Nitrogen isotope ratios and fatty acid composition as indicators of animal diets in belowground systems
Nitrogen isotope ratios and fatty acid composition as indicators of animal diets in belowground systems
复制标题
氮同位素比和脂肪酸组成作为地下系统动物饮食的指标
作者：
Liliane Ruess;M. Häggblom;R. Langel;S. Scheu
本研究通过微观实验分析了土壤真菌、微生物和中微生物之间的营养相互作用。研究了15N和脂肪酸（FAs）在两种（真菌和食草动物）和三种（真菌、线虫和线虫）食物链中的营养转移。与普遍接受的营养级联中15N富集的假设相反，实验显示，与饮食相比，消费者的15N富集、消耗或没有变化。造成这一现象的原因可能是：(1)氨系线虫的主要代谢途径与重同位素相似或贫化，而尿系线虫主要富集重同位素；(2)高蛋白饲粮（如捕食者）对15N有较高的偏移；(3)劣质食品改变15N分馏的补偿。脂质组成分析表明，磷脂一般不受影响，中性脂与食物来源的FA模式密切相关。膳食中脂肪酸转化为中性脂质发生了，宿主和消费者资料中相应的脂肪酸频率证明了这一点。此外，几种FAs仅在食物源中存在时才在食草动物中检测到。油酸表现出从真菌到线虫再到弹线虫的三个营养水平的转变。饮食中FAs的同化导致中性脂质谱更加多样化，即食物链中较高的动物比食物链中较低的动物含有更多的单个FAs。结果表明，单组C18和单组C20脂肪酸具有作为地下系统取食策略生物指示工具的潜力。我们认为，初级消费者在其中性脂质谱中没有或只有微量的单烯C20酸，而以真核生物为食的消费者将显示出相当高的频率。
This study analyses trophic interactions between soil fungi, micro- and mesofauna in microcosm experiments. The trophic shift of 15N and fatty acids (FAs) was investigated in different food chains, which comprised either two (fungi and grazers) or three (fungi, nematodes and Collembola) levels. Contrary to the widely accepted assumption of 15N enrichment in trophic cascades the experiments revealed enrichment, depletion or no change in 15N of consumers compared to their diet. Factors responsible for this pattern were suggested to be: (1) the main metabolic pathway used for N excretion in ammonotelic nematodes to be similar or depleted in the heavier isotope, and uricotelic Collembola mostly enriched in the heavier isotope; (2) a higher shift in 15N with a high-protein diet (e.g. for predators); (3) compensation due to low-quality food altering the fractionation of 15N. Analysis of the lipid composition showed phospholipids to be generally unaffected and neutral lipids closely related to the FA pattern of the food source. Dietary routing of FAs into neutral lipids occurred, as evidenced by corresponding frequencies of FAs in host and consumer profiles. Additionally, several FAs were only detected in the grazer when present in the food source. Oleic acid showed a shift over three trophic levels, from fungi to nematodes to Collembola. The assimilation of dietary FAs resulted in a more diverse neutral lipid profile, i.e. animals higher in the food chain contained more individual FAs compared to animals lower in the food chain. The results indicate that monoenoic C18 and monoenoic C20 FAs have the potential to act as tools for the bioindication of feeding strategies in belowground systems. We suggest that primary consumers will have no or only trace amounts of monoenoic C20 acids in their neutral lipid profile, whereas consumers feeding on a eukaryote diet will show a considerably higher frequency.