Interactions between limiting nutrients: Consequences for somatic and population growth of Daphnia magna

Abstract

To assess the significance of nutrient interactions on animal performance, life-history experiments were conducted with the freshwater herbivore Daphnia magna in a system with two limiting nutrients (i.e., sterols and polyunsaturated fatty acids [PUFAs]). By experimentally increasing the availability of dietary cholesterol either in the absence or in the presence of different dietary PUFAs we investigated whether dietary PUFAs differ in their potential to affect the sterol-limited growth kinetics of the herbivore. Sterol-limited growth responses of D. magna were characterized by a higher initial slope of the saturation curves and lower threshold concentrations in the presence of dietary PUFAs. This suggests significant interactions between limiting nutrients and lower dietary sterol requirements in the presence of dietary PUFAs. In contrast to somatic growth rates, which were significantly increased only by additional supplementation with eicosapentaenoic acid (EPA), population growth rates were significantly affected by additional supplementation with linoleic acid, a-linolenic acid, docosahexaenoic acid (DHA), and in particular EPA and arachidonic acid. The fatty acid composition of D. magna reflected that of their diet. However, animals fed the DHA supplemented diet contained considerable amounts of EPA, indicating a retro-conversion of DHA into EPA within their metabolism. The fitness of resource-limited consumers is strongly affected by complex interactions between limiting nutrients and this topic has to be considered in experimental and theoretical studies investigating nutrient-limited growth responses of animals to accurately assess consequences for population dynamics.

Dietary lipid quality affects temperature-mediated reaction norms of a freshwater key herbivore

“…The tendency of DHA to accumulate in winter-active poikilotherms suggests that this PUFA plays a crucial role in the adaptation to low temperatures (Hazel and Williams 1990;Smyntek et al 2008). However, as dietary DHA is effectively converted to EPA in Daphnia (von Elert 2002;Martin-Creuzburg et al 2010), the positive effect of DHA supplementation on population growth rates is presumably the result of an increased availability of EPA.…”

Growth Rate Hypothesis does not apply across colimiting conditions: cholesterol limitation affects phosphorus homoeostasis of an aquatic herbivore

“…attenuates the decline in body P content under P‐limiting conditions. Recent laboratory studies showed the colimitation of herbivore growth by P and PUFA (Becker & Boersma 2003) or sterols and PUFA (Martin‐Creuzburg, Sperfeld & Wacker 2009; Martin‐Creuzburg, Wacker & Basen 2010), but data on organism’s stoichiometry are lacking. Sterols are essential food components for herbivorous arthropods (Behmer & Nes 2003; Von Elert, Martin‐Creuzburg & Le Coz 2003), which have to metabolize dietary phytosterols into cholesterol (Svoboda & Thompson 1985).…”

Compound‐specific δ13C analyses reveal sterol metabolic constraints in an aquatic invertebrate

“…The sterols were analyzed by gas chromatography (GC) using a HP 6890 gas chromatograph (Agilent Technologies, Santa Clara, CA, USA) equipped with a flame ionization detector (FID) and a HP‐5 capillary column (Agilent Technologies). Details of the GC settings are given elsewhere . The sterols were quantified by comparison with the internal standard 5α‐cholestane using multipoint calibration curves generated for different sterols.…”