Parasites alter food-web topology of a subarctic lake food web and its pelagic and benthic compartments.

We compared three sets of highly resolved food webs with and without parasites for a subarctic lake system corresponding to its pelagic and benthic compartments and the whole-lake food web. Key topological food-web metrics were calculated for each set of compartments to explore the role parasites play in food-web topology in these highly contrasting webs. After controlling for effects from differences in web size, we observed similar responses to the addition of parasites in both the pelagic and benthic compartments demonstrated by increases in trophic levels, linkage density, connectance, generality, and vulnerability despite the contrasting composition of free-living and parasitic species between the two compartments. Similar effects on food-web topology can be expected with the inclusion of parasites, regardless of the physical characteristics and taxonomic community compositions of contrasting environments. Additionally, similar increases in key topological metrics were found in the whole-lake food web that combines the pelagic and benthic webs, effects that are comparable to parasite food-web analyses from other systems. These changes in topological metrics are a result of the unique properties of parasites as infectious agents and the links they participate in. Trematodes were key contributors to these results, as these parasites have distinct characteristics in aquatic systems that introduce new link types and increase the food web's generality and vulnerability disproportionate to other parasites. Our analysis highlights the importance of incorporating parasites, especially trophically transmitted parasites, into food webs as they significantly alter key topological metrics and are thus essential for understanding an ecosystem's structure and functioning.

Mitochondrial enzyme content in the muscles of high-performance fish: evolution and variation among fiber types.

Muscle mitochondrial content varies widely among fiber types and species. We investigated the origins of variation in the activity of the mitochondrial enzyme citrate synthase (CS), an index of mitochondrial abundance, among fiber types and species of high-performance fish (tunas and billfishes). CS activities varied up to 30-fold among muscles: lowest in billfish white muscle and highest in billfish heater organ. Among species, CS activities of red, white, and cardiac muscles of three tuna species were twofold greater than the homologous muscles of two billfish species. Because comparisons of CS amino acid sequences deduced from a combination of PCR methods argue against clade-specific differences in catalytic properties, CS activity reflects CS content among these five species. To assess the bases of these differences in CS activity, we looked at the relationship between CS activity (U/g muscle), nuclear content (DNA/g muscle), and CS transcript levels (CS mRNA/g RNA). Muscle CS activity differed by 10- to 30-fold when expressed per gram of muscle but only threefold when expressed per milligram of DNA. Thus it is nuclear DNA content, not fiber-type differences, in CS gene expression that may be the main determinant of CS activity in muscle. Conversely, evolutionary (tunas vs. billfishes) differences in CS arise from differences in posttranscriptional regulation, based on relationships between CS enzyme levels and CS mRNA assessed by quantitative competitive RT-PCR. These data argue that fiber-type differences can arise without major differences in fiber-type-specific regulation of the CS gene, whereas evolutionary differences may be largely due to posttranscriptional regulation.

Dietary proteins in the regulation of food intake and body weight in humans.

This review presents 4 lines of evidence supporting a role for proteins in the regulation of food intake and maintenance of healthy body weights. It is concluded that the protein content of food, and perhaps its source, is a strong determinant of short-term satiety and of how much food is eaten. Although the role of protein in the regulation of long-term food intake and body weight is less clear, the evidence reviewed suggests that further research to define its role is merited. Such research has the potential to lead to new functional foods, food formulations, and dietary recommendations for achieving healthy body weights.