WATER AND LIFE: EARTH ENVIRONMENTS AND PLANETARY POSSIBILITIES

Metabolically active life is linked to (and its distribution defined by) the presence of liquid water. Thus, even the most tolerant forms of life (extremophiles) apparently reach their limit(s) of activity when liquid water is effectively removed. This can happen by high or low temperatures, high salinity, or aridity. However, there are variations on this theme that should be kept in mind in the search for life in the Universe. First, survival of low water stress is a common feature of organisms generally referred to as prokaryotes (single celled anucleate life that includes Bacteria and Archaea). These “simple” organisms have few fragile parts, and adapt well to low water situations. In fact, microbiologists routinely freeze-dry bacterial cultures for long term storage. Bacterial spores, which remain viable for decades (perhaps much longer) use several molecular mechanisms to sequester water, rendering the cells resistant to heat and UV light. Second, episodic water may be sufficient for the long time existence and even prosperity of prokaryotic life. Many examples are known of earthly life that “wakes up” episodically to resume metabolic activity, repair damage that may have been incurred, and prepare for dormancy again. Of note are the Antarctic rock-dwelling (endolithic) microbes that are active for only a few days of the year. A fundamental question involving such organisms involves the upper limit of dormancy that can be endured before revival and repair are no longer possible. No well constrained numbers are available, and many factors might play into this, such as rates of amino acid racemization, or DNA damage by free radicals, in the absence of water. On the other hand, when water is present, the universal property of life to catalytically use energy sources and produce waste products (usually faster than the rate of diffusion) leads to very predictable patterns of redox nutrient distribution, perhaps the most universal biosignature on Earth. It seems inevitable that if life develops elsewhere in the presence of water, then similar patterns can be used as signatures of metabolic life, and their absence as a sign that life has not evolved to be a pervasive, metabolic part of the planet.