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Viruses are the most numerically abundant biological entities on Earth. As ubiquitous replicators of molecular information and agents of community change, viruses have potent effects on the life on Earth, and may play a critical role in human spaceflight, for life-detection missions to other planetary bodies and planetary protection. However, major knowledge gaps constrain our understanding of the Earth's virosphere: (1) the role viruses play in biogeochemical cycles, (2) the origin(s) of viruses and (3) the involvement of viruses in the evolution, distribution and persistence of life. As viruses are the only replicators that span all known types of nucleic acids, an expanded experimental and theoretical toolbox built for Earth's viruses will be pivotal for detecting and understanding life on Earth and beyond. Only by filling in these knowledge and technical gaps we will obtain an inclusive assessment of how to distinguish and detect life on other planetary surfaces. Meanwhile, space exploration requires life-support systems for the needs of humans, plants and their microbial inhabitants. Viral effects on microbes and plants are essential for Earth's biosphere and human health, but virus–host interactions in spaceflight are poorly understood. Viral relationships with their hosts respond to environmental changes in complex ways which are difficult to predict by extrapolating from Earth-based proxies. These relationships should be studied in space to fully understand how spaceflight will modulate viral impacts on human health and life-support systems, including microbiomes. In this review, we address key questions that must be examined to incorporate viruses into Earth system models, life-support systems and life detection. Tackling these questions will benefit our efforts to develop planetary protection protocols and further our understanding of viruses in astrobiology.
Kenneth M. Stedman, Department of Biology, Center for Life in Extreme Environments, Portland State University, PO Box 751, Portland, OR 97207-0751, USA,
Adam Clore, Department of Biology, Center for Life in Extreme Environments, Portland State University, PO Box 751, Portland, OR 97207-0751, USA,
Yannick Combet-Blanc, Laboratoire de Microbiologie IRD, Université de Provence, CESB/ESIL case 925, 163 avenue de Luminy, F-13288 Marseille Cedex 9, France
Biogeography, or the spatial distribution of biological diversity, has been studied since Darwin and Wallace in the 1800s. Their studies, and most later studies, concentrated on macroscopic organisms, mostly animals and plants, and many differences between species were observed, often correlated with geographical isolation. The theoretical basis for these differences was established later and is still being refined. The theories of island biogeography have been extremely influential in many fields of biology (Bell et al., 2005). Critical to biogeographical studies are comparable organisms from different locations with quantifiable diversity, often sequence diversity.
More recently, micro-organisms have been studied (Finlay, 2002), especially with the advent of molecular tools. Studies using enrichment cultures indicated that identical micro-organisms were present wherever they were collected (Smith et al., 1991); however, this is clearly biased due to the relatively small number of micro-organisms that can be cultivated (Pace, 1997). The advent of small-subunit (SSU) rRNA gene sequence analysis indicated that ‘everything is everywhere’, particularly for spore-forming bacteria (Roberts & Cohan, 1995). It was unclear whether this indicated that there was so much dispersal of these spore-forming organisms that they were identical throughout the world or whether it was general for bacteria due to their extremely large population sizes. For the most part, however, only one gene, generally the SSU rRNA gene, was investigated. Extremophiles are thought to have more barriers to dispersal than mesophilic organisms.
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