Fungi are diverse eukaryotic organisms, including yeasts, molds, and mushrooms. The relationship between fungi and oxygen is complex, depending entirely on the specific fungal species and the concentration of oxygen present. For most common fungi, atmospheric oxygen is required for their growth and survival. However, under specific conditions of extreme oxygen saturation, even typically oxygen-loving fungi can be killed.
Fungal Metabolism and Oxygen Requirements
The metabolic needs of fungi concerning oxygen fall into three distinct categories. Most fungi, such as many molds, are classified as obligate aerobes. These organisms absolutely require oxygen to perform cellular respiration, using it as the final electron acceptor in their energy production pathway to efficiently break down organic matter and grow.
Facultative anaerobes, including common yeasts like Saccharomyces cerevisiae, can adapt to different oxygen levels. While they thrive best using aerobic respiration, which generates significantly more energy, they can switch to fermentation pathways when oxygen is scarce. This adaptability allows organisms like yeast to survive in both oxygen-rich and oxygen-depleted settings, such as brewing and baking.
In contrast, a small number of fungi are obligate anaerobes, meaning oxygen is toxic and prevents their growth. These organisms, such as certain species of Chytridiomycota found in the digestive tracts of cattle, survive exclusively in environments devoid of oxygen. For the vast majority of fungi, the 21% oxygen concentration in the air is necessary for life, which is why simple exposure to the atmosphere does not kill them.
How Oxygen Becomes Toxic
Although many fungi require oxygen, excessively high concentrations can be toxic, even for obligate aerobes. This toxicity is mediated by the production of Reactive Oxygen Species (ROS), which are unstable, highly reactive molecules generated as byproducts of normal metabolic processes. These species include the superoxide radical (\(\text{O}_2^{\cdot-}\)), hydrogen peroxide (\(\text{H}_2\text{O}_2\)), and the hydroxyl radical (\(\text{OH}^{\cdot}\)).
Under normal conditions, ROS are generated primarily in the mitochondria during the transfer of electrons in the respiratory chain. These compounds readily react with and compromise the structure of cellular components, including DNA, proteins, and lipids. Fungi have developed defense mechanisms to neutralize these threats, utilizing specialized enzymes like Superoxide Dismutase (SOD) and Catalase.
Superoxide Dismutase converts the superoxide radical into less reactive hydrogen peroxide, which Catalase rapidly breaks down into harmless water and oxygen. However, when fungi are exposed to unnaturally high levels of oxygen, the rate of ROS production overwhelms these protective enzymes. This oxidative stress leads to irreparable cellular damage and ultimately causes the death of the fungal cell.
Hyperoxygenation as an Anti-Fungal Treatment
The mechanism of oxygen toxicity through ROS production is exploited in a medical procedure known as Hyperbaric Oxygen Therapy (HBOT). HBOT involves placing a patient in a sealed chamber where they breathe 100% oxygen at pressures greater than one atmosphere. This increased pressure forces a much higher concentration of oxygen to dissolve into the patient’s blood plasma and tissues, reaching levels unobtainable through normal breathing.
This therapy is used as an adjunctive measure for deep-seated or systemic fungal infections often resistant to standard antifungal medications. It treats conditions like mycetomas, which are chronic infections of the skin and underlying tissues, and severe invasive infections caused by organisms like Aspergillus or Mucorales. The success of HBOT is attributed to its ability to create a hostile, hyper-oxygenated environment deep within the infected tissues.
The elevated tissue oxygen levels generate a surge of Reactive Oxygen Species that overwhelms the fungal cells’ antioxidant defenses. This targeted oxidative stress kills the fungi directly and disrupts the low-oxygen microenvironments that protective fungal biofilms rely on to survive. HBOT is a specialized medical application where oxygen is used as a high-pressure, concentration-dependent therapeutic agent to combat serious infections.