Abstract
Four case studies are used to examine the relationships of water, ice nucleators and desiccation in the cold survival of invertebrates and the viability of frozen plant material: the freeze intolerant Antarctic springtail Cryptopygus antarcticus (Willem) (Collembola, Isotomidae), the freeze tolerant larvae of the fly Heleomyza borealis Boh. (Diptera: Heleomyzidae), the freeze intolerant Arctic springtail Onychiurus arcticus (Tullberg) (Collembola, Onychiuridae) and meristems of the currant Ribes ciliatum Humb. & Bonpl.(Grossulariaceae) from Mexico. Prevention of ice nucleation, lowering the water content by removal of osmotically active (freezable) water are critical features of the different cold survival strategies of the three species of invertebrates. In C. antarcticus, which desiccates rapidly by losing water via the cuticle to the atmosphere, the number of ice nucleators (and their activity) increases with lowered ambient temperature. During prolonged cold exposure ice nucleators are masked, but re-activated rapidly by water uptake in this species. Larval H. borealis do not readily desiccate and conserve their body water, 20-25% of it being bound (osmotically inactive). Experiments showed that a high proportion (c. 80%) of slowly cooled larvae survived exposure to -60°C. By comparison O. arcticus is able to sustain up to 40% loss of its body water and desiccation lowers its supercooling point to promote over winter survival. Dehydration leading to partial vitrification of currant (R. ciliatum) meristems improves their viability after cryopreservation in liquid nitrogen. From this comparison of four biological systems, it is concluded that the role of water and its activity at sub-zero temperatures are fundamental to the survival of freezing conditions by all the species studied. Although similar features exist in the four systems, no common basic mechanism was found.
Highlights
It has long been assumed that survival of sub-zero tem peratures, especially in freeze intolerant organisms, largely depends on the ability of the organism to super cool below the melting point temperature of its contained fluids (Zachariassen, 1985; Block, 1995)
Experiments conducted on the freeze intolerant Ant arctic springtail Cryptopygus antarcticus at Rothera Research Station, Adelaide Island, Antarctica investigated the relationship between the mean supercooling point (SCP) and ice nucleating activity (INA) of field-collected and temperature-acclimated samples in conjunction with water content and polyol assays (Block & Worland, 2001)
At one end of the spectrum of response is the dependence of the freeze intolerant Antarctic springtail Cryptopygus antarcticus for its survival on the elimination, masking or sequestration of potential INAs to avoid lethal freezing within the body
Summary
It has long been assumed that survival of sub-zero tem peratures, especially in freeze intolerant organisms, largely depends on the ability of the organism to super cool below the melting point temperature of its contained fluids (Zachariassen, 1985; Block, 1995). The quantity of contained water and its dissolved solutes, often in the form of polyhydric alcohols and sugars for the promotion of supercooling or cryoprotection, and its location and form (free or bound) are impor tant characteristics for the cold survival of an organism. Ice nucleation in such supercooled solutions is promoted by a range of particles and substances of biological or physical origin, whereas antinucleation processes include antifreeze proteins and other mechanisms to reduce the possibility of ice nuclei being formed (Zachariassen & Kristiansen, 2000). Reduction in water content through desiccation leads to an increase in cold hardiness for many species: the resultant smaller liquid volume, increased solute concentration, and an increase in the proportion of bound water relative to the total water content may be responsible
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