The Space-Faring Fungus: Aspergillus niger

                              

Breaking down the microbiology world one bite at a time


The Space-Faring Fungus: Aspergillus niger

Not everybody has the privilege of space travel. Only a few Astronauts, robots, and some research-relevant organisms get this opportunity. Yet, there can be some sneaky, uninvited guests who tag along – microbes. According to the Planetary Protection guidelines, every object must undergo thorough sanitizing or cleaning before leaving the earth’s surface. Every element of a new rover, lander, or spacesuit needs to be cleaned of any contaminants, be it biological or simply dirt. 

Some bacterial spores, however, survive this chemical treatment. Bacterial spores are a highly-protected cell type, resistant to the harsh conditions of radiation, vacuum, and high temperature. Bacillus subtilis is an example of spore-forming bacteria. Treating surfaces with UV light or peroxide are a few methods used to destroy bacterial spores. 

Unlike bacterial spores, which have been studied for their radiation resistance for years, fungal spores have attracted very little attention. Despite this, recent reports say that a fungus called Aspergillus niger (commonly known as Black mold) is a major contaminant on the International Space Station. A. niger is air-borne and colonizes new environments fast. This fungus is commonly known as a food contaminant, growing best in warm and humid conditions.

For instance, this fungus causes postharvest decay of fruits and vegetables like grapes, strawberries, and onions (Nair, 1985) In a similar vein, A. niger can cause respiratory problems for humans if it infects the respiratory tract. Conversely, A. niger could provide potential benefits. For example, A. niger has been significant as a cell factory in biotechnological applications, mainly for its capacity to produce high quantities of biological products like proteins and enzymes. For these reasons, it is crucial to study the behavior of A. niger in space conditions to avoid unwanted colonization of the fungus inside spacecrafts and instead exploit it for its biotechnological advantage. 

Dr. Marta Cortesão of the Space Biology Research group in Germany with other scientists attempted to research the radiation tolerance of A. niger spores. In this study, the researchers analyzed the radiation resistance of wild-type A. niger fungi and three mutants following exposure to levels of radiation that are usually encountered in space. They irradiated the fungi using UV-C radiation, X-Rays and cosmic radiation. X-Rays and UV-C rays are emitted by the Sun. On the other hand, cosmic radiation is produced as a result of supernova explosions or pulsars (Chancellor et al., 2018) and consists of helium and iron ions. 

What does radiation do to living cells? Scientists say that radiation can cause two types of damage – direct and indirect. Direct damage affects proteins, lipids, and DNA, the building blocks of cells. Indirect damage is caused due to the production of a reactive oxygen species (ROS) in large amounts. ROS are produced as a result of radiolysis, a process where radiation interacts with oxygen in the water to yield highly reactive molecules such as peroxide, superoxide, singlet oxygen, etc. Controlled levels of ROS are required for any cell’s proper function. But when there is excess production of ROS, oxidative stress can damage cellular organelles and can lead to apoptosis, or cell death. 

Figure 1: What radiation does to  Aspergillus niger. Direct and indirect damage (oxidative stress) are the two types of damage caused by radiation. The colors shown are for representation purposes only. Image created by the author withBiorender

Multiple factors contribute to radiation tolerance in any specific organism. The scientists created mutants of A. niger to understand the roles of three factors:  pigmentation, DNA repair, and polar growth (control of the size of the fungal colony). For each mutant, they deleted a single gene responsible for conferring each factor. The mutants were called ∆fwnA (pigmentation), ∆kusA (DNA repair), and ∆racA (polar growth).

Figure 2: Aspergillus niger mutants. The colors shown are for representation purposes only. Image created by the author withBiorender

X-Rays and cosmic radiation are types of ionizing radiation. Previous studies have shown that both ionizing radiation and hydrogen peroxide generate ROS and that pigmentation is involved in resistance to this oxidative stress. Following the treatment with X-Rays and cosmic rays, the scientists found that, contrary to what has previously been suggested, pigmentation did not have a role in resistance to space-like ionizing radiation, as the ∆fwnA pigmentation mutant showed growth just like the wild-type fungi. Dr. Cortesão and her group then incubated the ∆fwnA mutant in hydrogen peroxide. In this case, they found out that pigmentation is necessary for protection from H2O2-induced oxidative stress. Additionally, they found that the same mutant showed less growth after UV-C treatment, suggesting that pigmentation is crucial for resistance to UV-C radiation.

The DNA-repair mutant, ∆kusA, showed less growth in X-Rays, suggesting that X-Rays caused direct damage by DNA-breakage. Thus, DNA repair is crucial for protection against ionizing radiation. Finally, surface-associated growth, or biofilm formation, is an important feature of A. niger colonization. Thus, the researchers analyzed the ability of wild-type and mutant A. niger to form biofilms after radiation treatment. Their key finding was that biofilm formation in the ∆racA mutant was highly reduced compared to wild-type after UV-C treatment, which suggested that the gene controlling polar growth is essential for protection against UV-C radiation. Collectively, their results show that pigmentation protects A. niger against UV-C radiation, the DNA-repair pathway helps the fungi against ionizing radiation, and the gene controlling polar growth can be targeted for the prevention of colonization. 

This study shows us that A. niger is exceptionally resistant to radiation. It can withstand UV-C radiation levels more than Deinococcus radiodurans, a well-known radioresistant microbe. It would need treatment with more than 1000 Gy of UV-C radiation to curb the problem of A. niger colonization, which is higher than the expected exposure for a 360-day round trip to Mars (0.66 ± 0.12 Gy) (Cortesão et al., 2020).

In conclusion, space-like radiation alone cannot eliminate A. niger spores. Hence, there is a need for the Planetary Protection scientists to address such microbes that might cause biological contamination of planetary bodies without our knowledge. Yet, the high radiation resistance of A. niger will make it significant to Space Biotechnology in the future, allowing astronauts to produce essential substances like antibiotics, vitamins and enzymes onboard spacecraft. 


Link to the original post: Cortesão Marta, de Haas Aram, Unterbusch Rebecca, Fujimori Akira, Schütze Tabea, Meyer Vera, Moeller Ralf, 2020. Aspergillus niger Spores Are Highly Resistant to Space Radiation. Frontiers in Microbiology, Volume 11, https://doi.org/10.3389/fmicb.2020.00560

Additional sources:

  1. https://pubchem.ncbi.nlm.nih.gov/compound/Hydrogen-peroxide
  2. https://pubchem.ncbi.nlm.nih.gov/compound/5359597
  3. Committee on Space Research (COSPAR) » Panel on Planetary Protection (PPP) (cnes.fr)
  4. 18.14 Fungi as cell factories (davidmoore.org.uk)
  5. https://spaceplace.nasa.gov/supernova/en/
  6. https://www.atnf.csiro.au/outreach/education/everyone/pulsars/index.html
  7. Research – What is Singlet Oxygen? | Cal State LA

Featured image: https://commons.wikimedia.org/wiki/File:%D0%9A%D0%BE%D0%BD%D0%B8%D0%B4%D0%B8%D0%B8_aspergillus_niger_2.tif