects, population cycles are best known in geometrid moths, particularly the autumnal moth (Epirrita autumnata), which exhibits massive population outbreaks with approximately 10-year intervals that extend into the forest tundra[99]. In the tundra biome, no herbivorous insects are known to have population cycles[100]. However, the population dynamics of tundra invertebrates is poorly known due to the lack of long-term data. Soil invertebrates such as Collembola[101] sometimes exhibit large interannual fluctuations in population density. Large fluctuations in numbers have also been observed in arctic ungulate populations (reindeer/caribou and muskox), and seem to be the result of several biotic factors in combination with climatic variation[102].

Summary: Implications for animal responses to climate change

Terrestrial arctic animals possess many adaptations that enable them to persist in the arctic climate. Physiological and morphological traits in warm-blooded vertebrates (mammals and birds) include thick fur and feather plumages, short extremities, extensive fat storage before winter, and metabolic seasonal adjustments, while cold-blooded invertebrates have developed strategies of cold hardiness, high body growth rates, and pigmented and hairy bodies. Arctic animals can survive under an amazingly wide range of temperatures, including high temperatures. The short growing season represents a challenge for most arctic animals and life-history strategies have evolved to enable individuals to fulfill their life cycles under time constraints and high environmental unpredictability. The biotic environment (e.g., the ecosystem context) of arctic species is relatively simple with few enemies, competitors, and available food resources.

For those reasons, arctic animals have evolved fewer traits related to competition for resources, predator avoidance, and resistance to diseases and parasites than have their southern counterparts. Life cycles that are specifically adjusted to seasonal and multi-annual fluctuations in resources are particularly important because such fluctuations are very pronounced in terrestrial arctic environments. Many arctic animals possess adaptations for escaping unfavorable weather, resource shortages, or other unfavorable conditions through either winter dormancy or by selection of refuges at a wide range of spatial scales, including microhabitat selection at any given site, seasonal habitat shifts within landscapes, and long-distance seasonal migrations within or across geographic regions.

Based on the above general characteristics, if climate changes, terrestrial arctic animals are likely to be most vulnerable to the following conditions: higher summer temperatures that induce desiccation in invertebrates; climatic changes that interfere with migration routes and staging sites for long-distance migrants; climatic events that alter winter snow conditions and freeze–thaw cycles resulting in unfavorable temperature, oxygen, and CO2 conditions for animals below the snow and limited resource availability (e.g., vegetation or animal prey) for animals above the snow; climatic changes that disrupt behavior and life-history adjustments to the timing of reproduction and development that are currently linked to seasonal and multi-annual peaks in food resource availability; and the influx of new competitors, predators, parasites, and diseases.

Microorganisms (7.3.2.3)

As a group, microorganisms are highly mobile, can tolerate most environmental conditions, and have short generation times that can facilitate rapid adaptation to new environments associated with changes in climate and UV-B radiation levels.

Adaptations to cold

The ability to resist freezing (and to restore activity after thawing) and the ability to metabolize below the freezing point are fundamental microbial adaptations to cold climates prevailing at high latitudes.

Cell viability depends dramatically on the freezing rate, which defines the formation of intracellular water crystals[103]. Cold-adapted microbial species are characterized by remarkably high resistance to freezing due to the presence of specific intracellular compounds (metabolic antifreeze), stable and flexible membranes, and other adaptations. Lichens are extreme examples[104]: the moist thalli of such species as Xanthoria candelaria and Rhizoplaca melanophthalma fully tolerated gradual or rapid freezing to -196 °C, and even after being stored for up to several years, almost immediately resumed normal photosynthetic rates when warmed and wetted. For five to seven months of cold and continuous darkness, they remain green with intact photosynthetic pigments. However, freeze resistance is not a unique feature of arctic organisms.

The ability of microorganisms to grow and metabolize in frozen soils, subsoils, or water is generally thought to be insignificant. However, microbial growth and activity below the freezing point has been recorded in refrigerated food[105] as well as in arctic and antarctic habitats such as sea ice, frozen soil, and permafrost[106]. Such activity has important implications for ecosystem function. Field measurements of gas fluxes in Alaska and northern Eurasia revealed that winter CO2 emissions can account for up to half of annual CO2 emissions[107], implying significant cold-season activity in psychrophilic ("cold-loving") soil microbes. Soil fungi (including mycobionts, the symbiotic fungal component of lichens) have been considered as the most probable candidates for the majority of the tundra soil respiration occurring at temperatures below 0 °C[108] because their live biomass was estimated to be ten times larger than that of cohabiting bacteria.

Winter CO2 emissions have been also explained by other mechanisms (e.g., the physical release of summer-accumulated gases or abiotic CO2 formation due to cryoturbation[109]). Most recent studies[110] agree that microbial growth is limited at about -12 °C and that occasional reports of microbial activity below -12 °C (e.g., continuous photosynthesis in arctic and antarctic lichens down to -17 °C[111] and photosynthetic CO2 fixation at -24 °C[112]) were not carefully recorded and confirmed. Under laboratory conditions, Rivkina et al.[113] quantified microbial growth in permafrost samples at temperatures down to -20 °C. However, the data points below -12 °C turned out to be close to the detection limits of the highly sensitive technique that they employed. The authors concluded that nutrient uptake at -20 °C could be measured, but only transiently, "whereas in nature (i.e., under stable permafrost conditions)... the level of activity, if any, is not measurable ..." [114].

Recently, a new precise technique was applied to frozen soil samples collected from Barrow, Alaska, and incubated at a wide range of subzero temperatures under laboratory conditions [115]. The rate of CO2 production declined exponentially with temperature and unfrozen water content when the soil was cooled below 0 °C, but it remained surprisingly positive and measurable (e.g., 8 ng C/d/kg) at -39 °C. A range of experimental results and treatments confirmed that this CO2 production at very low temperatures was due to microbial respiration, rather than to abiotic processes. The demonstration that microorganisms can survive low temperatures suggests the possibility that ancient bacteria of distinctive genotypes trapped in permafrost will be released and become active during permafrost thawing[116]. However, the period over which ancient permafrost is likely to thaw will be significantly longer than the next 100 years (section 6.6.1.3). Dark pigmentation causes higher heat absorption in lichens, which is especially favorable in the cold polar environment[117].

Adaptations to drought

Freezing is always associated with a deficiency of available water. Thus, true psychrophilic organisms must also be "xerotolerant" (adapted to extremely dry environments). A number of plants and microorganisms in polar deserts, such as lichens, are termed "poikilohydrous", meaning that they tend to be in moisture equilibrium with their surroundings[118]. They have high desiccation tolerance and are able to survive water loss of more than 95% and long periods of drought. Rapid water loss inactivates the thallus, and in the inactive state the lichen is safe from heat-induced respiratory loss and heat stress[119]. In unicellular microorganisms, drought resistance can also be significant, although mycelial forms of microbial life (fungi and actinomycetes) seem to have a much higher resistance to drought due to their more efficient cytoplasm compartmentalization and spore formation.

Adaptations to mechanical disturbance

Wind, sand, and ice blasts, and seasonal ice oscillations, are characteristic features of arctic environments that affect the colonization and survival of organisms. Most lichens are adapted to such effects by forming a mechanically solid thallus firmly attached to the substrate. Windswept habitats such as hillsides can be favorable if they provide a suitably rough substrate and receive sufficient moisture from the air. In contrast, shallow depressions or small valleys, although more sheltered, are bare of lichens because snow recedes from them only for very brief periods each season or persists over several years. This phenomenon is one reason for the so-called trimline effect (a sharp delineation on rocks between zones with and without lichens[120]). The abrasive forces of the ice at the bottoms of glaciers may destroy all epilithic (rock-attached) lichen vegetation, but lichens once established are able to survive long periods of snow cover, and even glacial periods[121].

Adaptations to irradiance

Strong pigmentation is typical for numerous microorganisms inhabiting tundra and polar deserts, especially for those that are frequently or permanently exposed to sunlight at the soil surface (lichens and epiphytic bacteria). Pigments (melanin, melanoids, carotenoids, etc.) are usually interpreted as a protection against strong irradiation. Pigmentation may be constitutive for particular species or appear as a plastic response to irradiance, for example, originally colorless Cladonia and Cladina lichens quickly develop dark-pigmented thalli after exposure to higher levels of solar radiation[122]. Buffoni Hall et al.[123] demonstrated that in Cladonia arbuscula ssp. mitis an increase in phenolic substances is specifically induced by UV-B radiation, and that this increase leads to attenuation of the UV-B radiation penetrating into the thallus. The accumulation of the protective pigment parietin in Xanthoria parietina is induced specifically by UV-B radiation[124], while in Cladonia uncialis and Cladina rangiferina only UV-A radiation had a stimulating effect on the accumulation of usnic acid and atranorin, respectively. Photorepair of radiation-damaged DNA in Cladonia requires not only light, but also high temperature and a hydrated thallus[125]. As in higher plants, carotenoids protect algae, fungi, and lichens from excessive photosynthetically active radiation[126] and perhaps also have a role in protecting them from ultraviolet radiation. In contrast to higher plants, flavonoids do not act as screening compounds in algae, fungi, and lichens.

Braga et al.[127] surveyed the UV radiation sensitivity of conidia (spore-forming bodies) of 30 strains belonging to four species of the fungus Metarhizium, an important biological insecticide. Exposing the fungus to UV-B radiation levels within an ecologically relevant range revealed great differences between the strains, with strains from low latitudes generally more tolerant of UV-B radiation than strains from high latitudes.

Algae

Seven inter-related stress factors (temperature, water, nutrient status, light availability and/or UV radiation, freeze–thaw events, and growing-season length and unpredictability) are important for life in arctic terrestrial and shallow wetlands[128]. Cyanobacteria and algae have developed a wide range of adaptive strategies that allow them to avoid or at least minimize injury. Three main strategies for coping with life in arctic terrestrial and wetland habitats are avoidance, protection, and the formation of partnerships with other organisms[129]. Poikilohydricity (tolerance of desiccation) and shelter strategies are frequently interconnected, and when combined with cell mobility and the development of complex life cycles, afford considerable potential for avoidance. The production of intracellular protective compounds, which control the cell solute composition and viscosity (changes in the carbohydrate and polyols composition of the cell), together with changes in cell wall structures (production of multi-layered cell walls and mucilage sheets) are very common phenomena. The association of cyanobacteria and algae with fungi in lichens provides the benefit of physical protection.

Summary

Arctic microorganisms are not only resistant to freezing, but some can metabolize at temperatures down to -39 °C. During winter, this process could be responsible for up to 50% of annual CO2 emissions from tundra soils. Cold-tolerant microorganisms are usually also drought-tolerant. Microorganisms are tolerant of mechanical disturbance and high irradiance. Pigmentation protects organisms such as lichens from high irradiance, including UV radiation, and pigments can be present in considerable concentrations. Cyanobacteria and algae have developed a wide range of adaptive strategies that allow them to avoid, or at least minimize, UV injury. However, in contrast to higher plants, flavonoids do not act as screening compounds in algae, fungi, and lichens.

As a group, microorganisms are highly adaptive, can tolerate most environmental conditions, and have short generation times that can facilitate rapid adaptation to new environments associated with changes in climate and UV-B radiation levels.

Chapter 7: Arctic Tundra and Polar Desert Ecosystems

7.1 Introduction
7.2 Late-Quaternary changes in arctic terrestrial ecosystems, climate, and ultraviolet radiation levels
7.3 Species responses to changes in climate and ultraviolet-B radiation in the Arctic
                 7.3.1 Implications of current species distributions for future biotic change
                 7.3.2 General characteristics of arctic species and their adaptations in the context of changes in climate and ultraviolet-B radiation levels
                 7.3.3 Phenotypic responses of arctic species to changes in climate and ultraviolet-B radiation
                 7.3.4 Genetic responses of arctic species to changes in climate and ultraviolet-B radiation levels
                 7.3.5 Recent and projected changes in arctic species distributions and potential ranges
7.4 Effects of changes in climate and UV radiation levels on structure and function of arctic ecosystems in the short and long term
                 7.4.1 Ecosystem structure
                 7.4.2 Ecosystem function
7.5 Effects of climate change on landscape and regional processes and feedbacks to the climate system
7.6 Synthesis: Scenarios of projected changes in the four ACIA regions for 2020, 2050, and 2080
7.7 Uncertainties and recommendations

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Committee, I. (2012). General characteristics of arctic species and their adaptations in the context of changes in climate and ultraviolet-B radiation levels. Retrieved from http://www.eoearth.org/view/article/152937

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