A thin fungal layer was associated with fewer radiation counts on the ISS

Darkly pigmented fungi have been observed in environments with raised radiation, among them the area around Chernobyl. Their cell walls contain melanin, a family of pigments able to absorb some radiation and limit oxidative damage. Two different ideas grew from that fact: that melanised fungi tolerate difficult conditions well, and that a living, self-renewing layer might form part of a radiation shield.

On the International Space Station, Cladosporium sphaerospermum was grown beside an empty control chamber. Sensors beneath the culture and the control compared the dose over about a month. A fungal layer with a mean thickness of around 1.7 millimetres was associated with a reduction in the measurement of roughly two per cent. The fungus grew under flight conditions, but the experiment was small and does not constitute a finished shielding design for astronauts.

The authors extrapolated the result to thicker layers and to composite materials. Such a shield might have an advantage over a panel brought from Earth, because the organism can be multiplied on site and potentially regenerated. At the same time, several centimetres of wet biomass has mass, requires water and biological control, and cosmic radiation consists of various particles and energies. A result for a thin culture must not simply be converted into a promise of protection for a person.

The word "radiotrophy" calls for more caution still. Some studies suggest that radiation changes the properties of melanin and may affect growth, but no equivalent of photosynthesis has been demonstrated in which a fungus sustains itself on radiation as its main energy source. Cladosporium still needs nutrients. The most interesting, well-supported conclusion is more modest: a living, melanised layer measurably attenuated part of the radiation on the ISS and deserves further testing as a component of a material. That description preserves the difference between an observed result and an attractive hypothesis that has yet to be checked at a larger scale.

What the evidence actually shows

This was an intriguing feasibility experiment, not a finished radiation shield for astronauts. On the ISS, one side of a divided Petri dish contained Cladosporium sphaerospermum and the other only growth medium, with simple detectors beneath both sides. After biomass developed, the detector averaged 147 counts per minute below the fungus versus 151 below the control. The PIN detectors counted events but did not measure absorbed dose or energy spectrum; there was also only one orbital run, with one-sided geometry and a very thin layer. The data are consistent with modest attenuation by biomass, but they do not separate melanin’s contribution from water and other material. Flight growth was 1.21 ± 0.37 times the ground-control rate, yet the design could not separate radiation effects from microgravity. “Radiotrophy” and a practical living shield therefore remain hypotheses for further testing.