Ice nucleation in solutions and freeze-avoiding insects—homogeneous or heterogeneous?

Shear-Enhanced Ion Rejection during Seawater Freezing

Ion rejection during seawater freezing is the basis for freeze desalination. A high ion rejection rate is desired for improving the performance of freeze desalination. In this work, we propose a method to enhance the ion rejection rate through external shear, which is demonstrated through molecular dynamics (MD) simulations and experiments. MD simulations show that the ion rejection rate increases with an increasing shear rate. This is attributed to the disruption of the hydration bonds between ions and water molecules in the hydration shell caused by the shear. Consequently, the mobility of ions is increased, and the energy barrier is reduced at the ice-water interface such that ions have a greater chance of diffusing into the aqueous solution, leading to an enhanced ion rejection rate. The MD results in this work are qualitatively confirmed by experiments and provide insights into the enhancement of the ion rejection rate through external parameters.

Snow flies self-amputate freezing limbs to sustain behavior at sub-zero temperatures

Temperature profoundly impacts all living creatures. In spite of the thermodynamic constraints on biology, some animals have evolved to live and move in extremely cold environments. Here, we investigate behavioral mechanisms of cold tolerance in the snow fly (Chionea spp.), a flightless crane fly that is active throughout the winter in boreal and alpine environments of the northern hemisphere. Using thermal imaging, we show that adult snow flies maintain the ability to walk down to an average body temperature of -7°C. At this supercooling limit, ice crystallization occurs within the snow fly's hemolymph and rapidly spreads throughout the body, resulting in death. However, we discovered that snow flies frequently survive freezing by rapidly amputating legs before ice crystallization can spread to their vital organs. Self-amputation of freezing limbs is a last-ditch tactic to prolong survival in frigid conditions that few animals can endure. Understanding the extreme physiology and behavior of snow insects holds particular significance at this moment when their alpine habitats are rapidly changing due to anthropogenic climate change. VIDEO ABSTRACT.

Lethal and sublethal implications of low temperature exposure for three intertidal predators

Benthic invertebrate predators play a key role in top-down trophic regulation in intertidal ecosystems. While the physiological and ecological consequences of predator exposure to high temperatures during summer low tides are increasingly well-studied, the effects of cold exposure during winter low tides remain poorly understood. To address this knowledge gap, we measured the supercooling points, survival, and feeding rates of three intertidal predator species in British Columbia, Canada - the sea stars Pisaster ochraceus and Evasterias troschelii and the dogwhelk Nucella lamellosa - in response to exposure to sub-zero air temperatures. Overall, we found that all three predators exhibited evidence of internal freezing at relatively mild sub-zero temperatures, with sea stars exhibiting an average supercooling point of -2.50 °C, and the dogwhelk averaging approximately -3.99 °C. None of the tested species are strongly freeze tolerant, as evidenced by moderate-to-low survival rates after exposure to -8 °C air. All three predators exhibited significantly reduced feeding rates over a two-week period following a single 3-h sublethal (-0.5 °C) exposure event. We also quantified variation in predator body temperature among thermal microhabitats during winter low tides. Predators that were found at the base of large boulders, on the sediment, and within crevices had higher body temperatures during winter low tides, as compared to those situated in other microhabitats. However, we did not find evidence of behavioural thermoregulation via selective microhabitat use during cold weather. Since these intertidal predators are less freeze tolerant than their preferred prey, winter low temperature exposures can have important implications for organism survival and predator-prey dynamics across thermal gradients at both local (habitat-driven) and geographic (climate-driven) scales.

Physiological and biochemical differences in diapause and non-diapause pupae of Sericinus montelus (Lepidoptera: Papilionidae)

The swallowtail butterfly, Sericinus montelus Gray, is endemic to East Asia, has high ornamental value but faces an increased risk of extinction. To understand the overwintering strategies of this species, the dynamic changes in supercooling point (SCP) and water and biochemical contents of diapause-destined and non-diapause S. montelus pupae were investigated. The SCP of laboratory-reared diapause pupae was as low as -26°C compared to -24°C in diapause pupae in the field. Although there was no significant difference in total water content between diapause-destined and non-diapause pupae, the free water of diapause-destined pupae was significantly lower, and the bound water was significantly higher, than that of non-diapause pupae. Lipid, glycogen, and protein contents of diapause-destined pupae showed a downward trend, whereas the total sugar content showed the opposite trend after pupation. The glycogen content decreased rapidly during the initial stage of pupation, whereas the lipid content decreased significantly after 30 days of pupation, suggesting that diapause-destined pupae deplete glycogen stores during the pre-diapause period and then switch to using lipids during the diapause maintenance phase. Trehalose levels in diapause-destined pupae increased significantly and remained high after pupation. Meanwhile, the trehalose content of overwintering pupae during the diapause maintenance period was significantly higher than that of diapause termination pupae in the field. These results suggest that trehalose is the main cryoprotectant for overwintering pupae. Thus, diapausing S. montelus pupae appear to be freeze avoidant, accumulate trehalose as a cryoprotectant, and reduce the free water content to decrease the SCP, enhancing their cold tolerance.

Bioinspired Ice-Binding Materials for Tissue and Organ Cryopreservation

Cryopreservation of tissues and organs can bring transformative changes to medicine and medical science. In the past decades, limited progress has been achieved, although cryopreservation of tissues and organs has long been intensively pursued. One key reason is that the cryoprotective agents (CPAs) currently used for cell cryopreservation cannot effectively preserve tissues and organs because of their cytotoxicity and tissue destructive effect as well as the low efficiency in controlling ice formation. In stark contrast, nature has its unique ways of controlling ice formation, and many living organisms can effectively prevent freezing damage. Ice-binding proteins (IBPs) are regarded as the essential materials identified in these living organisms for regulating ice nucleation and growth. Note that controversial results have been reported on the utilization of IBPs and their mimics for the cryopreservation of tissues and organs, that is, some groups revealed that IBPs and mimics exhibited unique superiorities in tissues cryopreservation, while other groups showed detrimental effects. In this perspective, we analyze possible reasons for the controversy and predict future research directions in the design and construction of IBP inspired ice-binding materials to be used as new CPAs for tissue cryopreservation after briefly introducing the cryo-injuries and the challenges of conventional CPAs in the cryopreservation of tissues and organs.

Inhibition of Bacterial Ice Nucleators Is Not an Intrinsic Property of Antifreeze Proteins

Cold-adapted organisms use antifreeze proteins (AFPs) or ice-nucleating proteins (INPs) for the survival in freezing habitats. AFPs have been reported to be able to inhibit the activity of INPs, a property that would be of great physiological relevance. The generality of this effect is not understood, and for the few known examples of INP inhibition by AFPs, the molecular mechanisms remain unclear. Here, we report a comprehensive evaluation of the effects of five different AFPs on the activity of bacterial ice nucleators using a high-throughput ice nucleation assay. We find that bacterial INPs are inhibited by certain AFPs, while others show no effect. Thus, the ability to inhibit the activity of INPs is not an intrinsic property of AFPs, and the interactions of INPs and different AFPs proceed through protein-specific rather than universal molecular mechanisms.

Evidence for non-colligative function of small cryoprotectants in a freeze-tolerant insect

Freeze tolerance, the ability to survive internal ice formation, facilitates survival of some insects in cold habitats. Low-molecular-weight cryoprotectants such as sugars, polyols and amino acids are hypothesized to facilitate freeze tolerance, but their in vivo function is poorly understood. Here, we use a combination of metabolomics and manipulative experiments in vivo and ex vivo to examine the function of multiple cryoprotectants in the spring field cricket Gryllus veletis. Cold-acclimated G. veletis are freeze-tolerant and accumulate myo-inositol, proline and trehalose in their haemolymph and fat body. Injecting freeze-tolerant crickets with proline and trehalose increases survival of freezing to lower temperatures or for longer times. Similarly, exogenous myo-inositol and trehalose increase ex vivo freezing survival of fat body cells from freeze-tolerant crickets. No cryoprotectant (alone or in combination) is sufficient to confer freeze tolerance on non-acclimated, freeze-intolerant G. veletis. Given that each cryoprotectant differentially impacts survival in the frozen state, we conclude that small cryoprotectants are not interchangeable and likely function non-colligatively in insect freeze tolerance. Our study is the first to experimentally demonstrate the importance of non-colligative cryoprotectant function for insect freeze tolerance both in vivo and ex vivo, with implications for choosing new molecules for cryopreservation.

How crickets become freeze tolerant: The transcriptomic underpinnings of acclimation in Gryllus veletis

Some ectotherms can survive internal ice formation. In temperate regions, freeze tolerance is often induced by decreasing temperature and/or photoperiod during autumn. However, we have limited understanding of how seasonal changes in physiology contribute to freeze tolerance, and how these changes are regulated. During a six week autumn-like acclimation, late-instar juveniles of the spring field cricket Gryllus veletis (Orthoptera: Gryllidae) become freeze tolerant, which is correlated with accumulation of low molecular weight cryoprotectants, elevation of the temperature at which freezing begins, and metabolic rate suppression. We used RNA-Seq to assemble a de novo transcriptome of this emerging laboratory model for freeze tolerance research. We then focused on gene expression during acclimation in fat body tissue due to its role in cryoprotectant production and regulation of energetics. Acclimated G. veletis differentially expressed >3000 transcripts in fat body. This differential expression may contribute to metabolic suppression in acclimated G. veletis, but we did not detect changes in expression that would support cryoprotectant accumulation or enhanced control of ice formation, suggesting that these latter processes are regulated post-transcriptionally. Acclimated G. veletis differentially regulated transcripts that likely coordinate additional freeze tolerance mechanisms, including upregulation of enzymes that may promote membrane and cytoskeletal remodelling, cryoprotectant transporters, cytoprotective proteins, and antioxidants. Thus, while accumulation of cryoprotectants and controlling ice formation are commonly associated with insect freeze tolerance, our results support the hypothesis that many other systems contribute to surviving internal ice formation. Together, this information suggests new avenues for understanding the mechanisms underlying insect freeze tolerance.

Screening of cold tolerance in fifteen springtail species

Springtails (Collembola) are ubiquitous and help ecosystem processes such as the decomposition of dead plant material. Their ability to survive low winter temperatures is an important trait that partly defines their geographic distribution. The cold tolerances of 15 laboratory-reared species of springtails were investigated. Springtails were cold acclimated in the laboratory over two months in order to simulate a seasonal change in temperature during autumn. Springtails were then exposed to decreasing sub-zero temperatures and at the same time simulating the moisture conditions in frozen soil. The cold tolerance of the species reflected well the climate of region of origin. Differential scanning calorimetry of individual springtails showed that melting points of body fluids did not become lower due to long-term cold acclimation (from 20° to 1.5°C). However, both water content and melting point of two arctic species (Hypogastrura viatica and Protaphorura macfadyeni) decreased drastically during exposure to sub-zero temperatures indicating cryoprotective dehydration (CPD). These arctic species survived exposure to - 9 °C for two weeks and - 20 °C for at least one week using CPD. Four other subarctic or cool temperate species also used CPD and survived - 9 °C for weeks, whereas springtails in culture from less cool temperate regions had poor cold tolerance.

Effect of wettability on sessile drop freezing: when superhydrophobicity stimulates an extreme freezing delay

An increasing number of studies directed at supercooling water droplets on surfaces with different wettabilities have appeared in recent years. This activity has been stimulated by the recognition that water supercooling phenomena can be effectively used to develop methods for protecting outdoor equipment and infrastructure elements against icing and snow accretion. In this article, we discuss the nucleation kinetics of supercooled sessile water droplets on hydrophilic, hydrophobic, and superhydrophobic surfaces under isothermal conditions at temperatures of -8, -10, and -15 °C and a saturated water vapor atmosphere. The statistics of nucleation events for the ensembles of freezing sessile droplets is completed by the detailed analysis of the contact angle temperature dependence and freezing of individual droplets in a saturated vapor atmosphere. We have demonstrated that the most essential freezing delay is characteristic of the superhydrophobic coating on aluminum, with the texture resistant to contact with ice and water. This delay can reach many hours at T = -8 °C and a few minutes at -23 °C. The observed behavior is analyzed on the basis of different nucleation mechanisms. The dissimilarity in the total nucleation rate, detected for two superhydrophobic substrates having the same apparent contact angle of the water drop but different resistivities of surface texture to the contact with water/ice, is associated with the contribution of heterogeneous nucleation on external centers located at the water droplet/air interface.

Cross generation plasticity in cold hardiness is associated with diapause, but not the non-diapause developmental pathway, in the blowfly, Calliphora vicina

Predicting insect responses to global climate change involves understanding cross generation effects of temperature. The majority of temperate insects overwinter in a state of diapause, a pre-emptive response to winter conditions associated with increased cold hardiness. Diapause is often induced following maternal adult detection of an environmental cue signifying the onset of winter, whilst diapause is initiated in a subsequent life-stage/generation. Continued global warming will expose adults to higher late-autumn temperatures, whilst diapause life-stages will still experience prolonged winter-cold. The cross generation effect of temperature was investigated by acclimating adult Calliphora vicina, to present day (15°C) and future (20°C) late-autumn conditions and assessing cold-hardiness in diapause (D15 and D20) and non-diapause (ND15 and ND 20) progeny. A cross generation plasticity in cold hardiness was associated with D but not ND larvae. D15 larvae exhibited an enhanced ability to suppress the internal freezing (SCP = -18.9±0.9°C) compared to D20 (-15.3±0.8°C), and displayed a greater tolerance of prolonged exposure to -4°C (LT50 26.0± 1.0 days and 11.4±1.1 days, respectively) and -8°C (5.1±1.1 days and 3.0 ±1.1 days, respectively). These changes were associated with a reduced glucose content in D15 (2.4±0.3 g mg-1) compared to D20 (3.0±0.3 g mg-1) larvae. In conclusion, C. vicina adults exposed to warmer autumn conditions during diapause induction will produce larvae with a reduced cold hardiness capacity, which could negatively impact winter survival. Given that maternal regulation of diapause is common among temperate insects this could be a widespread phenomenon.

Cold tolerance and freeze-induced glucose accumulation in three terrestrial slugs

Cold tolerance and metabolic responses to freezing of three slug species common in Scandinavia (Arion ater, Arion rufus and Arion lusitanicus) are reported. Autumn collected slugs were cold acclimated in the laboratory and subjected to freezing conditions simulating likely winter temperatures in their habitat. Slugs spontaneously froze at about -4 °C when cooled under dry conditions, but freezing of body fluids was readily induced at -1 °C when in contact with external ice crystals. All three species survived freezing for 2 days at -1 °C, and some A. rufus and A. lusitanicus also survived freezing at -2 °C. (1)H NMR spectroscopy revealed that freezing of body fluids resulted in accumulation of lactate, succinate and glucose. Accumulation of lactate and succinate indicates that ATP production occurred via fermentative pathways, which is likely a result of oxygen depletion in frozen tissues. Glucose increased from about 6 to 22 μg/mg dry tissue upon freezing in A. rufus, but less so in A. ater and A. lusitanicus. Glucose may thus act as a cryoprotectant in these slugs, although the concentrations are not as high as reported for other freeze tolerant invertebrates.

Probability of freezing in the freeze-avoiding beetle larvae Cucujus clavipes puniceus (Coleoptera: Cucujidae) from interior Alaska

Freeze-avoiding insects must resist freezing or die. A suite of adaptations to low temperatures, including the production of antifreeze proteins, colligative antifreezes (polyols), and dehydration allows most individuals to prevent freezing below the lowest ambient temperatures experienced in situ; however, there can be a wide variance in the minimum temperatures that individuals of freeze-avoiding species reach before freezing. We used logistic regression to explore factors that affect this variance and to estimate the probability of freezing in larvae of the freeze-avoiding beetle Cucujus clavipes puniceus. We hypothesized that water content ≤0.5 mg mg(-1) dry mass would lead to deep supercooling (avoidance of freezing below -58°C). We found a significant interaction between water content and ambient below-snow temperature and a significant difference between individuals collected from two locations in Alaska: Wiseman and Fairbanks. Individuals collected in Wiseman deep supercooled with greater water content and to a greater range of ambient temperatures than individuals collected in Fairbanks, leading to significantly different lethal water contents associated with 50% probability of freezing.

Type I antifreeze proteins enhance ice nucleation above certain concentrations

In this study, we examined the effects that antifreeze proteins have on the supercooling and ice-nucleating abilities of aqueous solutions. Very little information on such nucleation currently exists. Using an automated lag time apparatus and a new analysis, we show several dilution series of Type I antifreeze proteins. Our results indicate that, above a concentration of ∼8 mg/ml, ice nucleation is enhanced rather than hindered. We discuss this unexpected result and present a new hypothesis outlining three components of polar fish blood that we believe affect its solution properties in certain situations.

Interactions between effects of environmental chemicals and natural stressors: a review

Ecotoxicological effect studies often expose test organisms under optimal environmental conditions. However, organisms in their natural settings rarely experience optimal conditions. On the contrary, during most of their lifetime they are forced to cope with sub-optimal conditions and occasionally with severe environmental stress. Interactions between the effects of a natural stressor and a toxicant can sometimes result in greater effects than expected from either of the stress types alone. The aim of the present review is to provide a synthesis of existing knowledge on the interactions between effects of "natural" and chemical (anthropogenic) stressors. More than 150 studies were evaluated covering stressors including heat, cold, desiccation, oxygen depletion, pathogens and immunomodulatory factors combined with a variety of environmental pollutants. This evaluation revealed that synergistic interactions between the effects of various natural stressors and toxicants are not uncommon phenomena. Thus, synergistic interactions were reported in more than 50% of the available studies on these interactions. Antagonistic interactions were also detected, but in fewer cases. Interestingly, about 70% of the tested chemicals were found to compromise the immune system of humans as judged from studies on human cell lines. The challenge for future studies will therefore be to include aspects of combined stressors in effect and risk assessment of chemicals in the environment.

Deep supercooling, vitrification and limited survival to –100°C in the Alaskan beetle Cucujus clavipes puniceus (Coleoptera: Cucujidae) larvae

Larvae of the freeze-avoiding beetle Cucujus clavipes puniceus (Coleoptera: Cucujidae) in Alaska have mean supercooling points in winter of –35 to –42°C, with the lowest supercooling point recorded for an individual of –58°C. We previously noted that some larvae did not freeze when cooled to –80°C, and we speculated that these larvae vitrified. Here we present evidence through differential scanning calorimetry that C. c. puniceus larvae transition into a glass-like state at temperatures <–58°C and can avoid freezing to at least –150°C. This novel finding adds vitrification to the list of insect overwintering strategies. While overwintering beneath the bark of fallen trees, C. c. puniceus larvae may experience low ambient temperatures of around –40°C (and lower) when microhabitat is un-insulated because of low snow cover. Decreasing temperatures in winter are correlated with loss of body water from summer high levels near 2.0 to winter lows near 0.4 mg mg–1 dry mass and concomitant increases in glycerol concentrations (4–6 mol l–1) and thermal hysteresis. Finally, we provide direct evidence that Cucujus from Wiseman, Alaska, survive temperatures to –100°C.

Freeze tolerance, supercooling points and ice formation: comparative studies on the subzero temperature survival of limno-terrestrial tardigrades

Many limno-terrestrial tardigrades live in unstable habitats where they experience extreme environmental conditions such as drought, heat and subzero temperatures. Although their stress tolerance is often related only to the anhydrobiotic state, tardigrades can also be exposed to great daily temperature fluctuations without dehydration. Survival of subzero temperatures in an active state requires either the ability to tolerate the freezing of body water or mechanisms to decrease the freezing point. Considering freeze tolerance in tardigrades as a general feature, we studied the survival rate of nine tardigrade species originating from polar, temperate and tropical regions by cooling them at rates of 9, 7, 5, 3 and 1°C h–1 down to –30°C then returning them to room temperature at 10°C h–1. The resulting moderate survival after fast and slow cooling rates and low survival after intermediate cooling rates may indicate the influence of a physical effect during fast cooling and the possibility that they are able to synthesize cryoprotectants during slow cooling. Differential scanning calorimetry of starved, fed and cold acclimatized individuals showed no intraspecific significant differences in supercooling points and ice formation. Although this might suggest that metabolic and biochemical preparation are non-essential prior to subzero temperature exposure, the increased survival rate with slower cooling rates gives evidence that tardigrades still use some kind of mechanism to protect their cellular structure from freezing injury without influencing the freezing temperature. These results expand our current understanding of freeze tolerance in tardigrades and will lead to a better understanding of their ability to survive subzero temperature conditions.

Physiological variation and phenotypic plasticity: a response to`Plasticity in arthropod cryotypes' by Hawes and Bale

In a recent publication, Hawes and Bale provide an extended discussion of phenotypic plasticity in the context of low temperature responses of animals. They argue that phenotypic plasticity may be partitioned phylogenetically at several levels and go on to explore these levels, and cold hardiness strategies that they term cryotypes, which in their view constitute cryotypic plasticity. Here we argue that this attempt to partition plasticity is misleading, that the term `genotypic plasticity' is potentially highly confusing and a misnomer for physiological variance, and that the term`superplasticity' should not be used. We also show that a definition of strategies as cryotypes is not useful and that the hypothesis about the relationship between evolutionary derivation and extent of plasticity in freeze-avoiding vs freeze-tolerant species is not supported by current evidence.

Is the strategy for cold hardiness in insects determined by their water balance? A study on two closely related families of beetles: Cerambycidae and Chrysomelidae

The strategy for cold-hardiness and water balance features of two closely related families of Coleoptera, Cerambycidae and Chrysomelidae, were investigated. Cerambycids were freeze-avoiding with low supercooling points, whereas chrysomelids froze at high temperatures and were tolerant to freezing. Hence, the two families have adopted different strategies for cold-hardiness. Due to their low trans-cuticular water permeability, the cerambycids have low rates of evaporative water loss. Chrysomelids have much higher trans-cuticular water permeability, but freezing brings their body fluids in vapour pressure equilibrium with ice and prevents evaporative water loss. The differences in cold-hardiness strategies and rates of water loss are likely to reflect the water content of the diets of the two families. Cerambycids feed on dry wood with low water content, causing a restrictive water balance. Chrysomelids feed on leaves with high water content and may use evaporation through the cuticle as a route of water excretion. Haemolymph ice nucleators help chrysomelids to freeze at a high temperature and thus to maximize the period they spend in the water saving frozen state. The diet-related differences in water balance may be the reason why the two families have developed different strategies for cold-hardiness.

Small Dendrobaena earthworms survive freezing better than large worms

Dendrobaena octaedra is a freeze tolerant earthworm widely distributed in boreal regions. Specimens collected in Sweden were cold acclimated and then frozen at -7 degrees C to examine the influence of body mass on survival of freezing. Results showed that survival was negatively correlated to body mass. Glycogen content of the worms was variable and seemed to decrease with increasing body mass consistent with the hypothesis that freeze survival is dependent on the ability to rapidly break down glycogen and accumulate high concentrations of glucose. The results suggest that large worms (subadults and adults) invest energy in production of cocoons at the expense of glycogen storage for cryoprotectant production, whereas juvenile worms increase their survival chances by investing energy in glycogen storage at the expense of growth as a preparation for winter.

Thermoperiodic acclimations enhance cold hardiness of the eggs of the migratory locust

Treatment of thermoperiods that simulate the patterns of natural occurrence is most efficient in enhancing cold hardiness. To examine the effects of different thermoperiods on cold hardiness of eggs in the migratory locust, Locusta migratoria (L.), the survival rates, cryoprotectant levels and three hsps expressions in mid-stage eggs (7-day-old) were measured after the eggs were subjected to three different thermoperiod regimes, : short (2 day), long (10 day), and nature-mimicking thermoperiodic acclimation. The thermoperiodic acclimations resulted in the highest egg survival rates in both the short and the long period acclimation groups in comparison with the groups treated with constant temperatures. The egg survival of nature-mimicking thermoperiod groups was significantly higher than those of constant temperature groups for the same acclimation duration. The survival rate of eggs under single daily thermoperiod was higher than that of multiple daily thermoperiods. The concentration of cryoprotectants (myo-inositol, trehalose, mannitol and sorbitol) and the expression levels of hsp20.5, hsp70, and hsp90 all increased in thermoperiodic acclimation eggs.

Cold hardiness in relation to trace metal stress in the freeze-avoiding beetle Tenebrio molitor

The antifreeze proteins (AFPs) are a family of proteins characterised by their ability to inhibit the growth of ice. These proteins have evolved as a protection against lethal freezing in freeze avoiding species. Metal stress has been shown to reduce the cold hardening in invertebrates, but no study has investigated how this type of stress affects the production of AFPs. This study demonstrates that exposure to cadmium (Cd), copper (Cu) and zinc (Zn) reduces the normal developmental increase in AFP levels in Tenebrio molitor larvae reared under summer conditions. Exposure to winter conditions, however stimulated the production of AFPs in the metal exposed larvae, and raised the concentrations of AFPs to normal winter levels. The reduced level of AFPs in metal-stressed animals acclimated to summer conditions seems to arise from alterations in the normal gene expression of AFPs. The results indicate that metal exposure may cause freeze avoiding insects to become more susceptible to lethal freezing, as they enter the winter with lowered levels of AFPs. Such an effect cannot be revealed by ordinary toxicological tests, but may nevertheless be of considerable ecological importance.

Effect of a mutation on the structure and dynamics of an α-helical antifreeze protein in water and ice

One strategy of psychrophilic organisms to survive subzero temperature is to produce antifreeze protein (AFPs), which inhibit the growth of macromolecular ice. To better understand the binding mechanism, the structure and dynamics of several AFPs have been studied by nuclear magnetic resonance (NMR) and X-ray crystallography. The results have shown that different organisms can use diverse structures (alpha-helix, beta-helix, or different globular folds) to achieve the same function. A number of studies have focused on understanding the relationship between the alpha-helical structure of fish type I AFP and its function as an inhibitor of ice growth. The results have not explained whether the 90% activity loss caused by the conservative mutation of two threonines to serines (Thr13Ser/Thr24Ser) is attributable to a change in protein structure in solution or in ice. We examine here the structure and dynamics of the winter flounder type I AFP and the mutant Thr13Ser/Thr24Ser in both solution and solid states using a wide range of NMR approaches. Both proteins remain fully alpha-helical at all temperatures and in ice, demonstrating that the activity change must therefore not be attributable to changes in the protein fold or dynamics but differences in surface properties.

Extra- and intracellular ice formation in mouse oocytes

The occurrence of intracellular ice formation (IIF) during freezing, or the lack there of, is the single most important factor determining whether or not cells survive cryopreservation. One important determinant of IIF is the temperature at which a supercooled cell nucleates. To avoid intracellular ice formation, the cell must be cooled slowly enough so that osmotic dehydration eliminates nearly all cell supercooling before reaching that temperature. This report is concerned with factors that determine the nucleation temperature in mouse oocytes. Chief among these is the concentration of cryoprotective additive (here, glycerol or ethylene glycol). The temperature for IIF decreases from -14 degrees C in buffered isotonic saline (PBS) to -41 degrees C in 1M glycerol/PBS and 1.5M ethylene glycol/PBS. The latter rapidly permeates the oocyte; the former does not. The initial extracellular freezing at -3.9 to -7.8 degrees C, depending on the CPA concentration, deforms the cell. In PBS that deformation often leads to IIF; in CPA it does not. The oocytes are surrounded by a zona pellucida. That structure appears to impede the growth of external ice through it, but not to block it. In most cases, IIF is characterized by an abrupt blackening or flashing during cooling. But in some cases, especially with dezonated oocytes, a pale brown veil abruptly forms during cooling followed by slower blackening during warming. Above -30 degrees C, flashing occurs in a fraction of a second. Below -30 degrees C, it commonly occurs much more slowly. We have observed instances where flashing is accompanied by the abrupt ejection of cytoplasm. During freezing, cells lie in unfrozen channels between the growing external ice. From phase diagram data, we have computed the fraction of water and solution that remains unfrozen at the observed flash temperatures and the concentrations of salt and CPA in those channels. The results are somewhat ambiguous as to which of these characteristics best correlates with IIF.

Effect of cooling rates on the cold hardiness and cryoprotectant profiles of locust eggs

To examine the relationship between cooling rate and cold hardiness in eggs of the migratory locust, Locusta migratoria, the survival rates and cryoprotectant levels of three embryonic developmental stages were measured at different cooling rates (from 0.05 to 0.8 degrees C min(-1)) in acclimated and non-acclimated eggs. Egg survival rate increased with decreasing cooling rate. The concentration of cryoprotectants (myo-inositol, trehalose, mannitol, glycerol, and sorbitol) increased in non-acclimated eggs, but varied significantly in response to different cooling rates in acclimated eggs. The acclimation process (5 degrees C for 3 days) did not increase eggs resistance to quick cooling ("plunge" cooling and 0.8 degrees C min(-1)). Earlier stage embryos were much more sensitive than later stage embryos to the same cooling rates. Time spent at subzero temperatures also had a strong influence on egg survival.