Collaborative Research: Iron and phosphorus balanced limitation of nitrogen fixation in the oligotrophic ocean

海洋蓝藻能够利用或“固定”大气中的氮气，因此提供了许多支持开放海洋食物链的基本营养氮。海洋学家通常认为，在任何特定的时间和地点，这些固氮蓝藻的生长都受到铁或磷供应的限制。然而，初步实验表明，当铁和磷同时缺乏时，这些固氮菌生长得最好。在这个项目中，研究人员将使用专门针对铁和磷限制的细胞指示器来确定这种类型的固氮蓝藻在控制热带大西洋海洋食物链的生产力方面有多重要。两名研究生将在南加州大学(USC)和伍兹霍尔海洋研究所接受培训，以及一名南加州大学的博士后研究员。教育外展工作将在洛杉矶市中心的一所高中进行，学生群体中超过98%的西班牙裔和非裔美国人，以及南加州大学全球环境微生物学课程中代表性不足的本科生。此外，本科生的两项研究经验将在暑期研究项目中得到指导，以帮助他们了解科学职业选择。研究人员将使用实验室和野外工作两种方法，研究这种独特的铁/磷平衡限制表型的生物学和生物地球化学后果。在这个项目的第一年，固氮蓝藻将在铁和/或磷限制下培养，然后应用蛋白质组学和转录组学来鉴定那些可能是铁/磷平衡限制的潜在诊断生物标志物的基因。初步工作已经在一种蓝藻中确定了一个有希望的候选生物标记物，即一个似乎与平衡限制下细胞大小减小有关的Ezra蛋白结构域，研究人员还确定了许多其他类似生物标记物的潜在候选者。在第二年，这些新的共限制生物标志物和其他先前验证的铁限制(ISIB)和磷限制(SphX)将被用于研究从相对高铁、低磷的北大西洋水域到相对高磷、低铁的南大西洋的研究巡航期间的平衡限制。这个实地考察部分将调查跨越自然铁/磷梯度的固氮蓝藻种群，以寻找平衡限制的遗传、蛋白质组和生理指标，并在船上培养实验中测试它们对铁和磷处理的反应。第三年将专门用于样本分析和出版物，探索海洋固氮剂对铁和磷同时限制的反应。

Marine cyanobacteria are able to use or "fix" atmospheric nitrogen gas, and so supply much of the essential nutrient nitrogen that supports open ocean food chains. Oceanographers have usually thought that the growth of these nitrogen-fixing cyanobacteria is limited at any particular time and place by the supply of either iron, or of phosphorus. Preliminary experiments have shown, though, that these nitrogen fixers instead grow best when both iron and phosphorus are scarce at the same time. In this project, the researchers will use cellular indicators that are specific for iron and phosphorus limitation to determine how important this type of "balanced limitation" of nitrogen-fixing cyanobacteria is in controlling the productivity of ocean food chains in the tropical Atlantic Ocean. Two graduate students will be trained at the University of Southern California (USC) and Woods Hole Oceanographic Institution, as well as a postdoctoral researcher at USC. Educational outreach efforts will take place at a Los Angeles inner city high school with a student body that is over 98% Hispanic and African-American, and with underrepresented undergraduates in the USC Global Environmental Microbiology course. In addition, two Research Experiences for Undergraduates students will be supervised for summer research projects to help them learn about science career options. The researchers will investigate the biological and biogeochemical consequences of this unique balanced iron/phosphorus-limited phenotype, using both laboratory and fieldwork approaches. During the first year of this project, the nitrogen-fixing cyanobacteria will be cultured under iron and/or phosphorus limitation, followed by application of proteomics and transcriptomics to identify genes that are potential diagnostic biomarkers for iron/phosphorus balanced limitation. Preliminary work has already identified one promising candidate biomarker in one cyanobacterium, an EzrA protein domain that appears to be associated with the cell size decreases seen specifically under balanced limitation, and the researchers have identified numerous other potential candidates for similar biomarkers. During the second year, these new co-limitation biomarkers and others previously validated for iron limitation (IsiB) and phosphorus limitation (SphX) will be used to investigate balanced limitation during a research cruise transecting from relatively high-iron, low-phosphorus North Atlantic waters, to the relatively high-phosphorus, low-iron South Atlantic. This fieldwork component will survey nitrogen fixing cyanobacteria populations across this natural iron/phosphorus gradient for genetic, proteomic, and physiological indicators of balanced limitation, as well as testing their responses to iron and phosphorus manipulations in shipboard incubation experiments. The third year will be devoted to sample analysis, and publications exploring the responses of oceanic nitrogen fixers to simultaneous limitation by both iron and phosphorus.