Life on the Edge: A Study of Cryobiosis in the Tardigrada

Abstract

<p> The search for life on other planets has brought with it a renewed interest in the study of extremophiles as it relates to cold-tolerance. The means for the elucidation of these studies has been through the use of analog-sites in the polar regions of Earth as approximations of extra-planetary environments. These extreme environments are typified by low annual temperatures, low levels of available liquid water, food and light. Despite these conditions, however, life prevails. Perhaps one of the most "extreme" organisms residing in these environments are tardigrades. These micrometazoans are capable of withstanding temperature extremes from 150°C to -276°C, pressure, X-ray radiation, dessication and salinity. Of relevance to this thesis is the ability of the tardigrade to withstand extreme low temperature, which they do by entering a cryobiotic, or latent state. Cryobiosis is seen as an extreme form of cold-tolerance, but apparently lacking a lower lethal temperature (LL T). Despite the incredible advantages that this strategy confers, cryobiosis remains poorly understood. This study provides a review of the literature on freeze tolerance and cryobiosis to bridge the two spheres of research, as well as clarifying the nomenclature used in these papers. Particular attention is paid to the terms of cryoprotective dehydration and cryobiosis, proposing that cryoprotective dehydration be thought of instead as a process leading to the latent state. Experiments were conducted to explore the relationship that time and temperature have on cryobiotic capability in the tardigrade Macrobiotus harmsworthii. Results showed that both time and temperature played a significant role above -80°C, with poor survivability at -20°C. At -80°C, however, time no longer appeared to play a role in viability. </p>