Vitamin supplements degrade during long-term storage under spaceflight conditions, so scientists are exploring in situ microbial production as a complementary strategy to keep nutrition quality up on long-duration missions. Microorganisms are attractive for bioregenerative life-support systems because they grow rapidly at high density in a small footprint and can produce vitamins, antioxidants, and other biomolecules needed for human health. But whether these microbes can keep functioning under the chronic, low-level radiation of a real Mars mission had barely been tested, since most radiobiology relies on acute exposures at far higher doses and dose rates.

The radiation environment of a Mars mission is roughly one hundred times stronger than what life experiences on Earth’s surface. On the Martian surface, where the thin atmosphere and the absence of a global magnetic field offer only partial shielding, the rate drops to about 9 microgray per hour, or roughly 27 microsieverts per hour, with secondary particles generated within the regolith adding to the mix. Although these chronic dose rates are low compared with the acute exposures of conventional radiobiology, they represent a continuous environmental pressure that could influence microbial physiology, DNA stability, and oxidative-stress responses over time.

The three test strains were selected from species on the European Food Safety Authority’s qualified presumption of safety and novel food lists, and all have roles in the European Space Agency’s MELiSSA life-support program. They were Debaryomyces hansenii DSM 3428, a yeast known for ergosterol production and B-vitamin contributions; Bacillus subtilis DSM 10, a spore-forming bacterium capable of riboflavin, or vitamin B2, biosynthesis; and Limosilactobacillus reuteri DSM 20016, an acidophilic, gut-associated lactic acid bacterium that can synthesize B-group vitamins.

For the first two organisms, the verdict was essentially a clean bill of health. Seven days of chronic irradiation at either dose rate left the final biomass yield of D. hansenii unchanged, with flow cytometry cell counts, optical density measurements normalized to dry weight, and intracellular riboflavin content all statistically indistinguishable from non-irradiated controls. Post-irradiation growth assays over 50 hours in liquid medium likewise showed no significant differences in growth rate, lag phase, or maximum biomass. B. subtilis fared the same: biomass yield, riboflavin measurements, and post-irradiation growth parameters were all indistinguishable from controls, indicating that chronic low-dose gamma irradiation did not impair the bacterium’s growth potential or its capacity to resume proliferation after exposure.

The proteomic analysis of B. subtilis reinforced this picture of resilience. Of 1,358 proteins identified, corresponding to roughly 32 percent of the theoretical proteome, fewer than 1 percent showed significant changes in abundance under either dose rate. At the higher dose rate, 104 proteins were differentially abundant, and STRING network clustering revealed a coherent shift: endospore coat proteins and manganese catalase increased in abundance, pointing toward sporulation-related protective processes, while prophage-related proteins were also upregulated. Meanwhile, proteins involved in amino acid and cofactor metabolism, chemotaxis and motility, and iron homeostasis and DNA repair were reduced, suggesting the bacterium dialed down energetically costly biosynthesis and repair in favor of structural protection. Notably, no proteins in the riboflavin biosynthesis pathway changed significantly, consistent with the stable biochemical measurements. At the lower dose rate, the response was even subtler, dominated by minor changes in metabolic and ribosomal proteins, which the authors interpret as a mild adaptive state rather than a stress response.

Limosilactobacillus reuteri told a different story. Both dose rates produced a clear, dose-dependent reduction in biomass yield, with lower cell concentrations by flow cytometry and reduced optical density normalized to dry weight, alongside possible changes in cell morphology or intracellular composition. The post-irradiation growth assay revealed a significantly prolonged lag phase in both irradiated groups, suggesting fewer viable cells or delayed recovery, although the 22 microgray per hour group ultimately reached a significantly higher maximum biomass than the control. Riboflavin production scaled with dry weight, meaning intracellular vitamin content per unit biomass was unchanged. At the proteomic level, the response was far broader than in B. subtilis: 38 proteins differed in abundance at the lower dose rate, about 2 percent of the detected proteome, and 234 at the higher rate, roughly 13 percent. The higher-dose response featured DNA damage sensing and SOS-response proteins, increased translation and ribosome-related proteins, and reduced abundance of carbon metabolism, cofactor biosynthesis, lipid metabolism, and amino acid transport proteins, a pattern the authors describe as resource reallocation toward genome maintenance at the expense of growth.

The authors caution that the study has important limitations. Cobalt-60 gamma rays do not reproduce the full complexity of the space radiation spectrum, which includes high-energy charged particles with different linear energy transfer and energy deposition patterns. The experiments were run under standard laboratory gravity, so any combined effects of radiation and altered gravity remain untested, even though simulated microgravity is known to affect microbial metabolism and stress responses. The aerobic agar-based cultures also differ from the liquid bioreactor conditions expected in closed life-support systems, and flow cytometry quantified total cell abundance rather than viability. Finally, the seven-day exposure window cannot capture long-term adaptation or evolutionary responses, which would require extended multigenerational experiments.

Even with those caveats, the central finding stands: intracellular riboflavin levels remained stable across all three strains under chronic low-dose gamma irradiation, preserving the metabolic capability that makes these organisms attractive for space food systems. The measured riboflavin concentrations were relatively low and do not represent optimized production yields, since the goal was to test whether biosynthesis capacity survived irradiation rather than to maximize output. Previous work has shown that riboflavin production can be substantially enhanced through cultivation optimization, and the authors note that further optimization and larger-scale studies will be needed to determine how much these microbes could actually contribute to astronaut nutrition. For now, the study provides a species-specific map of radiation tolerance that should guide candidate selection: D. hansenii and B. subtilis emerge as robust platforms for extraterrestrial bioprocessing, while L. reuteri, though more radiation-sensitive, remained viable and metabolically active, adapting through flexible regulation of DNA maintenance, envelope integrity, and core metabolism.

The broader significance lies in what the work rules out as much as what it confirms. Chronic exposure to gamma-simulated Martian-relevant dose rates did not critically impair the metabolism or growth potential of the two most resilient strains, and even the sensitive lactic acid bacterium kept its vitamin output per unit biomass intact.

Subject of Research: Effects of chronic Mars-mission-relevant gamma radiation dose rates on growth, riboflavin production, and proteomes of three food-grade microorganisms

Article Title: Molecular profile of micronutrient-producing microorganisms following Martian mission-relevant ionizing radiation dose rate exposure

Article References: Ellena, G., Mazzoli, A., Gupta, S., Spacova, I., Leys, N., Lebeer, S., & Mastroleo, F. (2026). Molecular profile of micronutrient-producing microorganisms following Martian mission-relevant ionizing radiation dose rate exposure. iScience, 29(11), Article 117979. https://doi.org/10.1016/j.isci.2026.117979

Image Credits: AI Generated

DOI: 10.1016/j.isci.2026.117979

Keywords: Mars, space radiation, microbiology, riboflavin, bioregenerative life support, Bacillus subtilis, Debaryomyces hansenii, Limosilactobacillus reuteri, proteomics, gamma irradiation, MELiSSA, astrobiology