This experimental study aimed to determine the activity of a near-UVA (405 nm) LED ceiling system against the SARS-CoV-2 virus. The ceiling system comprised 17 near-UVA LED lights with a radiant power of 1.1 W/each centred at 405 nm wavelength. A 96-multiwell plate, fixed to a wooden base, was inoculated with suspensions of VERO E6 cell cultures infected with SARS-CoV-2 virus and irradiated at a distance of 40 cm with a dose of 20.2 J/cm2 for 120 min. The collected suspensions were transferred to VERO cell culture plates and incubated for 3 days. The maximum measurable log reduction obtained, starting from a concentration of 107.2 TCID50/mL, was 3.0 log10 and indicated inhibition of SARS-CoV-2 replication by the near-UVA LED ceiling system. Near-UVA light at a 405-nm wavelength is emerging as a potential alternative treatment for localised infections and environmental decontamination because it is far less harmful to living organisms’ cells than UV-C irradiation.

In the past decade, various astrobiological studies on different lichen species investigated the impairment of viability and photosynthetic activity by exposure to simulated or real space parameters (as vacuum, polychromatic ultraviolet (UV)-radiation and monochromatic UVC) and consistently found high post-exposure viability as well as low rates of photosynthetic impairment (de Vera et al. 2003, 2004a; 2004b; de la Torre et al. 2010; Onofri et al. 2012; Sánchez et al. 2012, 2014; Brandt et al. 2014). To achieve a better understanding of the basic mechanisms of resistance, the present study subdued isolated and metabolically active photobionts of two astrobiologically relevant lichens to UVC254 nm, examined its effect on photosynthetic activity by chlorophyll a fluorescence and characterized the UVC-induced damages by quantum yield reduction and measurements of non-photochemical quenching. The results indicate a strong impairment of photosynthetic activity, photoprotective mechanisms and overall photobiont vitality when being irradiated in the isolated and metabolically active state. In conclusion, the present study stresses the higher susceptibility of photobionts towards extreme environmental conditions as UVC-exposure, a stressor that does not occur on the Earth. By comparison with previous studies, the present results highlight the importance of protective mechanisms in lichens, such as morphological–anatomical traits (Meeßen et al. 2013), secondary lichen compounds (Meeßen et al. 2014) and the symbiont's pivotal ability to pass into anhydrobiosis when desiccating.

We review the principles of ultraviolet (UV) irradiation, the inactivation of infectious agents by UV, and current applications for the control of microorganisms. In particular, wavelengths between 200 and 280 nm (germicidal UV) affect the double-bond stability of adjacent carbon atoms in molecules including pyrimidines, purines and flavin. Thus, UV inactivation of microorganisms results from the formation of dimers in RNA (uracil and cytosine) and DNA (thymine and cytosine). The classic application of UV irradiation is the inactivation of microorganisms in biological safety cabinets. In the food-processing industry, germicidal UV irradiation has shown potential for the surface disinfection of fresh-cut fruit and vegetables. UV treatment of water (potable and wastewater) is increasingly common because the process is effective against a wide range of microorganisms, overdose is not possible, chemical residues or by-products are avoided, and water quality is unaffected. UV has been used to reduce the concentration of airborne microorganisms in limited studies, but the technology will require further development if it is to gain wider application. For bioaerosols, the primary technical challenge is delivery of sufficient UV irradiation to large volumes of air, but the absence of UV inactivation constants for airborne pathogens under a range of environmental conditions (temperature, relative humidity) further compounds the problem.