Untitled Mushroom by Seana Gavin.
Over the last decade, climate change has intensified, leading to altered environmental conditions, pervasively affecting our planet’s ecosystems. Destructive human activities such as large-scale deforestation and pollution that reinforce climate change, have driven many species to the brink of extinction. Animals and plants have been relatively well studied and knowledge about these organisms enable work to conserve them. The less charismatic fungi kingdom, however, has been left out of most climate change studies (Medina, Rodriguez, & Magan, 2015). Relative to the plants and animals, very little is known about the fungi and we may be losing them faster than we are learning about them (Cheek, et al., 2020).
Fungi have long captivated mankind as objects of mystic and supernatural. Before humankind had completely understood fungi and their importance in our world, we already knew to employ fungi for various uses. A Neolithic corpse found preserved in glacial ice was found with a pouch carrying tinder fungus (Fomes fomentarius), that was most likely used to make fire (Peintner & Pöder, 2000). The Romans prayed to Robigus, the god of mildew, but were unfortunately not able to stop the fungal disease that led to famine and the decline of the empire (Boyd, 2005). Later in 1928, penicillin, the first antibiotic was an accidental discovery from contaminant mold growing on a petri dish (Ligon, 2004). Understanding of these organisms dawned with the invention of the microscope and hence the ability to study the fungi’s microscopic spores and cells. Now, we understand that fungi are a kingdom of their own, under the supergroup Opisthokonta. They are eukaryotes that feed by digesting and absorbing organic matter (osmotrophy) and can reproduce sexually or asexually via spores. Climate change has huge ramifications for our ecosystem, affecting fungi and all organisms alike. It is important to understand how these fungi are affected by climate change and how these organisms can in turn shape the course of climate change.
Fungi are a key component of the tropical forest ecosystem. Soil fungi communities control significant proportions of the labile nutrient which will otherwise be leeched quickly (Yang & Insam, 1991). Above ground, saprotrophic fungi also play an important role in mineralization and nutrient cycling. Saprotrophic fungi produce a cocktail of enzymes, capable of breaking down even tough cellulosic material, making it easier for other organisms to further breakdown the dead organic material (Baldrian & Valášková, 2008). Even at the atmosphere, fungi leave their mark. In tropical forests, mushrooms from the Basidiomycota phylum catapult basidiospores into the air (Hassett, Fischer, & Money, 2015). In the atmosphere, these spores allow water to accumulate on their surfaces, acting as giant cloud condensation nuclei, creating bigger droplets when smaller ones collide and combine (Hassett, Fischer & Money, 2015; Möhler et al., 2007). Given that these mushrooms depend on rain for the dispersal of their spores in the first place, changes in the climate to reduce rainfall could hold back the growth of these fungi and worse droughts through a feedback loop (Hassett, Fischer, & Money, 2015). Worryingly, it is unclear how fungi will respond to climate change and how stable and resilient they are to abiotic changes (de Oliveira, et al., 2020).
Fungal symbiosis is not just limited to plants but is extensive in higher organisms as well. Beneficial fungal mutualisms with animals include symbioses inside the digestive systems of invertebrates and vertebrates where the fungi participate in digestion. In ruminants, such fungi may help break down fiber (Orpin, 1975; Bauchop, 1981).Fungi also live in our gut where they interact with other gut microfauna to metabolize sugars (Finegold, Attebery and Sutter, 1974; Hallen-Adams, et al.,2015). Gut of invertebrates such as beetles has also been found to contain Ascomycota fungi, supplying digestive enzymes that aid with the digestion of plant material (Sung, Marshall, Mchugh, & Blackwell, 2003). More sinister symbioses include Aspergillus sydowii fungus attacking weak corals, causing their tissues to turn purple and die (see Fig. 1) (Smith et al., 1996; Geiser et al., 1998). The chytrid fungus, Batrachochytrium dendrobatidis uses amphibians as their host, bursting from their skin during sporulation, and is attributed as the main driver of amphibian decline (Bellard, Genovesi, & Jeschke, 2016). Though prevalent and tightly connected to our world and all that inhabit it, fungi have not been given much attention compared to animals and plants. Estimates suggest that there are between 2.2 and 3.8 million species of fungi in the world, which means only six percent of all fungal species have been described (Sheldrake, 2020). Understanding these extensive fungi-animal relationships is important for the complete understanding of the organisms they interact with.
**Figure 1. Sea fan coral (Gorgonia ventalina) infected with Aspergillus sydowii, multifocal purple annular lesions are indicative of infection (Source: Ernesto Weil).
Generally, it is still uncertain how fungi will respond to climate change (Cavicchioli, et al., 2019). However, it is certain that more studies are urgently needed to understand and be equipped to mitigate possible threats to and from fungi. Fungal diseases will drastically change ecosystems if left uncontained. Traditionally, infectious diseases have not been noted as a key driver of extinction given that obligate pathogens would normally co-evolve with rather than extirpate the host they depend on for their survival (De Castro & Bolker, 2004; Smith, Sax, & Lafferty, 2006). However, fungal infections can cause mortality rates approaching 100% like in Batrochocytrium dendrobatidis with amphibians, Pseudogymnoascus destructans in bats and Ophiostoma ulmi in elm trees due to the changing ecosystem and climate change (De Castro & Bolker, 2004; Frick, et al., 2015; Garner, et al., 2009).
Climate change can lead to an increase in the emergence of fungal diseases by affecting the distribution of invertebrate vectors or leading to water or temperature stresses on plants, making them more susceptible to infection (Elad & Pertot, 2014). An unusual increase in the frequency of heavy rain would also promote the spread of fungal pathogens (Anderson, et al., 2004). Additionally, plant species that grow more rapidly in warmer climates may in turn experience increased incidence of disease, since higher host density allows for increased pathogen transmission (Burdon & Chilvers, 1982). Fungi’s high reproductivity means that all individuals of the host population can be infected before the population numbers are too low to facilitate the spreading (Fischer, et al., 2012). Even if the fungal pathogen does not completely wipe out the host population, it can reduce population numbers severely, making the host population vulnerable to random or Allee effects (Stephens, Sutherland, & Freckleton, 1999). This is made worse by the fungi’s ability to survive outside their host, meaning that the fungi’s growth rate is independent of the host’s population size or density. The fungi can persist as durable spores in the environment or as a fre-living saprotrophs (Fischer, et al., 2012). In the ocean, Aspergillus fungi growth is also expected to increase with warming waters (Holmquist, Walker, & Stahr, 1983), further threatening the health of coral reefs.
Fungi can also play a role in accelerating ecosystem changes started by climate change. This is exemplified in a tropical montane cloud forest, where warming has resulted in the lifting of the cloud layer (Still, Foster, & Schneider, 1999; Lawton, Nair, Pielke Sr., & Welch, 2001). The warmer temperature means that the richness of soil fungi is likely to increase (Looby & Treseder, 2018). This will lead to an exponential increase in decomposition due to increased fungal activities of certain fungi (Benner, Vitousek, & Ostertag, 2010). Paired with the increased mortality of heat-sensitive flora and fauna, the break-down of organic matter can be expected to change the soil quality and increases oil respiration in the montane forest, releasing carbon in the process. Even in lowland forests, the increased temperature can facilitate fungi decomposition of soil carbon (Grieve, Proctor, & Cousins, 1990), turning these ecosystems into a carbon source, thus contributing further to warming and the decline of tropical montane ecosystems.
However, not all about fungi is bad news. The fungi might also have a role to play to keep aboveground carbon where they are. When it comes to soil-living mycorrhizal fungi, their branching filament or hyphae permeate the soil and require carbon in their synthesis. These mycorrhizal fungi could therefore act as carbon sequesters, unlike most other fungi (Treseder & Holden, 2013). Additionally, mycorrhizal fungi improve plant growth, contributing to the increase in carbon sequestration through an increase in plant biomass (Malyan, et al., 2019). However, not enough is known about this phenomenon, and more studies are required to understand how to minimize the loss of carbon capture by respiration to improve the carbons sequestration potential of fungi (Malyan, et al., 2019).
Drastic changes in climate may kill plants that are unable to adapt, causing other perturbations within the ecosystem when plant diversity is reduced. Incidences of biotic stress from insect pests and other pathogens could also increase due to climate change (Elad & Pertot, 2014). Fungi can contribute to plant survival since it is known that plants recruit rhizosphere fungi under stress for protection (Yi, Yang, Shim, & Ryu, 2011). These fungi have smaller genomes and generation times relative to the plants (Scott et al., 2019) meaning they are capable of faster evolution than plants and rapid adaptation to climate change (Grandaubert, Dutheil, & Stukenbrock, 2019; Suryanarayanan & Shaanker, 2021). More studies are required before we can understand how the fungi respond and contribute to climate change as a whole.
We have seen how fungi can be a friend of humanity but may also pose significant challenges. When it comes to food, mycotoxins or secondary metabolites produced by fungi is of concern. Mycotoxins can be toxic to us when ingested at low concentrations, posing a problem to human health when a toxic mushroom is consumed. Even if toxic mushrooms are avoided, mycotoxins can be ingested when other foods are contaminated by the mycotoxin while growing, post-harvest during storage or during transportation (Baert, et al., 2007). Known mycotoxins include aflatoxins produced by Aspergillus flavus growing on peanuts that could result in liver damage (Hedayati et al., 2007) or patulin produced by Penicillium expansum in contaminated apple juice (Marek, Annamalai, & Venkitanarayanan, 2003). Staple food crops can also be affected by fungal pathogens. Magnaporthe oryzae causes the rice blast disease which destroys 50 million tonnes of rice per annum, and is capable of host switching to wheat, posing a threat to South America’s agriculture industry (Maciel, 2011). The Cavendish banana is also poised to be wiped out by a strain of the Fusarium oxysporum fungus, which had previously wiped-out commercial Gros Michel banana farming (Hung, et al., 2018; Ploetz, 2015). Hence, climate change and the response of fungi to its effects will not only affect natural ecosystems but also agriculture and the livelihood of those who depend on it. Fungal diseases in oil palms and bananas are expected to increase with climate change, as the plants are increasingly stressed by the shifts in abiotic changes (Paterson, Sariah, & Lima, 2013). Philippines’ agriculture is already affected by mycotoxigenic fungi that reduce the quality and quantity of output, and some cases are expected to worsen with climate change (Salvacion, Pangga, & Cumagun, 2015). Stress to the food system due to climate change is going to threaten food security and worsen food inequality.
On the flip side, certain mycorrhizal fungi have also been proposed to bolster the resilience of staple food crops against climate change. Encouraging mutualistic relationships between introduced fungi and crops might confer the crops greater resilience against drought and other stressors such as the increasing salinity of crop fields due to saltwater intrusion into the water table, led by sea-level rise (Liu et al.,2016; Yadav et al. 2020).
When it comes to climate change, the funga can be a double-edged sword, and hence it is up to us to conserve and even employ fungi to help in mitigating the climate crisis. However, to be able to do this effectively, we first need to understand them. At present, mycology is still in the dark ages — whatever we know about them is merely the tip of the iceberg. It is estimated that in the region around Singapore, 70% of the macrofungi have yet to be discovered (Lee, et al., 2021). This severe lack of understanding of fungi means that they are left out of climate modeling. It is still unclear how factoring in fungi in climate models will tip the scales. Additionally, it is also unclear how many species we are losing to habitat loss and climate change, some of which might prove to be the silver bullets we greatly need to combat climate change.