Link between Energy Investment in Biosynthesis and Proteostasis: Testing the Cost-Quality Hypothesis in Insects

The energy requirement for biosynthesis plays an important role in an organism's life history, as it determines growth rate, and tradeoffs with the investment in somatic maintenance. This energetic trait is different between painted lady (Vanessa cardui) and Turkestan cockroach (Blatta lateralis) due to the different life histories. Butterfly caterpillars (holometabolous) grow 30-fold faster, and the energy cost of biosynthesis is 20 times cheaper, compared to cockroach nymphs (hemimetabolous). We hypothesize that physiologically the difference in the energy cost is partially attributed to the differences in protein retention and turnover rate: Species with higher energy cost may have a lower tolerance to errors in newly synthesized protein. Newly synthesized proteins with errors are quickly unfolded and refolded, and/or degraded and resynthesized via the proteasomal system. Thus, much protein output may be given over to replacement of the degraded new proteins, so the overall energy cost on biosynthesis is high. Consequently, the species with a higher energy cost for biosyntheses has better proteostasis and cellular resistance to stress. Our study found that, compared to painted lady caterpillars, the midgut tissue of cockroach nymphs has better cellular viability under oxidative stresses, higher activities of proteasome 20S, and a higher RNA/growth ratio, supporting our hypothesis. This comparative study offers a departure point for better understanding life history tradeoffs between somatic maintenance and biosynthesis.

The extremely low energy cost of biosynthesis in holometabolous insect larvae

The metabolic cost of growth, which quantifies the amount of energy required to synthesize a unit of biomass, is an important component of an animal's ontogenetic energy budget. Here we investigated this quantity as well as other energy budget variables of the larvae of a holometabolous insect species, Vanessa cardui (painted lady). We found that the high growth rate of this caterpillar cannot be explained by its metabolic rate and the percentage of the metabolic energy allocated to growth; the key to understanding its fast growth is the extremely low cost of growth, 336 Joules/gram of dry mass. The metabolic cost of growth in caterpillars is 15-65 times lower than that of the endothermic and ectothermic species investigated in previous studies. Our results suggest that the low cost cannot be attributed to its body composition, diet composition, or body size. To explain the "cheap price" of growth in caterpillars, we assumed that a high metabolic cost for biosynthesis resulted in a high "quality" of cells, which have fewer errors during biosynthesis and higher resistance to stressors. Considering the life history of the caterpillars, i.e., tissue disintegration during metamorphosis and a short developmental period and lifespan, we hypothesized that an energy budget that allocates a large amount of energy to biosynthesizing high quality cells would be selected against in this species. As a preliminary test of this hypothesis, we estimated the metabolic cost of growth in larvae of Manduca sexta (tobacco hornworm) and nymphs of Blatta lateralis (Turkestan cockroach). The preliminary data supported our hypothesis.

Shifting Balance of Protein Synthesis and Degradation Sets a Threshold for Larval Growth Under Environmental Stress

Exogenous environmental factors alter growth rates, yet information remains scant on the biochemical mechanisms and energy trade-offs that underlie variability in the growth of marine invertebrates. Here we study the biochemical bases for differential growth and energy utilization (as adenosine triphosphate [ATP] equivalents) during larval growth of the bivalve Crassostrea gigas exposed to increasing levels of experimental ocean acidification (control, middle, and high pCO₂, corresponding to ∼400, ∼800, and ∼1100 µatm, respectively). Elevated pCO₂ hindered larval ability to accrete both shell and whole-body protein content. This negative impact was not due to an inability to synthesize protein per se, because size-specific rates of protein synthesis were upregulated at both middle and high pCO₂ treatments by as much as 45% relative to control pCO₂. Rather, protein degradation rates increased with increasing pCO₂. At control pCO₂, 89% of cellular energy (ATP equivalents) utilization was accounted for by just 2 processes in larvae, with protein synthesis accounting for 66% and sodium-potassium transport accounting for 23%. The energetic demand necessitated by elevated protein synthesis rates could be accommodated either by reallocating available energy from within the existing ATP pool or by increasing the production of total ATP. The former strategy was observed at middle pCO₂, while the latter strategy was observed at high pCO₂. Increased pCO₂ also altered sodium-potassium transport, but with minimal impact on rates of ATP utilization relative to the impact observed for protein synthesis. Quantifying the actual energy costs and trade-offs for maintaining physiological homeostasis in response to stress will help to reveal the mechanisms of resilience thresholds to environmental change.

Metabolic rates are elevated and influenced by maternal identity during the early, yolk-dependent, post-hatching period in an estuarine turtle, the diamondback terrapin (Malaclemys terrapin)

Non-genetic maternal effects, operating through a female's physiology or behavior, can influence offspring traits and performance. Here we examined potential maternal influences on metabolic rates (MR) of offspring in an estuarine turtle, the diamondback terrapin (Malaclemys terrapin). Females and their eggs were collected from the field and the eggs incubated in the laboratory for subsequent measurement of MR of females, late-stage embryos, newly-hatched individuals that were nutritionally dependent on yolk, and older hatchlings that had depleted their yolk reserves and thus were independent of energetic contributions from the female. Female identity significantly affected MR of yolk-dependent hatchlings but, after yolk was depleted, MR of offspring converged and no longer reflected the maternal influence. Offspring from different females also differed in size, which influenced offspring MR and growth, but there was no correlation between female MR or size and offspring traits. MR of the older, yolk-independent hatchlings was lower overall than yolk-dependent hatchlings but correlated positively with growth rates and prior developmental rate (e.g. negatively correlated with time to hatching). Unlike another turtle species (snapping turtles), in which maternally-related differences in offspring MR were retained after yolk depletion, the maternal influence on offspring MR in diamondback terrapins is limited to early hatchling development and growth. The transient nature of the maternal effect, which was present only during the period that hatchlings were metabolizing yolk, suggests that variation among females in the composition of yolk deposited in eggs could be responsible for the differences observed in this study.

Metabolic Cost of Protein Synthesis in Larvae of the Pacific Oyster (Crassostrea gigas) Is Fixed Across Genotype, Phenotype, and Environmental Temperature

The energy made available through catabolism of specific biochemical reserves is constant using standard thermodynamic conversion equivalents (e.g., 24.0 J mg protein(-1)). In contrast, measurements reported for the energy cost of synthesis of specific biochemical constituents are highly variable. In this study, we measured the metabolic cost of protein synthesis and determined whether this cost was influenced by genotype, phenotype, or environment. We focused on larval stages of the Pacific oyster Crassostrea gigas, a species that offers several experimental advantages: availability of genetically pedigreed lines, manipulation of ploidy, and tractability of larval forms for in vivo studies of physiological processes. The cost of protein synthesis was measured in larvae of C. gigas for 1) multiple genotypes, 2) phenotypes with different growth rates, and 3) different environmental temperatures. For all treatments, the cost of protein synthesis was within a narrow range--near the theoretical minimum--with a fixed cost (mean ± one standard error, n = 21) of 2.1 ± 0.2 J (mg protein synthesized)(-1) We conclude that there is no genetic variation in the metabolic cost of protein synthesis, thereby simplifying bioenergetic models. Protein synthesis is a major component of larval metabolism in C. gigas, accounting for more than half the metabolic rate in diploid (59%) and triploid larvae (54%). These results provide measurements of metabolic cost of protein synthesis in larvae of C. gigas, an indicator species for impacts of ocean change, and provide a quantitative basis for evaluating the cost of resilience.

Validation of the flooding dose technique to determine fractional rates of protein synthesis in a model bivalve species, the blue mussel (Mytilus edulis L.)

For the first time, use of the flooding dose technique using (3)H-Phenylalanine is validated for measuring whole-animal and tissue-specific rates of protein synthesis in the blue mussel Mytilus edulis (61mm shell length; 4.0g fresh body mass). Following injection, the phenylalanine-specific radioactivities in the gill, mantle and whole-animal free pools were elevated within one hour and remained elevated and stable for up to 6h following injection of (3)H-phenylalanine into the posterior adductor muscle. Incorporation of (3)H-phenylalanine into body protein was linear over time following injection and the non-significant intercepts for the regressions suggested incorporation into body protein occurred rapidly after injection. These results validate the technique for measuring rates of protein synthesis in mussels. There were no differences in the calculated rates following 1-6h incubation in gill, mantle or whole-animal and fractional rates of protein synthesis from the combined time course data were 9.5±0.8%d(-1) for the gill, 2.5±0.3%d(-1) for the mantle and 2.6±0.3%d(-1) for the whole-animal, respectively (mean values±SEM). The whole-animal absolute rate of protein synthesis was calculated as 18.9±0.6mg protein day(-1). The use of this technique in measuring one of the major components of maintenance metabolism and growth will provide a valuable and convenient tool in furthering our understanding of the protein metabolism and energetics of this keystone marine invertebrate and its ability to adjust and respond to fluctuations, such as that expected as a result of climate change.

Experimental ocean acidification alters the allocation of metabolic energy

Energy is required to maintain physiological homeostasis in response to environmental change. Although responses to environmental stressors frequently are assumed to involve high metabolic costs, the biochemical bases of actual energy demands are rarely quantified. We studied the impact of a near-future scenario of ocean acidification [800 µatm partial pressure of CO2 (pCO2)] during the development and growth of an important model organism in developmental and environmental biology, the sea urchin Strongylocentrotus purpuratus. Size, metabolic rate, biochemical content, and gene expression were not different in larvae growing under control and seawater acidification treatments. Measurements limited to those levels of biological analysis did not reveal the biochemical mechanisms of response to ocean acidification that occurred at the cellular level. In vivo rates of protein synthesis and ion transport increased ∼50% under acidification. Importantly, the in vivo physiological increases in ion transport were not predicted from total enzyme activity or gene expression. Under acidification, the increased rates of protein synthesis and ion transport that were sustained in growing larvae collectively accounted for the majority of available ATP (84%). In contrast, embryos and prefeeding and unfed larvae in control treatments allocated on average only 40% of ATP to these same two processes. Understanding the biochemical strategies for accommodating increases in metabolic energy demand and their biological limitations can serve as a quantitative basis for assessing sublethal effects of global change. Variation in the ability to allocate ATP differentially among essential functions may be a key basis of resilience to ocean acidification and other compounding environmental stressors.

Effect of hypercapnic hypoxia and bacterial infection (Vibrio campbellii) on protein synthesis rates in the Pacific whiteleg shrimp,Litopenaeus vannamei

Estuarine species frequently encounter areas of simultaneously low dissolved O2(hypoxia) and high CO2(hypercapnia). Organisms exposed to hypoxia experience a metabolic depression that serves to decrease ATP utilization and O2demand during stress. This downregulation is typically facilitated by a reduction in protein synthesis, a process that can be responsible for up to 60% of basal metabolism. The added effects of hypercapnia, however, are unclear. Certain decapods also exhibit a metabolic depression in response to bacterial challenges, leading us to hypothesize that protein synthesis may also be reduced during infection. In the present study, we examined the effects of hypoxia (H), hypercapnic hypoxia (HH), and bacterial infection ( Vibrio campbellii) on tissue-specific (muscle and hepatopancreas) fractional protein synthesis rates ( ks) in Litopenaeus vannamei. We observed a significant decrease in ksin muscle after 24 h exposure to both H and HH, and in hepatopancreas after 24 h exposure to HH. Thus ksis responsive to changes in O2, and the combined effect of hypercapnic hypoxia on ksis more severe than hypoxia alone. These reductions in ksappear to be driven by changes in RNA translational efficiency ( kRNA), and not RNA capacity ( Cs). Bacterial infection, however, had no significant effect on ksin either tissue. These results suggest that crustaceans reduce metabolic demand during environmental hypoxia by reducing global protein synthesis, and that this effect is magnified when hypercapnia is concomitantly present. Conversely, an immune-mediated metabolic depression is not associated with a decrease in overall protein production.

Thermal dependence of clearance and metabolic rates in slow- and fast-growing spats of manila clam Ruditapes philippinarum

Thermal dependence of clearance rate (CR: l h(-1)), standard (SMR: J h(-1)) and routine metabolic rates (RMR: J h(-1)), were analyzed in fast (F)- and slow (S)-growing juveniles of the clam Ruditapes philippinarum. Physiological rates were measured at the maintenance temperature (17 °C), and compared with measurements performed at 10 and 24 °C after 16 h and 14 days to analyze acute and acclimated responses, respectively. Metabolic rates (both RMR and SMR) differed significantly between F and S seeds, irrespective of temperature. Mass-specific CRs were not different for F and S seeds but were significantly higher in F clams for rates standardized according to allometric size-scaling rules. Acute thermal dependency of CR was equal for F and S clams: mean Q 10 were ≈3 and 2 in temperature ranges of 10-17 and 17-24 °C, respectively. CR did not change after 2 weeks of acclimation to temperatures. Acute thermal effects on SMR were similar in both groups (Q 10 ≈ 1 and 1.6 in temperature ranges of 10-17 and 17-24 °C, respectively). Large differences between groups were found in the acute thermal dependence of RMR: Q 10 in F clams (≈1.2 and 1.9 at temperature ranges of 10-17 and 17-24 °C, respectively) were similar to those found for SMR (Q 10 = 1.0 and 1.7). In contrast, RMR of S clams exhibited maximum thermal dependence (Q 10 = 3.1) at 10-17 °C and become depressed at higher temperatures (Q 10 = 0.9 at 17-24 °C). A recovery of RMR in S clams was recorded upon acclimation to 24 °C. Contrasting metabolic patterns between fast and slow growers are interpreted as a consequence of differential thermal sensitivity of the fraction of metabolism associated to food processing and assimilation.

Influence of natural thermal gradients on whole animal rates of protein synthesis in marine gammarid amphipods

Although temperature is known to have an important effect on protein synthesis rates and growth in aquatic ectotherms held in the laboratory, little is known about the effects of thermal gradients on natural populations in the field. To address this issue we determined whole-animal fractional rates of protein synthesis (ks ) in four dominant species of gammarid amphipods with different distributions along the coasts of Western Europe from arctic to temperate latitudes. Up to three populations of each species were collected in the summer and ks measured within 48 h. Summer ks values were relatively high in the temperate species, Gammarus locusta, from Portugal (48°N) and Wales (53°N) and were maintained across latitudes by the conservation of translational efficiency. In sharp contrast, summer ks remained remarkably low in the boreal/temperate species G. duebeni from Wales, Scotland (58°N) and Tromsø (70°N), probably as a temporary energy saving strategy to ensure survival in rapidly fluctuating environments of the high intertidal. Values for ks increased in acclimated G. duebeni from Scotland and Tromsø showing a lack of compensation with latitude. In the subarctic/boreal species, G. oceanicus, summer ks remained unchanged in Scotland and Tromsø but fell significantly in Svalbard (79°N) at 5°C, despite a slight increase in RNA content. At 79°N, mean ks was 4.5 times higher in the circumpolar species G. setosus than in G. oceanicus due to a doubling in RNA content. The relationship between whole-animal protein synthesis rates and natural thermal gradients is complex, varies between species and appears to be associated with local temperatures and their variability, as well as changes in other environmental factors.

Molecular characterization of the MuRF genes in rainbow trout: Potential role in muscle degradation

Muscle growth is determined primarily by the balance between protein synthesis and degradation. When rates of protein synthesis are similar between individuals, protein degradation is critical in explaining differences in growth efficiency. Studies in mammals showed that muscle atrophy results from increased protein breakdown, and is associated with activation of the ubiquitin proteasome pathway, including induction of the muscle-specific ubiquitin protein ligase, MuRF1. Animals lacking MuRF1 are resistant to muscle atrophy. In fish, little is known about the role of the proteasome/MuRF pathway in muscle degradation. The objectives of this study were to: 1) clone and characterize MuRF genes in rainbow trout; and 2) determine expression of MuRF genes in association with starvation- and vitellogenesis-induced muscle atrophy in rainbow trout. We have identified full-length cDNA sequences for three MuRF genes (MuRF1, MuRF2, and MuRF3). These genes encode proteins with typical MuRF structural domains, including a RING-finger, a B-box and a Leucine-rich coiled-coil domain. RT-PCR analysis showed that MuRF genes are predominantly expressed in muscle and heart tissues. Real time PCR analysis revealed that expression of all MuRF genes is up-regulated during starvation and MuRF3 is up-regulated in vitellogenesis-associated muscle degradation. These results suggest that MuRF genes have an important role in fish muscle protein degradation. Further studies are warranted to assess the potential use of MuRF genes as tools to monitor fish muscle growth and degradation.

Ribosomal analysis of rapid rates of protein synthesis in the Antarctic sea urchin Sterechinus neumayeri

Previous research has shown that developing stages of the Antarctic sea urchin Sterechinus neumayeri have high rates of protein synthesis that are comparable to those of similar species living in much warmer waters. Direct measurements of the biosynthetic capacities of isolated ribosomes have not been reported for marine organisms living in the extreme-cold environment of Antarctica. Such measurements are required for a mechanistic understanding of how the critical and highly complex processes involved in protein synthesis are regulated in animals living in the coldest marine environment on Earth (< -1 degrees C). We tested the hypothesis that high rates of protein synthesis in the cold are a direct result of high biosynthetic capacities of ribosomes engaged in protein synthesis. Our results show that the rate at which ribosomes manufacture proteins (i.e., the peptide elongation rate) at -1 degrees C is surprisingly similar to rates measured in other sea urchin species at temperatures that are over 15 degrees C warmer. Average peptide elongation rates for a range of developmental stages of the Antarctic sea urchin were 0.36 codons s(-1) (+/- 0.05, SE). On the basis of subcellular rate determinations of ribosomal activity, we calculated stage-specific rates of protein synthesis for blastulae and gastrulae to be 3.7 and 6.5 ng protein h(-1), respectively. These findings support the conclusion that the high rates of biosynthesis previously reported for the Antarctic sea urchin are an outcome of high ribosomal activities.

Energetic cost of synthesizing proteins in Antarctic limpet, Nacella concinna (Strebel, 1908), is not temperature dependent

The energetic cost of protein synthesis is thought to account for a significant proportion of total metabolism. However, attempts to estimate the energetic cost of synthesizing proteins have resulted in surprisingly variable results, particularly for the small number of polar organisms studied, where cost estimates vary by two orders of magnitude. Much of this variability is probably the result of differing methodologies and experimental designs. Here we have used two different, carefully validated methods to measure the costs of protein synthesis in Antarctic limpets. One method, which utilized a specific protein synthesis inhibitor, was used to measure the cost of protein synthesis at two temperatures to test the hypothesis that the cost of protein synthesis varies with temperature. The cost of protein synthesis measured using the inhibitor cycloheximide was 13.95 +/- 0.77 micromol O2/mg protein, while correlation of absolute protein synthesis with oxygen consumption suggested the cost of protein synthesis was 19.58 micromol O2/mg protein. Water temperature did not alter the cost of protein synthesis in Nacella concinna (Student's t-test, P = 0.849, t = 0.19, df = 12). In a meta-analysis of literature values for the cost of protein synthesis there was no significant effect of temperature, but there was a significant relationship between the concentration of cycloheximide used to inhibit protein synthesis and the measured cost.

Cost of protein synthesis and energy allocation during development of antarctic sea urchin embryos and larvae

Cold environments represent a substantial volume of the biosphere. To study developmental physiology in subzero seawater temperatures typically found in the Southern Ocean, rates and costs of protein synthesis were measured in embryos and larvae of Sterechinus neumayeri, the Antarctic sea urchin. Our analysis of the "cost of living" in extreme cold for this species shows (1) that cost of protein synthesis is strikingly low during development, at 0.41 +/- 0.05 J (mg protein synthesized)(-1) (n = 16); (2) that synthesis cost is fixed and independent of synthesis rate; and (3) that a low synthesis cost permits high rates of protein turnover at -1 degrees C, at rates comparable to those of temperate species of sea urchin embryos developing at 15 degrees C. With a low synthesis cost, even at the highest synthesis rates measured (gastrulae), the proportion of total metabolism accounted for by protein synthesis in the Antarctic sea urchin was 54%-a value similar to that of temperate sea urchin embryos. In the Antarctic sea urchin, up to 87% of metabolic rate can be accounted for by the combined energy costs of protein synthesis and the sodium pump. We conclude that, in Antarctic sea urchin embryos, high rates of protein synthesis can be supported in extreme-cold environments while still maintaining low rates of respiration.

Protein metabolism in marine animals: the underlying mechanism of growth

Growth is a fundamental process within all marine organisms. In soft tissues, growth is primarily achieved by the synthesis and retention of proteins as protein growth. The protein pool (all the protein within the organism) is highly dynamic, with proteins constantly entering the pool via protein synthesis or being removed from the pool via protein degradation. Any net change in the size of the protein pool, positive or negative, is termed protein growth. The three inter-related processes of protein synthesis, degradation and growth are together termed protein metabolism. Measurement of protein metabolism is vital in helping us understand how biotic and abiotic factors affect growth and growth efficiency in marine animals. Recently, the developing fields of transcriptomics and proteomics have started to offer us a means of greatly increasing our knowledge of the underlying molecular control of protein metabolism. Transcriptomics may also allow us to detect subtle changes in gene expression associated with protein synthesis and degradation, which cannot be detected using classical methods. A large literature exists on protein metabolism in animals; however, this chapter concentrates on what we know of marine ectotherms; data from non-marine ectotherms and endotherms are only discussed when the data are of particular relevance. We first consider the techniques available to measure protein metabolism, their problems and what validation is required. Protein metabolism in marine organisms is highly sensitive to a wide variety of factors, including temperature, pollution, seasonality, nutrition, developmental stage, genetics, sexual maturation and moulting. We examine how these abiotic and biotic factors affect protein metabolism at the level of whole-animal (adult and larval), tissue and cellular protein metabolism. Available gene expression data, which help us understand the underlying control of protein metabolism, are also discussed. As protein metabolism appears to comprise a significant proportion of overall metabolic costs in marine organisms, accurate estimates of the energetic cost per unit of synthesised protein are important. Measured costs of protein metabolism are reviewed, and the very high variability in reported costs highlighted. Two major determinants of protein synthesis rates are the tissue concentration of RNA, often expressed as the RNA to protein ratio, and the RNA activity (k(RNA)). The effects of temperature, nutrition and developmental stage on RNA concentration and activity are considered. This chapter highlights our complete lack of knowledge of protein metabolism in many groups of marine organisms, and the fact we currently have only limited data for animals held under a narrow range of experimental conditions. The potential assistance that genomic methods may provide in increasing our understanding of protein metabolism is described.

Fixed metabolic costs for highly variable rates of protein synthesis in sea urchin embryos and larvae

Defining the physiological mechanisms that set metabolic rates and the`cost of living' is important for understanding the energy costs of development. Embryos and larvae of the sea urchin Lytechinus pictus(Verrill) were used to test hypotheses regarding differential costs of protein synthesis in animals differing in size, rates of protein synthesis, and physiological feeding states. For embryos, the rate of protein synthesis was 0.22±0.014 ng protein embryo-1 h-1 (mean ±s.e.m.) and decreased in unfed larvae to an average rate of 0.05±0.001 ng protein larva-1 h-1. Fed larvae had rates of synthesis that were up to 194 times faster than unfed larvae (9.7±0.81 ng protein larva-1 h-1). There was no significant difference, however, in the cost of protein synthesis between these larvae with very different physiological states. Furthermore, the cost of synthesis in the larval stages was also similar to costs measured for blastula and gastrula embryos of 8.4±0.99 J mg-1 protein synthesized. The cost of protein synthesis was obtained using both direct (`inhibitor') and indirect (`correlative') measurements; both methods gave essentially identical results. Protein synthesis accounted for up to 54±8% of metabolic rate in embryos. Percent of metabolism accounted for by protein synthesis in larvae was dependent on their physiological feeding state, with protein synthesis accounting for 16±4% in unfed larvae and 75±11% in fed larvae. This regulation of metabolic rate was due to differential rates of synthesis for a fixed energy cost per unit mass of protein synthesized. The cost of synthesizing a unit of protein did not change with increasing rates of protein synthesis. We conclude that the cost of protein synthesis is independent of the rate of synthesis, developmental stage, size and physiological feeding state during sea urchin development.

Metabolic Stoichiometry and the Fate of Excess Carbon and Nutrients in Consumers

Animals encountering nutritionally imbalanced foods should release elements in excess of requirements in order to maintain overall homeostasis. Quantifying these excesses and predicting their fate is, however, problematic. A new model of the stoichiometry of consumers is formulated that incorporates the separate terms in the metabolic budget, namely, assimilation of ingested substrates and associated costs, protein turnover, other basal costs, such as osmoregulation, and the use of remaining substrates for production. The model indicates that release of excess C and nonlimiting nutrients may often be a significant fraction of the total metabolic budget of animals consuming the nutrient-deficient forages that are common in terrestrial and aquatic systems. The cost of maintenance, in terms of not just C but also N and P, is considerable, such that food quality is important even when intake is low. Many generalist consumers experience short-term and unpredictable fluctuations in their diets. Comparison of model output with data for one such consumer, Daphnia, indicates that mechanisms operating postabsorption in the gut are likely the primary means of regulating excess C, N, and P in these organisms, notably respiration decoupled from biochemical or mechanical work and excretion of carbon and nutrients. This stoichiometrically regulated release may often be in organic rather than inorganic form, with important consequences for the balance of autotrophic and heterotrophic processes in ecosystems.

Metabolic depression and whole-body response to enforced starvation by Crassostrea gigas postlarvae

Physiological and biochemical measurements were performed on six oyster (Crassostrea gigas) cohorts, in order to: (a) investigate the whole-body response (growth, energy content, metabolic and excretion rates) of 2-week-old postlarvae (spat) to enforced (0-8 days) starvation, and (b) test the potential use of three aerobic enzyme systems as indices of physiological condition. Starvation resulted in exponential reduction of postlarval metabolic and excretion rates, as well as a linear decrease in enzyme activity. These response mechanisms effectively limited the loss of endogenous reserves after 2 days of starvation and maintained the oyster's functional integrity over prolonged (8 days) starvation. Proteins appeared to be selectively conserved during short-term (2 days) starvation, as suggested by a decrease in total protein content, while maintaining constant weight-specific enzyme activity. Postlarvae starved for 2 days exhibited relatively higher lipid losses, lower mortality and lower metabolism than metamorphosing stages, thus suggesting a greater buffering capacity to starvation in the former. The activity of the electron transport system may be a useful indicator of long-term stress or developmental condition of oyster postlarvae, while citrate synthase and cytochrome oxidase could be used as indicators of growth rate. None of these enzyme systems is recommended as an index of aerobic metabolism during short-term starvation.

In vivo downregulation of protein synthesis in the snail Helix apersa during estivation

Protein synthesis is downregulated during metabolic depression in a number of systems where the metabolic depression is effected by obvious extrinsic cues. The metabolic depression of the estivating land snail Helix apersa occurs in the absence of any obvious physiological stress and has an intrinsic component independent of temperature, pH, O(2) status, or osmolality. We show that this metabolic depression is accompanied by a downregulation of protein synthesis in vivo. The rate of protein synthesis decreases in two major tissues during estivation: to 23% and 53% of the awake rate in hepatopancreas and foot muscle, respectively. We show from calculations of the theoretical contribution of protein synthesis to total O(2) consumption that the depression of protein synthesis must be a significant, obligate, in vivo component of metabolic depression in H. aspersa.

High macromolecular synthesis with low metabolic cost in Antarctic sea urchin embryos

Assessing the energy costs of development in extreme environments is important for understanding how organisms can exist at the margins of the biosphere. Macromolecular turnover rates of RNA and protein were measured at -1.5 degrees C during early development of an Antarctic sea urchin. Contrary to expectations of low synthesis with low metabolism at low temperatures, protein and RNA synthesis rates exhibited temperature compensation and were equivalent to rates in temperate sea urchin embryos. High protein metabolism with a low metabolic rate is energetically possible in this Antarctic sea urchin because the energy cost of protein turnover, 0.45 joules per milligram of protein, is 1/25th the values reported for other animals.

Relations between variable rates of growth, metabolic costs and growth efficiencies in individual Sydney rock oysters (Saccostrea commercialis)

Rock oysters from a mass selection trial were compared with wild-caught (control) oysters of the same age to determine the physiological basis for faster growth rates amongst the selected individuals, and to describe the associated flexibility in phenotypic traits of feeding, metabolism and growth. In confirmation of earlier studies, fast growth was associated with faster rates of feeding, reduced metabolic rates and lower metabolic costs of growth. Selected individuals deposited more protein, at a lower metabolic cost, than the controls. Control oysters, however, deposited more lipid than the selected oysters, though the unit costs of lipid deposition did not differ between categories. The results indicated a wide plasticity of physiological rates and efficiencies and demonstrated how, by selection, interactions between physiological traits can serve to enhance growth. If differences in lipid deposition observed here were indicative of different rates of gametogenesis, then the results also suggest that selection alters the balance between growth and reproduction. Whether these differences can be termed compensatory with respect to the life history of the species remains to be determined, but the results indicate some of the ways in which physiological flexibility may be expressed to effect different patterns of energy allocation.

Physiological components of growth differences between individual oysters (Crassostrea gigas) and a comparison with Saccostrea commercialis

Pacific oysters (Crassostrea gigas) of identical age from two genetically distinct lines, one fast growing and the other slow growing, were held at three levels of ration and analysed for physiological traits to explain differences in their rates of growth. The data supported three hypotheses; faster growth was associated with faster rates of consumption of food, reduced metabolic rate at maintenance (i.e., at zero growth), and reduced metabolic costs of growth. A comparison with the Sydney rock oyster, Saccostrea commercialis, based on similar experiments on the two species, indicated that faster growth of Pacific oysters depended on similar physiological differences; the mean metabolic costs of growth, however, were similar in the two species. It is suggested that a general model for genetically linked differences in the growth rate of bivalve molluscs will need to include the processes of metabolic control rather than relying solely on an analysis of the individual components of the energetics of growth.

Cost of growth in cells and organisms: general rules and comparative aspects

In a crude fashion it can be said that metabolizable energy (M) is partitioned into metabolic work, paid for by 'oxidations' (R), and 'assimilation', i.e. production (P), so that M = R+P. However, a fraction of R is required to meet the expenses of production and if these expenses represent, Joule for Joule, a constant proportion of the amount produced, then Rt = Rm+cP, where Rt = total metabolic expenditures, Rm = metabolic expenditures for maintaining the non-producing organism, and cP = Rp = metabolic expenditures connected with the processes of production. The partitioning of metabolizable energy into R and P as well as into Rm and Rp may vary depending on the phylogeny and life-history of the species concerned and on ecological circumstances. Thus selection is expected to act on both ratios, R/P and Rm/Rp. By comparing the ratios P/(P+Rp) (the apparent efficiency of production) and Rp/P (the apparent metabolic cost of production) in different types of organisms, one finds that a value of P/(P+Rp) = 0.75, equal to 75% efficiency, 10 mgdbm/mmol ATP, and 16 mumolO2/mg dbm (when I mg identical to 22 J), can be used as a 'consensus value' for the average efficiency, or cost, of the transformation of metabolizable energy into production in a wide range of organisms, from bacteria to mammals. This value corresponds to about three times the theoretical cost of synthesizing the same amount of tissue on the basis of known biochemical principles. The reasons why the empirical costs of production are higher than the theoretical costs of synthesis by what appears to be a common factor may be quite different in bacteria, small ectothermic and large endothermic organisms. Deviations from the consensus value may be due to differences in energy density of the nutrients assimilated and the tissues synthesized. Further complications arise because of interactions between P, Rp, and Rm. In microorganisms the existence of a constant and a variable component of maintenance metabolism has been postulated, the latter decreasing with increasing rate of production. In small ectothermic metazoans, on the other hand, the nonlinear relationship between growth metabolism and growth rate has led to the speculation that above a critical value of Pg certain energy consuming functions of maintenance are suppressed and the energy thus gained used for fuelling growth processes. There is some evidence that, at least in ectothermic metazoans, the apparent cost of growth decreases with the rate of growth, reaching a low plateau of about 10 mumolO2/mgdbm at growth rates exceeding about 8 mgdbm/g/h.(ABSTRACT TRUNCATED AT 400 WORDS)