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

Abstract

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.

Increased carbon dioxide availability alters phytoplankton stoichiometry and affects carbon cycling and growth of a marine planktonic herbivore

“…Furthermore, A. tonsa do not store large amounts of lipids. Thus, the routes and mechanisms by which this disposal of excess C is achieved and the nature and fate of these C excretory products may vary between species, and, as we have seen, even between life stagers within species (Darchambeau et al 2003;Jensen and Hessen 2007;Plath and Boersma 2001;Sterner et al 1998;Anderson et al 2005;Hessen and Anderson 2008). The manner in which the consumers exude the C potentially affects both the consumers themselves and their environment.…”

“…The manner in which the consumers exude the C potentially affects both the consumers themselves and their environment. The preabsorption mechanisms available to consumers to regulate their carbon content favour the egestion of C in faecal pellets and the sequestration of this faecal matter (Anderson et al 2005). Excess C voided in this manner is an important driver of the biological pump.…”

“…The most straightforward way of dealing with this imbalance is of course not to take up the surplus carbon in the first place or to egest it directly from the gut as faeces. This carbon would then most likely be sequestered to the deep water layers and impact the strength of the biological pump (Anderson et al 2005). In addition, several post-absorptive regulation mechanisms exist.…”

Coupling of growth rate and body stoichiometry in Daphnia: a role for maintenance processes?

“…Thus, differences in overall protein synthesis demands (and thus RNA levels) may be set not only by growth-related demands but also by potential differences among species in the relative importance and pathways of protein retention and reclamation in overall metabolism. To our knowledge, no empirical studies of protein turnover have yet been performed for Daphnia, despite the importance of assumptions about protein turnover for emerging stoichiometric models of Daphnia growth and metabolism (Vrede et al, 2004;Anderson et al, 2005). Life histories vary widely among taxa.…”

Food type and temperature constraints on the fitness of a dominant freshwater shredder

“…According to stoichiometric regulation, digestion and absorption may be adjusted to favour the retention of a limiting element, with excess nutrients being released via excretion or respiration (Anderson et al, 2005). Consequently, the extent to which individuals can to compensate for this imbalance will ultimately determine the constraints on an individual's performance and survival, as well as alterations in the consumers' nutrient cycling (Sterner and Elser, 2002).…”