title: How Daphnia copes with excess carbon in its food.
abstract: Animals that maintain near homeostatic elemental ratios may get rid of excess ingested elements from their food in different ways. C regulation was studied in juveniles of Daphnia magna feeding on two Selenastrum capricornutum cultures contrasting in P content (400 and 80 C:P atomic ratios). Both cultures were labelled with (14)C in order to measure Daphnia ingestion and assimilation rates. No significant difference in ingestion rates was observed between P-low and P-rich food, whereas the net assimilation of (14)C was higher in the treatment with P-rich algae. Some Daphnia were also homogeneously labelled over 5 days on radioactive algae to estimate respiration rates and excretion rates of dissolved organic C (DOC). The respiration rate for Daphnia fed with high C:P algae (38.7% of body C day(-1)) was significantly higher than for those feeding on low C:P algae (25.3% of body C day(-1)). The DOC excretion rate was also higher when animals were fed on P-low algae (13.4% of body C day(-1)) than on P-rich algae (5.7% of body C day(-1)). When corrected for respiratory losses, total assimilation of C did not differ significantly between treatments (around 60% of body C day(-1)). Judging from these experiments, D. magna can maintain its stoichiometric balance when feeding on unbalanced diets (high C:P) primarily by disposing of excess dietary C via respiration and excretion of DOC.
authors.affiliation: URBO--Laboratory of Freshwater Ecology, Facultés Universitaires Notre-Dame de la Paix, Rue de Bruxelles 61, 5000 Namur, Belgium.
authors.forename: François
authors.initials: F
authors.lastname: Darchambeau
authors.forename: Per J
authors.initials: PJ
authors.lastname: Faerøvig
authors.forename: Dag O
authors.initials: DO
authors.lastname: Hessen
mesh_terms: D000222:Adaptation, Physiological; D000818:Animals; D002244:Carbon / Q000378:metabolism*; D003621:Daphnia / Q000502:physiology*; D004032:Diet; D004734:Energy Metabolism; D056890:Eukaryota / Q000737:chemistry; D006706:Homeostasis; D010758:Phosphorus / Q000378:metabolism
chemical_list: D010758:Phosphorus; D002244:Carbon