Cold hardiness in Entomobrya nivalis (Collembola, Entomobryidae): annual cycle of polyols and antifreeze proteins, and antifreeze triggering by temperature and photoperiod

In the Swiss Prealps Entomobrya nivalis hibernates in an inactive state, hidden under bark flakes on spruce. For freeze avoidance it relies on thermal hysteresis proteins (THPs) and polyols (mainly ribitol, with small amounts arabitol and threitol). Polyols are present only during the inactive state, THPs additionally protect during the transition phase in spring and autumn, when animals are still active but frosts may occur. Peak values were recorded in February/March for THPs (3.5 °C hysteresis between melting and freezing point) and for polyols (26 μg mg⁻¹ FW; hemolymph osmolality 680 mosmol l⁻¹). E. nivalis is able to control its hemolymph osmolality independently of body water content. Mean osmolality in summer was 350– 440 mosmol l⁻¹, in winter it was elevated to 650 mosmol l⁻¹, due to a synthesis mainly of ribitol. Body water content varied between 1.8 and 3.3 mg H₂O mg⁻¹ DW, depending on humidity conditions. Experiments on triggering of antifreeze synthesis showed the action of temperature and photoperiod as cues, but there was also evidence for an endogenous rhythm. No clear correlation between antifreeze concentration and supercooling ability could be established, suggesting that gut content or other parameters also play an inportant role. Accepted: 18 November 1995     

Cold acclimation and cryoprotectants in a freeze-tolerant Antarctic nematode, Panagrolaimus davidi

Panagrolaimus davidi is a freeze-tolerant Antarctic nematode which survives extensive intracellular freezing. This paper describes the development of culture techniques which provide clean samples, with a high degree of freeze tolerance and in sufficient quantities for the analysis of potential cryoprotectants. Cultures grown at 20 °C survived a short-term freezing stress but survival declined with the time spent frozen. Acclimation of cultures at 5 °C enhanced the long-term survival of freezing. Starvation, however, reduced the nematode's ability to survive short-term freezing. The principal cryoprotectants detected by gas chromatography were trehalose and glycerol. The levels of trehalose, but not those of glycerol, increased significantly after acclimation. Trehalose may stabilise membranes and protect them against the dehydrating effects of the osmotic stresses resulting from freeze concentration effects but other factors, such as recrystallisation inhibition, may be involved in long-term survival. Accepted: 7 March 2000     

Osmotic stress effects on the freezing tolerance of the antarctic nematodePanagrolaimus 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 non-frozen 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. Accepted: 5 May 1996     

Arctic Sea ice biota: design and evaluation of a mesocosm experiment

A mesocosm experiment (enclosure volume 220 l) was designed such that sea ice inhabited by Arctic Sea ice organisms was formed and maintained under natural conditions at 66°N in Rovaniemi, Finland. The experiment was run from natural freezing in December 1994 to melting in April 1995. The ice was inhabited by diatoms, chlorophyceae, heterotrophic flagellates, ciliates, nematodes and turbellarians. Biomass in the ice, expressed as Chlorophyll a concentration, was 20–110 μg l⁻¹; total cell densities varied from 5 × 10⁶ to 35 × 10⁶ cells l⁻¹. Amongst phototrophic organisms, a succession from a flagellate-dominated community (Chlamydomonas sp.) to a multi-species diatom-dominated community was observed. Typical Arctic species such as Nitzschia frigida and Melosira arctica were present in the ice. Bacterial concentration varied between 2 × 10⁸ and 7 × 10⁸ cells l⁻¹. At least two trophic levels were present in the ice. Received: 3 April 1997 / Accepted: 9 September 1997     

Isolation of freeze-tolerant laboratory strains of Saccharomyces cerevisiae from proline-analogue-resistant mutants

Since some amino acids, polyols and sugars in cells are thought to be osmoprotectants, we expected that several amino acids might also contribute to enhancing freeze tolerance in yeast cells. In fact, proline and charged amino acids such as glutamate, arginine and lysine showed a marked cryoprotective activity nearly equivalent to that of glycerol or trehalose, both known as major cryoprotectants for Saccharomyces cerevisiae. To investigate the cryoprotective effect of proline on the freezing stress of yeast, we isolated proline-analogue-resistant mutants derived from a proline-non-utilizing strain of S. cerevisiae. When cultured in liquid minimal medium, many mutants showed a prominent increase, two- to approximately tenfold, in cell viability compared to the parent after freezing in the medium at −20 °C for 1 week. Some of the freeze-tolerant mutants were found to accumulate a higher amount of proline, as well as of glutamate and arginine which are involved in proline metabolism. It was also observed that proline-non-utilizer and the freeze-tolerant mutants were able to grow against osmotic stress. These results suggest that the increased flux in the meta-bolic pathway of specific amino acids such as proline is effective for breeding novel freeze-tolerant yeasts. Received: 6 November 1996 / Accepted: 7 December 1996     

Dehydration and cold hardiness in the Arctic Collembolan Onychiurus arcticus Tullberg 1876

Specimens of the Arctic Collembolon Onychiurus arcticus were exposed to desiccation at several subzero temperatures over ice and at 0.5 °C over NaCl solutions. The effects of desiccation on water content (WC), body fluid melting point (MP), supercooling point (SCP) and survival were studied at several acclimation temperatures and relative humidities. Exposure to temperatures down to −19.5 °C caused a substantial and increasing dehydration. At the lowest exposure temperature unfrozen individuals lost 91.6% of the WC at full hydration but more than 80% of the individuals survived when rehydrated. Exposure at 0.5 °C to decreasing relative humidities (RH) from 100% to 91.3% caused increasing dehydration and increasing mortality. Survival of equally dehydrated individuals was higher at subzero temperatures than at 0.5 °C. Concurrent with the decline in WC a lowering of the MP was observed. Animals exposed to −3 °C and −6 °C over ice for 31 days had a MP of −3.8 and < −7.5 °C, respectively. Specimens from a laboratory culture had a mean SCP of −6.1 °C, and acclimation at 0 or −3 °C had little effect on SCPs. Exposure at −8.2 °C over ice for 8 days, however, caused the mean SCP to decline to −21.8 °C due to the severe dehydration of these individuals. Dehydration at 0.5 °C in 95.1 and 93.3% RH also caused a decline in SCPs to about −18 °C. Individuals that had been acclimated over ice at −12.4 °C or at lower temperatures apparently did not freeze at all when cooled to −30 °C, probably because all freezeable water had been lost. These results show that O. arcticus will inevitably undergo dehydration when exposed to subzero temperatures in its natural frozen habitat. Consequently, the MP and SCP of the Collembola are substantially lowered and in this way freezing is avoided. The increased cold hardiness by dehydration is similar to the protective dehydration mechanism described in earthworm cocoons and Arctic enchytraeids. Accepted: 5 January 1998     

Water influx and efflux in free-flying pigeons

We used tritium-labeled water to measure total body water, water influx (which approximated oxidative water production) and water efflux in free-flying tippler pigeons (Columba livia) during flights that lasted on average 4.2 h. At experimental air temperatures ranging from 18 to 27 °C, mean water efflux by evaporation and excretion [6.3 ± 1.3 (SD) ml · h⁻¹, n = 14] exceeded water influx from oxidative water and inspired air (1.4 ± 0.7 ml · h⁻¹, n = 14), and the birds dehydrated at 4.9 ± 0.9 ml · h⁻¹. This was not significantly different from gravimetrically measured mass loss of 6.2 ± 2.1 g · h⁻¹ (t = 1.902, n = 14, P>0.05). This flight-induced dehydration resulted in an increase in plasma osmolality of 4.3 ± 3.0 mosmol · kg⁻¹ · h⁻¹ during flights of 3–4 h. At 27 °C, the increase in plasma osmolality above pre-flight levels (ΔP _osm = 7.6±4.29 mosmol · kg⁻¹ · h⁻¹, n = 6) was significantly higher than that at 18 °C (ΔP _osm = 0.83±2.23 mosmol · kg⁻¹ · h⁻¹, (t = 3.43, n = 6, P < 0.05). Post-flight haematocrit values were on average 1.1% lower than pre-flight levels, suggesting plasma expansion. Water efflux values during free flight were within 9% of those in the one published field study (Gessaman et al. 1991), and within the range of values for net water loss determined from mass balance during wind tunnel experiments (Biesel and Nachtigall 1987). Our net water loss rates were substantially higher than those estimated by a simulation model (Carmi et al. 1992) suggesting some re-evaluation of the model assumptions is required. Accepted: 8 April 1997     

Adaptation and acclimation of growth and photosynthesis of five Antarctic red algae to low temperatures

Temperature requirements for growth, photosynthesis and dark respiration were determined for five Antarctic red algal species. After acclimation, the stenothermal species Gigartina skottsbergii and Ballia callitricha grew at 0 or up to 5 °C, respectively; the eurythermal species Kallymenia antarctica, Gymnogongrus antarcticus and Phyllophora ahnfeltioides grew up to 10 °C. The temperature optima of photosynthesis were between 10 and 15 °C in the stenothermal species and between 15 and 25 °C in the eurythermal species, irrespective of the growth temperature. This shows that the temperature optima for photosynthesis are located well below the optima from species of other biogeographical regions, even from the Arctic. Respiratory rates rose with increasing temperatures. In contrast to photosynthesis, no temperature optimum was evident between 0 and 25 °C. Partial acclimation of photosynthetic capacity to growth temperature was found in two species. B. callitricha and Gymnogongrus antarcticus acclimate to 0 °C, and 5 and 0 °C, respectively. But acclimation did in no case lead to an overall shift in the temperature optimum of photosynthesis. B. callitricha and Gymnogongrus antarcticus showed acclimation of respiration to 5 °C, and P. ahnfeltioides to 5 and 10 °C, resulting in a temperature independence of respiration when measured at growth temperature. With respect to the acclimation potential of the species, no distinction can be made between the stenothermal versus the eurythermal group. (Net)photosynthetic capacity:respiration (P:R) ratios showed in all species highest values at 0 °C and decreased continuously to values lower than 1.0 at 25 °C. In turn, the low P:R ratios at higher temperatures are assumed to determine the upper temperature growth limit of the studied species. Estimated daily carbon balance reached values between 4.1 and 30.7 mg C g⁻¹ FW day⁻¹ at 0 °C, 16:8 h light/dark cycle, 12–40 μmol m⁻² s⁻¹. Received: 4 November 1999 / Accepted: 7 March 2000     

Cold tolerance and dehydration in Enchytraeidae from Svalbard

When cooled in contact with moisture, eight species of arctic Enchytraeidae from Svalbard were killed by freezing within minutes or hours at −3 and −5 °C; an exception was Enchytraeus kincaidi which survived for up to 2 days. When the temperature approached 0 °C the enchytraeids apparently tried to escape from the moist soil. The supercooling capacity of the enchytraeids was relatively low, with mean supercooling points of −5 to −8 °C. In contrast, specimens of several species were extracted from soil cores that had been frozen in their intact state at −15 °C for up to 71 days. Compared to freezing in a moist environment, higher survival rates were obtained during cooling at freezing temperatures in dry soil. Survival was recorded in species kept at −3 °C for up to 35 days, and in some species kept at −6 °C for up to 17 days. Slow warming greatly increased survival rates at −6 °C . The results strongly suggest that arctic enchytraeids avoid freezing by dehydration at subzero temperatures. In agreement with this, weight losses of up to ca. 42% of fresh weight were recorded in Mesenchytraeus spp. and of up to 55% in Enchytraeus kincaidi at water vapour pressures above ice at −3 to −6 °C. All specimens survived dehydration under these conditions. Accepted: 12 December 1997     

Freezing avoidance of the Antarctic icefishes (Channichthyidae) across thermal gradients in the Southern Ocean

Biogeographic studies separate the Antarctic Notothenioid fish fauna into high- and low-latitude species. Past studies indicate that some species found in the high-latitude freezing waters of the High-Antarctic Zone have low-serum hysteresis freezing points, while other species restricted to the low-latitude seasonal pack ice zone have higher serum hysteresis freezing points above the freezing point of seawater (−1.9°C), but the relationship has not been systematically investigated. Freeze avoidance was quantified in 11 species of Antarctic icefishes by determining the hysteresis freezing points of their blood serum, in addition, the freezing point depression from serum osmolytes, the antifreeze activity from serum antifreeze glycoproteins (AFGPs), and the antifreeze activity from serum antifreeze potentiating protein were measured for each species. Serum hysteresis freezing point, a proxy for organismal freeze avoidance, decreased as species were distributed at increasing latitude (linear regression r ² 0.66, slope −0.046°C °latitude⁻¹), which appeared largely independent of phylogenetic influences. Greater freeze avoidance at high latitudes was largely a result of higher levels of antifreeze activity from serum AFGPs relative to those in species inhabiting the low-latitude waters. The icefish fauna could be separated into a circum High-Antarctic Group of eight species that maintained serum hysteresis freezing points below −1.9°C even when sampled from less severe habitats. The remaining three species with low-latitude ranges restricted to the waters of the northern part of the west Antarctic Peninsula and Scotia Arc Islands had serum hysteresis freezing points at or above −1.9°C due to significantly lower combined activity from all of their serum antifreeze proteins than found in the High-Antarctic Zone icefish.     

Osmoregulation and water balance in the springhare (Pedetes capensis)

Springhares are large rodents that live in arid and semi-arid regions of Africa. We deprived springhares of water for periods of up to 7 days to determine what physiological adaptations, if any, enable them to survive in arid regions without drinking. During water deprivation, springhares lost up to 30% body weight and produced a mean maximum urine concentration of 2548 mosmol kg⁻¹ with a maximum of 3076 mosmol kg⁻¹ in an individual animal. Haematocrit and plasma sodium and potassium concentrations were well regulated throughout water deprivation at 47.5 ± 3.8% and 132.6 ± 7.4 mmol l⁻¹ and 3.5 ± 0.7 mmol l⁻¹, respectively, while plasma osmolality increased slightly from 293 ± 12.5 mosmol kg⁻¹ to 324 ± 7.3 mosmol kg⁻¹. Springhares thus appeared to be good osmoregulators and were able to maintain plasma volume during 7 days of water deprivation. In addition to the production of a relatively concentrated urine, water loss was limited by the lowered solute load and faecal water loss achieved by a reduction in food consumption and by the production of very dry faeces. These abilities, together with a favourable burrow microclimate and nocturnal activity pattern, enable them to survive in arid regions. Accepted: 8 September 1998     

Stem respiration of ponderosa pines grown in contrasting climates: implications for global climate change

We examined the effects of climate and allocation patterns on stem respiration in ponderosa pine (Pinus ponderosa) growing on identical substrate in the cool, moist Sierra Nevada mountains and the warm, dry, Great Basin Desert. These environments are representative of current climatic conditions and those predicted to accompany a doubling of atmospheric CO₂, respectively, throughout the range of many western north American conifers. A previous study found that trees growing in the desert allocate proportionally more biomass to sapwood and less to leaf area than montane trees. We tested the hypothesis that respiration rates of sapwood are lower in desert trees than in montane trees due to reduced stem maintenance respiration (physiological acclimation) or reduced construction cost of stem tissue (structural acclimation). Maintenance respiration per unit sapwood volume at 15°C did not differ between populations (desert: 6.39 ± 1.14 SE μmol m⁻³ s⁻¹, montane: 6.54 ± 1.13 SE μmol m⁻³ s⁻¹, P = 0.71) and declined with increasing stem diameter (P = 0.001). The temperature coefficient of respiration (Q ₁₀) varied seasonally within both environments (P = 0.05). Construction cost of stem sapwood was the same in both environments (desert: 1.46 ± 0.009 SE g glucose g⁻¹ sapwood, montane: 1.48 ± 0.009 SE glucose g⁻¹ sapwood, P = 0.14). Annual construction respiration calculated from construction cost, percent carbon and relative growth rate was greater in montane populations due to higher growth rates. These data provide no evidence of respiratory acclimation by desert trees. Estimated yearly stem maintenance respiration was greater in large desert trees than in large montane trees because of higher temperatures in the desert and because of increased allocation of biomass to sapwood. By analogy, these data suggest that under predicted increases in temperature and aridity, potential increases in aboveground carbon gain due to enhanced photosynthetic rates may be partially offset by increases in maintenance respiration in large trees growing in CO₂-enriched atmospheres. Received: 4 November 1996 / Accepted: 23 January 1997     

Plasma arginine vasotocin and angiotensin II responses to body temperature elevation in the Pekin duck

The relationship between body temperature (T _b) and the plasma concentrations of arginine vasotocin (AVT) and angiotensin II (AII) was examined in conscious, adult Pekin ducks. Exposure of birds to an ambient temperature of 40 °C for 3 h increased T _b by about 1.5 °C and increased breathing rate five-fold. Plasma osmolality was elevated from the normothermic value of 294.9 ± 1.4 mosmol kg⁻¹ by about 8 mosmol kg⁻¹ Circulating AVT levels increased by about 2 pg ml⁻¹ from a basal concentration of 4.98 ± 0.15 pg ml⁻¹, a rise which could be accounted for by the change in osmotic status. Plasma AII concentrations were unchanged from the pre-heat exposure value of 31.8 ± 3.4 pg ml⁻¹. Time control birds, exposed only to an ambient temperature of 22 °C demonstrated no significant changes in any of the measured variables. The results suggest that an increased T _b has no direct effect on the circulating concentrations of AVT or AII in ducks. Accepted: 2 June 1997     

Thermal tolerance and acclimation response of larvae of the sub-Antarctic beetle Hydromedion sparsutum (Coleoptera: Perimylopidae)

Hydromedion sparsutum is a locally abundant herbivorous beetle on the sub-Antarctic island of South Georgia, often living in close association with the tussock grass Parodiochloa flabellata. Over a 4-day period in mid-summer when the air temperature varied from 0 to 20°C, the temperature in the leaf litter 5–10 cm deep at the base of tussock plants (the microhabitat of H. sparsutum) was consistently within the range of 5–7.5°C. Experiments were carried out to assess the ability of H. sparsutum larvae collected from this thermally stable environment to acclimate when maintained at lower (0°C) and higher (15°C) temperatures. The mean supercooling points (freezing temperature) of larvae collected in January and acclimated at 0°C for 3 and 6 weeks and 15°C for 3 weeks were all within the range of −2.6 to −4.6°C. Larvae in all treatment groups were freeze tolerant. Acclimation at 0°C significantly increased survival in a 15-min exposure at −8°C (from 27 to 96%) and −10°C (from 0 to 63%) compared with the field-fresh and 15°C-treated larvae. Similarly, survival of 0°C-acclimated larvae in a 72-h exposure at −6°C increased from 20 to 83%. Extending the acclimation period at 0°C to 6 weeks did not produce any further increase in cold tolerance. The concentrations of glucose and trehalose in larval body fluids increased significantly with low temperature acclimation. Larvae maintained at 15°C for 3 weeks (none survived for 6 weeks) were less able to survive 1-h exposures between 30 and 35°C than the 0°C-treated samples. Whilst vegetation and snow cover are an effective buffer against low winter temperatures in many polar insects, the inability of H. sparsutum larvae to acclimate or survive at 15°C suggests that protection against high summer temperatures is equally important for this species. Accepted: 2 August 1999     

Thermal tolerance limits in six weevil species (Coleoptera, Curculionidae) from sub-Antarctic Marion Island

Supercooling points, lower lethal temperatures, and the effect of short-term exposures to low temperatures were examined during both winter and summer in the adults of six weevil species from three different habitats on Marion Island. Upper lethal limits and the effects of short-term exposure to high temperatures were also examined in summer-acclimatized adult individuals of these species. Bothrometopus elongatus, B. parvulus, B. randi, Ectemnorhinus marioni, and E. similis were freeze tolerant, but had high lower lethal temperatures (−7 to −10°C). Seasonal variation in these parameters was not pronounced. Physical conditions of the habitat appeared to have little effect on cold hardiness parameters because the Ectemnorhinus species occur in very wet habitats, whereas the Bothrometopus species inhabit drier areas. The adults of these weevil species are similar to other high southern latitude insects in that they are freeze tolerant, but with high lower lethal temperatures. In contrast, Palirhoeus eatoni, a supra-littoral species, avoided freezing and had a mean supercooling point of −15.5 ± 0.94°C (SE) in winter and −11.8 ± 0.98°C in summer. Survival of a constant low temperature of −8°C also increased in this species from 6 h in summer to 27 h in winter. It is suggested that this strategy may be a consequence of the osmoregulatory requirements imposed on this species by its supra-littoral habitat. Upper lethal temperatures (31–34°C) corresponded closely with maximum microclimate temperatures in all of the species. This indicates that the pronounced warming, accompanied by the increased insolation that has been recorded at Marion Island, may reduce survival of these species. These effects may be compounded as a consequence of predation by feral house mice on the weevils. Received: 4 February 1997 / Accepted: 3 May 1997     

Structural and functional properties of sympagic communities in the annual sea ice at Terra Nova Bay (Ross Sea, Antarctica)

Studies on the chemical and biological properties of annual pack ice at a coastal station in Terra Nova Bay (74°41.72′S, 164°11.63′E) were carried out during austral spring at 3-day intervals from 5 November to 1 December 1997. Temporal changes of nutrient concentrations, algal biomasses, taxonomic composition, photosynthetic pigment spectra and P–E relationships were studied. Quantity, composition and degradation rates of organic matter in the intact sea ice were also investigated. In addition, microcosm experiments were carried out to evaluate photosynthetic and photo-acclimation processes of the sympagic flora in relation to different light regimes. High concentrations of ammonia were measured in four ice-cores (weighted mean values of the cores ranged from 4.3 ± 1.9 μM to 7.2 ± 3.4 μM), whereas nitrate and phosphate displayed high concentrations (up to 35.9 μM and 7.6 μM, respectively) only in the bottom layer (135–145 cm depth). Particulate carbohydrate and protein concentrations in the intact sea ice ranged from 0.5 to 2.3 mg l⁻¹ and 0.2 to 2.0 mg l⁻¹, respectively, displaying a notable accumulation of organic matter in the bottom colored layer, where bacterial enzymatic activities also reached the highest values. Aminopeptidase activity was extremely high (up to 19.7 μM l⁻¹ h⁻¹ ± 0.05 in the bottom layer), suggesting a rapid turnover rate of nitrogen–enriched organic compounds (e.g. proteins). By contrast, bacterial secondary production was low, suggesting that only a very small fraction of mobilized organic matter was converted into bacterial biomass (<0.01‰). The sympagic autotrophic biomass (in terms of chlorophaeopigments) of the bottom layer was high, increasing during the sampling period from 680 to 2480 μg l⁻¹. Analyses of pigments performed by HPLC, as well as microscope observations, indicated that diatoms dominated bottom communities. The most important species were Amphiprora sp. and Nitschia cfr. stellata. Bottom sympagic communities showed an average P ^B _max of 0.12 mgC mg Chl⁻¹ and low photoadaptation index (E _k=18 μE m⁻² s⁻¹, E _m=65 μE m⁻² s⁻¹). Results of the microcosm experiment also indicated that communities were photo-oxidized when irradiance exceeded 100 μE m⁻² s⁻¹. This result suggests that micro- autotrophs inhabiting sea ice might have a minor role in the pelagic algal blooms. Accepted: 4 August 1999     

Phytoplankton biomass and primary production in the marginal ice zone of the northwestern Weddell Sea during austral summer

During the austral summer of 1995, distributions of phytoplankton biomass (as chlorophyll a), primary production, and nutrient concentrations along two north-south transects in the marginal ice zone of the northwestern Weddell Sea were examined as part of the 8th Korean Antarctic Research Program. An extensive phytoplankton bloom, ranging from 1.6 to 11.2 mg m⁻³ in surface chlorophyll a concentration, was encountered along the eastern transect and extended ca. 180 km north of the ice edge. The spatial extent of the bloom was closely related to the density field induced by the input of meltwater from the retreating sea ice. However, the extent (ca. 200 km) of the phytoplankton bloom along the western transect exceeded the meltwater-influenced zone (ca. 18 km). The extensive bloom along the western transect was more closely related to local hydrography than to the proximity of the ice edge and the resulting meltwater-induced stability of the upper water column. In addition, the marginal ice zone on the western transect was characterized by a deep, high phytoplankton biomass (up to 8 mg Chl a m⁻³) extending to 100-m depth, and the decreased nutrient concentration, which was probably caused by passive sinking from the upper euphotic zone and in situ growth. Despite the low bloom intensity relative to the marginal ice zone in both of the transects, mean primary productivity (2.6 g C m⁻² day⁻¹) in shelf waters corresponding to the northern side of the western transect was as high as in the marginal ice zone (2.1 g C m⁻² day⁻¹), and was 4.8 times greater than that in open waters, suggesting that shelf waters are as highly productive as the marginal ice zone. A comparison between the historical productivity data and our data also shows that the most productive regions in the Southern Ocean are shelf waters and the marginal ice zone, with emerging evidence of frontal regions as another major productive site. Accepted: 27 September 1998     

Survival and metabolism of <Emphasis Type="Italic">Rana arvali</Emphasis>s during freezing

Freeze tolerance and changes in metabolism during freezing were investigated in the moor frog (Rana arvalis) under laboratory conditions. The data show for the first time a well-developed freeze tolerance in juveniles of a European frog capable of surviving a freezing exposure of about 72 h with a final body temperature of −3°C. A biochemical analysis showed an increase in liver and muscle glucose in response to freezing (respectively, 14-fold and 4-fold between 4 and −1°C). Lactate accumulation was only observed in the liver (4.1 ± 0.8 against 16.6 ± 2.4 μmol g⁻¹ fresh weight (FW) between 4 and −1°C). The quantification of the respiratory metabolism of frozen frogs showed that the aerobic metabolism persists under freezing conditions (1.4 ± 0.7 μl O₂ g⁻¹ FW h⁻¹ at −4°C) and decreases with body temperature. After thawing, the oxygen consumption rose rapidly during the first hour (6-fold to 16-fold) and continued to increase for 24 h, but at a lower rate. In early winter, juvenile R. arvalis held in an outdoor enclosure were observed to emerge from ponds and hibernate in the upper soil and litter layers. Temperature recordings in the substratum of the enclosure suggested that the hibernacula of these juvenile frogs provided sheltering from sub-zero air temperatures and reduced the time spent in a frozen state corresponding well with the observed freeze tolerance of the juveniles. This study strongly suggests that freeze tolerance of R. arvalis is an adaptive trait necessary for winter survival.     

Nitrogen uptake in the infiltration community, an ice algal community in Antarctic pack-ice

An infiltration community was the dominating ice algal community in pack-ice off Queen Maud Land, Southern Ocean, in January 1993. The community was dominated by autotrophic processes, and the most common species were the prymnesiophyte Phaeocystis antarctica and the diatoms Chaetoceros neglectus and Fragilariopsis cylindrus. The concentration of chlorophyll a was 1.3–47.9 μg l⁻¹, and the inner part of the community was nitrate depleted. Uptake rates of nitrate, nitrite, ammonium, urea and amino acids were measured using ¹⁵N. Nitrate was the major nitrogen source for ice algal growth (67 ± 6% nitrate uptake). It is suggested that % nitrate uptake in the infiltration community decreases during the growth season, from 92% during spring (literature data) to 67% during summer. Scalar irradiance in the infiltration community was high and variable. It reached ca. 2000 μmol m⁻² s⁻¹ at some locations, and nitrate uptake rate was potentially photoinhibited at irradiances >500 μmol m⁻² s⁻¹. Nitrate uptake rate in an average infiltration community (0.6 m of snow cover) was lowered by 13% over a 2-week period due to photoinhibition. Received: 16 December 1996 / Accepted: 5 January 1998     

Growth and bacteriocin production by Enterococcus faecium DPC1146 in batch and continuous culture

Production of the bacteriocin enterocin 1146 (E1146) by Enterococcus faecium DPC1146 was studied in batch and continuous fermentation. Growth was strongly inhibited by lactic acid. In batch fermentations maximum E1146 activity (2.8 MBU L⁻¹) was obtained in 9 h with 20 g L⁻¹ glucose. Increase in initial glucose concentration did not lead to a proportional increase in E1146 activity. A simple linear model was found to be adequate to explain the relationship between specific bacteriocin production rate and specific growth rate in batch fermentations with initial glucose concentration higher than 20 g L⁻¹. Maximum bacteriocin activity (2.9–3.2 MBU L⁻¹) was obtained in continuous fermentations at dilution rates between 0.12 and 0.17 h⁻¹ and specific bacteriocin production rate increased linearly with dilution rate. Received 31 July 1996/ Accepted in revised form 01 November 1996