Mycorrhizae and changing climate

Mycorrhizae and changing climate refers to the effects of climate change on mycorrhizae, a fungus which forms an endosymbiotic relationship between with a vascular host plant[1] by colonizing its roots, and the effects brought on by climate change. Climate change is any lasting effect in weather or temperature. It is important to note that a good indicator of climate change is global warming, though the two are not analogous[2]. However, temperature plays a very important role in all ecosystems on Earth, especially those with high counts of mycorrhiza in soil biota.

Mycorrhizae are one of the most widespread symbioses on the planet, as they form a plant-fungal interaction with nearly eighty percent of all terrestrial plants[3]. The resident mycorrhizae benefits from a share of the sugars and carbon produced during photosynthesis, while the plant effectively accesses water and other nutrients, such as nitrogen and phosphorus, crucial to its health[4]. This symbiosis has become so beneficial to terrestrial plants that some depend entirely on the relationship to sustain themselves in their respective environments. The fungi are essential to the planet as most ecosystems, especially those in the Arctic, are filled with plants that survive with the aid of mycorrhizae. Because of their importance to a productive ecosystem, understanding this fungus and its symbioses is currently an active area of scientific research.

History of mycorrhizae

First wave – Triassic

Mycorrhizae and their related symbioses have been around for millions of years – dating as far back as the Triassic Period (200–250 million years ago) and even older.[5] While there are still many gaps in the timeline of mycorrhizae, the oldest known forms of the fungal group can be dated back as far as 450 million years ago or older, where the first wave the eukaryotic fungi came about alongside the evolution of early land plants.[5] There are some later lineages that consisted only of arbuscular mycorrhizae until the early Cretaceous Period (75–140 million years ago) when the clade began to drastically branch off into various forms of mycorrhizae, most of which would be specialized to particular niches, environments, climates, and plants.[5] However, these lineages are separate from the lineages that other major types of mycorrhizae derived from. There are essential mycorrhizae that evolved from other symbioses such as Ascomycota, (which shares a phylum with Basidiomycota, another major mycorrhiza) which evolved to eventually become Ericoid mycorrhizae or Ectomycorrhizae.[6] Some of the derived families are more complex due to specialized or multifunctional roots, which were not present in earlier times before Pangaea. The climate of the environments these groups of mycorrhizae occupied (which developed on rocky surfaces) were arid, not allowing for much diversification in life due to fixed niches.[6] The downside to looking into the history of most fungi and plant symbioses is that typically, fungi do not preserve very well, so finding a fungal fossil of more ancient periods is not only difficult, but offers only specific information about the fungus and the environment in which it developed.[6]

Second wave – Cretaceous

This diversification in both plants and mycorrhizae brought about their second wave of evolution within the Cretaceous period, which introduced alongside arbuscular mycorrhizae three new types of mycorrhizae: orchid mycorrhizae, ericoid mycorrhizae, and ectomycorrhizae.[5] The taxonomic diversification of all plants with and without mycorrhizal symbiosis shows that 71% makes up arbuscular mycorrhizae, 10% makes up Orchidaceae, 2% make up ectomycorrhizae, and 1.4% make up ericoid mycorrhizae.[5] The defining feature of this wave of evolution was the consistency of root types (or in other words, the similarities shared between root types, though characteristically different for individual families or even species) within the families that allowed for appropriate symbiosis with the plants of the period.[7] The environments of this period had a radiation of angiosperms, showing a different reproductive strategy than before and providing distinct morphological traits for most varieties of plants as opposed to prior periods and before the K-Pg extinction event.[8] The climate that allowed for these developments could be described as relatively warm, leading to higher sea levels and shallow inland bodies of water.[8] These areas were occupied mostly by reptiles that fed on animals, and insects that fed on plants, showing a more complex ecosystem than was present in the Triassic period and further pushing evolution in plants and mycorrhizae via ever-present natural selection. There is plenty of plant evidence to support most of these findings; however, the information necessary to form hypotheses regarding the mycorrhizae of the time, as well as other related symbioses, is incredibly limited as the fossilization of such individuals is very rare.

Third wave – Paleogene

The third wave of evolutionary diversification began in the Paleogene Period (24–75 million years ago) and is closely linked with change in climate and soil conditions.[5] The conditions that caused these changes are mostly due to an increase in disturbed niches and environments and the warming of global ecosystems, causing a shift in mycorrhizal types in plants within more complex soils.[8] This wave consists of lineages of plants with root morphologies that are often inconsistent with the previously mentioned families from the second wave.[5] These would be referred to as "New Complex Root Clades," due to the complexities that would arise in peculiar environments between ectomycorrhizal and nonmycorrhizal plants.[5] While both the second and third waves are linked to climate change, the defining feature of the third wave is the increased variability within the families and complexities in plant-fungus associations.[8] These stretches of diversification were brought about by an initially hot and humid climate, but became cooler and drier over time, forcing genetic drift.[8]

These three waves are what help divide and organize most of the mycorrhizae timeline without getting into specific genera and species. While it is important to mention the distinction of these fungal types and their differences, it is equally important to recognize their counterpart plant diversification as well. There are a number of notable nonmycorrhizal plants that speciate during the Cretaceous Period—while there was a spread in mycorrhizal plants, there was also a spread in nonmycorrhizal plants. This all helps play into a clearly picture of the distribution of plants and their symbiotic fungi over the course of an Earth's history.

The effect of climate on plants and mycorrhizae

There are various effects that a changing climate can have on the numerous species found within an ecosystem. This includes plants and their symbiotic relations. As it is understood, any particular mycorrhiza is expected to be both present and abundant in any of its respective niches so long as the environment can support its growth. However, sustainable environments are becoming uncommon due to the effects of a warming, changing climate. It is important to note that the relationship between the vascular host plant and mycorrhizae is mutualistic. This means global environmental change first affects the host plant, which in turn impacts the mycorrhizae in a very similar way. Essentially, if the host plant experiences environmental stress, this will be passed along to the mycorrhizae, which could have negative consequences[9].

Arbuscular mycorrhizae, the most common form of mycorrhizae which are widespread "essential components of soil biota in natural and agricultural ecosystems"[10], are used as a benchmark for the impacts of climate change on mycorrhizae in the following sections.

Increasing temperatures and excess CO2

The temperature of the globe is steadily rising due to human activity, where the majority of the blame can be placed on anthropogenic production of pollutant gases. The most common gas that is produced by both artificial and natural means is CO2, and its heavy collective concentration in the atmosphere traps a large amount of heat underneath the atmosphere.[11] The heat affects fungi differently depending on what genus, species or strain they are; while some fungi suffer at certain temperatures, others thrive in them.[11] This depends on which environments the fungi are most often found in. However, temperature also plays a vital role in availability of water and nutrients as the hotter climates will have an easier time absorbing nutrients but are also threatened by denaturation of proteins.[11] If the soil is dried by excessive heat, the hyphae of the mycorrhizae as well as the plant root hairs will have far more difficulty obtaining both water and the nutrients to sustain their interactions.[11]

While temperature may play a key role in fungal and plant growth, there is equally as much dependence on the amount of CO2 that is absorbed. The amount of CO2 within the soil is different from the amount that is in the air; the presence of this CO2 is a vital part of many plant cycles (such as photosynthesis) and due to the properties of plant-fungus symbiosis taking place in roots, mycorrhizae are affected as well. When plants are exposed to higher levels of CO2, they tend to take advantage of it and grow faster.[11] This also increases the allocation of carbon to the plant's roots rather than the plant's shoots, which is beneficial to the symbiotic mycorrhizae.[11] There is an increase in the amount of space that the roots can occupy and thus the cycle of trade between the plant and the fungi increases, showing potential for further growth and taking advantage of the available resources until the feedback becomes neutral.[11] The allocated CO2 that is provided to the mycorrhizae also allows them to grow at an increased rate at higher levels, meaning the hyphae of the fungi will also expand, however the direct benefits seem to cease there in accordance to the mycorrhizae, alone.[11] "Despite significant effects on root carbohydrate levels, there were generally no significant effects on mycorrhizal colonization."[11] This means that while the plant may grow larger, the mycorrhizae will grow proportionally larger with the growth of the plant. In other words, the mycorrhizae's growth is caused by the growth of the plant; the opposite cannot be proven true even though these environmental factors affect both the mycorrhizae and the plant. CO2 should not be thought of as entirely beneficial: its main contribution is to photosynthetic processes but the plant relies on it while the essential sugars that the mycorrhizae require can only be provided by the plant; they cannot be extracted directly from the soils.[11] The effects CO2 has on the environment are detrimental in the long run as it is a vital contributor to the problem of greenhouse gases and loss of territory in which plants and their respective mycorrhizae grow.

Mycorrhizae in Arctic Regions

While it may seem like a barren landscape, the Arctic is actually home to huge populations of animals, plants, and fungi. The plants in these regions depend on their relationship with mycorrhizae, and without it, would not fare as well as they do in such harsh conditions. In Arctic regions, nitrogen and water are harder for plants to obtain as the ground is frozen, which makes mycorrhizae crucial to their fitness, health, and growth.[12]

Climate change has been recognized to affect Arctic regions more drastically than non-Arctic regions, a process known as Arctic Amplification. There seem to be more positive feedback loopsthan negativeoccurring in the Arctic as a result of this, which causes faster warming and further unpredictable change that will affect its ecosystems[13]. Since mycorrhizae tend to do better in cooler temperatures, warming could have a detrimental effect on overall health of colonies[14].

Since these ecosystems offer soil with sparse, easily accessed nutrients, it is critical for shrubs and other vascular plants to obtain such nutrients through their symbiosis with mycorrhiza[15]. If these relationships are placed under too much stress, a positive feedback loop could occur causing a decrease in the terrestrial plant and fungi populations because of harsher and potentially drier environments[16].

Biogeographic movement of plants and mycorrhizae

"Fungi may appear to have limited geographical distributions, but dispersal per se plays no role in determining such distributions."[17] The limitations of animals and plants is different from that of fungi. Fungi tend to grow where there are already plants and probably animals because many of them are symbiotic in nature and the rely on very specific environments in order to grow. Plants on the other hand must rely on separate elements in order to spread, like the wind or other animals, and when seeds are planted the environments must still be sufficient enough to help them grow.[17] Arbuscular mycorrhizae are the best example of this as it is found nearly anywhere where plants are growing in the wild. However, with changing climate comes change in environments. As climates warm or cool, plants tend to "move", that is – they exhibit biogeographic movement.[17] Some habitats no longer remain viable to certain plants but then other previously hostile environments may become more hospitable to the same species.[17] Once again, if a plant occupies an environment where mycorrhizae can grow and form a symbiosis with the plant, it will likely occur with seldom exceptions.

Not all fungi can grow in the same places though, distinct types of fungi are necessary to consider. Even though some fungi can have a massive area of dispersal, they still succumb to the same barriers that most species do. Some elevations are too high or too low and limit the capacity to disperse spores, favoring similar elevation as opposed to an increase inclining or declining elevation.[17] Some biomes are too wet or too dry for a plant to not only move to but grow and survive in, or the fungi that occupy one climate do not function as efficiently (if at all) in another climate, limiting the dispersal even more.[17] There are other factors that will mediate the dispersal of fungi, creating boundaries that can cause speciation between fungal communities, such as distance, bodies of water, strength or direction of wind, even animal interactions There are "structural differences, such as mushroom height, spore shape and size of the Buller’s drop, that determine dispersal distances."[17] Morphological reproductive traits such as these play a big role in dispersal, and if there is a barrier that isolates or eliminates these, such as a river or a lack of soil which can support mycorrhizal interactions due to something like falling pH levels from acid rain, essential tactics for germination become obsolete as the offspring do not survive and thus, the population cannot grow or move. Vertical transmission of mycorrhizae does not exist, so to move past these barriers requires alternative means of horizontal transmission.[18] Endemism in mycorrhizal fungi is due to the limitations of how fungal species can spread within their respective niches and home ranges, noticeably widespread within these areas.[18]

While the changing climates keep these fungi from spreading, they also illustrate essential points. There is a greater degree of phylogenetic similarities between fungal communities at similar latitudes and they exhibit just as much similarity between themselves as do plant communities.[17] Tracking one species of plant will help narrow down the specific movement of the mycorrhizae that are commonly associated with the plant species. Alaskan trees for example tend to move north as climate changes because tundra regions are becoming more hospitable and allows for these trees to grow there.[7] Mycorrhizae will follow but which ones in specific is difficult to measure. While vegetation above ground is easier to see and varies less over a larger region, soil contents vary widely within a much smaller region. This makes it difficult to pinpoint exact movements of particular fungi which may be in competition with one another, however these Alaskan trees have obligate endomycorrhizal symbiotes in great quantities, so accounting for their movement is easier.[19] The measurements showed that there were varying distributions of not only the ectomycorrhizal fungi in trees, but the ericoid mycorrhizae, orchid mychorrhize, and arbuscular mycorrhizae in shrubs and fruit plants.[19] They found that of the measurable ectomycorrhizal species richness and density, "– the colonization of seedlings declines with increased distance from forest edge for both native and invasive tree species across fine spatial scales."[19] Thus, the greatest inhibitor of forest expansion is actually the mycorrhizae that prioritize a host's growth rather than their establishment (planting of the seed). The nutrients in the soil cannot sustain the complete growth of a tree within the perimeters of the amount of nutrient absorption that a mycorrhizae (that focuses on growth rather than establishment) will allow.[19] The mycorrhizae which help a plant's establishment will aid the species (and in turn themselves) the most, by maintaining a healthy and balanced intake of nutrients. Species that are moving away from the equator due to change in climate likely experience the best benefits when establishing mycorrhizae infect their roots and spread to other offspring.[19]

Effects on environmental health

CO2 gases are only one of the most common gases to enter the atmosphere and circulate within several natural cycles essential to the preservation of life on a daily basis; however, there are a plethora of other harmful emissions that can be produced by industrial activity.[20] These gaseous molecules negatively affect the phosphorus cycle, carbon cycle, water cycle, nitrogen cycle, and many others that keep ecosystems in check. Mycorrhizal fungi can be affected most heavily by the absorption of unnatural chemicals that can be found in the soils near man-made facilities such as factories, which give off many pollutants that can enter the ecosystem through many means, one of the worst being acid rain, which can precipitate sulfur and nitrogen oxides into the soils and harm or kill plants in its path.[20] This is just one example of how extreme the harsh side effects of pollution can affect the environment, there is evidence that agricultural activities are also heavily affected by negative human influences. The advantage of having a mycorrhizal community in an agricultural setting is that the plants survive and obtain nutrients from their environment more easily.[20] These mycorrhizae are indirectly and directly exposed to the same effects that human activity stresses upon their respective plants; the most common fungi being arbuscular mycorrhizae – specifically, the pollutants of the Earth's atmosphere.[20]

The most common industrial air pollutants that are introduced into the atmosphere include, but are not limited to, SO2, NO-x, and O3 molecules.[20] These gases all negatively impact mycorrhizal and plant development and growth. The most notable effects that these gases have on the mycorrhizae include "– a reduction in viable mycorrhizae propagules, the colonization of roots, degradation in connections between trees, reduction in the mycorrhizal incidence in trees, and reduction in the enzyme activity of ectomycorrhizal roots."[20] Root growth and mycorrhizal colonization are important to note as these directly influence how well the plant can uptake essential nutrients, affecting how well it survives more so than the other adverse effects.[20] Changing climates are correlated with the production of air pollutants, therefore these results are of significance to the understanding of how, not only mycorrhizae, but their symbiotic plant-host interactions are affected as well.

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