The flow of energy in aquatic food webs is often constrained by the low transfer efficiency of organic carbon to higher trophic levels. In particular, at the phytoplankton—zooplankton interface the predominance of nutritional inadequate food sources can lead to a decoupling of primary and secondary production. Nutritional inadequacy can be attributed to numerous factors, such as toxicity or morphological properties that prevent ingestion or digestion. Furthermore, the absence of essential elemental and biochemical nutrients, such as polyunsaturated fatty acids (PUFAs) and sterols, has been shown to constrain growth and reproduction of herbivorous consumers .
Our research is focused primarily on understanding the ecological significance of essential biochemicals in aquatic food webs. We investigate the trophic transfer of these nutrients from primary producers to higher trophic levels, explore life history consequences of dietary deficiencies using various consumers, and try to assess potential effects on food web processes.
PUFA are indispensable structural components of cell mebranes and are signifcantly involved in regulating important membrane properties, such as membrane fluidity and permeability. Restructuring the lipid composition of biological membranes in response to changing temperatures is a major strategy by which ectotherms maintain vital physiological functions of cell membranes. Exposure to cold temperatures can cause a significant increase in the concentration of long chain PUFA in body tissue of Daphnia. Recently, we have shown that an adequate dietary supply with long chain PUFA is important particularly in the cold, implying that Daphnia exposed to cold temperatures have higher dietary PUFA requirements (Martin-Creuzburg et al. 2012). This example suggests that the significance of dietary PUFA for animal performance depends on the prevailing environmental conditions.
Besides their significance for membrane properties, PUFA serve as precursors for eicosanoids, locally acting signaling molecules, which in both vertebrates and invertebrates are known to be important mediators in reproduction and in the immune systems. Animals are incapable of synthesizing long chain PUFA, such as eicosapentaenoic acid (EPA, 20:5n-3) and arachidonic acid (ARA, 20:4n-6), from low-molecular-weight precursors and thus rely on an adequate dietary supply with these essential nutrients to cover their physiological demands. It has been shown repeatedly that the dietary supply with EPA and ARA primarily supports reproduction of Daphnia (e.g. Martin-Creuzburg et al. 2009, 2010) and recently it has been suggested that the resistance of Daphnia to pathogenic infections is also affected by the dietary PUFA supply (Schlotz et al. 2013). Considering that genes involved in eicosanoid signaling, such as the cyclooxygenase gene (Cox), coding for a central enzyme in the eicosanoid pathway, are highly responsive to the dietary PUFA supply (Schlotz et al. 2012), we propose that the effects of dietary PUFA on reproduction and immunity of Daphnia are regulated via the action of eicosanoids. However, further studies are required to understand the influence of dietary PUFA on the eicosanoid signaling pathway.
It is generally accepted that all arthropods are incapable of synthesizing sterols de novo. Moreover, there is evidence that also nematodes, rotifers, and bivalves are either incapable or at least have limited capacities to synthesize sterols and thus rely on an adequate dietary sterol supply (e.g. Wacker & Martin-Creuzburg 2012; Basen et al. 2012). Sterols are indispensable structural components of cell membranes, serve as precursors for steroid hormones, such as the molt-inducing ecdysteroids (Martin-Creuzburg et al. 2007), and are required for developmental processes. A deficiency in dietary sterols severely constrains various life history traits of Daphnia (Martin-Creuzburg et al. 2005). Cyanobacteria and heterotrophic bacteria, in contrast to eukaryotic algae, generally do not contain sterols. Thus, a limitation of aquatic consumers by sterols is most likely when the seston is dominated by prokaryotic food sources (Martin-Creuzburg et al. 2008, 2011).
Cholesterol is the predominant sterol in most animals. Thus, unlike carnivorous species, the herbivore Daphnia cannot rely on a dietary source of cholesterol since this sterol is often hardly represented in plant material. Instead, plants and algae contain several types of phytosterols that differ from cholesterol by additional substituents or by the positions and/or number of double bonds in the side chain or in the sterol nucleus. Herbivores can either use the sterols present in their diet directly or they have to metabolize them to cholesterol to meet the requirements for growth and development. By supplementing different types of phytosterols to a sterol-free diet we have shown that phytosterols differ in their suitability to support growth and reproduction of Daphnia and that not all phytosterols are suitable precursors for cholesterol (Martin-Creuzburg and Von Elert 2004; Martin-Creuzburg et al. 2014).
To determine the role of essential lipids in Daphnia nutrition, we use a variety of analytical methods, such as gas chromatography (GC), gas chromatography—mass spectrometry (GC-MS), and high performance liquid chromatography (HPLC).