title: Does excess dietary carbon affect respiration of Daphnia?
abstract: Like many invertebrate herbivores, Daphnia frequently face diets with excess carbon (C) relative to elements like phosphorus (P), and with limited ability to store C-rich compounds. To cope with this relative surplus of C they may either regulate the net uptake of C or dispose of excess assimilated C via increased release of dissolved organic carbon or CO(2). Here we investigate whether juvenile Daphnia magna use respiration as a means of stoichiometrically regulating excess C. Growth rate and respiration were measured under different algal food qualities (P-replete and P-depleted algae). Growth rate was strongly reduced by P-depleted food, implying a stoichiometric disposal of excess ingested C. Respiration rates of feeding animals were measured after short- (0.5 h), medium- (12 h) and long- (five days) term acclimation to P-limited food. The respiration rates of animals during active feeding were not affected by the acclimation period per se, whereas food quality had a significant effect; respiration rates of feeding animals increased slightly in individuals receiving low-P food under all acclimation regimes. Respiration was also measured on nonfeeding and fasting animals that had been acclimated for five days to P-limited food. Respiration rates of these animals were strongly affected by feeding conditions but not by food quality; feeding individuals had higher respiration rates than those deprived of food, which again had higher respiration than fasting animals. Although animals grown on low-P food had strongly reduced growth and thus were expected to have decreased respiration rates due to reduced growth-related costs, this seems to be canceled out by increased stoichiometric respiration under P-deficiency. These results indicate that D. magna partly releases excess C as CO(2), but other means of stoichiometric regulation most likely add to this.
authors.affiliation: Department of Biology, University of Oslo, P.O. Box 1066, Blindern, 0316 Oslo, Norway. t.c.jensen@bio.uio.no
authors.forename: Thomas C
authors.initials: TC
authors.lastname: Jensen
authors.forename: Dag O
authors.initials: DO
authors.lastname: Hessen
mesh_terms: D000818:Animals; D002244:Carbon / Q000378:metabolism*; D000460:Chlorophyta; D003621:Daphnia / Q000254:growth & development / Q000378:metabolism*; D004032:Diet; D004734:Energy Metabolism; D005508:Food Deprivation; D010101:Oxygen Consumption; D013997:Time Factors
chemical_list: D002244:Carbon