Calcium influence on phosphorus regulation in Daphnia magna: implications for phosphorus cycling

Abstract

We examined the influence of environmental calcium on phosphorus biokinetics and regulation in Daphnia magna. When the Ca concentration in culturing media increased from 0.5 to 200 mg l -1 , the specific P content of D. magna feeding on P-sufficient algae decreased significantly from 1.43 to 1.05% of dry weight. There was a significant negative relationship between the specific Ca and P contents (% dry wt) of daphnids. However, measured biokinetic parameters, including the assimilation efficiency (AE) of dietary P, the weight-specific ingestion rate (IR), the uptake of inorganic P from water, and the efflux rate constant (k e ) of P, were similar over the wide range of Ca concentrations, with some exceptions at the lowest Ca level (0.5 mg l -1 ). Diet was the dominant source of P for daphnids, and only 1 to 2% of P was incorporated from water, based on the biokinetic calculation. With the elevation of the Ca level from 0.5 to 200 mg l -1 , the proportion of P lost through molting by D. magna feeding on P-sufficient diets increased 2.5-fold, whereas the dissolved release decreased 1.9-fold. P released into the dissolved phase is readily re-utilized by primary producers, while P contained within the shed molts sinks out of the epilimnion and becomes at least temporarily unavailable to primary producers; thus, the effects of Ca on molting and dissolved release have significant implications for P cycling in freshwater lakes. Our results suggest that the availability of P to primary producers can be reduced by sedimentation of P-rich daphnid molts in high-Ca lakes, and Ca concentration in the water needs to be considered in studying the stoichiometric regulation of P in freshwater cladocerans. KEY WORDS: Phosphorus regulation · Phosphorus cycling · Phosphorus biokinetics · Effects of calcium · Daphnia magna Resale or republication not permitted without written consent of the publisherThe relative importance of different routes of phosphorus loss from Daphnia magna (via excretion, reproduction and molting) is affected by the calcium level in the water. Photo: Ke Pan OPEN PEN ACCESS CCESSAquat Biol 5: [1][2][3][4][5][6][7][8][9][10][11] 2009 of P is distributed within the carapace, possibly bound with Ca. Sterner et al. (1993) found that D. obtusa on a P-deficient diet has problems in molting. Shedding molts accounts for ~70 and ~50% of P loss from juvenile and adult D. magna over a 3 to 4 d period, respectively (He & Wang 2007). The link between Ca and P in D. magna is also indicated by the crystalline calcium phosphate composition of the shell of its ephippia (Kawasaki et al. 2004). Ca levels in the water considerably affect the calcification of the carapace of daphnids (Alstad et al. 1999) and, thus, their total Ca content (Jeziorski & Yan 2006, Tan & Wang 2008. It is therefore tempting to ask whether the content and distribution of P in daphnids is also affected by the aqueous Ca level, considering the close association of these 2 elements in these organisms. Among the various freshwater zooplankton s...

Effects of calcium and phosphorus limitation on the nutritional ecophysiology ofDaphnia

“…Elemental limitation resulted in large changes in consumer body stoichiometry and growth rates in animals grown across nutrient gradients. Our results confirm previous daphnid studies showing changes in body Ca and P content in animals grown under low nutrient supplies (He and Wang ; Tan and Wang ). However, we found that dietary P was the most significant variable controlling daphnid elemental content, explaining the majority of variation in consumer body Ca : P ratios.…”

Interspecies differences in calcium content and requirement in four freshwater cladocerans explained by biokinetic parameters

“…The medium was renewed every 2 d. The green alga C. reinhardtii was added as food at the dose of 1.0-2.5 3 10 6 cells individual 21 d 21 , depending on the cladoceran species and life stage of the cladocerans. The food abundance level was considered enough to ensure the optimal growth of these four species because 10 6 cells individual 21 d 21 was enough for the healthy growth of a larger species, D. magna (Tan and Wang 2009a).…”

“…are recognized as having higher Ca content than the nondaphnid cladocerans (Waervå gen et al 2002;Jeziorski and Yan 2006), and counterintuitively, smaller Daphnia species have lower Ca content than larger sized daphnids, despite their higher surface-to-volume ratio (Waervå gen et al 2002). In Daphnia magna, the differences in Ca content between juveniles and adults or among populations cultured in different Ca environments were successfully explained by the physiologically based biokinetic parameters (e.g., influx rate, J, and efflux rate constant, k e , of Ca [Tan and Wang 2009b]). Specifically, a higher Ca content originated from a higher J or a lower k e of Ca, or both.…”

Carbon and phosphorus linkages in Daphnia growth are determined by growth rate, not species or diet

“…Body P content is not a static, species-specific trait. Instead, body P content is a property which varies as a function of the environment, maintenance costs, growth, C availability, food C : P and the degree of stoichiometric homeostasis (Kyle et al 2006;Shimizu & Urabe 2008;Tan & Wang 2009). The degree of stoichiometric homeostasis is defined as resistance to change of animal stoichiometry (e.g.…”