Nitrogen Fixing Prokaryotes in Corals: Is Nitrogen Fixation a Core Function of the Coral Microbiome?

珊瑚礁为人类提供了至关重要的服务，并日益受到气候变化的威胁。但关于珊瑚礁的许多重要问题仍然没有答案，比如它们令人难以置信的高生物多样性是如何发展和维持的。众所周知，形成珊瑚礁的珊瑚是由许多不同种类的细菌和藻类组成的，这些细菌和藻类生活在珊瑚的组织中，但大多数微生物的功能仍然未知。报道的微生物功能之一是称为固氮的过程。固氮是陆地上一个非常重要的过程。例如，农田里的紫花苜蓿的根部有微生物，它们可以从空气中吸收氮，并在水中补充土壤中流失的氮。海洋中的氮循环不太为人所知，特别是在氮含量较低的热带水域，尽管已知珊瑚礁系统中的氮主要来自固氮。最近在珊瑚组织中发现了固氮细菌，这就引出了这个项目的首要问题：这种固氮对珊瑚和周围环境的贡献有多重要？这项研究将与一个教育项目相结合，通过让与科学家密切合作的本科生参与其中，培养参与者的批判性思维和合作学习能力。将招收来自代表性不足背景的学生；这些本科生将成为现有本科生研究机会计划的一部分，在进行自己的原创研究项目时，他们将获得指导、经济支持和密集的写作支持。此外，该校将与退伍军人办公室共同开发一个新项目，以吸引退伍军人进入校园的科学领域。最后，将在当地的公立高中组织外展活动，并计划一些关于珊瑚礁生态学和生物学的公共环境教育讲座。关于原核生物在造礁珊瑚的生理、生物化学和生态学中的作用，人们的认识还存在很大的空白。该项目将评估不同的原核生物群落如何影响珊瑚全息生物体内氮的生物地球化学，特别是固氮作用，以及固氮作用如何促进分泌粘液中溶解有机氮（DON）的含量。通过固氮和释放DON引入“新氮”可能对珊瑚周围邻近底栖生物群落氮的生物地球化学产生重大影响。高通量测序工具将用于量化太平洋和加勒比珊瑚中这些共生体的分类和功能多样性，以及珊瑚组织中nifH基因的分类多样性。研究人员将测量来自澳大利亚、夏威夷和库拉索岛的几种珊瑚物种的固氮率，以及固定氮在珊瑚全胞体所有隔间中的转运情况，并确定“新氮”对释放到环境中的黏液DON的贡献。这些结果将增加我们对珊瑚共生原核生物的分类和功能多样性的认识。该项目还将有助于指导未来对珊瑚礁上珊瑚衍生氮的生物地球化学循环的研究。

Coral reefs provide crucial services for people and are becoming increasingly threatened by climate change. But many important questions about coral reefs remain unanswered, such as how their incredibly high biodiversity has developed and is maintained. It is well known that reef-forming corals consist of many different groups of bacteria and algae that live in the tissues of corals, but the function of most of these microbes remain unknown. One of the reported microbial functions is the process called nitrogen fixation. Nitrogen fixation is better known as a very important process on land. For example, alfalfa in agricultural fields has microbes associated with its roots that can capture nitrogen from the air and replace nitrogen lost from soils in water. The nitrogen cycle in the oceans is less well known, especially in tropical waters where nitrogen levels are low, although nitrogen in coral reef systems is known to derive mainly from nitrogen fixation. Recently nitrogen-fixing bacteria have been discovered in the tissues of corals, which begs the overarching question of this project: how important is the contribution of this fixed nitrogen to corals and to the surrounding environment? This research will integrate with an educational program that fosters critical thinking and cooperative learning among participants by involving undergraduate students who will work closely with the scientists. Students from underrepresented backgrounds will be recruited; the undergraduates will become part of an existing undergraduate research opportunities program where they receive mentoring, financial support, and intensive writing support while conducting original research projects of their own. Also a new program will be developed with the university's veterans office to attract and transition veterans into science fields across campus. Finally, outreach activities will be organized at local public high schools, and some public environmental education lectures about the ecology and biology of coral reefs are planned.Significant gaps in understanding exist about the role of prokaryotes in the physiology, biochemistry and ecology of reef-forming corals. This project will assess how different prokaryotic communities affect the biogeochemistry of nitrogen, specifically nitrogen fixation, within the coral holobiont and how nitrogen fixation contributes to the dissolved organic nitrogen (DON) content of excreted mucus. The introduction of "new nitrogen" though nitrogen fixation and release of DON could have significant impacts on the biogeochemistry of nitrogen in the adjacent benthic communities surrounding corals. High throughput sequencing tools will be used to quantify the taxonomic and functional diversity of these symbioses in corals from the Pacific and Caribbean, as well as the taxonomic diversity of nifH genes in the tissues of corals. Rates of nitrogen fixation on several coral species from Australia, Hawaii and Curacao will be measured, along with the translocation of fixed nitrogen to all compartments of the coral holobiont, and the contribution of "new nitrogen" to the DON of mucus released to the environment determined. The results will increase our understanding of the taxonomic and functional biodiversity of symbiotic prokaryotes in corals. This project will also help guide future studies on the biogeochemical cycling of coral-derived nitrogen on coral reefs.