Role of the Diatom Mitochondria in Cellular Nitrogen Metabolism

目前的估计表明，地球上每年光合作用产生的有机物质约有一半发生在海洋环境中水柱的透光层，使海洋成为全球碳循环的一个主要组成部分。现代海洋中几乎所有的水华形成浮游植物，包括硅藻，都是叶绿素c（Chl c）谱系的成员。现在公认的是，含叶绿素c的藻类有着共同的祖先，其中真核生物包裹并驯化了红藻，将来自两种生物的生化实体结合在一起。这一谱系随后的多样化产生了生态上占主导地位的浮游植物的惊人多样性。最终，海洋生态系统中的碳输出是由浮游植物对氮的同化作用控制和平衡的。在海洋浮游植物中，硅藻往往是最敏感的混合事件，其中充满营养的水被注入到阳光照射的真光区，但这种竞争优势的细胞基础知之甚少。关于光合真核生物中氮代谢的全球知识状态主要基于来自一些充分研究的模式绿色藻类和维管植物的生物化学和基因组序列数据。基因组序列，生理和生化研究，基因和蛋白质表达数据表明，硅藻和其他叶绿素藻类利用各种生化成分以前只观察到后生动物或细菌。例如，一个完整的后生动物样尿素循环似乎与细菌和红色或绿色藻类起源的基因以功能扩展和迄今未观察到的方式协同作用。在后生动物中，尿素循环具有与细胞解毒和输出固定氮相关的功能。尿素降解酶的存在强烈表明硅藻的另一种功能。这项研究正在解决的假设是，在海洋硅藻，这是经常受到氮限制，尿素循环和相关的线粒体酶功能作为无机碳和氮循环和重新包装枢纽;最终用于将分解代谢衍生的NH 4+重新分配为精氨酸和其它非氨基酸氮化合物。基于硅藻反向遗传学的最新进展和其他最近开发的基因组研究资源，关于线粒体在海洋硅藻氮同化和代谢中的作用的基本假设正在评估中。更广泛的影响人为的沿海有机和无机氮负荷预计将增加，气候变化-据预测，海洋混合层深度和混合频率的变化将改变营养物质向真光层的输送。因此，一个关键的研究重点是促进预测硅藻和其他叶绿素藻类将如何应对不断变化的频率和强度的营养物质输送事件。这项研究将有助于更准确地描述海藻中的氮代谢，这对预测生态系统建模至关重要。作为研究项目的一部分，将招募圣地亚哥埃斯孔迪多联合高中学区的一名高中生物教师从事与拟议研究相关的特定主题。在完成他/她的带薪实习后，与PI合作，教师将设计与海洋基因组学的快速新兴领域相关的课堂活动和课程设置，还包括海洋生物地球化学的相关主题，供下一学年使用。

Current estimates indicate that approximately half of the annual photosynthetic production of organic matter on Earth takes place in the euphotic zone of the water column in marine environments, making the ocean a major component of the global carbon cycle. Nearly all of the bloom-forming phytoplankton in the modern ocean, including the diatoms, are members of the chlorophyll c (Chl c) lineage. It is now well accepted that chlorophyll c-containing algae share a common ancestry where a eukaryote enveloped and domesticated a red algae, incorporating biochemical entities from both organisms. The subsequent diversification of this lineage has yielded an astounding diversity of ecologically dominant phytoplankton. Ultimately carbon export in marine ecosystems is governed and balanced by assimilation of nitrogen by phytoplankton. Among marine phytoplankton, diatoms are often the most responsive to mixing events where nutrient laden water is injected into the sunlit euphotic zone, yet the cellular basis for this competitive advantage is poorly understood. The global state-of-knowledge concerning nitrogen metabolism in photosynthetic eukaryotes is largely based on biochemistry and genome sequence data from a few well-studied model green algae and vascular plants. Genome sequences, physiological and biochemical studies, and gene and protein expression data indicate that diatoms and other Chl c algae utilize a variety of biochemical components previously only observed in metazoans or bacteria. A complete metazoan-like urea cycle, for example, appears to function in concert with genes of bacterial and red or green algal origin in a functionally expanded and heretofore unobserved fashion. In metazoans the urea cycle has a function related to cellular detoxification and export of fixed nitrogen. The presence of the urea degrading enzyme urease strongly suggests an alternative function in diatoms.This research is addressing the hypothesis that in marine diatoms, which are frequently subjected to nitrogen limitation, the urea cycle and associated mitochondrial enzymes function as an inorganic carbon and nitrogen recycling and repackaging hub; ultimately serving to redistribute catabolically derived NH4+ to arginine and other non-amino-acid nitrogen compounds. Based on recent advancements in diatom reverse genetics and other recently developed resources for genome enabled research, basic hypothesis concerning the role of mitochondria in nitrogen assimilation and metabolism in marine diatoms is being evaluated.Broader ImpactsAnthropogenic coastal loading of organic and inorganic nitrogen is expected to increase and climate change-induced shifts in oceanic mixed layer depth and mixing frequency are predicted to alter the delivery of nutrients into the euphotic zone. As a result, a key research focus is to facilitate prediction of how diatoms and other Chl c algae will respond to changing frequency and intensity of nutrient delivery events. This research will contribute to a more accurate depiction of nitrogen metabolism in marine algae, which is critically important for predictive ecosystem modeling.As part of the research project, a high school biology teacher from the Escondido Union High School District in San Diego will be recruited to work on a specific topic related to the proposed research. Upon completion of his/her paid internship, in collaboration with the PIs, the teacher will design a classroom activity and curriculum installment related to the rapidly emerging field of marine genomics, also including associated topics in marine biogeochemistry, for use the following school year.