Impact of diazotroph nitrogen on zooplankton nutrition in the ocean

由于气候变化的前景，固定氮气的蓝藻现存量预计将增加。很好地理解氮固定和浮游动物固氮(重固氮)向较高营养层的氮传输机制的耦合，对于正确评估蓝藻数量增加对海洋氮循环和海洋生产力的影响具有重要意义。与其他初级生产者相比，蓝藻被认为是食物链的死胡同，因为只有很少的浮游动物物种被发现直接以蓝藻为食。蓝藻抵抗浮游动物放牧的原因包括其毒性、大小和营养质量差。据推测，蓝藻水华在重氮氮以氨或溶解的有机氮的形式释放之前必须腐烂，并必须通过微生物环路转移到浮游动物，然后进入更高的营养水平。然而，来自稳定同位素测量的数据表明，浮游动物直接捕食的程度必须比之前认为的更高，最新的化合物特定稳定同位素分析结果首次表明，固定氮的蓝藻可能是浮游动物氨基酸氮的一个重要数量来源。该项目的目的是利用多种GC-C-IRMS、GC-MS和EA-IRMS方法对热带北大西洋和墨西哥湾这两个由不同蓝藻群落主导的海域的直接放牧过程进行氮分析，并确定直接放牧的潜在调节因素。

N2-fixing cyanobacteria standing stocks are expected to increase due to prospect climate changes. A good comprehension for the coupling of N2-fixation and the transport mechanisms of nitrogen from N2-fixation (diazotroph nitrogen) by zooplankton in the water column and into higher trophic levels is of elemental importance to correctly assess the consequences of increasing cyanobacteria stocks for the marine nitrogen cycle and the productivity of the oceans. In contrast to other primary producers, cyanobacteria are regarded as dead end of the food-chain because only few zooplankton species were found to directly graze on cyanobacteria. The reasons for the resistance of cyanobacteria against zooplankton grazing include their toxicity, size and poor nutritional quality. It is assumed that cyanobacteria blooms have to decay before the diazotroph nitrogen is released as ammonium or dissolved organic nitrogen and has to be transferred through the microbial loop into zooplankton and subsequently into higher trophic levels. However, data from stable isotope measurements imply that direct grazing by zooplankton must occur to a higher extent than formerly thought and most recent results from compound specific stable isotope analysis show for the first time that N2-fixing cyanobacteria may be a quantitatively significant source for amino acid nitrogen in zooplankton. The aim of this project is to explore the process of direct grazing in two sea areas, the tropical North Atlantic and the Gulf of Mexico, which are dominated by different cyanobacteria communities using a multiple GC-C-IRMS, GC-MS und EA-IRMS approach for nitrogen analysis and to identify the underlying regulating factors of direct grazing.