After discussing the evolutionary implications of bioenergetics, energy budget management and regulation, the collection of reviews now turns to consider metabolic strategies employed by organisms in a wide range of environments.
Discussing the upper metabolic limits that animals can maintain for extended periods, Theunis Piersma from the University of Groningen, The Netherlands describes the extreme metabolic outputs maintained by polar explorers, athletes and long-distance migrants (p. 295). Explaining that trained human athletes can only maintain metabolic rates that are tens of times their basal metabolic rates (BMR) for a few seconds or minutes, Piersma points out that even polar explorers and Tour de France cyclists can only maintain metabolic rates that are 4–5 times their BMR over sustained periods. However, migrating shorebirds routinely exceed this apparent limit, expending 8–10 times their BMR as they cover thousands of kilometres to reach their breeding grounds. Explaining that the birds fuel these extreme endurance feats by breaking down their fat stores and even their own organs, Piersma suggests that shorebirds may be able to sustain these high-performance levels because they do not have to guard against predators and infection as these threats are minimal in the birds' native environments. He suggests that animals may have evolved ‘laziness’ to protect from potentially fatal metabolic and tissue damage incurred by extreme performance and says, ‘there is scope for experimental studies in which relationships between energy expenditure levels, wear and tear and survival in well-described ecological contexts are investigated’.
So far, the collection has focused on metabolic energy expenditure by organisms, but energy must be taken in to balance the energy budget, and this must be acquired either by eating food or adopting a photosynthetic lodger; i.e. symbiosis. Mary Rumpho and colleagues from the University of Maine and Rutgers University, USA describe a ‘solar-powered’ sea slug, Elysia chlorotica, which consumes Vaucheria litorea and incorporates chloroplasts from the alga into cells lining the sea slug's digestive tract (p. 303). Living as a plant, the sea slug is provided with carbon and energy by the chloroplasts. However, Rumpho explains that chloroplast genomes ‘encode a small percent of the predicted 1000–5000 proteins required to sustain the full metabolic capacity of the plastid,’ and she suggests that long-term chloroplast function is sustained by a combination of unusual plastid stability, very limited horizontal gene transfer and, possibly, long-term maintenance of algal DNA, RNA and proteins in the sea slug.
Although most life forms on the planet's surface ultimately depend on the sun for energy, chemoautotrophs exploit alternative energy sources. Without access to the sun, some deep-sea hydrothermal vent species have struck up symbiotic relationships with chemoautotrophs. However, the hosts may pay a high metabolic price for this convenient relationship. ‘Chemoautotrophy is very demanding of oxygen, and a previous study suggests that up to 80% of oxygen uptake is driven by symbiont metabolism,’ say Jim Childress from the University of California, Santa Barbara and Peter Girgius from Harvard University, USA (p. 312). Measuring the oxygen consumption rates of the tube worm, Riftia pachyptila, and its symbionts, Childress and Girgius report that the symbiont consumes 13.5 times more oxygen than its host. Explaining that, ‘the ability of the animal hosts to support these high oxygen demands is a critical determinant of the rates of carbon fixation that can be achieved,’ the duo describe how Riftia delivers oxygen and hydrogen sulphide to the symbiont by high-affinity haemoglobin. They suggest that the high oxygen demands of chemoautotrophic symbionts have prevented cnidarians from striking up symbiotic relationships with chemoautotrophs.
Remaining on the theme of bioenergetics in the ocean, Brad Seibel from the University of Rhode Island, USA discusses the effects of oceanic oxygen minimum zones on the species that inhabit them (p. 326). Defining two critical oxygen thresholds – one threshold where all animals have to make specific adaptations in oxygen uptake to sustain metabolism and a second threshold that is the oxygen partial pressure below which animals cannot adapt to utilise oxygen – Seibel outlines the adaptations that permit organisms to inhabit oxygen minimum zones. Explaining that many creatures migrate daily to depth and pass though oxygen minimum zones, Seibel points out that climate change could drop oxygen levels below the level where midwater species can extract oxygen, resulting in a drastic ecosystem shift from ‘an ecosystem dominated by diverse midwater fauna to one dominated by diel migrant biota that must return to surface waters at night’.
Concluding the section on metabolic responses, Bente Pedersen from the University of Copenhagen, Denmark discusses the role of muscles in metabolic regulation (p. 337). Muscle is one of the major metabolic organs and it has long been appreciated that exercise protects against diseases of inactivity. Pedersen's discovery of a cytokine produced by contracting muscle in 2000 was the first identification of the elusive ‘exercise factor’ that could regulate metabolic change in other organs in response to exercise. IL-6, the first of these so-called ‘myokines’, stimulates glucose uptake and fat oxidation in response to exercise, while IL-15 reduces visceral fat, which is a potential source of inflammation and is implicated in metabolic diseases such as type-2 diabetes, cardiovascular disease and several cancers. Pedersen concludes that ‘skeletal muscle is an endocrine organ producing and releasing myokines, which work in a hormone-like fashion, exerting specific endocrine effects on other organs,’ and adds that, ‘myokines may contribute to mediate exercise-induced protection against several chronic diseases’.