The influence of diet on the distribution of nitrogen isotopes in animals was investigated by analyzing animals grown in the laboratory on diets of constant nitrogen isotopic composition. 
The isotopic composition of the nitrogen in an animal reflects the nitrogen isotopic composition of its diet. The δ^(15)N values of the whole bodies of animals are usually more positive than those of their diets. Different individuals of a species raised on the same diet can have significantly different δ^(15)N values. The variability of the relationship between the δ^(15)N values of animals and their diets is greater for different species raised on the same diet than for the same species raised on different diets. Different tissues of mice are also enriched in ^(15)N relative to the diet, with the difference between the δ^(15)N values of a tissue and the diet depending on both the kind of tissue and the diet involved. The δ^(15)N values of collagen and chitin, biochemical components that are often preserved in fossil animal remains, are also related to the δ^(15)N value of the diet. 
The dependence of the δ^(15)N values of whole animals and their tissues and biochemical components on the δ^(15)N value of diet indicates that the isotopic composition of animal nitrogen can be used to obtain information about an animal's diet if its potential food sources had different δ^(15)N values. The nitrogen isotopic method of dietary analysis probably can be used to estimate the relative use of legumes vs non-legumes or of aquatic vs terrestrial organisms as food sources for extant and fossil animals. However, the method probably will not be applicable in those modern ecosystems in which the use of chemical fertilizers has influenced the distribution of nitrogen isotopes in food sources. 
The isotopic method of dietary analysis was used to reconstruct changes in the diet of the human population that occupied the Tehuacan Valley of Mexico over a 7000 yr span. Variations in the δ^(15)C and δ^(15)N values of bone collagen suggest that C_4 and/or CAM plants (presumably mostly corn) and legumes (presumably mostly beans) were introduced into the diet much earlier than suggested by conventional archaeological analysis.

Influence of Diet On the Distribtion of Nitrogen Isotopes in Animals

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Modern Applied Statistics With S

Paleophysiology and End-Permian Mass Extinction

Ecotoxicological effects of selected cyanobacterial secondary metabolites: a short review.

The egg the exoskeleton growth and moulting the epidermis musculature the nervous system the pseudocoelom the secretory-excretory system the digestive system the reproductive system nematode pathology.

The Structure of Nematodes

A key process in freshwater plankton food webs is the regulation of the efficiency of energy and material transfer. Cyanobacterial carbon (C) in particular is transferred very inefficiently to herbivorous zooplankton, which leads to a decoupling of primary and secondary production and the accumulation of cyanobacterial biomass, which is associated with reduced recreational quality of water bodies and hazards to human health. A recent correlative field study suggested that the low transfer efficiency of cyanobacterial C is the result of the absence of long-chain polyunsaturated fatty acids (PUFA) in the diet of the zooplankton. By supplementation of single-lipid compounds in controlled growth experiments, we show here that the low C transfer efficiency of coccal and filamentous cyanobacteria to the keystone herbivore Daphnia is caused by the low sterol content in cyanobacteria, which constrains cholesterol synthesis and thereby growth and reproduction of the herbivore. Estimations of sterol requirement in Daphnia suggest that, when cyanobacteria comprise more than 80% of the grazed phytoplankton, growth of the herbivore may be limited by sterols and Daphnia may subsequently fail to control phytoplankton biomass. Dietary sterols therefore may play a key role in freshwater food webs and in the control of water quality in lakes dominated by cyanobacteria.

/pdf/absence-of-sterols-constrains-carbon-transfer-between-4383bbhc4w.pdf

Absence of sterols constrains carbon transfer between cyanobacteria and a freshwater herbivore (Daphnia galeata)

In past decades, a considerable amount of research has been conducted to elucidate the factors that affect the carbon transfer across the cyanobacteria-Daphnia interface. It is well accepted that cyanobacteria are a nutritionally inadequate food source for cladocerans, but the underlying mechanisms are still controversial. Morphological properties, toxicity, and the absence of essential lipids, i.e., polyunsaturated fatty acids (PUFAs) and sterols, are discussed as the most important factors that account for this nutritional inadequacy. Here, we conducted standardized growth experiments with the herbivore Daphnia magna feeding on coccal, filamentous, and putatively toxic cyanobacterial strains comprising the genera Synechococcus, Anabaena, Aphanizomenon, and Microcystis and on a cyanobacterial mixture containing strains of these genera. The relative importance of PUFAs and sterols in determining the food quality of cyanobacteria for Daphnia was assessed by supplementation of eicosapentaenoic acid- and/or cholesterol-containing liposomes to the cyanobacterial carbon. We provide evidence that somatic growth of daphnids on coccal as well as on filamentous cyanobacteria is primarily constrained by the absence of sterols, provided that the cyanobacterial carbon is readily ingested and nontoxic. The absence of PUFAs in cyanobacteria appears to be of minor importance for somatic growth but potentially affects egg production in Daphnia. Thus, the absence of sterols has to be considered a major food-quality constraint that potentially affects the efficiency of carbon transfer across the cyanobacteria-Daphnia interface.

https://aslopubs.onlinelibrary.wiley.com/doi/pdf/10.4319/lo.2008.53.2.0456

Nutritional constraints at the cyanobacteria—Daphnia magna interface: The role of sterols

1. In nature, food conditions change temporally and force consumers into trade-offs during resource allocation. In particular, under poor food conditions, for example during cyanobacterial blooms, herbivores have to optimize their resource allocation to maximize fitness, and face two decisions: (i) an individual might attempt to allocate acquired essential resources to reproductive tissues or use them for its own maintenance; and (ii) an individual might decide to optimize the chemical quality of its eggs. 2. As cyanobacteria feature a deficiency in some essential lipids that leads to a decline in the growth and fecundity of Daphnia, an important freshwater herbivore, we investigated Daphnia magna's Strauss allocation of lipids such as polyunsaturated fatty acids (PUFAs) and cholesterol during an experimental non-toxic cyanobacterial bloom. 3. Generally, we found a substantial maternal investment of the particularly important omega-3 (n-3) PUFAs, in particular eicosapentaenoic acid (EPA), into the eggs. The concentration of EPA was 2·4-fold higher in eggs than in somatic tissue, a cumulative effect, which was not significantly changed under cyanobacterial food conditions. 4. Under poor conditions, D. magna not only decreased the number of eggs produced but, in principle, reduced the previously high concentrations of EPA in both eggs and somatic tissues to a similar degree. In contrast to EPA, the concentrations of α-linolenic acid and cholesterol, although lower than EPA, were more homeostatic in eggs than in somatic tissues, in which concentrations decreased. 5. When food quality was improved, D. magna were able to recover completely the fatty acid concentrations in their somatic tissues and eggs. 6. This study shows that the content of particular lipids in its food clearly affects resource allocation in D. magna, and suggests that cholesterol is important for somatic growth, while PUFAs are primarily needed for reproduction. As a decreasing investment of essential lipids into eggs implies a reduced fitness of the animals' progeny under poor food conditions, this could have a strong impact on population dynamics, which might also be valid for other species.

/pdf/allocation-of-essential-lipids-in-daphnia-magna-during-4qp4mkhp1m.pdf

Allocation of essential lipids in Daphnia magna during exposure to poor food quality

The absence of essential biochemical nutrients, such as polyunsaturated fatty acids or sterols, has been considered as a mechanism determining trophic interactions between the herbivore Daphnia and its phytoplankton food source. Here, we experimentally quantify the sensitivity of two Daphnia species to decreasing amounts of dietary sterols by measuring variations in life history traits. The two species Daphnia magna and D. galeata were fed different mixtures of the sterol-containing green alga Scenedesmus obliquus and the sterol-free cyanobacterium Synechococcus elongatus; a higher proportion of Synechococcus in the food is equivalent to a decrease in dietary sterols. To address the significance of sterol limitation, the Daphnia species were also fed Synechococcus supplemented with cholesterol. In both species, somatic and population growth rates, maternal dry mass, the number of viable offspring, and the probability of survival were significantly reduced with the lower availability of sterols. A high correlation between the sterol content of the mixed diet and the somatic and population growth rates was found, and growth on cholesterol-supplemented Synechococcus fitted well into this correlation. Somatic growth of first-clutch neonates grown on 100% Synechococcus exhibited a pattern similar to that of somatic growth of their mothers grown on the different food regimes, which demonstrated the significance of maternal effects for sterol-limited population growth. Daphnia galeata had a twofold higher incipient limiting sterol level than D. magna, which indicated interspecific differences in sterol requirements between the two Daphnia species. The results suggest a strong impact of dietary sterols on life history traits and therefore, population dynamics of the keystone species Daphnia.

/pdf/life-history-consequences-of-sterol-availability-in-the-5ffk28bbbn.pdf

Life history consequences of sterol availability in the aquatic keystone species Daphnia.

Empirical data providing evidence for a colimitation of an herbivore by two or more essential nutrients are scarce, particularly in regard to biochemical resources. Here, a graphical model is presented, which describes the growth of an herbivore in a system with two potentially limiting resources. To verify this model, life-history experiments were conducted with the herbivore Daphnia magna feeding on the picocyanobacterium Synechococcus elongatus, which was supplemented with increasing amounts of cholesterol either in the presence or the absence of saturating amounts of eicosapentaenoic acid (EPA). For comparison, D. magna was raised on diets containing different proportions of S. elongatus and the cholesterol- and EPA-rich eukaryotic alga Nannochloropsis limnetica. Somatic and population growth of D. magna on a sterol- and EPA-deficient diet was initially constrained by the absence of sterols. With increased sterol availability, a colimitation by EPA became apparent and when the sterol requirements were met, the growth-limiting factor was shifted from a limitation by sterols to a limitation by EPA. These data imply that herbivores are frequently limited by two or more essential nutrients simultaneously. Hence, the concept of colimitation has to be incorporated into models assessing nutrient-limited growth kinetics of herbivores to accurately predict demographic changes and population dynamics.

/pdf/colimitation-of-a-freshwater-herbivore-by-sterols-and-eih6bjcf5z.pdf

Dominik Martin-Creuzburg

Papers

Absence of sterols constrains carbon transfer between cyanobacteria and a freshwater herbivore (Daphnia galeata)

Nutritional constraints at the cyanobacteria—Daphnia magna interface: The role of sterols

Allocation of essential lipids in Daphnia magna during exposure to poor food quality

Life history consequences of sterol availability in the aquatic keystone species Daphnia.

Colimitation of a freshwater herbivore by sterols and polyunsaturated fatty acids.