Novel sulfur-oxidizing streamers thriving in perennial cold saline springs of the Canadian high Arctic

The perennial springs at Gypsum Hill (GH) and Colour Peak (CP), situated at nearly 80°N on Axel Heiberg Island in the Canadian high Arctic, are one of the few known examples of cold springs in thick permafrost on Earth. The springs emanate from deep saline aquifers and discharge cold anoxic brines rich in both sulfide and sulfate. Grey-coloured microbial streamers form during the winter months in snow-covered regions of the GH spring run-off channels (−1.3°C to 6.9°C, ∼7.5% NaCl, 0–20 p.p.m. dissolved sulfide, 1 p.p.m. dissolved oxygen) but disappear during the Arctic summer. Culture- and molecular-based analyses of the 16S rRNA gene (FISH, DGGE and clone libraries) indicated that the streamers were uniquely dominated by chemolithoautotrophic sulfur-oxidizing Thiomicrospira species . The streamers oxidized both sulfide and thiosulfate and fixed CO₂ under in situ conditions and a Thiomicrospira strain isolated from the streamers also actively oxidized sulfide and thiosulfate and fixed CO₂ under cold, saline conditions. Overall, the snow-covered spring channels appear to represent a unique polar saline microhabitat that protects and allows Thiomicrospira streamers to form and flourish via chemolithoautrophic, phototrophic-independent metabolism in a high Arctic winter environment characterized by air temperatures commonly below −40°C and with an annual average air temperature of −15°C. These results broaden our knowledge of the physical and chemical boundaries that define life on Earth and have astrobiological implications for the possibility of life existing under similar Martian conditions.     

Differential responses of thermal tolerance to acclimation in the sub-Antarctic rove beetle Halmaeusa atriceps

Abstract. Investigations of the responses to acclimation of upper and lower lethal limits and limits to activity in insects have focused primarily on Drosophila. In the present study, Halmaeusa atriceps (Staphylinidae) is examined for thermal tolerance responses to acclimation, and seasonal acclimatization. In summer and winter, lower lethal temperatures of adults and larvae are approximately −7.6 ± 0.03 and −11.1 ± 0.06 °C, respectively. Supercooling points (SCPs) are more variable, with winter SCPs of −5.4 ± 0.4 °C in larvae and −6.3 ± 0.8 °C in adults. The species appears to be chill susceptible in summer and moderately freeze tolerant in winter, thus showing seasonal acclimatization. Similar changes cannot be induced solely by acclimation to low temperatures in the laboratory. Upper lethal temperatures show a weaker response to acclimation. There are also significant responses to acclimation of critical thermal limits. Critical thermal minima vary between −3.6 ± 0.2 and −0.6 ± 0.2 °C in larvae, and from −4.1 ± 0.1 to −0.8 ± 0.2 °C in adults. By contrast, critical thermal maxima vary much less within adults and larvae. These findings are in keeping with the general pattern found in insects, although this species differs in several respects from others found on Marion Island.     

Survival of rapidly fluctuating natural low winter temperatures by High Arctic soil invertebrates

The extreme polar environment creates challenges for its resident invertebrate communities and the stress tolerance of some of these animals has been examined over many years. However, although it is well appreciated that standard air temperature records often fail to describe accurately conditions experienced at microhabitat level, few studies have explicitly set out to link field conditions experienced by natural multispecies communities with the more detailed laboratory ecophysiological studies of a small number of ‘representative’ species. This is particularly the case during winter, when snow cover may insulate terrestrial habitats from extreme air temperature fluctuations. Further, climate projections suggest large changes in precipitation will occur in the polar regions, with the greatest changes expected during the winter period and, hence, implications for the insulation of overwintering microhabitats. To assess survival of natural High Arctic soil invertebrate communities contained in soil and vegetation cores to natural winter temperature variations, the overwintering temperatures they experienced were manipulated by deploying cores in locations with varying snow accumulation: No Snow, Shallow Snow (30 cm) and Deep Snow (120 cm). Air temperatures during the winter period fluctuated frequently between +3 and −24 °C, and the No Snow soil temperatures reflected this variation closely, with the extreme minimum being slightly lower. Under 30 cm of snow, soil temperatures varied less and did not decrease below −12 °C. Those under deep snow were even more stable and did not decline below −2 °C. Despite these striking differences in winter thermal regimes, there were no clear differences in survival of the invertebrate fauna between treatments, including oribatid, prostigmatid and mesostigmatid mites, Araneae, Collembola, Nematocera larvae or Coleoptera. This indicates widespread tolerance, previously undocumented for the Araneae, Nematocera or Coleoptera, of both direct exposure to at least −24 °C and the rapid and large temperature fluctuations. These results suggest that the studied polar soil invertebrate community may be robust to at least one important predicted consequence of projected climate change.     

Temperature-dependent conduction properties in Arctic fish peripheral nerves

The conduction properties of peripheral nerves from the Arctic fish species Arctic eelpouts (Lycodes sp.), snake blenny (Lumpenus lampretaeformis) and polar cod (Boreogadus saida), permanently adapted to low temperatures, were studied. Nerves of these fishes have two types of fibres, characterised by extracellular compound action potentials with fast (7 m/s) and slow (4 m/s) conduction velocities, as measured at 12 °C. The temperature dependence of the conduction velocity was bimodal, changing its slope at about 16 °C. The Q ₁₀ above 16 °C was 1.12–1.49, while below 16 °C it was 1.82–2.16. Irreversible deterioration of the nerve was observed at temperatures around 19–27 °C. A comparison with data previously obtained from Mediterranean fishes indicates that Arctic fishes have similar temperature sensitivity of nerve conduction and a slight vertical displacement on the conduction velocity-temperature curves, which is insufficient to compensate the decrease of the conduction velocity at their physiological temperature, the conduction velocity of Arctic fishes being about one-half of that of temperate fishes. This suggests that this neurophysiological function is not fully cold-adapted in these Arctic fish species. Accepted: 3 June 2000     

MICROCLIMATE VERSUS PREDATION RISK IN ROOST AND COVERT SELECTION BY BOBWHITES

Abstract: Knowledge of factors that influence habitat selection by wildlife leads to better understanding of habitat ecology and management. Therefore, we compared microclimate and predation risk as factors influencing the selection of stopping points (mid-day coverts, nocturnal roosts) by northern bobwhites (Colinus virginianus). Stopping points were located using radiomarked bobwhites in the Texas Panhandle, USA, during 2002–2003. We obtained blackbody temperatures of microclimates and assessed predation risk (angles of obstruction for aerial predators, vegetation profiles for terrestrial predators) at stopping points and paired random points. Summer coverts showed fewer degree-minutes of hyperthermic exposure (blackbody temperatures >39°C; x̄ = 655.0, SE = 4.1 for coverts, x̄ = 2,255.5, SE = 4.9 for random; 1200–1600 hr) and a lower risk to predators (e.g., 95% confidence intervals [CIs] of angles of obstruction = 87.8–90.8° for coverts, 55.9–70.6° for random). Summer roost temperatures were similar to paired random sites (x̄ = −13.9°C, SE = 0.6 for roost, x̄ = 13.9°C, SE = 0.7 for random) as were winter roost temperatures (x̄ = −1.3°C, SE = 0.7 for roosts, x̄ = −1.4°C, SE = 0.8 for random). There were minor issues of habitat selection of winter or summer roosts based on predation risk (e.g., 95% CIs of vegetation profiles of summer roosts and random sites did not overlap at lower strata). We concluded other selection factors likely exist for winter roosts because microclimate and predation risk assessments between winter roosts and random sites showed no difference. Similarly, other selection factors may exist for summer roosts, as they showed only a weak difference in terrestrial predation risk and no difference in microclimate in comparison to random sites. We concluded microclimate was the primary selection factor for coverts because prevention of hyperthermia necessitated that bobwhites select cooler microclimates within the study area.     

Effects of extended and repeated exposures to low temperature on mortality of the peach-potato aphid Myzus persicae

Abstract.

• 1 The survival of adult and first-instar Myzus persicae reared at 20°C and 10°C was investigated after brief (1 min) exposure in the absence of plant material to temperatures between −5°C and −25°C, and extended exposures on plants of 1–10 days at a constant 5°C, 3°C and −5°C and a 24 h cycling regime between 5°C (18 h) and −5°C (6 h).
• 2 Life stage, rearing temperature, period of exposure and temperature regime all had a significant effect on the ability of aphids to survive cold. The effects of life stage and rearing temperature were most noticeable following exposure to cycling temperatures and extended exposures at −5°C, and least apparent after 1 min exposures at lower sub-zero temperatures.
• 3 Mortality following exposure to temperatures cycling between −5°C and 5°C was greater than that at 3°C (the mean of the cycling temperatures) and less than at a constant −5°C, suggesting that when temperatures fluctuate by a few degrees around 0°C the minimum temperature may affect survival to a greater extent than the mean.
• 4 These results suggest that an overwintering population of acclimated M.persicae would persist without significant mortality after a period of 7–10 days with −5°C frosts each night.

     

Thermal habitat of adult Atlantic salmon Salmo salar in a warming ocean

The year-round thermal habitat at sea for adult Atlantic salmon Salmo salar (n = 49) from northern Norway was investigated using archival tags over a 10 year study period. During their ocean feeding migration, the fish spent 90% of the time in waters with temperatures from 1.6–8.4°C. Daily mean temperatures ranged from −0.5 to 12.9°C, with daily temperature variation up to 9.6°C. Fish experienced the coldest water during winter (November–March) and the greatest thermal range during the first summer at sea (July–August). Trends in sea-surface temperatures influenced the thermal habitat of salmon during late summer and autumn (August–October), with fish experiencing warmer temperatures in warmer years. This pattern was absent during winter (November–March), when daily mean temperatures ranged from 3.4–5.0°C, in both colder and warmer years. The observations of a constant thermal habitat during winter in both warmer and colder years, may suggest that the ocean distribution of salmon is flexible and that individual migration routes could shift as a response to spatiotemporal alterations of favourable prey fields and ocean temperatures.     

A Comparison of the Effects of Temperature on the Growth of Fusarium oxysporum f. sp. narcissi in Solid and Liquid Media

The effect of different temperatures on mycelial growth of Fusarium oxysporum f. sp. narcissi in liquid and on solid medium was compared. Differences between optimum and maximum temperatures using these substrates were considerable and, could lead to different interpretations of the parasitic growth of the pathogen in vascular tissues, for example, in liquid culture Fusarium grew at 45°C whereas it did not on solid media. Initially F. oxysporum f. sp. narcissi may, be encouraged to grow faster in warm air drying systems than those using lower temperatures. The rates of diffusion between 17°C and 35°C are not markedly different. The speed with which moisture is removed from the internal tissues of the remaining roots is governed more by air flow over the bulbs than by temperature.     

Some like it cold: Temperature‐dependent habitat selection by narwhals

The narwhal (Monodon monoceros) is a high‐Arctic species inhabiting areas that are experiencing increases in sea temperatures, which together with reduction in sea ice are expected to modify the niches of several Arctic marine apex predators. The Scoresby Sound fjord complex in East Greenland is the summer residence for an isolated population of narwhals. The movements of 12 whales instrumented with Fastloc‐GPS transmitters were studied during summer in Scoresby Sound and at their offshore winter ground in 2017–2019. An additional four narwhals provided detailed hydrographic profiles on both summer and winter grounds. Data on diving of the whales were obtained from 20 satellite‐linked time‐depth recorders and 16 Acousonde? recorders that also provided information on the temperature and depth of buzzes. In summer, the foraging whales targeted depths between 300 and 850 m where the preferred areas visited by the whales had temperatures ranging between 0.6 and 1.5°C (mean = 1.1°C, SD = 0.22). The highest probability of buzzing activity during summer was at a temperature of 0.7°C and at depths > 300 m. The whales targeted similar depths at their offshore winter ground where the temperature was slightly higher (range: 0.7–1.7°C, mean = 1.3°C, SD = 0.29). Both the probability of buzzing events and the spatial distribution of the whales in both seasons demonstrated a preferential selection of cold water. This was particularly pronounced in winter where cold coastal water was selected and warm Atlantic water farther offshore was avoided. It is unknown if the small temperature niche of whales while feeding is because prey is concentrated at these temperature gradients and is easier to capture at low temperatures, or because there are limitations in the thermoregulation of the whales. In any case, the small niche requirements together with their strong site fidelity emphasize the sensitivity of narwhals to changes in the thermal characteristics of their habitats.     

Strategies of freeze avoidance in larvae of the goldenrod gall moth,Epiblema scudderiana: Winter profiles of a natural population

Larvae of the goldenrod gall moth, Epiblema scudderiana (Clemens) utilize a freeze-avoidance strategy for winter survival. Cold-hardiness adaptations of an outdoor population of the species were profiled over the 1984–1985 winter. Over the autumn months supercooling points of the larvae dropped from −13.9±2.3°C to −37.8±2.8°C (the lowest winter temperature recorded was −26°C), water content of the larvae decreased from 57.2±1.2 to 24.8±1.6% of fresh weight, and glycerol content of the larvae rose to an average of 2030 μmol/g wet weight or 18.7% of fresh weight. All parameters stabilized over the mid-winter months. Glycerol production was largely accounted for by the loss of stored glycogen while lipid and protein reserves remained nearly constant over the winter months. Supercooling-point depression and glycerol systhesis both appeared to be initiated after the first overnight exposures to subzero temperatures. Highest rates of glycerol production, about 60 μmol g⁻¹ d⁻¹, were achieved with mean daily temperatures of about 0°C and subzero nights. Glycerol content was rapidly cleared in the spring but only 20% of the resulting carbon was restored as glycogen.     

Growth and mortality of Arctic charr and European whitefish reared at low temperatures

This comparative study explores how low temperatures affect the mortality and growth of first generation hatchery-reared progeny of subarctic populations of Arctic charr (Salvelinus alpinus L.) and European whitefish (Coregonus lavaretus L.). Replicate fish groups where held under simulated natural light regimes (70°N) at three constant temperatures (1, 3 and 6°C). The mortality of Arctic charr was low (≤1.4%) at all temperature treatments, whereas the mortality of whitefish increased with decreasing temperature from 6% at 6°C to 33% at 1°C. The Arctic charr exhibited higher growth rates than whitefish at all three temperature regimes. All groups of Arctic charr increased in weight, whereas whitefish held at 1°C did not gain weight throughout the experimental period of 133 days. Arctic charr exhibited a large intraspecific variability in growth leading to large variations in size-structure, whereas whitefish in contrast showed very homogenous growth and size-structure patterns; a dissimilarity probably related to species-specific differences in antagonistic behaviour. Evidently, Arctic charr are more cold water adapted than whitefish and are able to maintain growth at extremely low temperatures. Arctic charr thus appear to be the most suitable species for aquaculture at low water temperatures.     

Adaptations to the Arctic: low-temperature development and cold tolerance in the free-living stages of a parasitic nematode from Svalbard

For nematodes with a direct life cycle, transmission is highly dependent on temperature-related development and survival of the free-living stages. Therefore, in the Arctic, where the winter lasts from October to May, nematode transmission is expected to be focused in the short summer season, yet there is strong evidence that as well as focussing egg output during winter months, the nematode parasite, Marshallagia marshalli, infects Svalbard reindeer during the Arctic winter when temperatures are persistently below freezing. To investigate the potential for development and survival of eggs and infective third-stage larvae in winter and therefore the possibility of for winter transmission, we ran a series of low-temperature laboratory experiments. These provide five key insights into the transmission and survival of the free-living stages of M. marshalli: (1) eggs hatched at temperatures as low as 2 °C, but not below 0 °C, (2) eggs were viable and developed after being exposed to sub-zero temperatures for up to 28 months, (3) infective-stage larvae survived for up to 80 days at 5 °C, (4) infective-stage larvae could survive rapid exposure to temperatures below ?30 °C, and (5) desiccation resistance may be important for long-term larval survival at low temperatures. Together, these results indicate that eggs deposited during the winter are highly tolerant of prevailing environmental conditions and have the potential for rapid development with the onset of spring. It is therefore likely that the parasite remains in the egg stage in the faeces during the winter of deposition, hatch and develop into the infective larval stage in the summer, remaining viable on the tundra until the reindeer host returns to the winter feeding grounds the following winter.     

Upper thermal limits of cardiac function for Arctic cod Boreogadus saida,a key food web fish species in the Arctic Ocean

The objective of this study was to determine the upper thermal limits of Arctic cod Boreogadus saida by measuring the response of maximum heart rate (f_Hmax) to acute warming. One set of fish were tested in a field laboratory in Cambridge Bay (CB), Nunavut (north of the Arctic Circle), and a second set were tested after air transport to and 6 month temperature acclimation at the Vancouver Aquarium (VA) laboratory. In both sets of tests, with B. saida acclimated to 0° C, f_Hmax increased during acute warming up to temperatures considerably higher than the acclimation temperature and the near‐freezing Arctic temperatures in which they are routinely found. Indeed, f_Hmax increased steadily between 0·5 and 5·5° C, with no significant difference between the CB and VA tests (P > 0·05) and with an overall mean ± s.e. Q₁₀ of 2·4 ± 0·5. The first Arrhenius breakpoint temperature (T_AB) for f_Hmax was also statistically indistinguishable for the two sets of tests (mean ± s.e. 3·2 ± 0·3 and 3·6 ± 0·3° C), suggesting that the temperature optimum for B. saida could be reliably measured after live transport to a more southerly laboratory location. Continued warming above 5·5° C revealed a large variability among individuals in the upper thermal limits that triggered cardiac arrhythmia (T_arr), ranging from 10·2 to 15·2° C with mean ± s.e. 12·4 ± 0·4° C (n = 11) for the field study. A difference did exist between the CB and VA breakpoint temperatures when the Q₁₀ value decreased below 2 (the Q₁₀ breakpoint temperature; T_QB) at 8·0 and 5·5° C, respectively. These results suggest that factors, other than thermal tolerance and associated cardiac performance, may influence the realized distribution of B. saida within the Arctic Circle.     

Physiological flexibility: the key to success and survival for Antarctic fairy shrimps in highly fluctuating extreme environments

1. The anostracan fairy shrimp Branchinecta gaini inhabits one of the most hostile environments on earth, living in pools and lakes in Antarctica. Between January 2002 and January 2003 temperatures in two pools where B. gaini are extremely abundant on Adelaide Island ranged from ?18.6 to ?15.7 °C in winter, to 19.4 to 17.1 °C in summer, whilst air temperatures ranged from ?34 to 6.3 °C. 2. Branchinecta gaini survives winter as cysts, but endures large summer temperature fluctuations as adults. Cysts froze between ?24.4 and ?25.7 °C. In experiments adults survived 0–10 °C with no mortality for 1 week, 25 °C for nearly 48 h with 50% mortality, and at 32 °C complete mortality occurred in <1 h. 3. Oxygen consumption (M?O₂) in B. gaini approximately doubled for every 10 °C temperature rise (Q₁₀ = 2.04) up to 20 °C where it reached a peak. Females had, on average 19% higher M?O₂ than males. Females also had greater metabolic scopes, (maximum–minimum M?O₂ across temperatures was ×3.6 for females, ×3.1 for males). 4. Ventilation frequency increased linearly with temperature, and did not decline at 25 °C, indicating animals were ‘trying’ progressively harder to supply oxygen to tissues, and oxygen deficiency was the probable cause of death. Females had a higher ventilation frequency than males (8.6–17.1% higher) and they also exhibited greater scope to raise ventilation frequency (×2.4 for females versus ×1.5 for males). 5. Great metabolic flexibility allows B. gaini to exploit extreme, highly fluctuating environments, and larger ventilatory and respiratory scopes allow females to survive higher temperatures than males. Because of this flexibility their prospects for coping with physical environmental change are high.     

DISTRIBUTION,PHYLOGENY, AND GROWTH OF COLD‐ADAPTED PICOPRASINOPHYTES IN ARCTIC SEAS1

Our pigment analyses from a year‐long study in the coastal Beaufort Sea in the western Canadian Arctic showed the continuous prevalence of eukaryotic picoplankton in the green algal class Prasinophyceae. Microscopic analyses revealed that the most abundant photosynthetic cell types were Micromonas‐like picoprasinophytes that persisted throughout winter darkness and then maintained steady exponential growth from late winter to early summer. A Micromonas (CCMP2099) isolated from an Arctic polynya (North Water Polynya between Ellesmere Island and Greenland), an ice‐free section, grew optimally at 6°C–8°C, with light saturation at or below 10 μmol photons·m^?2·s^?1 at 0°C. The 18S rDNA analyses of this isolate and environmental DNA clone libraries from diverse sites across the Arctic Basin indicate that this single psychrophilic Micromonas ecotype has a pan‐Arctic distribution. The 18S rDNA from two other picoprasinophyte genera was also found in our pan‐Arctic clone libraries: Bathycoccus and Mantoniella. The Arctic Micromonas differed from genotypes elsewhere in the World Ocean, implying that the Arctic Basin is a marine microbial province containing endemic species, consistent with the biogeography of its macroorganisms. The prevalence of obligate low‐temperature, shade‐adapted species in the phytoplankton indicates that the lower food web of the Arctic Ocean is vulnerable to ongoing climate change in the region.     

Survival of Heleomyza borealis (Diptera, Heleomyzidae) larvae down to − 60°C

Heleomyza borealis Boh. (Diptera, Heleomyzidae) overwinters as larvae in Arctic habitats, where they may experience winter temperatures below ? 15°C. The larvae freeze at c.? 7°C but in acclimation experiments 80% survived when exposed to ? 60°C. Of the larvae exposed to between ? 4 and ? 15°C, only 3% pupated. However, when cooled to ? 20°C this increased to 44%, with 4% emerging as adults. Larvae maintained at 5°C contained low levels of glycerol, sorbitol and trehalose, which did not increase with acclimation to low temperatures. However, levels of fructose increased from 6.1 μg mg^?1 fw in control animals to 17 μg mg^?1 fw when exposed to ? 2°C for 1 week. Larval body water (2.2 ± 0.1 g/g dw, mean ± SD, n = 100) and lipid content (0.22 ± 0.002 g/g fw, mean ± SE) showed no significant change during acclimation to low temperatures. Larvae maintained at a constant 5°C survived for over 18 months with little loss of body mass (from 7.5 ± 1.2 to 7.0 ± 1.2 mg fw, mean ± SD, n = 20), but none pupated. Heleomyza borealis larvae appear to feed and grow until they reach a body mass of about 7.5 mg and then become dormant. They remain in this state until they experience a low temperature stimulus (< ? 15°C) followed by a warm period (≈ 5°C). This ensures that the larvae pupate and adults emerge in early summer, allowing the maximum growing period before the following winter. Heleomyza borealis are adapted to survive long winters in a dormant larval state. They have a low metabolic rate, can conserve body water even at subzero temperatures but do not synthesize large quantities of cryoprotectants.     

Aeromycoflora in the Central Milk Dairy of Calcutta, India

Intramural aeromycological survey was performed at the Central Milk Dairy, Calcutta, covering eight locations within the Dairyusing Burkard personal volumetric air sampler. The locations were butter cold storage (−2 °C), cold store (8 °C), packaging section (23 °C), milk processing section (24 °C), reconstituent of skimmed milk (24 °C), quality control lab (25 °C), raw milk reception (28 °C) and loading dock (26 °C). A number of fungal spores, conidia and mycelia were recorded in different rooms: the highest spore quantity was recorded in the packaging section (23 °C) and the minimum at the butter cold store (−2 °C). The dominant spores consisted of Aspergillus niger, A flavus,Cladosporium sp., Fusarium sp., Curvularia sp.,Alternaria sp., Torula sp., Myrotheciumsp., Helminthosporium sp., Periconia sp.,Nigrospora sp. and Pithomyces sp. This revised version was published online in June 2006 with corrections to the Cover Date.     

The neotropical shrub Lupinus elegans,from temperate forests,may not adapt to climate change

Considering that their distribution is limited to altitudinal gradients along mountains that are likely to become warmer and drier, climate change poses an increased threat to temperate forest species from tropical regions. We studied whether the understorey shrub Lupinus elegans, endemic to temperate forests of west‐central Mexico, will be able to withstand the projected temperature increase under seven climate change scenarios. Seeds were collected along an altitudinal gradient and grown in a shade‐house over 7 months before determining their temperature tolerance as electrolyte leakage. The plants from colder sites tolerated lower temperatures, i.e. the temperature at which half of the maximum electrolyte leakage occurred (LT₅₀), ranged from −6.4 ± 0.7 to −2.4 ± 0.3 °C. In contrast, no pattern was found for tolerance to high temperature (LT₅₀ average 42.8 ± 0.3 °C). The climate change scenarios considered here consistently estimated an increase in air temperature during the present century that was higher for the maximum air temperature than for the mean or minimum. In particular, the anomaly from the normal maximum air temperature at the study region ranged from 2.8 °C by 2030 to 5.8 °C by 2090. In this respect, the inability of L. elegans to adapt to increasingly higher temperatures found here, in addition to a possible inhibition of reproduction caused by warmer winters, may limit its future distribution.     

High Temperatures and Net CO2 Uptake, Growth, and Stem Damage for the Hemiepiphytic Cactus Hylocereus undatus1

Hylocereus undatus, which is native to tropical forests experiencing moderate temperatures, would not be expected to tolerate the extremely high temperatures that can be tolerated by cacti native to deserts. Nevertheless, total daily net CO₂ uptake by this hemiepiphytic cactus, which is widely cultivated for its fruits, was optimal at day/night air temperatures of 30/20°C, temperatures that are higher than those optimal for daily net CO₂ uptake by cacti native to arid and semiarid areas. Exposure to 35/25°C for 30 weeks led to lower net CO₂ uptake than at 10 weeks; exposure to 40/30°C led to considerable necrosis visible on the stems at 6 weeks and nearly complete browning of the stems by 19 weeks. Dry mass gain over 31 weeks was greatest for plants at 30/20°C, with root growth being especially noteworthy and root dry mass gain representing an increasing percentage of plant dry mass gain as day/night air temperatures were increased. Viability of chlorenchyma cells, assayed by the uptake of the vital stain neutral red into the central vacuoles, was decreased 50 percent by a one‐hour treatment at 55°C compared with an average of 64°C for 18 species of cacti native to deserts. The lower high‐temperature tolerance for H. undatus reflected its low high‐temperature acclimation of only 1.4°C as growth temperatures were raised by 10°C compared with an average acclimation of 5.3°C for the other 18 species of cacti. Thus, this tropical hemiepiphytic cactus is not adapted to day/night air temperatures above ca 40/30°C, although its net CO₂ uptake is optimal at the relatively high day/night air temperatures of 30/20°C.     

A marine Chlamydomonas sp. emerging as an algal model

The freshwater microalga Chlamydomonas reinhardtii, which lives in wet soil, has served for decades as a model for numerous biological processes, and many tools have been introduced for this organism. Here, we have established a stable nuclear transformation for its marine counterpart, Chlamydomonas sp. SAG25.89, by fusing specific cis‐acting elements from its Actin gene with the gene providing hygromycin resistance and using an elaborated electroporation protocol. Like C. reinhardtii, Chlamydomonas sp. has a high GC content, allowing reporter genes and selection markers to be applicable in both organisms. Chlamydomonas sp. grows purely photoautotrophically and requires ammonia as a nitrogen source because its nuclear genome lacks some of the genes required for nitrogen metabolism. Interestingly, it can grow well under both low and very high salinities (up to 50 g · L^‐1) rendering it as a model for osmotolerance. We further show that Chlamydomonas sp. grows well from 15 to 28°C, but halts its growth at 32°C. The genome of Chlamydomonas sp. contains some gene homologs the expression of which is regulated according to the ambient temperatures and/or confer thermal acclimation in C. reinhardtii. Thus, knowledge of temperature acclimation can now be compared to the marine species. Furthermore, Chlamydomonas sp. can serve as a model for studying marine microbial interactions and for comparing mechanisms in freshwater and marine environments. Chlamydomonas sp. was previously shown to be immobilized rapidly by a cyclic lipopeptide secreted from the antagonistic bacterium Pseudomonas protegens PF‐5, which deflagellates C. reinhardtii.