Ecophysiological studies on citrate-synthase: (I) enzyme regulation of selected crustaceans with regard to temperature adaptation

Hotter is not (always) better: Embracing unimodal scaling of biological rates with temperature

Rate-temperature scaling relationships have fascinated biologists for nearly two centuries and are increasingly important in our era of global climate change. These relationships are hypothesized to originate from the temperature-dependent kinetics of rate-limiting biochemical reactions of metabolism. Several prominent theories have formalized this hypothesis using the Arrhenius model, which characterizes a monotonic temperature dependence using an activation energy E. However, the ubiquitous unimodal nature of biological temperature responses presents important theoretical, methodological, and conceptual challenges that restrict the promise for insight, prediction, and progress. Here we review the development of key hypotheses and methods for the temperature-scaling of biological rates. Using simulations, we examine the constraints of monotonic models, illustrating their sensitivity to data nuances such as temperature range and noise, and their tendency to yield variable and underestimated E, with critical consequences for climate change predictions. We also evaluate the behaviour of two prominent unimodal models when applied to incomplete and noisy datasets. We conclude with recommendations for resolving these challenges in future research, and advocate for a shift to unimodal models that better characterize the full range of biological temperature responses.

Hepatic citrate synthase suppression in the freeze-tolerant wood frog (Rana sylvatica)

The wood frog, Rana sylvatica endures whole body freezing for weeks/months while overwintering at subzero temperatures. Survival of long-term freezing requires not only cryoprotectants but also strong metabolic rate depression (MRD) and reorganization of essential processes in order to maintain a balance between ATP-producing and ATP-consuming processes. Citrate synthase (CS) (E.C. 2.3.3.1) is an important irreversible enzyme of the tricarboxylic acid (TCA) cycle and forms a crucial checkpoint for many metabolic processes. Present study investigated the regulation of CS from wood frog liver during freezing. CS was purified to homogeneity by a two-step chromatographic process. Kinetic and regulatory parameters of the enzyme were investigated and, notably, demonstrated a significant decrease in the V_max of the purified form of CS from frozen frogs as compared to controls when assayed at both 22 °C and 5 °C. This was further supported by a decrease in the maximum activity of CS from liver of frozen frogs. Immunoblotting also showed changes in posttranslational modifications with a significant decrease in threonine phosphorylation (by 49 %) for CS from frozen frogs. Taken together, these results suggest that CS is suppressed and TCA flux is inhibited during freezing, likely to support MRD survival of harsh winters.

Mechanistic Temperature-Size Rule Explanation Should Reconcile Physiological and Mortality Responses to Temperature

AbstractThe temperature-size rule is one of the universal rules in ecology and states that ectotherms in warmer waters will grow faster as juveniles, mature at smaller sizes and younger ages, and reach smaller maximum body sizes. Many models have unsuccessfully attempted to reproduce temperature-size rule-consistent life histories by using two-term (anabolism and catabolism) Pütter-type growth models, such as the von Bertalanffy. Here, we present a physiologically structured individual growth model, which incorporates an energy budget and optimizes energy allocation to growth, reproduction, and reserves. Growth, maturation, and reproductive output emerge as a result of life-history optimization to specific physiological rates and mortality conditions. To assess which processes can lead to temperature-size rule-type life histories, we simulate 42 scenarios that differ in temperature and body size dependencies of intake, metabolism, and mortality rates. Results show that the temperature-size rule can emerge in two ways. The first way requires both intake and metabolism to increase with temperature, but the temperature-body size interaction of the two rates must lead to relatively faster intake increase in small individuals and relatively larger metabolism increase in large ones. The second way requires only higher temperature-driven natural mortality and faster intake rates in early life (no change in metabolic rates is needed). This selects for faster life histories with earlier maturation and increased reproductive output. Our model provides a novel mechanistic and evolutionary framework for identifying the conditions necessary for the temperature-size rule. It shows that the temperature-size rule is likely to reflect both physiological changes and life-history optimization and that use of von Bertalanffy-type models, which do not include reproduction processes, can hinder our ability to understand and predict ectotherm responses to climate change.

Metabolic Responses of Pacific Crown-of-Thorns Sea Stars (Acanthaster sp.) to Acute Warming

AbstractClimate change and population irruptions of crown-of-thorns sea stars (Acanthaster sp.) are two of the most pervasive threats to coral reefs. Yet there has been little consideration regarding the synergies between ocean warming and the coral-feeding sub-adult and adult stages of this asteroid. Here we explored the thermosensitivity of the aforementioned life stages by assessing physiological responses to acute warming. Thermal sensitivity was assessed based on the maximal activity of enzymes involved in aerobic (citrate synthase) and anaerobic (lactate dehydrogenase) metabolic pathways, as well as the standard metabolic rate of sub-adult and adult sea stars. In both life stages, citrate synthase activity declined with increasing temperature from 15 °C to 40 °C, with negligible activity occurring >35 °C. On the other hand, lactate dehydrogenase activity increased with temperature from 20 °C to 45 °C, indicating a greater reliance on anaerobic metabolism in a warmer environment. The standard metabolic rate of sub-adult sea stars increased with temperature throughout the testing range (24 °C to 36 °C). Adult sea stars exhibited evidence of thermal stress, with metabolic depression occurring from 33 °C. Here, we demonstrate that crown-of-thorns sea stars are sensitive to warming but that adults, and especially sub-adults, may have some resilience to short-term marine heatwaves in the near future.

The tricarboxylic acid cycle in L₃ Teladorsagia circumcincta: metabolism of acetyl CoA to succinyl CoA

Nematodes, like other species, derive much of the energy for cellular processes from mitochondrial pathways including the TCA cycle. Previously, we have shown L₃ Teladorsagia circumcincta consume oxygen and so may utilise a full TCA cycle for aerobic energy metabolism. We have assessed the relative activity levels and substrate affinities of citrate synthase, aconitase, isocitrate dehydrogenase (both NAD+ and NADP+ specific) and α-ketoglutarate dehydrogenase in homogenates of L₃ T. circumcincta. All of these enzymes were present in homogenates. Compared with citrate synthase, low levels of enzyme activity and low catalytic efficiency was observed for NAD+ isocitrate dehydrogenase and especially α-ketoglutarate dehydrogenase. Therefore, it is likely that the activity of these to enzymes regulate overall metabolite flow through the TCA cycle, especially when [NAD+] limits enzyme activity. Of the enzymes tested, only citrate synthase had substrate affinities which were markedly different from values obtained from mammalian species. Overall, the results are consistent with the suggestion that a full TCA cycle exists withinL₃ T. circumcincta. While there may subtle variations in enzyme properties, particularly for citrate synthase, the control points for the TCA cycle inL₃ T. circumcincta are probably similar to those in the tissues of their host species.

Physiology and metabolism of Northern krill (Meganyctiphanes norvegica Sars)

Advances in our understanding of the physiology and metabolism of Northern krill, Meganyctiphanes norvegica have been sporadic but significant. Despite problems with keeping M. norvegica in good condition in the laboratory, those who have tried, and succeeded, have contributed to a better knowledge of krill biology and challenged our understanding of some basic biological processes. Most recent work has been concentrated in the fields of digestive physiology, lipid biochemistry, respiration and anaerobiosis, metabolic properties, and pollutants. M. norvegica is capable of digesting an opportunistic, omnivorous diet, showing some digestive enzyme polymorphism and high levels of enzyme activity, the latter varying with season. It also seems capable of digesting cellulose and hemicelluloses, for example, laminarin. The biochemical composition of krill is relatively well known with some recent extensive work focusing on the previously little studied lipid and fatty acid composition, particularly with reference to reproduction, overwintering energy storage and as a nutrition marker. A high aerobic metabolism (but poor anaerobic capacity) is characteristic of M. norvegica, and how this is affected by temperature, low O(2), and season has attracted some attention, particularly in the context of diel vertical migration (DVM) across pronounced pycnoclines. Despite determining high metabolic turnover rates and a high physiological plasticity for this species, we know little of the regulative potential of metabolites, particularly their modulative effect on enzyme activity. Certainly a modest ability to maintain aerobic metabolism when encountering hypoxia, and little or no ability to osmoregulate in hyposaline conditions, does not prevent DVM in adults of this species. The ability to maintain aerobic metabolism develops early in ontogeny at about furcilia III (i.e. concurrent with first DVM behaviour). The respiratory pigment of M. norvegica, haemocyanin, has a low O(2) affinity and high temperature sensitivity (although temperature has the opposite effect on O(2) binding than found for nearly every other haemocyanin). Also surprising is the apparent use of haemocyanin as an energy source/store. While recent work has focused on physiological effects, the ecophysiological effects of transuric elements and trace metals, the effects of pollution generally are widely understudied.

Effects of Body Size and Temperature on Population Growth

For at least 200 years, since the time of Malthus, population growth has been recognized as providing a critical link between the performance of individual organisms and the ecology and evolution of species. We present a theory that shows how the intrinsic rate of exponential population growth, rmax, and the carrying capacity, K, depend on individual metabolic rate and resource supply rate. To do this, we construct equations for the metabolic rates of entire populations by summing over individuals, and then we combine these population-level equations with Malthusian growth. Thus, the theory makes explicit the relationship between rates of resource supply in the environment and rates of production of new biomass and individuals. These individual-level and population-level processes are inextricably linked because metabolism sets both the demand for environmental resources and the resource allocation to survival, growth, and reproduction. We use the theory to make explicit how and why rmax exhibits its characteristic dependence on body size and temperature. Data for aerobic eukaryotes, including algae, protists, insects, zooplankton, fishes, and mammals, support these predicted scalings for rmax. The metabolic flux of energy and materials also dictates that the carrying capacity or equilibrium density of populations should decrease with increasing body size and increasing temperature. Finally, we argue that body mass and body temperature, through their effects on metabolic rate, can explain most of the variation in fecundity and mortality rates. Data for marine fishes in the field support these predictions for instantaneous rates of mortality. This theory links the rates of metabolism and resource use of individuals to life-history attributes and population dynamics for a broad assortment of organisms, from unicellular organisms to mammals.

Citrate synthase and pyruvate kinase activities during early life stages of the shrimp Farfantepenaeus paulensis (Crustacea, Decapoda, Penaeidae): effects of development and temperature

Energy metabolism in early life stages of the shrimp Farfantepenaeus paulensis subjected to temperature reduction (26 and 20 degrees C) was determined using the activities of citrate synthase (CS) and pyruvate kinase (PK). At both temperatures, weight-specific activity of CS decreased throughout the ontogenetic development from protozoea II (PZ II) to postlarva XII-XIV (PL XII-XIV). PK activity reached a pronounced peak in PL V-VI, followed by a further decrease in PL XII-XIV. Temperature reduction produced variation in oxygen consumption rates (QO(2)), ammonia-N excretion and in enzyme activities. Ammonia-N excretion was higher at 20 degrees C in mysis III (M III), PL V-VI and PL XII-XIV, resulting in substantially lower O:N ratios in these stages. QO(2) was increased in protozoea II (PZ II) and mysis I (M I) at 26 degrees C, while no difference in QO(2) was detected in the subsequent stages at either temperature. This fact coincided with higher CS and PK activities in M III, PL V-VI and PL XII-XIV at 20 degrees C compared with 26 degrees C. Regressions between individual enzyme activities and dry weight exhibited slope values of 0.85-0.92 for CS and 1.1-1.2 for PK and temperature reduction was reflected by higher slope values at 20 than at 26 degrees C for both enzymes. Weight-specific CS activity was positively correlated with QO(2) at 20 and 26 degrees C, and may thus be used as an indicator of aerobic metabolic rate throughout the early stages of F. paulensis. The variation in enzyme activities is discussed in relation to possible metabolic adaptations during specific ontogenetic events of the F. paulensis life cycle. Here, the catalytic efficiency of energy-metabolism enzymes was reflected in ontogenetic shifts in behaviour such as larval settlement and the adoption of a benthic existence in early postlarvae. In most cases, enhanced enzyme activities appeared to counteract negative effects of reduced temperature.

Scaling metabolism from organisms to ecosystems

Understanding energy and material fluxes through ecosystems is central to many questions in global change biology and ecology. Ecosystem respiration is a critical component of the carbon cycle and might be important in regulating biosphere response to global climate change. Here we derive a general model of ecosystem respiration based on the kinetics of metabolic reactions and the scaling of resource use by individual organisms. The model predicts that fluxes of CO2 and energy are invariant of ecosystem biomass, but are strongly influenced by temperature, variation in cellular metabolism and rates of supply of limiting resources (water and/or nutrients). Variation in ecosystem respiration within sites, as calculated from a network of CO2 flux towers, provides robust support for the model's predictions. However, data indicate that variation in annual flux between sites is not strongly dependent on average site temperature or latitude. This presents an interesting paradox with regard to the expected temperature dependence. Nevertheless, our model provides a basis for quantitatively understanding energy and material flux between the atmosphere and biosphere.

Phthalocyanine-photosensitized inactivation of a pathogenic protozoan, Acanthamoeba palestinensis

Incubation of Acanthamoeba palestinensis cells with a tetracationic phthalocyanine (RLP068) at concentrations ranging between 0.2 and 1.0 microM, caused a ready uptake of the photosensitizer with recoveries of the order of 0.5-2.5 nmol per mg of cell protein. The amount of cell-bound phthalocyanine did not appreciably change with incubation times ranging between 0.5 and 3 h. Fluorescence microscopic investigations showed an obvious accumulation of the phthalocyanine at the level of the vacuolar membranes. A nearly complete photoinduced cell death occurred upon irradiating A. palestinensis cells with 600-700 nm light with a total energy of 15-30 J cm(-2) using 1.0 microM RLP068 in the incubation medium. DAPI staining of the photosensitized cells indicates significant damage of the nucleus. On the other hand, photosensitization of the protozoan cells does not directly involve the mitochondria as shown by the lack of photoinduced decrease in the activity of typical mitochondrial enzymes, such as NADH dehydrogenase and citrate synthase.

Effects of size and temperature on developmental time

Body size and temperature are the two most important variables affecting nearly all biological rates and times. The relationship of size and temperature to development is of particular interest, because during ontogeny size changes and temperature often varies. Here we derive a general model, based on first principles of allometry and biochemical kinetics, that predicts the time of ontogenetic development as a function of body mass and temperature. The model fits embryonic development times spanning a wide range of egg sizes and incubation temperatures for birds and aquatic ectotherms (fish, amphibians, aquatic insects and zooplankton). The model also describes nearly 75% of the variation in post-embryonic development among a diverse sample of zooplankton. The remaining variation is partially explained by stoichiometry, specifically the whole-body carbon to phosphorus ratio. Development in other animals at other life stages is also described by this model. These results suggest a general definition of biological time that is approximately invariant and common to all organisms.

Effects of size and temperature on metabolic rate

We derive a general model, based on principles of biochemical kinetics and allometry, that characterizes the effects of temperature and body mass on metabolic rate. The model fits metabolic rates of microbes, ectotherms, endotherms (including those in hibernation), and plants in temperatures ranging from 0 degrees to 40 degrees C. Mass- and temperature-compensated resting metabolic rates of all organisms are similar: The lowest (for unicellular organisms and plants) is separated from the highest (for endothermic vertebrates) by a factor of about 20. Temperature and body size are primary determinants of biological time and ecological roles.

Effects of temperature on the respiration rates and the kinetics of citrate synthase in two species of Idotea (Isopoda, Crustacea)

The two species of isopods, Idotea baltica (Pallas) and Idotea emarginata (Fabricius), co-occur frequently near Helgoland, North Sea, occupying different ecological niches. Respiration rates and kinetic properties of citrate synthase (CS) were compared in these species in order to identify possible mechanisms of temperature adaptation. Specimens were acclimated to 5 and 15 degrees C prior to further investigations. Respiration rates were measured under normoxic conditions at 5, 10 and 15 degrees C. CS was partly purified chromatographically and influences of temperature, pH, substrate saturation and ATP-concentration on enzyme activity were examined. In both species, rising temperatures led to linearly increasing oxygen consumption, with estimated Q10 values between 3.2 and 4.2. Only I. baltica showed an effect of short term acclimation: warm adapted animals had always higher respiration rates than cold adapted ones. In I. emarginata, the acclimation temperature had no effect on oxygen consumption. Furthermore, its CS slightly indicates higher affinity to oxaloacetic acid when specimens were adapted to 15 degrees C compared to those maintained at 5 degrees C. Any effect of the experimental temperature on CS in I. baltica was negligible. The results are discussed in view of the different habitats occupied by the species compared.

Effect of temperature on the activities of glucose-6-phosphate dehydrogenase and hexokinase in entomopathogenic nematodes (Nematoda: Steinernematidae)

The kinetic properties of two metabolic enzymes, glucose-6-phosphate dehydrogenase and hexokinase, were studied in four strains of entomopathogenic nematodes that had been recycled for two years at various temperatures: Steinernema feltiae NF strain, Steinernema feltiae Umeå strain, Steinernema carpocapsae All strain, Steinernema riobravis TX strain. The recycling temperatures influenced the activities of glucose-6-phosphate dehydrogenase and hexokinase in an adaptive fashion in all the strains. At each assay temperature (5-35 degrees C), the maximum specific activity of both the enzymes was greater in the nematodes that had been recycled at lower temperatures than in those reared at higher temperatures. In three enzyme-nematode strain combinations, the lowest K(m) values measured at each assay temperature occurred in nematodes that had been recycled at the lower temperatures. However, the assay temperatures at which the minimum K(m) values occurred were > or = 15 degrees C. The capacities of these nematodes to adjust to different recycling temperatures is discussed in relation to the physiological mechanisms involved.