Fish introductions reveal the temperature dependence of species interactions

A major area of current research is to understand how climate change will impact species interactions and ultimately biodiversity. A variety of environmental conditions are rapidly changing owing to climate warming, and these conditions often affect both the strength and outcome of species interactions. We used fish distributions and replicated fish introductions to investigate environmental conditions influencing the coexistence of two fishes in Swedish lakes: brown trout (Salmo trutta) and pike (Esox lucius). A logistic regression model of brown trout and pike coexistence showed that these species coexist in large lakes (more than 4.5 km²), but not in small, warm lakes (annual air temperature more than 0.9–1.5°C). We then explored how climate change will alter coexistence by substituting climate scenarios for 2091–2100 into our model. The model predicts that brown trout will be extirpated from approximately half of the lakes where they presently coexist with pike and from nearly all 9100 lakes where pike are predicted to invade. Context dependency was critical for understanding pike–brown trout interactions, and, given the widespread occurrence of context-dependent species interactions, this aspect will probably be critical for accurately predicting climate impacts on biodiversity.     

Effects of temperature on the coupled activities of the vanadate-sensitive proton pump from maize root microsomes

The mechanism by which proton transport is coupled to ATP hydrolysis by vanadate-sensitive pumps is poorly understood. The effects of temperature on the activities of the vanadate-sensitive ATPase from maize (Zea mays) roots were assessed to provide insight into the coupling mechanism. The initial rate of proton transport had a bell-shaped dependence on temperature with an optimal range between 20 and 30°C. However, the rate of vanadate-sensitive ATP hydrolysis increased as the temperature was raised from 4 to 43°C. The differential sensitivity of proton transport to temperatures above 30°C was also observed when the ATPase was reconstituted into dioleoylphosphatidylcholine vesicles. Inhibition of proton transport with temperatures above 30°C was associated with higher rates of proton leakage from the membranes. In addition, proton transport was more inhibited than ATP hydrolysis at temperatures below 10°C. Reduced rates of proton transport at lower temperatures were not associated with higher rate of proton conductivity across the membranes. Therefore, the preferential inhibition of proton transport at temperatures below 10°C may reflect an effect of temperature on the coupling between proton transport and ATP hydrolysis within the vanadate-sensitive ATPase.     

Topographic and vegetation drivers of thermal heterogeneity along the boreal–grassland transition zone in western Canada: Implications for climate change refugia

Climate change refugia are areas that are relatively buffered from contemporary climate change and may be important safe havens for wildlife and plants under anthropogenic climate change. Topographic variation is an important driver of thermal heterogeneity, but it is limited in relatively flat landscapes, such as the boreal plain and prairie regions of western Canada. Topographic variation within this region is mostly restricted to river valleys and hill systems, and their effects on local climates are not well documented. We sought to quantify thermal heterogeneity as a function of topography and vegetation cover within major valleys and hill systems across the boreal–grassland transition zone.Using iButton data loggers, we monitored local temperature at four hills and 12 river valley systems that comprised a wide range of habitats and ecosystems in Alberta, Canada (N = 240), between 2014 and 2020. We then modeled monthly temperature by season as a function of topography and different vegetation cover types using general linear mixed effect models.Summer maximum temperatures (T _max) varied nearly 6°C across the elevation gradient sampled. Local summer mean (T _mean) and maximum (T _max) temperatures on steep, north‐facing slopes (i.e., low levels of potential solar radiation) were up to 0.70°C and 2.90°C cooler than highly exposed areas, respectively. T _max in incised valleys was between 0.26 and 0.28°C cooler than other landforms, whereas areas with greater terrain roughness experienced maximum temperatures that were up to 1.62°C cooler. We also found that forest cover buffered temperatures locally, with coniferous and mixedwood forests decreasing summer T _mean from 0.23 to 0.72°C and increasing winter T _min by up to 2°C, relative to non‐forested areas.Spatial predictions of temperatures from iButton data loggers were similar to a gridded climate product (ClimateNA), but the difference between them increased with potential solar radiation, vegetation cover, and terrain roughness.Species that can track their climate niche may be able to compensate for regional climate warming through local migrations to cooler microsites. Topographic and vegetation characteristics that are related to cooler local climates should be considered in the evaluation of future climate change impacts and to identify potential refugia from climate change.     

Development of a Threshold Model to Predict Germination of Populus tomentosa Seeds after Harvest and Storage under Ambient Condition

Effects of temperature, storage time and their combination on germination of aspen (Populus tomentosa) seeds were investigated. Aspen seeds were germinated at 5 to 30°C at 5°C intervals after storage for a period of time under 28°C and 75% relative humidity. The effect of temperature on aspen seed germination could not be effectively described by the thermal time (TT) model, which underestimated the germination rate at 5°C and poorly predicted the time courses of germination at 10, 20, 25 and 30°C. A modified TT model (MTT) which assumed a two-phased linear relationship between germination rate and temperature was more accurate in predicting the germination rate and percentage and had a higher likelihood of being correct than the TT model. The maximum lifetime threshold (MLT) model accurately described the effect of storage time on seed germination across all the germination temperatures. An aging thermal time (ATT) model combining both the TT and MLT models was developed to describe the effect of both temperature and storage time on seed germination. When the ATT model was applied to germination data across all the temperatures and storage times, it produced a relatively poor fit. Adjusting the ATT model to separately fit germination data at low and high temperatures in the suboptimal range increased the models accuracy for predicting seed germination. Both the MLT and ATT models indicate that germination of aspen seeds have distinct physiological responses to temperature within a suboptimal range.     

Thermal performance of the Chagas disease vector,Triatoma infestans,under thermal variability

Vector-borne diseases (VBD) are particularly susceptible to climate change because most of the diseases’ vectors are ectotherms, which themselves are susceptible to thermal changes. The Chagas disease is one neglected tropical disease caused by the protozoan parasite, Trypanosoma cruzi. One of the main vectors of the Chagas disease in South America is Triatoma infestans, a species traditionally considered to be restricted to domestic or peridomestic habitats, but sylvatic foci have also been described along its distribution. The infestation of wild individuals, together with the projections of environmental changes due to global warming, urge the need to understand the relationship between temperature and the vector’s performance. Here, we evaluated the impact of temperature variability on the thermal response of T. infestans. We acclimated individuals to six thermal treatments for five weeks to then estimate their thermal performance curves (TPCs) by measuring the walking speed of the individuals. We found that the TPCs varied with thermal acclimation and body mass. Individuals acclimated to a low and variable ambient temperature (18°C ± 5°C) exhibited lower performances than those individuals acclimated to an optimal temperature (27°C ± 0°C); while those individuals acclimated to a low but constant temperature (18°C ± 0°C) did not differ in their maximal performance from those at an optimal temperature. Additionally, thermal variability (i.e., ± 5°C) at a high temperature (30°C) increased performance. These results evidenced the plastic response of T. infestans to thermal acclimation. This plastic response and the non-linear effect of thermal variability on the performance of T. infestans posit challenges when predicting changes in the vector’s distribution range under climate change.     

Population Dynamics of Aphids on Cereals: Digging in the Time-Series Data to Reveal Population Regulation Caused by Temperature

Aphid populations show periodic fluctuations and many causes are attributed to their dynamic. We investigated the regulation by temperature of the aphid populations composed of Metopolophium dirhodum, Sitobion avenae, and Rhopalosiphum padi on winter wheat using a 24 years long time series data. We computed the sum of daily temperatures above 5°C, the threshold temperature for aphid development, and the sum of daily temperatures within the [0(threshold for wheat development),5] °C interval. Applying Generalised Additive Model framework we tested influences of temperature history expressed via degree days before the start of the aphid immigration on the length of their occurrence. We aimed to estimate the magnitude and direction of this influence, and how far to the past before the start of the aphid season the temperature effect goes and then identify processes responsible for the effect. We fitted four models that differed in the way of correcting for abundance in the previous year and in specification of temperature effects. Abundance in the previous year did not affect the length of period of aphid population growth on wheat. The temperature effect on the period length increased up to 123 days before the start of the current season, i.e. when wheat completed vernalization. Increased sum of daily temperatures above 5°C and the sum of daily temperatures within the [0,5] °C interval both shortened the length of period of aphid population growth. Stronger effect of the latter suggests that wheat can escape from aphid attacks if during winter temperatures range from 0 to 5°C. The temperature influence was not homogeneous in time. The strongest effect of past temperature was about 50 to 80 and 90 to 110 days before the beginning of the current aphid season indicating important role of termination of aphid egg dormancy and egg hatching.     

Incubation Temperature Effects on Hatchling Performance in the Loggerhead Sea Turtle (Caretta caretta)

Incubation temperature has significant developmental effects on oviparous animals, including affecting sexual differentiation for several species. Incubation temperature also affects traits that can influence survival, a theory that is verified in this study for the Northwest Atlantic loggerhead sea turtle (Caretta caretta). We conducted controlled laboratory incubations and experiments to test for an effect of incubation temperature on performance of loggerhead hatchlings. Sixty-eight hatchlings were tested in 2011, and 31 in 2012, produced from eggs incubated at 11 different constant temperatures ranging from 27°C to 33°C. Following their emergence from the eggs, we tested righting response, crawling speed, and conducted a 24-hour long swim test. The results support previous studies on sea turtle hatchlings, with an effect of incubation temperature seen on survivorship, righting response time, crawling speed, change in crawl speed, and overall swim activity, and with hatchlings incubated at 27°C showing decreased locomotor abilities. No hatchlings survived to be tested in both years when incubated at 32°C and above. Differences in survivorship of hatchlings incubated at high temperatures are important in light of projected higher sand temperatures due to climate change, and could indicate increased mortality from incubation temperature effects.     

Shifting thermal regimes influence competitive feeding and aggression dynamics of brook trout (Salvelinus fontinalis) and creek chub (Semotilus atromaculatus)

The natural distributions of freshwater fish species are limited by their thermal tolerances via physiological constraints and increased interspecific competition as temperatures shift toward the thermal optima of other syntopic species. Species may mediate stress from temperature change physiologically, behaviorally, or both; but these changes may compromise competitive advantages through effects on feeding and social behavior. In the Appalachian Mountains of North America, creek chub (Semotilus atromaculatus) are found in warm‐water and cold‐water streams and overlap in range with brook trout (Salvelinus fontinalis) across lower thermal maxima, where they compete for food and space. As stream temperatures continue to increase due to climate change, brook trout are under increasing thermal stress which may negatively affect their ability to compete with creek chub. To examine the influences of temperature on competitive interactions between these species, we observed feeding behavior, aggression, and habitat use differences at three temperatures approaching brook trout thermal maxima (18°C, 20°C, and 22°C) among dyad pairs for all combinations of species in experimental flow‐through tanks. We also examined feeding and habitat use of both species under solitary conditions. We found as temperature increased, feeding and aggression of brook trout were significantly reduced in the presence of creek chub. Creek chub pairs were more likely to occupy benthic areas and refugia while brook trout pairs used surface water more. Space use patterns significantly changed by pairing treatment. Aggression and space use shifts allowed increased exploitative and interference competition from creek chub when paired with brook trout that was not present in conspecific pairs. The decreased dominance of a top predator may lead to diverse impacts on stream community dynamics with implications for the future range restriction of brook trout and demonstrate possible mechanisms to facilitate competitive advantages of warm water generalist species under thermal stress.     

Potential effects of warmer worms and vectors on onchocerciasis transmission in West Africa

Development times of eggs, larvae and pupae of vectors of onchocerciasis (Simulium spp.) and of Onchocerca volvulus larvae within the adult females of the vectors decrease with increasing temperature. At and above 25°C, the parasite could reach its infective stage in less than 7 days when vectors could transmit after only two gonotrophic cycles. After incorporating exponential functions for vector development into a novel blackfly population model, it was predicted that fly numbers in Liberia and Ghana would peak at air temperatures of 29°C and 34°C, about 3°C and 7°C above current monthly averages, respectively; parous rates of forest flies (Liberia) would peak at 29°C and of savannah flies (Ghana) at 30°C. Small temperature increases (less than 2°C) might lead to changes in geographical distributions of different vector taxa. When the new model was linked to an existing framework for the population dynamics of onchocerciasis in humans and vectors, transmission rates and worm loads were projected to increase with temperature to at least 33°C. By contrast, analyses of field data on forest flies in Liberia and savannah flies in Ghana, in relation to regional climate change predictions, suggested, on the basis of simple regressions, that 13–41% decreases in fly numbers would be expected between the present and before 2040. Further research is needed to reconcile these conflicting conclusions.     

Temperature Characteristics of Excitation in Space-Clamped Squid Axons

Temperature characteristics of excitability in the squid giant axon were measured for the space-clamped axon with the double sucrose gap technique. Threshold strength-duration curves were obtained for square wave current pulses from 10 µsec to 10 msec and at temperatures from 5°C to 35°C. The threshold change of potential, at which an action potential separated from a subthreshold response, averaged 17 mv at 20°C with a Q¹⁰ of 1.15. The average threshold current density at rheobase was 12 µa/cm² at 20°C with a Q¹⁰ of 2.35 compared to 2.3 obtained previously. At short times the threshold charge was 1.5·10^-8 coul/cm². This was relatively independent of temperature and occasionally showed a minimum in the temperature range. At intermediate times and all temperatures the threshold currents were less than for both the single time constant model and the two factor excitation process as developed by Hill. FitzHugh has made computer investigations of the effect of temperature on the excitation of the squid axon membrane as represented by the Hodgkin-Huxley equations. These are in general in good agreement with our experimental results.     

Ultraviolet Inactivation and Photoproducts of Transforming DNA Irradiated at Low Temperatures

Solutions of Haemophilus influenzae transforming DNA were irradiated at temperatures ranging from 25°C to - 196°C. Temperature dependence of the formation of thymine-containing dimers was closely correlated with inactivation of transforming activity; in general, both dimerization and inactivation decreased with decreasing temperature. The fraction of nonphotoreactivable damage increased with increasing dose at low temperatures. The nonphotoreactivable spore-type photoproduct was formed at low temperatures with a maximum at - 100°C, a temperature at which the nonphotoreactivable biological inactivation was also a maximum. Intrastrand cross-linking, like dimer formation, decreased with decreasing irradiation temperature.     

Influence of Temperature on Intra- and Interspecific Resource Utilization within a Community of Lepidopteran Maize Stemborers

Competition or facilitation characterises intra- and interspecific interactions within communities of species that utilize the same resources. Temperature is an important factor influencing those interactions and eventual outcomes. The noctuid stemborers, Busseola fusca and Sesamia calamistis and the crambid Chilo partellus attack maize in sub-Saharan Africa. They often occur as a community of interacting species in the same field and plant at all elevations. The influence of temperature on the intra- and interspecific interactions among larvae of these species, was studied using potted maize plants exposed to varying temperatures in a greenhouse and artificial stems kept at different constant temperatures (15°C, 20°C, 25°C and 30°C) in an incubator. The experiments involved single- and multi-species infestation treatments. Survival and relative growth rates of each species were assessed. Both intra- and interspecific competitions were observed among all three species. Interspecific competition was stronger between the noctuids and the crambid than between the two noctuids. Temperature affected both survival and relative growth rates of the three species. Particularly at high temperatures, C. partellus was superior in interspecific interactions shown by higher larval survival and relative growth rates. In contrast, low temperatures favoured survival of B. fusca and S. calamistis but affected the relative growth rates of all three species. Survival and relative growth rates of B. fusca and S. calamistis in interspecific interactions did not differ significantly across temperatures. Temperature increase caused by future climate change is likely to confer an advantage on C. partellus over the noctuids in the utilization of resources (crops).     

Thermal thresholds as predictors of seed dormancy release and germination timing: altitude-related risks from climate warming for the wild grapevine Vitis vinifera subsp. sylvestris

Background and Aims

The importance of thermal thresholds for predicting seed dormancy release and germination timing under the present climate conditions and simulated climate change scenarios was investigated. In particular, Vitis vinifera subsp. sylvestris was investigated in four Sardinian populations over the full altitudinal range of the species (from approx. 100 to 800 m a.s.l).

Methods

Dried and fresh seeds from each population were incubated in the light at a range of temperatures (10–25 and 25/10 °C), without any pre-treatment and after a warm (3 months at 25 °C) or a cold (3 months at 5 °C) stratification. A thermal time approach was then applied to the germination results for dried seeds and the seed responses were modelled according to the present climate conditions and two simulated scenarios of the Intergovernmental Panel on Climate Change (IPCC): B1 (+1·8 °C) and A2 (+3·4 °C).

Key Results

Cold stratification released physiological dormancy, while very few seeds germinated without treatments or after warm stratification. Fresh, cold-stratified seeds germinated significantly better (>80 %) at temperatures ≥20 °C than at lower temperatures. A base temperature for germination (T_b) of 9·0–11·3 °C and a thermal time requirement for 50 % of germination (θ₅₀) ranging from 33·6 °Cd to 68·6 °Cd were identified for non-dormant cold-stratified seeds, depending on the populations. This complex combination of thermal requirements for dormancy release and germination allowed prediction of field emergence from March to May under the present climatic conditions for the investigated populations.

Conclusions

The thermal thresholds for seed germination identified in this study (T_b and θ₅₀) explained the differences in seed germination detected among populations. Under the two simulated IPCC scenarios, an altitude-related risk from climate warming is identified, with lowland populations being more threatened due to a compromised seed dormancy release and a narrowed seed germination window.     

Climate Warming May Facilitate Invasion of the Exotic Shrub Lantana camara

Plant species show different responses to the elevated temperatures that are resulting from global climate change, depending on their ecological and physiological characteristics. The highly invasive shrub Lantana camara occurs between the latitudes of 35°N and 35°S. According to current and future climate scenarios predicted by the CLIMEX model, climatically suitable areas for L. camara are projected to contract globally, despite expansions in some areas. The objective of this study was to test those predictions, using a pot experiment in which branch cuttings were grown at three different temperatures (22°C, 26°C and 30°C). We hypothesized that warming would facilitate the invasiveness of L. camara. In response to rising temperatures, the total biomass of L. camara did increase. Plants allocated more biomass to stems and enlarged their leaves more at 26°C and 30°C, which promoted light capture and assimilation. They did not appear to be stressed by higher temperatures, in fact photosynthesis and assimilation were enhanced. Using lettuce (Lactuca sativa) as a receptor plant in a bioassay experiment, we also tested the phytotoxicity of L. camara leachate at different temperatures. All aqueous extracts from fresh leaves significantly inhibited the germination and seedling growth of lettuce, and the allelopathic effects became stronger with increasing temperature. Our results provide key evidence that elevated temperature led to significant increases in growth along with physiological and allelopathic effects, which together indicate that global warming facilitates the invasion of L. camara.     

High Temperature Increases the Masculinization Rate of the All-Female (XX) Rainbow Trout “Mal” Population

Salmonids are generally considered to have a robust genetic sex determination system with a simple male heterogamety (XX/XY). However, spontaneous masculinization of XX females has been found in a rainbow trout population of gynogenetic doubled haploid individuals. The analysis of this masculinization phenotype transmission supported the hypothesis of the involvement of a recessive mutation (termed mal). As temperature effect on sex differentiation has been reported in some salmonid species, in this study we investigated in detail the potential implication of temperature on masculinization in this XX mal-carrying population. Seven families issued from XX mal-carrying parents were exposed from the time of hatching to different rearing water temperatures ((8, 12 and 18°C), and the resulting sex-ratios were confirmed by histological analysis of both gonads. Our results demonstrate that masculinization rates are strongly increased (up to nearly two fold) at the highest temperature treatment (18°C). Interestingly, we also found clear differences between temperatures on the masculinization of the left versus the right gonads with the right gonad consistently more often masculinized than the left one at lower temperatures (8 and 12°C). However, the masculinization rate is also strongly dependent on the genetic background of the XX mal-carrying families. Thus, masculinization in XX mal-carrying rainbow trout is potentially triggered by an interaction between the temperature treatment and a complex genetic background potentially involving some part of the genetic sex differentiation regulatory cascade along with some minor sex-influencing loci. These results indicate that despite its rather strict genetic sex determinism system, rainbow trout sex differentiation can be modulated by temperature, as described in many other fish species.     

Relationships among Isoprene Emission Rate, Photosynthesis, and Isoprene Synthase Activity as Influenced by Temperature

Isoprene emissions from the leaves of velvet bean (Mucuna pruriens L. var utilis) plants exhibited temperature response patterns that were dependent on the plant's growth temperature. Plants grown in a warm regimen (34/28°C, day/night) exhibited a temperature optimum for emissions of 45°C, whereas those grown in a cooler regimen (26/20°C, day/night) exhibited an optimum of 40°C. Several previous studies have provided evidence of a linkage between isoprene emissions and photosynthesis, and more recent studies have demonstrated that isoprene emissions are linked to the activity of isoprene synthase in plant leaves. To further explore this linkage within the context of the temperature dependence of isoprene emissions, we determined the relative temperature dependencies of photosynthetic electron transport, CO₂ assimilation, and isoprene synthase activity. When measured over a broad range of temperatures, the temperature dependence of isoprene emission rate was not closely correlated with either the electron transport rate or the CO₂ assimilation rate. The temperature optima for electron transport rate and CO₂ assimilation rate were 5 to 10°C lower than that for the isoprene emission rate. The dependence of isoprene emissions on photon flux density was also affected by measurement temperature in a pattern independent of those exhibited for electron transport rate and CO₂ assimilation rate. Thus, despite no change in the electron transport rate or CO₂ assimilation rate at 26 and 34°C, the isoprene emission rate changed markedly. The quantum yield of isoprene emissions was stimulated by a temperature increase from 26 to 34°C, whereas the quantum yield for CO₂ assimilation was inhibited. In greenhouse-grown aspen leaves (Populus tremuloides Michaux.), the high temperature threshold for inhibition of isoprene emissions was closely correlated with the high temperature-induced decrease in the in vitro activity of isoprene synthase. When taken together, the results indicate that although there may be a linkage between isoprene emission rate and photosynthesis, the temperature dependence of isoprene emission is not determined solely by the rates of CO₂ assimilation or electron transport. Rather, we propose that regulation is accomplished primarily through the enzyme isoprene synthase.     

Variation among Species in the Temperature Dependence of the Reappearance of Variable Fluorescence following Illumination

The relationship between the thermal dependence of the reappearance of chlorophyll variable fluorescence following illumination and temperature dependence of the apparent Michaelis constant (K_m) of NADH hydroxypyruvate reductase for NADH was investigated in cool and warm season plant species. Brancker SF-20 and SF-30 fluorometers were used to evaluate induced fluorescence transients from detached leaves of wheat (Triticum aestivum L. cv TAM-101), cotton (Gossypium hirsutum L. cv Paymaster 145), tomato (Lycopersicon esculentum cv Del Oro), bell pepper (Capsicum annuum L. cv California Wonder), and petunia (Petunia hybrida cv. Red Sail). Following an illumination period at 25°C, the reappearance of variable fluorescence during a dark incubation was determined at 5°C intervals from 15°C to 45°C. Variable fluorescence recovery was normally distributed with the maximum recovery observed at 20°C in wheat, 30°C in cotton, 20°C to 25°C in tomato, 30 to 35°C in bell pepper and 25°C in petunia. Comparison of the thermal response of fluorescence recovery with the temperature sensitivity of the apparent K_m of hydroxypyruvate reductase for NADH showed that the range of temperatures providing fluorescence recovery corresponded with those temperatures providing the minimum apparent K_m values (viz. the thermal kinetic window).     

Influence of Temperature on Multiplication and Egg Hatching of Longidorus africanus

Longidorus africanus multiplication on tomato was highest at 29 °C. Few nematodes were recovered after 6 weeks at soil temperatures of 35 °C or below 23 °C. The time to egg hatching was shortest and the percentage of eggs hatching was highest at 29 °C. The minimum temperature and the heat sum above this temperature required for egg development were calculated to be 14.3 °C and 94.08 degree-days, respectively. The thermal times required for egg development by L. africanus and L. elongatus were nearly identical. For both species the product of the base temperature and the heat sum was near constant, and at a temperature of 22.3 °C the rates of egg development were equal.     

Respiration and Alternative Oxidase in Corn Seedling Tissues during Germination at Different Temperatures

Respiration rates of Zea mays L. seedling tissues grown at 30 and 14°C were measured at 25°C at different stages of seedling growth. Accumulation of heat units was used to define the developmental stages to compare respiration between the two temperatures. At both temperatures, respiration rates of most tissues were highest at the youngest stages, then declined with age. Respiration rates of mesocotyl tissue were the most responsive to temperature, being nearly twofold higher when grown at 14 compared to 30°C. Alternative pathway respiration increased concomitantly with respiration and was higher in mesocotyls grown in the cold. When seedlings were started at 30 then transferred to 14°C, the increase in alternative pathway respiration due to cold was not observed unless the seedlings were transferred before 2 days of growth. Seedlings transferred to 14°C after growth at 30°C for 2 days had the same alternative oxidase capacity as seedlings grown at 30°C. Seedlings grown at 14°C for 10 to 12 days, then transferred to 30°C, lost alternative pathway respiratory capacity over a period of 2 to 3 days. Western blots of mitochondrial proteins indicated that this loss of capacity was due to a loss of the alternative oxidase protein. Some in vitro characteristics of mitochondria were determined. The temperature optimum for measurement of alternative oxidase capacity was 15 to 20°C. At 41°C, very little alternative oxidase was measured, i.e., the mitochondrial oxygen uptake was almost completely sensitive to cyanide. This inactivation at 41°C was reversible. After incubation at 41°C, the alternative oxidase capacity measured at 25°C was the similar to when it was measured at that temperature directly. Isolated mitochondria lost alternative oxidase capacity at the same rate when incubated at 41°C as they did when incubated at 25°C. Increasing the supply of electrons to isolated mitochondria increased the degree of engagement of the alternative pathway, whereas lower temperature decreased the degree of engagement. Lower temperatures did not increase the degree of engagement of the pathway in intact tissues. We interpret these observations to indicate that the greater capacity of alternative oxidase in cold-grown seedlings is a consequence of development at these low temperatures which results in elevated respiration rates. Low temperature itself does not cause greater capacity or engagement of the alternative oxidase in mitochondria that have developed under warm temperatures. Our hypothesis would be that the low growth temperatures require the seedlings to have a higher respiration rate for some reason, e.g., to prevent the accumulation of a toxic metabolite, and that the alternative pathway functions in that respiration.     

Thermal Inactivation of Nonproteolytic Clostridium botulinum Type E Spores in Model Fish Media and in Vacuum-Packaged Hot-Smoked Fish Products

Thermal inactivation of nonproteolytic Clostridium botulinum type E spores was investigated in rainbow trout and whitefish media at 75 to 93°C. Lysozyme was applied in the recovery of spores, yielding biphasic thermal destruction curves. Approximately 0.1% of the spores were permeable to lysozyme, showing an increased measured heat resistance. Decimal reduction times for the heat-resistant spore fraction in rainbow trout medium were 255, 98, and 4.2 min at 75, 85, and 93°C, respectively, and those in whitefish medium were 55 and 7.1 min at 81 and 90°C, respectively. The z values were 10.4°C in trout medium and 10.1°C in whitefish medium. Commercial hot-smoking processes employed in five Finnish fish-smoking companies provided reduction in the numbers of spores of nonproteolytic C. botulinum of less than 10³. An inoculated-pack study revealed that a time-temperature combination of 42 min at 85°C (fish surface temperature) with >70% relative humidity (RH) prevented growth from 10⁶ spores in vacuum-packaged hot-smoked rainbow trout fillets and whole whitefish stored for 5 weeks at 8°C. In Finland it is recommended that hot-smoked fish be stored at or below 3°C, further extending product safety. However, heating whitefish for 44 min at 85°C with 10% RH resulted in growth and toxicity in 5 weeks at 8°C. Moist heat thus enhanced spore thermal inactivation and is essential to an effective process. The sensory qualities of safely processed and more lightly processed whitefish were similar, while differences between the sensory qualities of safely processed and lightly processes rainbow trout were observed.