|Rinehart, Joseph - Joe|
Submitted to: Experimental Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/6/2007
Publication Date: 1/1/2008
Citation: Elnitsky, M.A., Hayward, S.A., Rinehart, J.P., Denlinger, D.L., Lee, R.E. 2008. Cryoprotective dehydration and the resistance to inoculative freezing in the Antarctic midge, Belgica antarctica. Journal of Experimental Biology. 211:524-530. Interpretive Summary: Antarctica, being the coldest and driest continent on earth, contains some of the earth’s most inhospitable environments. Although an abundance of animals have adapted to life in the sea in this part of the world, few animals have adapted to living on the continent itself. One exception is the midge Belgica antarctica, which at ¼ of an inch long, is Antarctica’s largest free-living, year-round inhabitant. Previously, our research team has separately studied the cold tolerance and desiccation tolerance of this insect. These studies showed that this insect is freeze tolerant (able to survive ice forming inside its body) and very desiccation tolerant (able to survive even when much of the water has been removed from its body). This study investigates how these two processes may be linked during the extended Antarctic winter. The results indicate that this insect is able to undergo “cryoprotective dehydration”, which means that a slow loss of body water leads to increased tolerance of cold temperature exposure. Hence, two of the major forms of stress encountered by this insect (low temperatures and low humidity) are used in combination to protect this insect from the rigors of living on the harsh continent.
Technical Abstract: During winter, larvae of the Antarctic midge, Belgica antarctica (Diptera, Chironomidae), must endure 7–8 months of continuous subzero temperatures, encasement in a matrix of soil and ice, and severely desiccating conditions. This environment, along with the fact that larvae possess a high rate of water loss and are extremely tolerant of desiccation, may promote the use of cryoprotective dehydration as a strategy for winter survival. This study investigates the capacity of larvae to resist inoculative freezing and undergo cryoprotective dehydration at subzero temperatures. Slow cooling to –3°C in an environment at equilibrium with the vapor pressure of ice reduced larval water content by ~40% and depressed the body fluid melting point more than threefold to –2.6°C. This melting point depression was the result of the concentration of existing solutes (i.e. loss of body water) and the de novo synthesis of osmolytes. By day 14 of the subzero exposure, larval survival was still >95%, suggesting larvae have the capacity to undergo cryoprotective dehydration. However, under natural conditions the use of cryoprotective dehydration may be constrained by inoculative freezing as result of the insect’s intimate contact with environmental ice. During slow cooling within a substrate of frozen soil, the ability of larvae to resist inoculative freezing and undergo cryoprotective dehydration was dependent upon the moisture content of the soil. As detected by a reduction of larval water content, the percentage of larvae that resisted inoculative freezing increased with decreasing soil moisture. These results suggest that larvae of the Antarctic midge have the capacity to resist inoculative freezing at relatively low soil moisture contents and likely undergo cryoprotective dehydration when exposed to subzero temperatures during the polar winter.