Litter quality and stream food webs: a new paradigm for understanding interactions between microbes and invertebrates.

秋天落入溪流和河流中的树叶可以为生活在树叶中和周围的生物提供重要的食物资源。当树叶进入水中时，细菌和真菌会迅速侵占它们并开始分解它们。树叶和微生物定居者为生活在水中的昆虫幼虫提供食物，而昆虫幼虫反过来又为鱼类提供食物，并最终在羽化后为鸟类、蝙蝠和蜥蜴提供食物。这项研究将测试不同树木的叶子如何影响溪流和河流生态系统的食物链。研究人员将评估不同树种的树叶的命运，以确定这些树叶是主要被微生物分解者转化为二氧化碳，还是主要是昆虫幼虫的食物来源。科学家们一直认为，分解较快的树叶是昆虫更好的食物来源，这项研究挑战了这一假设，因为它们的特殊化学成分表明，分解较慢的树叶是昆虫更好的食物来源。这项研究还将确定哪些微生物在特定的叶片类型上定植，以及叶片类型和它们的微生物定殖者如何影响吃它们的昆虫的营养质量。了解不同的树种如何影响生活在溪流中和附近的动物，将指导管理者种植最有益的树木。研究人员还将研究溪流的温度和水质如何影响昆虫如何利用树叶。这可以帮助土地管理人员了解某些溪流条件是否会使树叶成为更好的昆虫资源。该研究项目将培训本科生和研究生，并与高中教师和学生合作。研究结果将与致力于改善河流健康的政府和非政府组织分享。研究人员将使用多种最先进的技术来测试不同树叶物种如何向微生物和昆虫转移能量和营养。首先，研究人员将在温室里种植12种不同的植物，使用13C和15N来标记将被喂养给昆虫的叶子。在树叶衰老后，研究人员将收集树叶，并在亚利桑那州和新罕布夏州的溪流中孵化它们，利用稳定的同位素特征追踪树叶碳和氮向微生物和昆虫的转移。吃掉溪流中树叶的昆虫将被带回实验室，在那里它们将被用同位素比质谱仪进行分析，以准确地测量树叶中有多少碳和氮最终进入了它们的组织。其次，研究人员将使用稳定同位素探测和DNA测序来研究哪些微生物是昆虫的良好食物来源，哪些微生物通过迅速耗尽储存在树叶中的碳和氮与昆虫竞争。第三，科学家将构建溪流中胚层来控制温度和水质，以测试溪流条件如何影响树叶的营养质量。

Leaves that fall into streams and rivers in autumn can provide an important food resource for organisms that live in and around them. When leaves enter the water, bacteria and fungi rapidly colonize them and begin to decompose them. The leaves and microbial colonizers provide food for insect larvae that live in the water, which in turn, provide food for fish, and ultimately upon emergence, birds, bats and lizards. This study will test how leaves from different trees impact the food webs of stream and river ecosystems. The researchers will assess the fate of leaves from different tree species to determine whether the leaves are primarily converted into carbon dioxide by the microbial decomposers or whether the leaves are primarily a food resource for insect larvae. Scientists have assumed that leaves that decompose quickly are a better food source for insects and this research challenges that assumption by suggesting that leaves that decompose more slowly, because of their particular chemical composition, are a better food resource for insects. The research will also determine which microbes colonize specific leaf types and how leaf types along with their microbial colonizers affect the nutritional quality for insects that eat them. Understanding how different tree species affect the animals living in and near streams will guide managers to plant trees that are most beneficial. The researchers will also study how the temperature and water quality of streams affects how insects use leaves. This could help land managers understand if certain stream conditions make leaves a better resource for insects.  This research project will train undergraduate and graduate students and work with high school teachers and students. Results will be shared with the governmental and non-governmental organizations that are working to improve stream health.The researchers will use multiple state-of-the-art techniques to test how different leaf species transfer energy and nutrients to microbes and insects. First, investigators will grow twelve different plant species in the green house using 13C and 15N to label the leaves that will be feed to insects. After the leaf senescence, the researchers will collect the leaves and incubate them in streams in Arizona and New Hampshire, using the stable isotopic signatures to trace the transfer of leaf carbon and nitrogen to microbes and insects. Insects that eat the leaves in the stream will be brought back to the laboratory where they will be analyzed with an isotope ratio mass spectrometer to measure exactly how much of the leaf carbon and nitrogen ends up in their tissues. Second, the researchers will use stable isotope probing and DNA sequencing to study which microbes are a good food source for insects versus which microbes compete with insects by quickly using up the carbon and nitrogen stored in leaves. Third, scientists will construct stream mesocosms to control temperature and water quality to test how stream conditions affect the nutritional quality of leaves.