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Abstract

Perchlorates have been identified on the surface of Mars. This has prompted speculation of what their influence would be on habitability. We show that when irradiated with a simulated Martian UV flux, perchlorates become bacteriocidal. At concentrations associated with Martian surface regolith, vegetative cells of Bacillus subtilis in Martian analogue environments lost viability within minutes. Two other components of the Martian surface, iron oxides and hydrogen peroxide, act in synergy with irradiated perchlorates to cause a 10.8-fold increase in cell death when compared to cells exposed to UV radiation after 60 seconds of exposure. These data show that the combined effects of at least three components of the Martian surface, activated by surface photochemistry, render the present-day surface more uninhabitable than previously thought, and demonstrate the low probability of survival of biological contaminants released from robotic and human exploration missions.

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Conflict of interest statement

The authors declare that they have no competing interests.

Figures

Figure 1
Figure 1
Effects of UVC-irradiated Mg(ClO4)2 on B. subtilis viability. UV = UVC irradiated control; Perchlorate = 0.6 wt% Mg(ClO4)2. p < 0.05 was considered statistically significant (*) among groups; error bars are + s.d. (n = 3).
Figure 2
Figure 2
(a) Effects of UVC-irradiated Mg(ClO4)2 in rock analogues, under anaerobic conditions, polychromatic light and low temperature. Rock analogues exposed to aerobic environment (30 & 60 seconds);Liquid and rock analogue exposed to anaerobic environment (60 s); Liquid system exposed to polychromatic light (10 s); Liquid system chilled to 4 °C whilst irradiated (30 & 60 seconds); UV = UVC irradiated control; Perchlorate = 0.6 wt% Mg(ClO4)2. p < 0.05 was considered statistically significant (*) among groups; error bars are + s.d. (n = 3). (b) Effects of UVC-irradiated Mg(ClO4)2 at low temperature, 1–4 minute exposure. Liquid system chilled to 4 °C whilst irradiated. UV = UVC irradiated control; Perchlorate = 0.6 wt% Mg(ClO4)2. p < 0.05 was considered statistically significant (*) between or among groups; error bars are + s.d. (n = 3).
Figure 3
Figure 3
Influence of increased perchlorate concentration on bacteriocidal effects under UV irradiation. Mg(ClO4)2 at representative measured Martian concentration (0.6 wt %,), 1, 2.5 and 5 wt% (30 & 60 s); UV = UVC irradiated control (30 & 60 s). p < 0.05 was considered statistically significant (*) between and among groups; error bars are + s.d. (n = 3).
Figure 4
Figure 4
Effects of UVC-irradiated sodium/calcium perchlorate and sulfate on B. subtilis viability. UV = UVC irradiated control at given distance from light source; Ca = 0.6 wt% Ca(ClO4)2 at given distance from light source; Na = 0.6 wt% NaClO4 at given distance from light source; perchlorate = 0.6 wt % Mg(ClO4)2; sulfate = 30 wt% MgSO4. Letters shared in common between or among the groups indicate no significant difference (p > 0.05); error bars are + s.d. (n = 3). Vertical grey line indicates separate experiment with different control.
Figure 5
Figure 5
Perchlorate-induced bacteriocidal effects in the presence of other components of the Martian surface (hematite and hydrogen peroxide) after 30 and 60 seconds UV exposure. UV = UV irradiated control; H = 1 g/L hematite; HP = 10 mM hydrogen peroxide; ClO4 = 0.6 wt% Mg(ClO4)2. Letters shared in common between or among the groups indicate no significant difference (p > 0.05); error bars are + s.d. (n = 3). Vertical grey lines serve as visual separation of single, double and triple combinations.
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References

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