The Microbiology of Tardigrades and its Potential Role for Astrobiological Research

Abstract

Tardigrades (water bears) are polyextremophilic, cosmopolitan eukaroytic metazoans that are able to survive different types of extreme conditions, ranging from extreme temperature ranges from -273 to >100°C, 1,000 times more ionizing radiation than most other animals such as humans, and complete desiccation for long periods. Tardigrades are one of the few groups of organisms on our planet that are capable of reversibly suspending their metabolism and entering into a state of cryptobiosis, where metabolism has been lowered to immeasurable levels at water content below 1% of the normal hydration. Due to these traits, they belong to one of the few species on planet Earth that can survive some of the extreme conditions found in outer space, and may thus serve as important model organisms to explore the survival rate of species from planet Earth on other planetary bodies. Although the biology and the survival strategies of tardigrades have been studied in greater detail, our knowledge about the endogenous microbiology of tardigrades and its potential role for its survival is rather scarce. However, we anticipate that a combined research on the biology in general and the microbiology of tardigrades could possibly add interesting insights into the survival strategies of tardigrades. For example, to obtain novel insights into the role of prokaryotes as a food source for tardigrades in general, as fundamental parts of the intestinal gut flora that may help degrade ingested substrates, secrete valuable biochemical compounds like vitamins, or as pathogens that may decrease their survival possibilities. Another intriguing question is if prokaryotes in tardigrades shipped to outer space would also survive, and if synergistic interactions may take place that could influence the survival possibilities of both tardigrades and the prokaryotes in an alien system. For example, would it be possible to feed/infect tardigrades with chemolithoautotrophic species that are able to metabolize different types of inorganic compunds and would these be able to grow again on inorganic particles after the cryptobiotic state and thus serve as a continuous food source for the tardigrades in an alien environment? In order to trace such fascinating, though rather speculative issues, several basic studies and method developments must be performed so that reliable tools for microbial involvement under different stress reactions can be used in these kinds of astrobiological experiments. We have started to compare different analytical tools to explore the microbiological composition and dynamics in tardigrades, ranging from nucleic acid based tools and PCR for sequencing and quantification of 16S rRNA genes, microcalorimetric energy measurements as a nondisruptive indicator for responses to different conditions, to microscopic studies for in situ visualization of prokaryotes on the tardigrades. We are currently in the process of evaluating these methods on tardigrades exposed to different stress conditions and fed with different microorganisms in order to explore if tardigrades are dependent on a specific microflora for optimal survival. In future, we plan to design elaborate experiments to explore further the effect of the microflora on the survival rates of tardigrades under different simulated extreme conditions on Earth, as well as in outer space.