Impact of diazotroph nitrogen on zooplankton nutrition in the ocean

由于未来的气候变化，预计固定氮的蓝藻存量将增加。了解浮游动物对氮的固定和氮固定（重氮营养态氮）在水体和更高营养水平的运输机制的耦合，对于正确评估蓝藻种群增加对海洋氮循环和海洋生产力的影响至关重要。与其他初级生产者相比，蓝藻被认为是食物链的死角，因为只有很少的浮游动物物种被发现直接以蓝藻为食。蓝藻对浮游动物产生抗性的原因包括其毒性、体积和营养质量差。据推测，在重氮营养氮作为铵或溶解的有机氮释放之前，蓝藻繁殖必须腐烂，并且必须通过微生物循环转移到浮游动物中，随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，浮游动物直接放牧的程度一定比以前认为的要高，最近化合物特定稳定同位素分析的结果首次表明，固定n2的蓝藻可能是浮游动物中氨基酸氮的定量重要来源。本项目旨在利用GC-C-IRMS、GC-MS和EA-IRMS等多种方法对北大西洋和墨西哥湾这两个蓝藻群落为主的海域进行直接放牧过程的研究，并探讨直接放牧的潜在调控因素。

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.