Cold tolerance strategies in nematodes

Terrestrial nematodes from the Maritime Antarctic

Background

Soil nematodes are one of the most important terrestrial faunal groups in Antarctica, as they are a major component of soil micro-food webs. Despite their crucial role in soil processes, knowledge of their species diversity and distribution is still incomplete. Taxonomic studies of Antarctic nematodes are fragmented, which prevents assessment of the degree of endemicity and distribution of the species, as well as other aspects of biogeography.

New information

The present study is focused on the nematode fauna of one of the three Antarctic sub-regions, the Maritime Antarctic and summarises all findings published up to April 2023. A species list that includes 44 species, belonging to 21 genera, 16 families and eight orders is provided. A review of the literature on terrestrial nematodes inhabiting the Maritime Antarctic showed that the sites are unevenly studied. Three islands (Signy, King George and Livingston Islands) revealed highest species richness, probably due to the highest rates of research effort. Most species and four genera (Antarctenchus, Pararhyssocolpus, Amblydorylaimus and Enchodeloides) are endemic, proving that nematode fauna of the Maritime Antarctic is autochthonous and unique. Several groups of islands/sites have been revealed, based on their nematode fauna. The study showed that species with a limited distribution prevailed, while only two species (Plectusantarcticus and Coomansusgerlachei) have been found in more than 50% of the sites. Based on the literature data, details on species localities, microhabitat distribution, plant associations and availability of DNA sequences are provided.

The functional role and diversity of soil nematodes are stronger at high elevation in the lesser Himalayan Mountain ranges

Soil nematodes are a foremost component of terrestrial biodiversity; they display a whole gamut of trophic guilds and life strategies, and by their activity, affect major ecosystem process, such as organic matter degradation and carbon cycling. Based on nematodes' functional types, nematode community indices have been developed, and can be used to link variation in nematodes community composition and ecosystem processes. Yet, the use of these indices has been mainly restricted to anthropogenic stresses. In this study, we propose to expand the use of nematodes' derived ecological indices to link soil and climate properties with soil food webs, and ecosystem processes that all vary along steep elevation gradients. For this purpose, we explored how elevation affects the trophic and functional diversity of nematode communities sampled every 300 m, from about 1,000 m to 3,700 m above sea level, across four transects in the lesser Himalayan range of Jammu and Kashmir. We found that (a) the trophic and functional diversity of nematodes increases with elevation; (b) differences in nematodes communities generate habitat-specific functional diversity; (c) the maturity index (ΣMI) increases with elevation, while the enrichment index decreases, indicating less mature and less productive ecosystems, enhanced fungal-based energy flow, and a predominant role of nematodes in generating carbon influxes at high-elevation sites. We thus confirm that the functional contribution of soil nematodes to belowground ecosystem processes, including carbon and energy flow, is stronger at high elevation. Overall, this study highlights the central importance of nematodes in sustaining soil ecosystems and brings insights into their functional role, particularly in alpine and arctic soils.

Temperature-dependent development and freezing survival of protostrongylid nematodes of Arctic ungulates: implications for transmission

BACKGROUND

Umingmakstrongylus pallikuukensis and Varestrongylus eleguneniensis are two potentially pathogenic lungworms of caribou and muskoxen in the Canadian Arctic. These parasites are currently undergoing northward range expansion at differential rates. It is hypothesized that their invasion and spread to the Canadian Arctic Archipelago are in part driven by climate warming. However, very little is known regarding their physiological ecology, limiting our ability to parameterize ecological models to test these hypotheses and make meaningful predictions. In this study, the developmental parameters of V. eleguneniensis inside a gastropod intermediate host were determined and freezing survival of U. pallikuukensis and V. eleguneniensis were compared.

METHODS

Slug intermediate hosts, Deroceras laeve, were collected from their natural habitat and experimentally infected with first-stage larvae (L1) of V. eleguneniensis. Development of L1 to third-stage larvae (L3) in D. laeve was studied at constant temperature treatments from 8.5 to 24 °C. To determine freezing survival, freshly collected L1 of both parasite species were held in water at subzero temperatures from -10 to -80 °C, and the number of L1 surviving were counted at 2, 7, 30, 90 and 180 days.

RESULTS

The lower threshold temperature (T0) below which the larvae of V. eleguneniensis did not develop into L3 was 9.54 °C and the degree-days required for development (DD) was 171.25. Both U. pallikuukensis and V. eleguneniensis showed remarkable freeze tolerance: more than 80% of L1 survived across all temperatures and durations. Larval survival decreased with freezing duration but did not differ between the two species.

CONCLUSION

Both U. pallikuukensis and V. eleguneniensis have high freezing survival that allows them to survive severe Arctic winters. The higher T0 and DD of V. eleguneniensis compared to U. pallikuukensis may contribute to the comparatively slower range expansion of the former. Our study advances knowledge of Arctic parasitology and provides ecological and physiological data that can be useful for parameterizing ecological models.

Sea ice meiofauna distribution on local to pan-Arctic scales

Arctic sea ice provides microhabitats for biota that inhabit the liquid-filled network of brine channels and the ice-water interface. We used meta-analysis of 23 published and unpublished datasets comprising 721 ice cores to synthesize the variability in composition and abundance of sea ice meiofauna at spatial scales ranging from within a single ice core to pan-Arctic and seasonal scales. Two-thirds of meiofauna individuals occurred in the bottom 10 cm of the ice. Locally, replicate cores taken within meters of each other were broadly similar in meiofauna composition and abundance, while those a few km apart varied more; 75% of variation was explained by station. At the regional scale (Bering Sea first-year ice), meiofauna abundance varied over two orders of magnitude. At the pan-Arctic scale, the same phyla were found across the region, with taxa that have resting stages or tolerance to extreme conditions (e.g., nematodes and rotifers) dominating abundances. Meroplankton, however, was restricted to nearshore locations and landfast sea ice. Light availability, ice thickness, and distance from land were significant predictor variables for community composition on different scales. On a seasonal scale, abundances varied broadly for all taxa and in relation to the annual ice algal bloom cycle in both landfast and pack ice. Documentation of ice biota composition, abundance, and natural variability is critical for evaluating responses to decline in Arctic sea ice. Consistent methodology and protocols must be established for comparability of meiofauna monitoring across the Arctic. We recommend to (1) increase taxonomic resolution of sea ice meiofauna, (2) focus sampling on times of peak abundance when seasonal sampling is impossible, (3) include the bottom 30 cm of ice cores rather than only bottom 10 cm, (4) preserve specimens for molecular analysis to improve taxonomic resolution, and (5) formulate a trait-based framework that relates to ecosystem functioning.

Meio- and Macrofaunal Communities in Artificial Water-Filled Tree Holes: Effects of Seasonality, Physical and Chemical Parameters, and Availability of Food Resources

Objectives

In this study we investigated the dynamics of meiofaunal and macrofaunal communities in artificial water-filled tree holes. The abundances and, for the first time, biomasses and secondary production rates of these communities were examined. The experimental set-up consisted of 300 brown plastic cups placed in temperate mixed forests and sampled five times over a period of 16 months to determine the impact of (i) seasonal events, (ii) physicochemical parameters, and (iii) food resources on the tree hole metazoans.

Outcomes

Metazoan organisms, especially the meiofauna (rotifers and nematodes) occupied nearly all of the cups (> 99%) throughout the year. Between 55% and 99% of the metazoan community was represented by rotifers (max. 557,000 individuals 100 cm^-2) and nematodes (max. 58,000 individuals 100 cm^-2). Diptera taxa, particularly Dasyhelea sp. (max. 256 individuals 100 cm^-2) dominated the macrofaunal community. Macrofauna accounted for the majority of the metazoan biomass, with a mean dry weight of 5,800 μg 100 cm^-2 and an annual production rate of 20,400 μg C 100 cm^-2, whereas for meiofauna mean biomass and annual production were 100 μg 100 cm^-2 and 5,300 μg C 100 cm^-2, respectively. The macrofaunal taxa tended to show more fluctuating population dynamic while the meiofaunal dynamic was rather low with partly asynchronous development. Seasonality (average temperature and rain intervals) had a significant impact on both meiofauna and macrofauna. Furthermore, bottom-up control (chlorophyll-a and organic carbon), mainly attributable to algae, was a significant factor that shaped the metazoan communities. In contrast, physicochemical water parameters had no evident influence. 23.7% of organism density distribution was explained by redundancy analysis (RDA) indicating a high dynamic and asynchrony of the systems.

Intracellular freezing in the infective juveniles of Steinernema feltiae: an entomopathogenic nematode

Taking advantage of their optical transparency, we clearly observed the third stage infective juveniles (IJs) of Steinernema feltiae freezing under a cryo-stage microscope. The IJs froze when the water surrounding them froze at -2°C and below. However, they avoid inoculative freezing at -1°C, suggesting cryoprotective dehydration. Freezing was evident as a sudden darkening and cessation of IJs' movement. Freeze substitution and transmission electron microscopy confirmed that the IJs of S. feltiae freeze intracellularly. Ice crystals were found in every compartment of the body. IJs frozen at high sub-zero temperatures (-1 and -3°C) survived and had small ice crystals. Those frozen at -10°C had large ice crystals and did not survive. However, the pattern of ice formation was not well-controlled and individual nematodes frozen at -3°C had both small and large ice crystals. IJs frozen by plunging directly into liquid nitrogen had small ice crystals, but did not survive. This study thus presents the evidence that S. feltiae is only the second freeze tolerant animal, after the Antarctic nematode Panagrolaimus davidi, shown to withstand extensive intracellular freezing.

Cold tolerance in sealworm (Pseudoterranova decipiens) due to heat-shock adaptations

Third-stage larvae ofPseudoterranova decipienscommonly infect whitefish such as cod, and the parasite can be transferred to humans through lightly prepared (sushi) meals. Because little is known about the nematode's cold tolerance capacity, we examined the nematode's ability to supercool, and whether or not cold acclimation could induce physiological changes that might increase its ability to tolerate freezing conditions. Even if third-stagePseudoterranova decipienslarvae have some supercooling ability, they show no potential for freezing avoidance because they are not able to withstand inoculative freezing. Still, they have the ability to survive freezing at high subzero temperatures, something which suggests that these nematodes have a moderate freeze tolerance. We also show that acclimation to high temperatures triggers trehalose accumulation to an even greater extent than cold acclimation. Trehalose is a potential cryoprotectant which has been shown to play a vital role in the freeze tolerance of nematodes. We suggest that the trehalose accumulation observed for the cold acclimation is a general response to thermal stress, and that the nematode's moderate freeze tolerance may be acquired through adaptation to heat rather than coldness.

Cold tolerance of an Antarctic nematode that survives intracellular freezing: comparisons with other nematode species

Panagrolaimus davidi is an Antarctic nematode with very high levels of cold tolerance. Its survival was compared with that of some other nematodes (P. rigidus, Rhabditophanes sp., Steinernema carpocapsae, Panagrellus redivivus and Ditylenchus dipsaci) in both unacclimated samples and those acclimated at 5 degrees C. Levels of recrystallization inhibition in homogenates were also compared, using the splat-cooling assay. The survival of P. davidi after the freezing of samples was notably higher than that of the other species tested, suggesting that its survival ability is atypical compared to other nematodes. In general, acclimation improved survival. Levels of recrystallization inhibition were not associated with survival but such a relationship may exist for those species that are freezing tolerant.

Environmental constraints on life histories in Antarctic ecosystems: tempos, timings and predictability

Knowledge of Antarctic biotas and environments has increased dramatically in recent years. There has also been a rapid increase in the use of novel technologies. Despite this, some fundamental aspects of environmental control that structure physiological, ecological and life-history traits in Antarctic organisms have received little attention. Possibly the most important of these is the timing and availability of resources, and the way in which this dictates the tempo or pace of life. The clearest view of this effect comes from comparisons of species living in different habitats. Here, we (i) show that the timing and extent of resource availability, from nutrients to colonisable space, differ across Antarctic marine, intertidal and terrestrial habitats, and (ii) illustrate that these differences affect the rate at which organisms function. Consequently, there are many dramatic biological differences between organisms that live as little as 10 m apart, but have gaping voids between them ecologically. Identifying the effects of environmental timing and predictability requires detailed analysis in a wide context, where Antarctic terrestrial and marine ecosystems are at one extreme of the continuum of available environments for many characteristics including temperature, ice cover and seasonality. Anthropocentrically, Antarctica is harsh and as might be expected terrestrial animal and plant diversity and biomass are restricted. By contrast, Antarctic marine biotas are rich and diverse, and several phyla are represented at levels greater than global averages. There has been much debate on the relative importance of various physical factors that structure the characteristics of Antarctic biotas. This is especially so for temperature and seasonality, and their effects on physiology, life history and biodiversity. More recently, habitat age and persistence through previous ice maxima have been identified as key factors dictating biodiversity and endemism. Modern molecular methods have also recently been incorporated into many traditional areas of polar biology. Environmental predictability dictates many of the biological characters seen in all of these areas of Antarctic research.

The environmental physiology of Antarctic terrestrial nematodes: a review

The environmental physiology of terrestrial Antarctic nematodes is reviewed with an emphasis on their cold-tolerance strategies. These nematodes are living in one of the most extreme environments on Earth and face a variety of stresses, including low temperatures and desiccation. Their diversity is low and declines with latitude. They show resistance adaptation, surviving freezing and desiccation in a dormant state but reproducing when conditions are favourable. At high freezing rates in the surrounding medium the Antarctic nematode Panagrolaimus davidi freezes by inoculative freezing but can survive intracellular freezing. At slow freezing rates this nematode does not freeze but undergoes cryoprotective dehydration. Cold tolerance may be aided by rapid freezing, the production of trehalose and by an ice-active protein that inhibits recrystallisation. P. davidi relies on slow rates of water loss from its habitat, and can survive in a state of anhydrobiosis, perhaps aided by the ability to synthesise trehalose. Teratocephalus tilbrooki and Ditylenchus parcevivens are fast-dehydration strategists. Little is known of the osmoregulatory mechanisms of Antarctic nematodes. Freezing rates are likely to vary with water content in Antarctic soils. Saturated soils may produce slow freezing rates and favour cryoprotective dehydration. As the soil dries freezing rates may become faster, favouring freezing tolerance. When the soil dries completely the nematodes survive anhydrobiotically. Terrestrial Antarctic nematodes thus have a variety of strategies that ensure their survival in a harsh and variable environment. We need to more fully understand the conditions to which they are exposed in Antarctic soils and to apply more natural rates of freezing and desiccation to our studies.

The response of Anisakis larvae to freezing

Anisakis third stage larvae utilize a variety of fish as intermediate hosts. Uncooked fish are rendered safe for human consumption by freezing. Larvae freeze by inoculative freezing from the surrounding medium but can survive freezing at temperatures down to -10 degrees C. This ability may be aided by the production of trehalose, which can act as a cryoprotectant, but does not involve recrystallization inhibition. Monitoring of fish freezing in commercial blast freezers and under conditions which simulate those of a domestic freezer, indicate that it can take a long time for all parts of the fish to reach a temperature that will kill the larvae. This, and the moderate freezing tolerance of larvae, emphasizes the need for fish to be frozen at a low enough temperature and for a sufficient time to ensure that fish are safe for consumption.

Parasites and low temperatures

Low temperatures affect the rate of growth, development and metabolism of parasites and when temperatures fall below 0 degrees C may expose the parasite to the potentially lethal risk of freezing. Some parasites have mechanisms, such as diapause, which synchronise their life cycle with favourable seasons and the availability of hosts. Parasites of endothermic hosts are protected from low temperatures by the thermoregulatory abilities of their host. Free-living and off-host stages, however, may be exposed to subzero temperatures and both freezing-tolerant and freeze-avoiding strategies of cold hardiness are found. Parasites of ectothermic hosts may be exposed to subzero temperatures within their hosts. They can rely on the cold tolerance adaptations of their host or they may develop their own mechanisms. Exposure to low temperatures may occur within the carcass of the host and this may be of epidemiological significance if the parasite can be transmitted via the consumption of the carcass.

RE. Cold hardiness in two helminth parasites of the freeze-tolerant wood frog, Rana sylvatica

Wood frogs, Rana sylvatica, tolerate the freezing of their body tissues as an overwintering adaptation. Various parasites infect wood frogs of northern populations, but nothing is known about their strategies for surviving within a frozen host. We examined winter-conditioned wood frogs that were experimentally exposed to 0°C (nonfrozen) or –4°C (frozen) to determine whether endoparasites survive the freezing of their host. We found no differences in the prevalence or intensity of adult lungworms Rhabdias ranae (Nematoda) or of larvae of an unidentified species of digenetic trematode between these groups. Live individuals of both species were observed in hosts that recovered from experimental freezing at –4°C. Within the host, R. ranae also tolerated exposure to –5°C, a temperature near the lower limit of survival of the wood frog. Cryostage experiments showed that, like its host, R. ranae was highly susceptible to inoculative freezing and tolerant of the freezing of its tissues. Rhabdias ranae frozen in vitro in the presence or absence of 250 mM glucose, the cryoprotectant used by wood frogs, recovered from a 10-h exposure to –4°C. The mechanism of cold tolerance used by larval trematodes was not investigated; however, we hypothesize that freeze avoidance by supercooling may be important in this species. Freeze-tolerant anurans, such as the wood frog, are useful subjects in the study of coevolution of thermal tolerance in parasites and their host.

Partial desiccation induced by sub-zero temperatures as a component of the survival strategy of the Arctic collembolan Onychiurus arcticus (Tullberg)

The mechanism by which the freeze susceptible Arctic collembolan Onychiurus arcticus survives winter temperatures of -25 degrees C in the field is not fully understood but exposure to sub-zero temperatures (e.g. -2.5 degrees C) is known to induce dehydration and lower the supercooling point (SCP). In this study, changes in the water status and certain biochemical parameters (measured in individual Collembola) during a 3-week exposure to decreasing temperatures from 0 to -5.5 degrees C were studied. Osmotically active and inactive body water contents were measured by differential scanning calorimetry (DSC), water soluble carbohydrates by high performances liquid chromatography (HPLC) and glycogen by enzymatic assays. The activity of trehalase and trehalose 6-phosphate synthase were also measured. During the experiment, total water content decreased from 70 to 40% of fresh weight, mostly by the loss of osmotically active water with only a small reduction in the osmotically inactive component. The SCP decreased from -7 to -17 degrees C. Analysis of the results shows that if O. arcticus is exposed to -7 degrees C in the presence of ice, all osmotically active water would be lost due to the vapour pressure gradient between the animals supercooled body fluids and the ice. Under these conditions the estimated SCP would reach a minimum of c. -27 degrees C, but the Collembola may never freeze as all the osmotically active water has been lost, the animal becoming almost anhydrobiotic. Trehalose concentration increased from 0.9 to 94.7&mgr;g mg(-1)fw while glycogen reserves declined from 160 to 7.7 nmol glucose equivalents mg(-1) protein. Trehalase activity declined as the temperature was reduced, while trehalose 6-phosphate activity peaked at 0 degrees C. By adopting a strategy of near anhydrobiosis induced by sub-zero temperatures, O. arcticus, which was previously thought to be poorly adapted to survive severe winter temperatures, is able to colonise high Arctic habitats.

Differential scanning calorimetry studies on an Antarctic nematode (Panagrolaimus davidi) which survives intracellular freezing

Differential scanning calorimetry was used to characterize thermal events associated with freezing and melting of suspensions and extracts of Panagrolaimus davidi, an Antarctic nematode which can survive intracellular freezing. Nematode suspensions produced a single freezing exotherm with a shoulder on the peak representing the freezing of the nematodes. A shoulder on the peak of melting endotherms indicates the melting of the nematodes and of the water surrounding them. Exotherms were also detected from individual nematodes mounted in liquid paraffin. The freezing of nematodes was very rapid and in marked contrast to that of freezing-tolerant insects and vertebrates, which take hours or days to freeze. Eighty-two percent of the nematodes' body water froze. High levels of survival were obtained in nematodes exposed to temperatures down to -40 degrees C. No additional thermal events were observed after the freezing event and before the melting of samples cooled to -40 degrees C, indicating no changes in the proportion of body water frozen. Ice nucleating activity is present in nematode suspensions but not in supernatants from nematode extracts. No thermal hysteresis activity was detected in nematode extracts.

Osmotic stress effects on the freezing tolerance of the antarctic nematode Panagrolaimus davidi

The freezing and freezing survival of the Antarctic nematode Panagrolaimus davidi after exposure to solutions of different osmotic concentrations has been examined using a thermoelectric cooling stage and multi-specimen cooling block to see if there is any evidence that freeze-induced desiccation prevents inoculative freezing. The nematodes froze in all the test solutions used (up to 1138 mosmol.l-1) and at all cooling rates and nucleation temperatures tested. Freezing survival was at its maximum in 0.1 mol.l-1 NaCl in artificial tap water after 1 h exposure to the test solution and in artificial tap water after 24 h exposure. Hyperosmotic and hyposmotic stress adversely affected the nematodes' ability to survive freezing. In nonfrozen controls survival declined with increasing osmolality of the test solution. Measurements of the osmolality of water extracted from a variety of moss samples indicate that the nematodes are exposed to an osmotic concentration of about 9 mosmol.l-1 in their natural habitat. This is close to that of artificial tap water. Our experiments, and measurements of freeze concentration effects in the literature, indicate that freeze-induced desiccation is unlikely to prevent inoculative freezing and the survival of nematodes over the winter.