Exposure of phototrophs to 548 days in low\ud Earth orbit: microbial selection pressures in\ud outer space and on early earth

摘要

An epilithic microbial community was launched into low Earth orbit, and exposed to conditions in\udouter space for 548 days on the European Space Agency EXPOSE-E facility outside the International\udSpace Station. The natural phototroph biofilm was augmented with akinetes of Anabaena cylindrica\udand vegetative cells of Nostoc commune and Chroococcidiopsis. In space-exposed dark controls,\udtwo algae (Chlorella and Rosenvingiella spp.), a cyanobacterium (Gloeocapsa sp.) and two bacteria\udassociated with the natural community survived. Of the augmented organisms, cells of A. cylindrica\udand Chroococcidiopsis survived, but no cells of N. commune. Only cells of Chroococcidiopsis were\udcultured from samples exposed to the unattenuated extraterrestrial ultraviolet (UV) spectrum\ud(4110nm or 200 nm). Raman spectroscopy and bright-field microscopy showed that under these\udconditions the surface cells were bleached and their carotenoids were destroyed, although cell\udmorphology was preserved. These experiments demonstrate that outer space can act as a selection\udpressure on the composition of microbial communities. The results obtained from samples exposed\udto 4200nm UV (simulating the putative worst-case UV exposure on the early Earth) demonstrate the\udpotential for epilithic colonization of land masses during that time, but that UV radiation on anoxic\udplanets can act as a strong selection pressure on surface-dwelling organisms. Finally, these\udexperiments have yielded new phototrophic organisms of potential use in biomass and oxygen\udproduction in space exploration.