Survival Through Synthesis: The Chemical Tactics of Fungi

Keywords: Enzymes, Metabolism, Antibiotics, Bioremediation, Thermal Shock, Chemical Adaptations

Abstract

This paper investigates the survival strategies of fungi through their chemical tactics and metabolic processes. We explore how fungi utilize complex biochemical pathways, such as glycolysis and the pentose phosphate pathway, to generate energy and synthesize vital compounds, including antibiotics like penicillin, which are crucial for their defense mechanisms and survival. The role of enzymes in fungal metabolism will be examined, highlighting their contributions to degrading environmental pollutants, facilitating nutrient absorption, and their role in fungi’s survival. Additionally, we discuss the ecological significance of compounds like psilocybin and psilocin, including their potential therapeutic applications and interactions within ecosystems. By understanding these chemical tactics, we reveal how fungi adapt to environmental stresses and contribute to ecological balance, emphasizing their crucial role in both natural and engineered ecosystems.

Introduction

Fungi are nature's recyclers, breaking down organic matter and recycling nutrients back into the soil, a process that supports plant life and, consequently, all terrestrial food chains. Beyond their ecological contributions though, fungi are constantly fighting for survival, needing to survive harsh environments and outcompete other organisms. Fungi possess an arsenal of chemical defenses, ranging from metabolic byproducts and adaptive enzymes to lethal compounds. With the use of these chemical tools, fungi overcome the many challenges that seek to remove it from their ecosystems.

Fungi exhibit remarkable metabolic versatility, allowing them to thrive in diverse environments. For instance, the yeast Saccharomyces cerevisiae, commonly used in baking and brewing, has been harnessed for centuries to produce bread, beer, and wine. However, fungal metabolism is more than just a means to produce food; rather, it is the source of various survival strategies. Metabolism defines how fungi can survive in aerobic or anaerobic environments. It is the production pathway for alcohols, antibiotics, which can be used to eliminate competition, and is the source of hallucinogens, which can be used as a defense strategy.

Fungi’s ability to synthesize a wide array of bioactive compounds makes them sophisticated chemical tacticians. Using their chemical synthesis abilities, fungi can produce potent antibiotics like penicillin, as well as psychoactive compounds such as psilocybin and psilocin, found in certain mushroom species. Additionally, fungi can adapt to extreme environmental conditions through unique biochemical pathways, enabling them to degrade pollutants and absorb heavy metals, making them invaluable allies in bioremediation efforts. Overall, whether synthesizing bioactive compounds to outcompete rivals, adapting biochemically to harsh climates, or absorbing and breaking down pollutants, fungi exemplify a unique mastery over their chemical surroundings.

Metabolism

Fungal metabolism occupies a central role in ecological and industrial processes, driving the degradation of organic pollutants and facilitating the absorption of heavy metals from the environment. Fungi not only contribute to the biogeochemical cycles of carbon and nitrogen, but also play a vital role in the production of fermented beverages, biofuels, antibiotics, and sterols. In biotechnology, yeast species stand at the forefront of recombinant DNA technology, synthesizing human therapeutic proteins such as insulin.

Fig. 1. Fungal metabolism in ecosystem and industry: (A) fungal processes facilitate biogeochemical cycles (Behera & Das, 2023) and (B) diverse fungal metabolic enzymes offer many industrial uses (Hyde et al., 2019).

As chemoorganotrophs, fungi rely on the oxidation of environmental nutrients to sustain their complex biosynthetic pathways, enabling them to thrive in diverse ecological niches (Kavanagh, 2005).

Fig. 2. Photograph showing the saprotrophic lifestyle of several fungi. Stereum fasciatum (middle), Trametes hirsuta (right) and Lenzites betulina (left) are capable of decomposing cellulose and lignin into their glucose and phenolic subunits (Photographed by Felix Gamper).

The structure of the fungal cell wall is highly specialized; its porousness allows the absorption of molecules up to 300 daltons while selectively retaining larger molecules around 700 daltons (Kavanagh, 2005). This structural feature limits fungi’s absorption of basic macromolecular components, necessitating extracellular breakdown of these larger structures. To achieve this, fungi have evolved hydrolytic enzymes and active transport mechanisms, allowing them to extract essential nutrients such as monosaccharides (typically 180 daltons) and amino acids (up to 200 daltons). These mechanisms are integral to their survival and offer insights into their bioremediation and mycoremediation capacities.

Anabolic processes require energy input. Fungal biosynthesis relies on interconnected pathways as glycolysis and the pentose phosphate pathway (PPP). Glycolysis is primarily catabolic in nature, after an energy investment phase it yields NADPH and ATP, at the cost of converting glucose to pyruvate. Further energy is extracted from the products of glycolysis in respiration and fermentation. PPP also produces NADPH during the oxidation of glucose-6-phosphate, the product of phosphorylation of glucose on the sixth carbon during the energy investment step. However, the pathway does not further break down the molecule, instead it produces ribose-5-phosphate, which can be used in nucleotide production. Side pathways produce different products. The Shikimate pathway produces aromatic precursors to aromatic amino acids such as tryptophan, but also to many other secondary metabolites. (Masi, Mach, & Mach-Aigner, 2021). Even the synthesis of vitamins and some antibiotics originates from these precursors (Roberts et al., 2002). Together, these two processes illustrate the balance within the organism’s metabolism. Depending on cellular needs, food can be directed towards energy production or the building of molecules.

Fig. 3. Glycolysis and Pentose Phosphate Pathways (Masi, Mach, & Mach-Aigner, 2021). Tandem processes up- and down- regulated to meet the fungal cells' energetic and synthetic needs.

While these tandem processes are performed by other organisms, they are extremely important to fungi who actively use their metabolism to change the environment around them. PPP is used in fungi to manage oxidative stress, support cell wall biosynthesis and to produce secondary metabolites (Masi, Mach, & Mach-Aigner, 2021).

While fungi have mitochondria, and many rely on respiration for the bulk of their energy, anaerobic pathways remain essential to fungi's life cycles. Fermentation allows fungi to survive under various conditions—at different oxygen concentrations, with diverse carbon sources—and to produce important secondary metabolites.

Alcohol production is a key survival mechanism of Saccharomyces cerevisiae. In low oxygen conditions, the Krebs cycle, left without a final electron acceptor, oxygen, cannot function. As a result, pyruvate produced in glycolysis is further metabolized into ethanol. Yet yeasts may also produce alcohol in environments with a surplus of oxygen as a mechanism evolved to prevent colonization of food sources by other organisms more sensitive to ethanol. Evolving self-harming traits to control competition is common in the microbiotic world, known as the Crabtree effect (Pfeiffer & Morley, 2014).

Fig. 4. The relationship between glycolysis, respiration and fermentation. The Krebs (TCA) Cycle requires oxygen for oxidative phosphorylation, while Fermentation is anaerobic. Despite breaking down pyruvate, fermentation produces no further ATP, only NADH (Pfeiffer & Morley, 2014).

Enzymes

Fungi's success is largely attributable to the complex synthesis they can perform from decomposing materials. This process is enabled by enzymes, which work by reducing the activation energy needed for a particular chemical reaction. Fungal enzyme function can be broadly characterized as lytic, oxidoreductive, transferring, isomerizing, and ligating.

Lytic enzymes are essential to the carbon catabolizing processes, cleaving bonds between macromolecule subunits. Next, oxidoreductases play a role in catabolizing and synthesis while addressing environmental dangers. Fungi are exposed to reactive oxygen species produced during respiration, radiation exposure, and by plant defense mechanisms. By managing peroxides, some fungi can even use these mechanisms to degrade complex substrates like lignin (Conesa, Punt & Hondel, 2002). Furthermore, fungi can reduce nitrate to nitrite and to ammonium, which they can then use for organic synthesis or exchange with other organisms. Similarly, fungi with nitrogenase enzymes can reduce elemental nitrogen to ammonium; in fact, around 80% of terrestrial plants associate with nitrogen-reducing mycorrhizal fungi, demonstrating how fungi's unique chemical characteristics open up certain mutualistic niches (Di Martino et al., 2022). Finally, transferases, enzymes responsible for transferring functional groups, and isomerases, enzymes that ensure the particular chirality of synthesized molecules, enable the synthesis of specific molecules. In tandem with ligating enzymes, fungal synthesis can be very specific and complex. For example, antibiotics underline how specificity draws the line between organisms' life and death; with even a small change in chemical structure enhancing or reducing the killing potency of the compound. Through enzymatic activity, evolution has refined fungal metabolism into a survival toolkit for species’ ecosystems.

Chemical Defense on the Environment

A fascinating characteristic of fungi is their diverse range of enzymes and metabolites utilized to solve many environmentally imposed problems. Particularly, fungi has the ability to degrade compounds such as polyethylene, highlighting their potential in addressing plastic pollution. Fungi can also degrade pharmaceutically active compounds (PhACs), making them valuable for bioremediation efforts. Additionally, fungi play a crucial role in mitigating pesticide pollution through the production of specific enzymes that break down harmful pesticides, reducing their toxicity and environmental impact.

Heavy metal contamination is another significant environmental problem that fungi can help address through mechanisms like biosorption and bioaccumulation, absorbing and detoxifying heavy metals from polluted environments. This multifaceted capability of fungi offers sustainable solutions to some of the most pressing environmental issues, contributing to the restoration of soil and water quality and promoting healthier ecosystems.

Bioremediation

Bioremediation involves removing pharmaceutically active compounds (PhACs) from contaminated environments due to their harmful effects on ecosystems and human health (Amobonye et al., 2023). PhACs have a low octanol/water partition coefficient, making them highly mobile and resistant to biodegradation, particularly in wastewater (Samal et al., 2022). PhACs’ hydrophilic nature allows them to dissolve easily in water, facilitating their transport through aqueous environments without significant retention. Moreover, PhACs often contain stable chemical moieties, such as aromatic rings and halogenated groups (see Fig. 5), which are resistant to microbial degradation. These structural features hinder the breakdown processes typically carried out by microbial communities in wastewater treatment systems, leading to the persistence of PhACs in the environment (Bangia et al., 2023).

Fig. 5. Chemical structure of Fluoxetine commonly known as Prozac and a PhAC (aromatic rings are circled in red and halogens, here fluorine, are highlighted in blue).

Fungal bioremediation is an emerging solution regarding PhAC contamination; based on the use of high recoverability and reusability fungi (Kuppan et al., 2024). In other words, these organisms must be able to thrive in diverse and extreme environments, relying not only on pollutants as a nutritional substrate, but also on other compounds or on trophic interactions with plants for sustenance. Fungi perfectly meet this criterion– in fact, our physics paper: The Fungal Kingdom: Mechanisms of Adaptation and Interaction with the Physical World, addresses and explains how fungi interact with their surroundings.

White Rot Fungi

White rot fungi, classified as Basidiomycetes, are known for their ability to degrade lignin using extracellular enzymes such as laccases and peroxidases. This special feature is what makes them useful for reducing pollutants like polycyclic aromatic hydrocarbons and synthetic dyes through bioremediation. Some notable species include Pleurotus ostreatus, Phanerochaete chrysosporium, Trametes versicolor (fig. 6 (f)), Ganoderma lucidum (fig. 6 (d)) and Irpex lacteus (fig. 6 (e)) (Mir-Tutusaus et al., 2018).

Fig. 6. White-rot fungi of species (a) Abortiporus biennis, (b) Ceriporiopsis subvermispora, (c) Coriolopsis trogii, (d) Ganoderma applanatum, (e) Irpex lacteus and (f) Trametes versicolor. (Ginns, 2017).

White rot fungi secrete the enzymes laccase, lignin peroxidase, manganese peroxidase (MnP) and versatile peroxidase. These enzymes are classified as lignin modifying enzymes (LME) (Fig. 7). Lignin peroxidase (LiP) oxidizes di-methoxylated aromatic compounds to radical cations, often leading to lignin polymerization in vitro. The enzyme's catalytic cycle involves veratryl alcohol, which is oxidized to radical cations that mediate lignin oxidation. LiP activity is regulated by phenolic compounds, which can transiently inactivate the enzyme, suggesting a feedback control mechanism. This regulation favors degradative ring-opening reactions rather than polymerization. In the presence of low concentrations of hydrogen peroxide and veratryl alcohol, LiP can also catalyze the partial depolymerization of synthetic hardwood lignin, highlighting its potential in bioremediation (Harvey et al., 1992). MnP system generates reactive radicals that break down recalcitrant lignin and other complex pollutants (Hofrichter, 2002). Versatile peroxide degrades complex and recalcitrant pollutants as it shows both LiP-like and MnP-like activities, allowing it to break down a wide range of aromatic and phenolic compounds. It oxidizes non-phenolic substrates, such as those found in lignin, via radical formation (similar to LiP), and converts Mn²⁺ to Mn³⁺, which in turn oxidizes organic pollutants (a typical MnP function). Additionally, versatile peroxidase can degrade synthetic dyes such as Reactive Black 5, which are commonly found in industrial wastewater and resistant to many other microbial degradation processes (Camarero et al., 1999).

Fig. 7. Degradation of organic pollutants by white rot fungi (WRF) and their oxidative and extracellular ligninolytic systems (Zhuo & Fan, 2021).

Mycoremediation comapred to other types of Bioremediation

Similar to fungi, plants and bacteria also play significant roles in bioremediation, each with unique advantages and limitations. For instance, hyperaccumulator plants like Thlaspi caerulescens can absorb heavy metals from soil, and bacteria such as Pseudomonas putida can degrade hydrocarbons (Medić & Karadžić, 2022; Milner & Kochian, 2008). While fungi go through mycoremediation (Fig. 8-B), plants go through phytoremediation (Fig. 8-D); they absorb, accumulate, and detoxify pollutants via their roots and shoots; this makes them especially good at degrading metals just like fungi. This mechanism earned them the title of liver of nature (Nabi & Dar, 2022). While phytoremediation is a captivating subject to study, it falls short as it is generally slower and limited to a plant’s root zone. Root systems extend only a few feet into the soil and restrict their ability to access and remediate contaminants located deeper underground. Interestingly, combining fungi and plants in bioremediation can enhance the overall effectiveness of the process. In fact, fungi can degrade complex organic pollutants and heavy metals, making them more accessible for uptake by plant roots. This synergistic approach leverages the strengths of both organisms: the deep soil penetration and enzymatic capabilities of fungi, and the stabilization and accumulation abilities of plants (Tomer et al., 2021). Bacterial bioremediation (seen in Fig. 8 A), rather, competes with fungi in the biodegradation of PhACs and hydrocarbons. They are often used in synergy with mycoremediation due to their limitation: dependency on some environmental factors such as pH, temperature, and presence of particular nutrients (Thirumalaivasan et al., 2024).

Fig. 8. An overview of bioremediation approaches (in the case study of cadmium contamination into soil by (Kumar et al., 2021)).

Fungus Biodegradation of Polyethylene

Polyethylene (PE) is widely utilized as a synthetic polymer, especially known as plastic, due to its strong and resistant nature against deterioration. These strengths, however, cause it to be a major environmental contaminant, as PE builds up in garbage dumps and ecosystems, endangering animals and biodiversity (Britannica, T., 2024). Nonetheless, specific types of fungi, notably Aspergillus flavus, Aspergillus oryzae, and Penicillium simplicissimum, have exhibited significant capabilities in breaking down PE, presenting a hopeful solution to this enduring issue (Bautista-Zamudio et al., 2023). Biodegradation of PE by fungi seems to be a combination of physical and chemical processes. The initial step involves the colonization of the PE surface by fungal hyphae (see Fig. 9 1) to increase the surface area of the polymer and create a point of access to enzymatic action (fungal hyphae systems are discussed in: The Fungal Kingdom: Mechanisms of Adaptation and Interaction with the Physical World).

Fig. 9. Steps involved in the biodegradation process performed by white-rot fungi (Bautista-Zamudio et al., 2023).

The enzymes involved in this process are cutinases, lipases, and lignocellulolytic enzymes like laccases and peroxidases (Srikanth et al., 2022), similarly to mycoremediation. These enzymes start the degradation of PE by adding oxygen to the polymer structure. In other words, oxidative degradation occurs (Fig. 9 3). This results in the creation of functional groups such as carbonyls and hydroxyls, which enhance the polymer's hydrophilicity and lower its molecular weights (Srikanth et al., 2022). The smaller fragments with low molecular weight are easier to be broken down by enzymes (Fig. 9 4), eventually resulting in complete biodegradation into carbon dioxide and water.

A remarkable fungi species is Cladosporium halotoleran (See Fig. 10) that has the ability to break down high-density PE (HDPE) by binding to the plastic through its hyphae and releasing oxidoreductase enzymes (Di Napoli et al., 2023).

Fig. 10. Scanning Electron Microscopy (SEM) of (A) Typical HDPE particles, (B) hyphae of Cladosporium halotolerans, and (C) fungus and HDPE particles at low magnification, and (D) fungus and HDPE particles at high magnification. Scale bars: 50µ (A, B, D). 100µ (C) (Di Napoli et al., 2023).

Thermal Stress Adaptations

Most fungi are mesophiles who thrive in temperatures of 20 to 30℃ but can survive in environments between 5 to 35℃. Similarly, most other organisms prefer these ranges because high temperatures cause their proteins to denature and aggregate (Fig. 11 A). Denaturation refers to the unfolding of proteins and the loss of their three-dimensional shape which leaves them dysfunctional. Denatured proteins can form aggregates which are structures of accumulated dysfunctional proteins. Despite this, certain fungi have developed strategies to help them tolerate and even thrive in hot environments. For instance, the Chaetomium thermophilus can exist in composts that reach 58℃ due to high substrate decomposition rates. For these fungi, the ability to tolerate high temperatures is critical for survival and competition with other organisms. Thermotolerant and thermophilic fungi can occupy extreme niches that other creatures simply cannot tolerate (Dix and Webster, 1995). However, heat-shock adaptations are also critical for mesophilic fungi, which often face precarious situations. Since fungi have limited locomotion, escaping environmental stressors is not a viable survival strategy. This section will discuss a two-step defense protocol found in Saccharomyces cerevisiae (brewer’s yeast) that helps them persevere through heat-shock. However, these mechanisms can be applied to other fungi as well.

Fig. 11. Thermal stress response in S. cerevisiae (Singer and Lindquist, 1998).

Damage Mitigation

Firstly, brewer’s yeast produces trehalose as the primary line of defense against denaturation. This sugar is composed of two glucose molecules linked at their 1-carbons. Initially, it was hypothesized that yeast produces trehalose to serve as an energy reserve, similar to glycogen. However, studies have shown that S. cerevisiae prefers to metabolize glycogen during starvation and only use trehalose as a last resort. This preference shows that the disaccharide has a primary biological function that is not simply energy storage. Moreover, in normal conditions, yeast cells only produce trehalose during the stationary (non-growth) and spore phases. However, heat stress causes the sugar to be produced during the growth phase as well. In these circumstances, trehalose can amount to 20% of dry cell weight (Singer and Lindquist, 1998). This is a substantial amount, which indicates that trehalose must play a non-negligible role in heat protection. Indeed, a study by De Virgillio et al. showed that elevated trehalose levels were correlated to cell survival in adverse conditions. The researchers compared wild type yeast to cells with defective trehalose synthases and incubated them at 52℃ for 20 minutes and at 54℃ for 4 minutes. The mutant cells showed 100 times and 800 times less survival rates respectively than wild types (De Virgillio et al., 1994). Trehalose is proposed to mitigate denaturation and aggregation by surrounding proteins, similar to how dogs herd sheep (Fig. 11 B). Unlike denaturation protection mechanisms in other organisms, trehalose does not inhibit unfolding by trapping structural water within the protein. Instead, the disaccharide forms hydrogen bonds with sensitive amino acid groups, thereby inhibiting their interaction with other unstable species (Allison et al., 1999). This strategy ensures that protein shape is conserved while already denatured enzymes are blocked from forming aggregates with each other (Singer and Lindquist, 1998). However, these mechanisms do not allow the cell to resume normal functions, rather they simply mitigate the damage. For a viable stress response, the trehalose requires a teammate: Heat-shock proteins (Singer and Lindquist, 1998).

Cell Recovery

Heat-shock proteins (HSPs) work with trehalose to recover cell function after thermal stress. Most HSPs act as janitors by degrading proteins that have been irreversibly damaged or as stabilizing agents that inhibit denaturation and aggregation during heat-shock (Singer and Lindquist, 1998). However, certain HSPs, like the Hsp104, can reactivate damaged enzymes. This unique ability was demonstrated by Parsell et al., who conducted a study which compared wild-type yeast cells to mutants that lack Hsp104s. First, both groups were made to produce bioluminescent luciferase enzymes. Then, the cells were incubated at 37℃, heat-shocked at 44℃, and allowed to recover at 25℃. The ability to produce new proteins was blocked in the mutant cells. Surprisingly, the WT cells were able to reactivate 70% of luciferases in the next 2 hours, where these mutants could only recover about 5% (Parsell et al., 1994). The results demonstrate that Hsp104s are vital for the heat-shock response as trehalose in brewer’s yeast. Moreover, this enzyme seems to be the more elegant design solution to thermal stress. Instead of simply degrading useless proteins and making new ones, Hsp104s fix them. S. cerevisiae may have developed this mechanism because fixing broken enzymes could demonstrate a lower energy investment than building anew. In contrast, other fungi and organisms prefer Hsp70s, which lack the disaggregating abilities of Hsp104s. The former enzyme acts in a similar way to trehalose by stabilizing proteins during heat stress. This dichotomy illustrates that both approaches are viable and depend on the specific needs of the organism. Mechanisms that utilize Hsp104s could be better adapted to heat shocks, while those that use Hsp70s could function better during prolonged heat exposure (Parsell et al., 1994).

Chemical Weaponry

Although fungi face many challenges in surviving their ecological environments, fungi are always fighting a constant war of survival and competition with other organisms. Fungi come well-equipped to their battles, wielding chemical weapons like antibiotics and hallucinogens. These fascinating weapons allow fungi to have effective means of survival and propagation against both microscale and macroscale organisms, ensuring fungi will have a chance to continue in the game of life.

Antibiotics

Imagine that one day your skin just stopped repairing itself. Quickly, your skin deteriorates, becoming fragile and leaky, with any small impact causing lesions. Eventually, your skin becomes so weak that it bursts, and your organs leak out! You might imagine that there was some catastrophic bioweapon of mass destruction let loose – and for bacteria, this is often the reality they face when encountering fungi; the factories of these destructive bioweapons. These “bioweapons” are antibiotics, one of the most critical chemical weapons produced by fungi to kill or reduce competitor microorganisms, ensuring the survival and propagation of a fungal colony (Grenni et al, 2018).

The first antibiotic was discovered in 1928 by Alexander Fleming, produced by a mold of the genus Penicillium (Gaynes, 2017). While Fleming was talking to a colleague, he noticed that on an agar plate, an invading fungus was surrounded by a zone where no bacteria grew (see Fig. 12). After isolating Penicillium, Fleming obtained an extract from it, determining this extract was what caused the region to be bacteria free. After medical testing, it was determined that the active ingredient of this extract, penicillin, had the capacity to destroy bacteria. (Gaynes, 2017) Many new antibiotics would be discovered from this point on, often found being produced by different species of fungus.

Fig. 12. Alexander Fleming’s Agar dish. Around the Penicillium colony at the top of the agar plate, there are no staphylococcal bacteria colonies due to the presence of penicillin antibiotic (Brown, 2024).

Fungi produce antibiotics through their metabolic processes, specifically fermentation (Binod et al., 2023). Antibiotic creation starts with the synthesis of amino acid precursors, which can be created as a result of metabolic reactions or other biosynthetic pathways. With enough precursor concentration, specialized enzymes catalyze chains of reactions, called pathways, to form intermediates and products. At the end of the pathways, the antibiotic product is formed, ready to be used by fungi (Brakhage, 2006).

Biosynthesis of Penicillin

The synthesis of penicillin is one of the most understood biosynthetic pathways of antibiotic creation in fungi. It occurs over three major steps, recruiting three or four main enzymes (Brakhage, 2006; (Mestrovic, 2023). Due to its well understood nature, it is a model pathway for understanding antibiotic biosynthesis.

The pathway begins with the amino acid precursors L-α-aminoadipic acid, L-cysteine and L-valine (ACV). Fungi can obtain L-cysteine and L-valine from their own respective pathways, and L-α-aminoadipic acid, an intermediate produced in the L-lysine biosynthetic pathway (Brakhage, 2006). With the presence of the precursors, the first catalytic step begins; the enzyme ACV synthase chemically condenses the three precursors into the tripeptide ACV (Mestrovic, 2023). Next, the second enzyme isopenicillin N synthase causes oxidative ring closure of the ACV molecule. In other words, the isopenicillin N synthase causes a four-membered beta-lactam ring as well as a five-membered thiazolidine ring (see Fig. 13). The resulting compound is isopenicillin N, or IPN, which has weak antibiotic activity. From here, the pathway can diverge depending on the fungi. For Penicillium and other penicillium-producing fungi, the third step is catalyzed by acyl coenzyme A, where a lateral L-alpha-aminoadipate chain is replaced by a hydrophobic lateral chain. Comparatively, fungi of genus Acremonium converts IPN to penicillin N in a two-enzyme system involving the enzymes acyl-CoA-synthetase and acyl-CoA-racemase (Mestrovic, 2023). Although there are different types of penicillins each with their own structural differences, caused by slight differences in the biosynthesis processes of various fungi, in general all penicillins have a reactive β-lactam ring system and reactive cyclic amide group (Fernandes et al., 2012).

While various fungi may synthesize different antibiotics with vastly different pathways, fungi generally synthesize antibiotics through a similar process to that of penicillin; that being through catabolic reactions of amino acid precursors forming antibiotic products.

Fig. 13. The biosynthetic pathway of penicillin (Mestrovic, 2023).

Chemical Mechanism of Antibiotics

Just as fungi are diverse, so are the antibiotics they produce. Despite the diversity of antibiotics, they generally work in one of the five ways: inhibition of cell wall synthesis; impairment of the cytoplasmic membrane; inhibition of nucleic acid synthesis; inhibition of protein synthesis; and metabolic antagonist action (Fernandes et al., 2012). β-lactam antibiotics, including antibiotics like penicillin and amoxicillin, use the mode of action ‘inhibition of cell wall synthesis’ to kill bacteria.

β-lactams themselves are also diverse; in fact, synthesis of β-lactams is actually widespread in nature (Brakhage, 1998). However, their common feature, the β-lactam ring (shown in Fig. 14), is what plays the defining role in antibacterial activity. β-lactam rings are composed of a four-membered ring structure containing a carbonyl group and an amide group (Nangia et al., 1996). Since the bond angles of β-lactam rings are less than 109.5°, they experience ring strain, a type of bond instability, which is further exacerbated by being fused to another ring structure (Gevorg, 2024). The high instability makes β-lactam rings readily hydrolyzed, meaning their bonds can be broken by water (Iwata et al., 1990). When hydrolyzed, the carbonyl-amide group can covalently bond to the serine residues of enzymes, termed penicillin-binding-proteins (PBPs) (Nangia et al., 1996). PBPs are crucial for the biosynthesis and linkage of peptidoglycans, a key component of bacterial cell walls. With the covalent linkage of β-lactams, PBP activity is inhibited, causing the loss of vitality in bacterial cell walls, and eventually, death by cell lysis (Pandey & Cascella, 2023).

Fig. 14. Structure of the β-lactam ring (Nangia et al., 1996).

Hallucinogens

While fungi can secrete antibiotic compounds to actively remove microorganism competitors, some species also use a fascinating strategy to deter, or interestingly, encourage, microorganism predation– mind manipulation! More specifically, various fungi can produce metabolites with psychoactive and hallucinogenic properties (Meyer & Slot, 2023), such as psilocybin or lysergic acid (LSD) (Cleveland Clinic, 2024). While the ecological role of hallucinogens is not yet fully understood, several hypotheses exist regarding how fungi may benefit from their production (Meyer & Slot, 2023).

Psilocybin and Psilocin

Psilocybin is a naturally occurring tertiary amine found in many species of fungi (Glatfelter et al., 2022). It is the prodrug of psilocin, meaning once ingested, psilocybin becomes activated as psilocin. Psilocin is a four-substituted simple tryptamine, meaning its structure is built around a fused double ring consisting of a benzene ring and a pyrrole, as well as a 2-carbon side chain and a hydroxy group substituent on position 4 (Greene, 2013; PubChem, 2005). Psilocin induces psychedelic effects by acting as a serotonin agonist; since psilocin’s structure is chemically very similar to serotonin, serotonin receptors bind with psilocin instead of serotonin causing psychedelic effects (Glatfelter et al., 2022). Notably, the differences between psilocin and serotonin are the primary amine of serotonin being replaced with a tertiary amine, and the indole ring of psilocybin is substituted on position 5 for serotonin instead of position 4 (See Fig. 15) (Zanjani et al., 2023).

Fig. 15. The differences in the structures of psilocin and serotonin (Zanjani et al., 2023).

Ecological Role of Psilocybin

While antibiotics prove to be an effective means to control competition on a microorganism scale, fungi often need ways to defend themselves from macro-scale aggressors. Fortunately for fungi, the synthesis of hallucinogens, such as psilocybin, are one of many design solutions fungi use to defend themselves with! The most straightforward way fungi weaponize psilocybin is using it as a form of direct toxicity against fungivorous species (Meyer & Slot, 2023). In mouse models, ingestion of large doses of psilocin (293 mg of psilocin per kg of mouse body weight) caused mortality, and even low doses (10 mg/kg) could cause oxidative DNA damage to neural tissues. Similarly for invertebrates, while not extensively tested, psilocin ingestion likely causes fatal oxidative damage (Meyer & Slot, 2023). Fungi also use psilocybin as a deterrent from ingestion by modulating gut microbiomes of fungivorous species (Meyer & Slot, 2023). Psilocybin can suppress appetite by regulating the expression of dual oxidase, an enzyme responsible for regulating gut microbiomes (Zeng et al., 2022). Thus, consumption of psilocybin-producing fungi reduces the appetite for fungi. In a similar vein, psilocybin can cause immune system depression, directly causing selection against mushroom consumption. Psilocybin downregulates and upregulates various immunomodulatory factors that are crucial for defense against bacterial infections in Drosophila (Meyer & Slot, 2023). Furthermore, psilocybin further discourages fungal consumption by disruption of social behavior, particularly for colonial organisms. Hallucinogens like LSD produce asocial behavior in ants such as decreased group formation, food sharing, and nest return rates of foragers, limiting colony growth. Psilocybin may have similar effects on colonial insects, selecting for psilocybin-producing-fungi avoidant behavior (Meyer & Slot, 2023).

While fungi use psilocybin to protect against predation for the above reasons, a complementary hypothesis suggests that some fungi use psilocybin to facilitate interactions with animal vectors for spore dispersal. For example, in a study conducted by Hibicke and Nichols (Hibicke & Nichols, 2022), fruit flies were demonstrated to travel more, which would allow for greater spore dispersal in a naturalistic setting (Meyer & Slot, 2023).

Overall, fungi cleverly implement the use of chemical weapons to gain a competitive advantage in its respective niches. When dealing with competition in the form of microorganisms, fungi can produce antibiotics such as penicillin to kill off bacteria. For larger organisms, fungi can use hallucinogens, such as psilocybin, as a solution to deter predation, or sometimes encourage predation for greater propagation. Despite the effectiveness of these design solutions, with the diverse array of fungi, there are still many more chemical weapons that fungi use to survive and compete with in their environments.

Conclusion

Fungi live in a dangerous and hostile world. They must avoid both predators and environmental stresses. However, compared to other organisms, they are limited in their access to conventional means of survival. Fungi do not possess legs or wings they can use to flee, nor do they have any conventional weapons like claws or teeth that they can employ to defend themselves. Yet, they exist in virtually every ecosystem on earth. It is clear that the chemical solutions they have developed for attack and defense are just as effective as more conventional methods.

Firstly, fungi have developed complex metabolic pathways and enzymes that allow them to break down various substances in their environment. These design solutions are critical for the ecosystem and for fungi themselves who use them as both offensive and defensive measures. For instance, white rot employs a variety of enzymes to break down lignin for nutrients. Curiously, the same solutions are also used by the fungus to degrade organic pollutants such as dyes and aromatics. By protecting itself from harmful substances, white rot also contributes to bioremediation and mitigation of harm done by human activity. Moreover, fungi like brewer’s yeast utilize heat-shock enzymes to help them survive thermal stress. This adaptation is critical for efficient decomposition because it allows certain fungi to colonize composts with high internal temperatures.

Similarly, fungi also employ synthesis of molecules to ensure their survival. They can attack other organisms by producing chemical weapons, such as antibiotics, which kill bacterial pathogens and competitors. Fungi can also protect themselves by synthesizing hallucinogens that discourage predation. They do not let the lack of physical weapons stop them from defending themselves and their territory. Likewise, fungi also fight inanimate foes like thermal stress using synthesis. Brewer’s yeast produces sugar trehalose that allows it to withstand heat by inhibiting denaturation and aggregation of proteins.