Collaborative Research: The Role of Iron-oxidizing Bacteria in the Sedimentary Iron Cycle: Ecological, Physiological and Biogeochemical Implications

铁是地球上最丰富的元素之一，也是生命的基本元素。尽管铁储量丰富，但并不总是生物可用的。例如，在海洋的水柱中，铁很容易被氧化，沉淀或下沉到沉积物中。这可能会导致开放海洋中铁的缺乏，而铁通常是构成海洋食物网基础的浮游植物生长的主要限制营养物质。当海洋沉积物被生物利用时，它可以成为海洋铁的主要来源。有趣的是，一组细菌，铁氧化细菌（FeOB），可以直接使用铁作为能量来源来促进它们的生长，并可能控制海洋其他部分铁的可用性。虽然这一群体在热液喷口可能大量存在，但人们对它们在海洋沉积物中的丰度或活动知之甚少。这些细菌在控制铁从沉积物到水柱的通量中起重要作用吗？为了回答这个问题，将对美国东西海岸的沉积物进行分析，以表征和量化FeOB的多样性和丰度。此外，一系列的实验室实验将旨在了解它们在控制从沉积物到海洋的铁通量方面所起的具体作用，以及确定它们可以生长的氧气下限这一技术上具有挑战性的问题。这项工作与我们对海水中看似次要的成分铁的生物控制如何影响整个海洋的生产力的理解有关。值得注意的是，据预测，气候变化对海洋环境的影响是降低海洋中的氧气含量。这可能对沉积铁循环产生深远的影响，并可能导致更多的铁输入，这反过来可能缓解海洋某些区域的铁限制，从而提高二氧化碳的固定速度，并从大气中吸收二氧化碳。该项目将为博士后科学家、研究生和本科生提供培训。公众宣传将包括一个由学生发起的展览，题为“铁与地球上生命的进化”，在哈佛自然历史博物馆举办，为本科生培训和宣传提供了一个独特的机会。该提案的中心假设是，FeOB在海洋沉积环境中比以前认识到的更为普遍，并且通过限制沉积物中溶解铁（dFe）的释放，在控制沉积物进入水柱的铁通量方面发挥了实质性作用。对缅因湾近岸地区和加利福尼亚海岸附近的蒙特利峡谷的横断面进行调查，将获得沉积泥的岩心，并使用敏感技术观察FeOB和假定的铁还原细菌的垂直分布，以检测它们的存在和相对丰度。沉积物将用于一种新型反应器系统，该系统将允许精确控制O2水平和铁浓度，以测量不同氧气方案下铁循环的动力学。具有不同氧亲和力的FeOB纯培养物将在生物反应器中进行测试，并与高灵敏度质谱仪相结合，以确定在铁上生长的不同FeOB对O2利用的下限，从而为其在低氧环境中的活性和分布提供机制见解。

Iron is one of the most abundant elements on Earth and is an essential element for life. Despite its abundance, iron is not always biologically available. For example, in the water column of the ocean, iron is easily oxidized and precipitates or sinks to the sediments. This can result in there being such a deficit of iron in the open ocean that it is often the primary limiting nutrient for the growth of phytoplankton that form the base of the marine food web. Marine sediments can be a major source of iron to the ocean, when it is made biologically available. Interestingly, one group of bacteria, the iron-oxidizing bacteria (FeOB), can use iron directly as an energy source to fuel their growth, and may govern the availability of iron to other parts of the ocean. While this group can be abundant at hydrothermal vents, little is known about their abundance or activity in marine sediments. Are these bacteria playing an important role in controlling the flux of iron from the sediments to the water column? To answer this, sediments on the east and west coasts of the United States will be analyzed to characterize and quantitate the diversity and abundance of FeOB. In addition, a series of laboratory experiments will be aimed at understanding the specific role they play in controlling iron flux from the sediments to the ocean, as well as the technically challenging question of determining the lower limit of oxygen at which they can grow. This work has relevance to our understanding of how biological control of a seemingly minor constituent in seawater, iron, could have implications for productivity of the entire ocean. Notably, a predicted impact of climate change on marine environments is to decrease oxygen levels in the ocean. This could have a profound influence on the sedimentary iron cycle, and possibly lead to greater inputs of iron, which could in turn alleviate iron-limitation in some regions of the ocean, thereby enhancing the rate of CO2-fixation and draw down of CO2 from the atmosphere. This project will provide training for a postdoctoral scientist, graduate students and undergraduates. Public outreach will include a student initiated exhibit, entitled "Iron and the evolution of life on Earth" at the Harvard Museum of Natural History providing a unique opportunity for undergraduate training and outreach. The central hypothesis of this proposal is that FeOB are more common in marine sedimentary environments than previously recognized, and play a substantive role in governing the iron flux from the sediments into the water column by constraining the release of dissolved iron (dFe) from sediments. A survey of near shore regions in the Gulf of Maine, and a transect along the Monterey Canyon off the coast of California will obtain cores of sedimentary muds and look at the vertical distribution of FeOB and putative Fe-reducing bacteria using sensitive techniques to detect their presence and relative abundance. Sediments will be used in a novel reactor system that will allow for precise control of O2 levels and iron concentration to measure the dynamics of the iron cycle under different oxygen regimens. Pure cultures of FeOB with different O2 affinities will be tested in a bioreactor coupled to a highly sensitive mass spectrometer to determine the lower limits of O2 utilization for different FeOB growing on iron, thus providing mechanistic insight into their activity and distribution in low oxygen environments.