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Keywords: Space, Radiation-Shield, Fungus, Cladosporium sphaerospermum, Radiotrophic, ISRU
A Self-Replicating Radiation-Shield for Human Deep-Space Exploration: Radiotrophic Fungi can
Attenuate Ionizing Radiation aboard the International Space Station
Graham K. Shunk1,2*, Xavier R. Gomez1,3, Nils J. H. Averesch4,5*
1 Higher Orbits “Go For Launch!” Program
2 North Carolina School of Science and Mathematics, Physics Department, Durham NC, United States
3 Department of Systems Engineering, University of North Carolina at Charlotte, NC, United States
4 Center for the Utilization of Biological Engineering in Space
5 Department of Civil and Environmental Engineering, Stanford University, Stanford, CA, United States
* To whom correspondence should be directed: nils.averesch@uq.net.au; graham1118@gmail.com
Abstract
The greatest hazard for humans on deep-space exploration missions is radiation. To protect
astronauts venturing out beyond Earth’s protective magnetosphere and sustain a permanent presence
across the solar system, advanced passive radiation protection is highly sought after. Due to the complex
nature of space radiation, there is likely no one-size-fits-all solution to this problem, which is further
aggravated by up-mass restrictions. In search of innovative radiation-shields, biotechnology appeals with
suitability for in-situ resource utilization (ISRU), self-regeneration, and adaptability.
Certain fungi thrive in high-radiation environments on Earth, such as the contamination radius of
the Chernobyl Nuclear Power Plant. Analogous to photosynthesis, these organisms appear to perform
radiosynthesis, utilizing the pigment melanin to harvest gamma-radiation and generate chemical energy. It
is hypothesized that the dissipation of radiation by these organisms translates to a radiation shield. Here,
growth of Cladosporium sphaerospermum and its capability to attenuate ionizing radiation, was studied
aboard the International Space Station (ISS) over a period of 30 days, as an analog to habitation on the
surface of Mars. At full maturity, radiation beneath a ≈ 1.7 mm thick lawn of the dematiaceous
radiotrophic fungus (180° protection radius) was 2.17±0.25% lower as compared to the negative control.
Based on an estimated growth advantage in space of ~ 20%, the reduction of radiation was attributed to
the fungus’ radiotropism and melanin-content. This was supported by calculations based on Lambert's
law, where the melanin content of the biomass could be approximated to 8.6±0.9% [w/w]. The analysis of
the experimental data was further complemented by an attenuation analysis subject to a Martian radiation
environment scenario that put the shielding capacity of melanized (bio)materials into perspective.
Compatible with ISRU, bio-based melanin-containing composites are promising as a means for radiation
shielding while reducing overall up-mass, as is compulsory for future Mars-missions.
Abbreviations
B, buildup-factor; BoPET, biaxially-oriented polyethylene terephthalate; CDW, cell dry-weight;
CPM, counts per minute; CWW, cell wet-weight; DHN, 1,8-dihydroxynaphthalene; GCR, galactic
cosmic radiation; HDPE, high-density polyethylene; HZE, high atomic number and energy; ISRU, in-situ
resource utilization; ISS, international space station; LAC, linear attenuation coefficient; LEO, low earth-
orbit; MAC, mass attenuation coefficient; PDA, potato dextrose agar; RT, room temperature.
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1
Introduction
Background
With concrete efforts to return humans to the Moon by 2024 under the Artemis program and
establish a permanent foothold on the next rock from Earth by 2028, humankind reaches for Mars as the
next big leap in space exploration 1. In preparation for prolonged human exploration missions venturing
past Earth-orbit and deeper into space, the required capabilities significantly increase 2. While advanced
transportation solutions led by the private and public sectors alike (Starship, New Glenn, SLS/Orion) are
pivotal and have already reached high technological readiness, life support systems as well as crew health
and performance capabilities are equally essential. Therefore, any mission scenario such as ‘Design
Reference Architecture 5.0 3 or ‘Mars Base Camp’ 4 (with up to 1000 days of crewed usage), must include
innovative solutions that can meet the needs and address the hazards of prolonged residence on celestial
surfaces 4.
The foremost threat to the short- and long-term health of astronauts on long-duration deep-space
missions is radiation 5,6. Over one year, the average person on Earth is dosed with about 6.2 mSv 7, while
the average astronaut on the International Space Station (ISS) is exposed to an equivalent of
approximately 144 mSv 8, and one year into a three-year mission to Mars, an astronaut would already
have accumulated some 400 mSv, primarily from Galactic Cosmic Radiation (GCR) 9. While the
particular health effects of interplanetary radiation exposure have still not been fully assessed 10, it is clear
that adequate protection against space-radiation is crucial for missions beyond Earth-orbit, but is more
than any other factor restricted by up-mass limitations 11. Both active and passive radiation-shielding, the
latter investigating inorganic as well as organic materials, have been and are extensively studied 12,13.
In-Situ Resource Utilization (ISRU) will play an integral role to provide the required capabilities,
as well as to break the supply chain from Earth and establish sustainable methods for space exploration
because once underway there virtually is no mission-abort scenario 14. For ISRU, biotechnology holds
some of the most promising approaches 15,16,17, posing useful for providing nutrition 18, producing raw-
materials 19 and consumables 20, and potentially even “growing” radiation shielding 21,22.
Among all domains of life exist extremophiles that live and persist in highly radioactive
environments, including bacteria, fungi, as well as higher organisms such as insects 23,24. Certain fungi
can even utilize ionizing radiation through a process termed radiosynthesis 25, analogous to how
photosynthetic organisms turn energy from visible light into chemical energy 26,27. Large amounts of
melanin in the cell walls of these fungi protect the cells from radiation damage while also mediating
electron-transfer, thus allowing a net energy gain 28. Melanized fungi have been found to thrive in highly
radioactive environments such as the cooling ponds of the Chernobyl Nuclear Power Plant 27, where
radiation levels are three to five orders of magnitude above normal background levels 29. Additionally,
they populate the interiors of spacecraft in low Earth orbit (LEO), where exposure to ionizing radiation is
also intensified 27. Black molds and their spores have been found to remain viable even after exposure to
several months’ worth of Space radiation (or an equivalent radiation dose) 30. How these organisms
protect themselves from radiation damage has been the subject of intense study; melanins have also
specifically been explored as biotechnological means for radiation shielding 31,32.
Here, we explore the opportunity to utilize the dissipation of radiation by melanized fungi as part
of a multi-faceted solution to passive radiation-shielding for ISRU 18,33. In an additional analysis based on
the concept of linear attenuation (Lambert’s law) 34, we assess the potential of melanized fungus, as well
as the specific contribution of melanin, to provide adequate shielding against cosmic radiation.
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2
Concept
This experiment tested the capability of Cladosporium sphaerospermum (a melanized,
radiotrophic fungus 21, cf. supplementary material 1, section A for details) to attenuate ionizing gamma-
radiation in space. The preference of C. sphaerospermum for environments with extreme radiation-levels
on Earth (e.g. Chernobyl) is well documented 35. Consequently, it has been hypothesized that similar
proliferation occurs in response to the high radiation environment off-Earth and that such melanized fungi
can be utilized for radioprotection in Space 32. The objective of this experiment was to conduct a proof-of-
principle study on a single payload, utilizing basic flight hardware for an autonomous experiment in the
unique radiation environment of the ISS. This offers the opportunity to test the fungus’ response to, and
ability to shield from cosmic radiation.
Materials & Methods
Experimental Setup
Space Tango (Space Tango, Inc., Lexington, KY, USA) was contracted for experimental design
and construction (terrestrial logistics and on-orbit operations) 36. The initial concept for the experimental
design was utilized by Space Tango for assembly of the flight-hardware and implementation aboard the
ISS within TangoLab™ facilities. The flight-hardware was housed in a 4”×4”×8” double unit standard-
size CubeLab™ hardware module (cf. supplementary material 1, figure S1) and consisted of the
following main components: two Raspberry Pi 3 Model B+ (Raspberry Pi Foundation, Caldecote,
Cambridgeshire, UK) single-board computers, EP-0104 DockerPi PowerBoard (Adafruit Industries, New
York, NY, USA), Pocket Geiger Type5, X100-7 SMD (Radiation Watch, Miyagi, Japan), Raspberry Pi
Camera v2 (Raspberry Pi Foundation, Caldecote, Cambridgeshire, UK) and light source (0.8 W LED-
strip) for imaging, DHT22 integrated environmental sensor suite (Aosong Electronics Co. Ltd, Huangpu
District, Guangzhou, China) for temperature and humidity readings, a real-time WatchDog™ timer
(Brentek International Inc., York, PA, USA), and D6F-P0010A1 (Omron Electronics LLC, Hoffman
Estates, IL, USA) electronic flow-measurement system. One Raspberry Pi (“auxiliary-computer”) running
Raspbian v10.18 was dedicated to photography, lighting, temperature, humidity, and electronic flow
measurement (EFM) readings, while the second Raspberry Pi (“flight-computer”) controlled radiation
measurements, stored in a probed Logger Memobox (Fluke Corporation, Everett, WA, USA). The
assembled flight-hardware was calibrated and vetted before flight; in particular consistency of the two
Geiger counters was confirmed so that no deviation existed between them.
Cladosporium sphaerospermum (ATCC® 11289™) was obtained from Microbiologics
(St. Cloud, Minnesota, USA), catalog no. 01254P. Further details on the employed microorganism can be
found in supplementary material 1, section A. Growth medium was potato dextrose agar “PDA” from
Carolina Biological (Burlington, North Carolina, USA) obtained as “Prepared Media Bottle” (approx.
composed of 15 g/L agar, 20 g/L glucose, and 4 g/L starch). A total of 20 mL PDA (dyed with orange 1)
was used to fill the two compartments of a split Petri dish (100×15 mm). The agar plate was sealed with
parafilm post-inoculation. With a total height of the Petri dish of 15 mm and a 75 cm2 surface area, the
thickness of the PDA was ~ 13.33 mm, leaving an approx. 1.67 mm gap for the fungal growth layer.
Cellular mass density of fungal biomass (1.1 g/cm3) 37 was assumed to remain unaffected by
microgravity. The hardware was fully assembled before the inoculation of the agar with
C. sphaerospermum to minimize latent growth of the fungus while in transit. A block-chart of the
experimental flight hardware setup is given in figure 1 while further details are provided in supplementary
material 1, section B.
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Figure 1: Block-chart of the experimental flight hardware setup. The single (split) Petri dish
accommodated both the experiment (agar + fungus), as well as the negative-control (agar only). The two
radiation sensors were situated in parallel directly beneath the Petri dish (one for each side). Note that
“shielding” is one-sided only (for simplicity of experimental setup) since radiation is ubiquitous in space.
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Vetting for Cold-Stow
The response of Cladosporium sphaerospermum to cold-storage was determined in a preliminary
experiment. A total of six Petri dishes containing PDA were inoculated with the fungus, five were stored
at 4°C with one kept at room temperature (RT) as control. Plates were removed sequentially after 1, 5, 10,
15 and 20 days. Fungal growth on each plate was monitored at RT and compared to the control.
On-Orbit Implementation
The equipment was packaged with the inoculated Petri dish before flight, accommodated in a 2U
CubeLab™ (Sealed) 38, between Dec 2018 and Jan 2019. Lead-time before the launch was 2 days in cold-
stow. Transition time to LEO was 69 hours and 20 minutes (time to “power-on” after inoculation),
transported to the ISS in cold storage (4°C) on SpaceX mission CRS-16.
On orbit, the experiment was oriented such that the Petri dish and Geiger counters faced away
from Earth. Pictures of the Petri dish were taken in 30-minute intervals (as set by the watchdog time) for
576 h, resulting in 1139 images. Temperature, humidity and radiation were measured every ~ 110 seconds
throughout the 718 h run-time of the experiment. The three temperature sensors recorded 27772
measurements each. Radiation was measured incrementally; 23607 radiation and noise-counts were
recorded for each Geiger counter. The lab was active aboard the ISS for 30 days with data downlinked on
average every 3 days before power was cut and the lab awaited return to Earth in June 2019.
Ground-Control
In addition to the preflight growth test and on-orbit negative control, Earth-based growth
experiments were performed postflight, replicating the conditions of the flight-experiment without
radiation. The same methods and techniques were utilized when preparing the cultures on solid medium.
A time-dependent temperature profile analogous to the on-orbit experiment was replicated and collection
of graphical data was performed in the same intervals to record the growth behavior.
Evaluation of Growth
Photographs of the cultures were processed with MATLAB 39 to derive the average brightness
values for congruent subsections of each image (cf. supplementary material 1, section C and figure S2), as
proxy for biomass formation. These average brightness values were then normalized so as to render the
relative optical density (OD) ranging from zero to one (cf. supplementary material 2) over time. With
exponential regression, this allowed growth rates “k” to be determined. Based on these, a relative
difference between kexp for the on-orbit experiment and the ground-control, kctrl, was estimated.
Evaluation of On-Orbit Radiation
Cumulative radiation counts, derived from the counts recorded by the Geiger counters of control-
and fungus-side throughout the course of the on-orbit experiment, were plotted over time. The relative
difference in radiation attenuation (in percent) between negative-control and fungus-side was determined
based on the slope of linear regressions in different phases, which were defined based on the relative OD.
Phase 1, the initial phase, was defined for a relative OD below 5% of the maximum, corresponding to the
first 10 h. Phase 2, the growth phase, was defined for a relative OD between 5% and 95% of the
maximum, correspondingly starting 10 h after the start (t0), and ending 60 h after t0. Phase 3, the
stationary phase, includes the remaining data from 60 – 718 h after t0, accordingly corresponding to a
relative OD greater than 95% of the total maximum.
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Further, total radiation was quantified in dose equivalents, with an estimate correlation between
counts-per-minute (CPM) and radiation dose for the PocketGeiger Type5 of 4 CPM ≈
0.075 ± 0.025 µSv/h 40, based on the total cumulative recorded counts over the 30 day runtime of the
experiment (cf. supplementary material 2).
Radiation Attenuation Analysis
Applying Lambert's law, the concept of linear attenuation served to evaluate the experimental
data, under the assumption that no alpha- and beta-particles could reach the experiment, as neither would
be able to penetrate the hull of the ISS 41. Focusing on ionizing electromagnetic radiation, the analysis
determined the attenuation capacity of the fungal biomass within the absorption spectrum of the employed
Geiger counters (cf. supplementary material 1, section B & D).
To interpret the results in light of a more relevant (real-world / Space radiation environment)
scenario, experimental findings were used to inform a theoretical analysis. This relied on the extensive
resources that exist for attenuation coefficients of different materials (and mixtures thereof) over a range
of photon energies by means of the NIST X-ray Attenuation Databases 42. Particularly, MACs for the
respective compounds and mixtures (cf. supplementary material 3) were derived from NIST-XCOM for
total coherent scattering. The non-linear influence of secondary radiation was respected by means of
buildup-factors43 and equivalent kinetic energies of subatomic and elementary particles in electronvolts
(eV) were used in the calculations. The correlations and assumptions, as well as the specific workflow
used in the attenuation analyses are described in detail in supplementary material 1, section D & E.
Results & Discussion
Pre-Flight - Cold-Stow Growth-Test
The ‘Cold-Stow’ experiment showed that for all refrigerated sample-plates there was insignificant
fungal growth immediately upon removal from incubation at 4°C (no fungal growth prior to t0) for all
trialed timeframes. Furthermore, all samples exhibited similar growth once at ambient temperature,
regardless of the time spent in cold storage.
Growth Advantage On-Orbit
From the point the hardware was powered on aboard the ISS, the temperature rose sharply from
the initial 22°C, reaching 30°C within 4 hours and stabilized after 8 hours around 31.5±2.4°C for the rest
of the experiment (cf. supplementary material 2).
Many dimorphic fungi are characterized by slow growth and require up to 14 days for significant
biomass formation to occur at an optimum temperature around 25°C 44. In the on-orbit lab,
C. sphaerospermum reached maximum growth already after 2 days, as discernible from the
photographical data (cf. supplementary material 1, figure S2) and derived growth curves, shown in figure
2. Comparison with the ground-controls indicates that the fungus experienced faster-than-normal growth
aboard the ISS. In respect to the two separate control experiments, the growth rate in the on-orbit
experiment was 1.2-fold higher (based on kflight = 0.194 h-1 and kground = 0.161 h-1, determined for the on-
orbit experiment and ground-control average, respectively, as per the data shown in figure 2B). It is
further worth mentioning, that in preliminary ground-control experiments often poor growth was
observed at 30°C, as compared to RT (data not shown), with only sporadic coverage of the PDA with
fungal colonies (cf. supplementary material 1, figure S3). The inconsistency in growth among the ground-
controls is also reflected by the rather large deviations between the individual replicates of the two
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experiments depicted in figure 2 (as per the error bars representing standard deviation). However, in no
case growth higher than in the on-orbit experiment was observed (cf. raw data in supplementary material
2). The observation that higher than optimum incubation temperatures rapidly hindered growth, as has
been reported before 45, further strengthens the conclusion that the Space environment benefited the
fungus. The growth advantage in Space may be attributable to the utilization of ionizing radiation of the
Space environment by the fungus as a metabolic support function, as has been reported for other high-
radiation environments: it has previously been shown that C. sphaerospermum can experience up to three
times faster growth than normal with gamma-rays 500 times as intense as normal 28,46.[
1
]
Figure 2: Growth curves (initial 100 h shown) for C. sphaerospermum on PDA, depicted by means of
connected data-points of relative optical densities. The curves were consolidated to t0 as the onset of
exponential growth. While the on-orbit experiment (green) was a single run, the ground-control growth-
experiment (purple) was conducted three times (two replicates in each of the three runs). The error bars
show the standard deviation between the three separate runs.
Radiation Attenuation On-Orbit
Given the specific absorption range of the radiation-sensor (cf. supplementary material 1, section
B), the observed radiation levels were within the expected range for LEO. Independent of the absolute
radiation levels, only the relative difference in ionizing events recorded beneath each side of the Petri dish
was significant for the experiment (congruence of the radiation sensors had been ensured before flight,
1
Measured by the dose equivalent, which is 144 mSv/a 8 for the ISS and 2.4 – 6.2 mSv for Earth 7,47. The
radiation on the ISS is about 20 - 60 times stronger than the average background on Earth, however 80%
of this is attributable to GCR, which is mostly composed of particle radiation. Hence the fraction of
(gamma-) radiation utilizable by the fungus may not be equivalently significant.
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data not shown). The radiation levels directly beneath the side of the Petri dish inoculated with
C. sphaerospermum decreased throughout the experiment (as compared to the negative-control),
particularly correlating with biomass formation: while there was no significant difference over the first 12
– 24 h, a constant attenuation was observable in the later stage of the experiment (cf. figure 3). While no
further gain in optical density was observed past two days into the experiment (cf. figure 2), the mark of
240 h onward was chosen to determine the attenuation capacity, in order to be conservative. This allowed
an additional eight days of maturation before fungal growth was considered complete and melanin content
to be constant. The linearity of the trendlines (figure 3B) supports this conclusion. The full dataset and
additional plots of the complete radiation data over the whole course of the experiment can be found in
supplementary material 2.
The periodic, slight fluctuations of the radiation, especially recognizable in figure 3A, may be
explainable with the orbit time / position of the ISS relative to celestial bodies and orientation of the
experiment especially in respect to the sun (either towards or facing away from it). This is supported by
the observation that these fluctuations coincide for the recordings of both Geiger counters throughout the
entire experiment. For single spikes (as particularly prominent in the incremental plot of radiation events
in supplementary material 2), solar events (flare / particle event) are also plausible explanations.
Over the first 24 h of the experimental trial, radiation levels beneath the fungal-growth side of the
Petri dish were by average only 0.5% lower. Towards the end of the experiment, an almost 5-fold increase
in radiation attenuation could be observed relative to the control side, with average radiation levels 2.42%
lower than those of the negative-control (as per the difference of the trendlines’ slopes given in figure 3).
This shows a direct relationship between the amount of fungal biomass (and putatively the melanin
content thereof) and the dissipated ionizing radiation. With a baseline difference between the two sensors
of 0.5% (considering that the fungus may have contributed to this), the observed radiation attenuating
capacity can be stated as 2.17±0.25%. Since radiation is ubiquitous in Space and only one side of the
Geiger counter was shielded by the fungus, it is postulated that only half of the radiation was blocked.
Therefore, it can be extrapolated that the fungus would reduce total radiation levels (of the measured
spectrum) by 4.34±0.5% were it fully surrounding an object. Considering the thin fungal lawn, this shows
the significant ability of C. sphaerospermum to shield against space radiation.
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Figure 3 A & B: Cumulative radiation counts of the control and the fungus over time. A: initial
phase (0 – 24 h), B: final ⅔ of the experiment (240 – 718 h). For the sake of legitimacy, the cumulative
count of ionizing events (radiation) was scaled-down three orders of magnitude (×1000-1). While in
section ‘A’ the regression lines almost coincide, a significant difference in the slope is evident in section
‘B’ (where the fungus was fully matured), corresponding to an attenuation of the transmitted radiation.
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Radiation Attenuation in Light of ISRU on Mars
The average rates of ionizing events over the whole course of the experiment were 48.9 and
47.8 CPM for the negative-control and the fungus-side, respectively (cf. supplementary material 2). Based
on these numbers, dose equivalents of 0.916 µSv/h and 0.872 µSv/h were estimated. These, together with
a maximum thickness of 1.67 mm of the fungal lawn, were the primary figures used in the attenuation
analysis. The linear attenuation coefficient (LAC) of C. sphaerospermum, 𝜇Fungus, provides a measure for
the fungus’ capacity to shield against ionizing radiation and further allowed the estimation of the
biomass’ melanin content. From this, the thickness of melanized fungus that could negate a certain
radiation dose equivalent was approximated, to put the shielding potential into perspective. Relevant
calculations can be found in supplementary material 1, section D & E.
The LAC of C. sphaerospermum at 10 keV was determined to be 𝜇Fungus = 10.5±5.9 cm-1, and
with a density of
⍴
m ≈ 1.1 g/cm3 for wet microbial biomass 48 the MAC for the fungus was derived as
𝜇Fungus/
⍴
m = 9.51 cm2/g. The experimental attenuation coefficient is, however, only valid for the specific
gamma-energy absorption range of the employed Geiger counters (~ 10 keV, cf. supplementary material
1, section B). An approximate LAC for melanized biomass at any energy can be determined if the
composition of fungal biomass, in particular the melanin content, is known. Here, it was estimated that
about 8.6% [wmelanin/wCWW] of the accumulated C. sphaerospermum biomass were composed of melanin
(cf. supplementary material 1, section D for derivation).[
2
] The high melanin content of
C. sphaerospermum is potentially a metabolic response to the strong radiation environment of the ISS,
analogous to other studies on the fungus in ionizing environments.
Based on the empirical elemental formula for fungal biomass 51 and the water content of fungus
microorganisms 49 the theoretical MAC of melanized fungal biomass at 150 MeV (the average cumulative
energy of the Martian radiation environment) 52 was determined (cf. supplementary material 3). This
allowed 𝜇Fungus and the shielding potential of melanized biomass to be compared in a deep Space
environment equivalent scenario to other (theoretical) attenuation capacities of common aerospace
construction materials and those considered for passive shielding against Space radiation (cf. table S1,
supplementary material 1). Both forms of the pigment that are common in melanized fungi (eumelanin
and DHN-melanin) are comparatively effective radiation attenuators (𝜇Melanin = 0.046 cm-1 at 150 MeV).
Therefore, the predicted high attenuation capacity of melanized fungi at 150 MeV (𝜇Fungus = 0.023 cm-1,
compared to non-melanized fungal biomass with 𝜇Biomass = 0.016 cm-1) is a result of the high LAC of
melanin.
Materials with large LACs typically have high densities and are therefore heavy (like e.g. steel,
cf. supplementary material 3). C. sphaerospermum, however, just like melanin, has a comparatively (for
organic compounds) large innate LAC and a low density, which is desirable in aerospace. Unlike steel or
aluminum, melanin may be available from ISRU through biotechnology. Due to the saprotrophic nature
of the fungus, it grows on virtually any carbon-based biomass, which on Mars could for instance be
cyanobacterial lysate (as previously proposed 53) and/or organic waste. The nature of radiotrophic fungi
also makes them inevitably/necessarily radioresistant, and thus effectively a self-regenerating and -
replicating radiation shield. As biomass has large water content, the fungus may pose an excellent passive
radiation shield for GCR in particular.
2
The melanin content of 8.6% [wmelanin/wCWW] for wet biomass is approx. equivalent to 21.5%
[wmelanin/wCDW] for dry biomass (based on 60% water content 49), which seems realistic in lieu of reported
values for microscopic fungi of 11.2% 37 and 31.5% 50 [wmelanin/wCDW].
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Regardless of how effective a radiation attenuator may be, passive radiation shielding is
ultimately always limited by mass 10. To increase the density and thus the LAC, fungal biomass or
melanin itself could be integrated with in-situ resources that are abundant at destination, such as regolith.
In a case study we estimated that a ~ 2.3 m layer of melanized fungal biomass (8.6% [wmelanin/wCWW]
melanin-content) would be needed to lower Martian radiation levels to those on Earth (from 234 mSv/a to
6.2 mSv/a) 52,7, whereas an equimolar composite of melanin and Martian regolith would only require a
~ 1 m thick layer for the same reduction of radiation. For comparison, in case of pure Martian regolith,
about 1.3-times the thickness would be required to absorb the respective dose equivalent. When
conducting the same analysis with purely non-melanized fungal biomass, a thickness of ~ 3.5 m is
required for the same shielding effect (reduction of radiation by 97%). Table 1 provides an overview of
the inputs and outputs of this basic assessment, details can be found in supplementary material 1, section
E.
Table 1: Comparison of radiation attenuating capacity of in-situ resources on Mars with melanized
composites. Attenuation coefficients for the compounds were obtained from the NIST XCOM database
42, with buildup-factors generated by the RadPro Calculator 54, based on molecular formulas and/or
densities for the respective materials as referenced, unless trivial or noted otherwise.
Material and literature
source for molecular
formula and density
Mass Attenuation
Coefficient ‘𝜇/
⍴
’
[cm2/g] at 150
MeV *
Linear Attenuation
Coefficient ‘𝜇’ [cm-1]
at relative material
density
Required thickness
[cm] to reduce
Martian radiation
levels by ~ 97% $
Water
0.0149
0.0149
367
Martian regolith 55
0.0268
0.0407
128
Martian regolith + melanin
(equimolar mixture)56
0.0288
0.0518
106
Non-melanised fungus§ 51
0.0141
0.0155
351
Melanized fungus#
0.0213
0.0234
232
* cumulative radiation environment on the surface of Mars 52; $ from 234 mSv/a, the average radiation
dose on Mars 52, to 6.2 mSv/a the average radiation dose on Earth 7; § based on an empirical elemental
formula for the biomass of baker’s yeast 57, adjusted for 60% water content 49; # based on 91.4% [w/w]
non-melanized fungal biomass (baker’s yeast 57, adjusted for 60% water content 49), and 8.6% [w/w]
melanin content (DHN-melanin) as per supplementary material 3.
Various melanins and melanin-based composites have been explored as means to attenuate
radiation, and synergistic improvements to radiation attenuating capacity could be achieved: the MAC
curve of a melanin-bismuth composite, for instance, is about double that of lead at energies in a low MeV
range 58. Another example of an enhanced, melanin-based radiation protection agent that is potentially
bio-based is selenomelanin: it was found that under increased radiation, nanoparticles of the compound
could efficiently protect cells against cell cycle changes 59. In future, blends or layers of melanin with
other materials, analogous to the concept of Martian ‘biolith’ 60, may yield composites that more
efficiently shield against cosmic rays. Advanced additive manufacturing technologies such as 3D
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bioprinting, may ultimately also allow the creation of smart ‘living composite’ materials that are adaptive,
self-healing and largely autonomous 61. This will, however, require extensive further theoretical as well as
experimental studies. Section G in supplementary information 1 contains additional remarks on this topic.
Conclusion
Through the design of a subtle yet simple experimental setup, implemented as a small single
payload, it could be shown that the melanized fungus C. sphaerospermum can be cultivated in LEO,
while subject to the unique microgravity and radiation environment of the ISS. Growth characteristics
further suggested that the fungus not only adapts to but thrives on and shields against Space radiation,
analogous to Earth-based studies. It was found that a microbial lawn of less than 2 mm thickness already
decreased the measured radiation levels by at least 1.92% and potentially up to 4.84%. Attenuation of
radiation was consistent over the whole course of the 30-day experiment, allowing a scenario-specific
linear attenuation coefficient for C. sphaerospermum to be determined. This was further used to
approximate the melanin content of the biomass, which corresponded well with literature, serving to
explain the significant reduction in radiation levels by the fungal lawn. Based on the melanin content, the
theoretical radiation attenuation capacity of melanized fungal biomass could be put into perspective at
constant equivalent ionizing radiation energy levels resembling deep-space conditions: melanized
biomass and melanin containing composites were ranked effective radiation attenuators, emphasizing the
great potential melanin holds as component of radiation shields to protect from Space radiation.
Being a living organism, C. sphaerospermum self-replicates from microscopic amounts, which
opens the door for ISRU through biotechnology and significant savings in up-mass. Often nature has
already developed blindly obvious yet surprisingly effective solutions to engineering and design problems
faced as humankind evolves - C. sphaerospermum and melanin could thus prove to be invaluable in
providing adequate protection of explorers on future missions to the Moon, Mars and beyond.
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Acknowledgments
As members of ‘Team Orion’ Srikar Kaligotla, Finn Poulin, and Jamison Fuller were
instrumental for the conception and planning of the experiment. We cordially extend our thanks to the
Higher Orbits Foundation for providing funding for this project through the ‘Go For Launch!’ program,
and to Space Tango, for the technical solution, logistics, and implementation of the experiment.
The team would like to specifically express deep gratitude to Michelle Lucas of Higher Orbits,
for her guidance and enthusiastic encouragement and to Matthew Clobridge of the Durham County Public
Library for bringing the program into reach for the team in the first place, as well as to Gentry Barnett of
Space Tango. We would also like to thank Dr. Robert Gotwals for his guidance on the scientific process,
and Dr. Jonathan Bennet, who introduced the team to calculations used to interpret the results.
We want to make known our appreciation for our colleagues who supported the research every
step of the way and provided encouragement throughout the composition of the manuscript. Finally, we
wish to thank our families and friends for their support and encouragement throughout this study.
Author contributions
GKS and XG, in conjunction with colleagues Srikar Kaligotla, Finn Poulin, and Jamison Fuller,
conceived of the idea for the study in 2018 and composed the proposal for funding. GKS wrote the initial
draft of this paper with support from XG. NJHA joined the team in early 2020, re-visiting and analyzing
the data, revised and refined the manuscript. All authors have read and approved the final version of the
manuscript. The authors declare no competing interests.
(which was not certified by peer review) is the author/funder. All rights reserved. No reuse allowed without permission.
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