EAGER: Nitrogen fixing bacterial endosymbioses in aboveground conifer tissue

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An important building block of DNA and proteins, nitrogen (N) often limits plant growth in natural ecosystems. Still, the source of N to northern conifer forests is a long-standing ecological mystery. In these ecosystems, N accumulation in the soil and plants is much higher than expected given the known N input sources. Bacteria called endophytes, which live inside plant tissue, may provide part of the explanation. Although some forest conifers can grow in extremely nitrogen limited soil, they are not generally believed to cooperate with bacteria that can reduce, or 'fix' atmospheric N2. This project is designed to determine if endophytic bactera fix N in the aboveground tissues of forest pines. To do this, the investigators will use a method called Nanometer-Scale Secondary Ion Mass Spectrometer (NanoSIMS), an instrument suited to measure, visualize and quantify the distribution of elements and their stable isotopes within cells. The investigators will expose pine needles to the stable isotope 15N, which only makes up a fraction of naturally occurring N. NanoSIMS will then be used to visualize endophytic bacterial cells that have taken up 15N, as well as nearby plant cells containing 15N. The researchers expect this project to provide the first quantitative and direct demonstration that bacterial endophytes provision host plants with fixed atmospheric N, and identify a potential source for missing N inputs to forests. The amount of N available to forest trees affects how much carbon is stored on land and how much remains in the atmosphere, therefore, discovery of a new forest N input pathway will improve our ability to predict future climate-change. The project will provide scientific training to a diverse group of students at UC Merced, a university with a large percentage of students from underrepresented backgrounds, and to K-8 students in a rural, low-income area of California.