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，也是生命所必需的元素。尽管铁含量丰富，但并不总是生物可利用的。例如，在海洋的水柱中，铁很容易被氧化并沉淀或下沉到沉积物中。这可能导致在开放的海洋中出现如此严重的铁缺乏，以至于它往往是构成海洋食物网基础的浮游植物生长的主要限制养分。海洋沉积物可以成为海洋铁的主要来源，当它在生物上可用时。有趣的是，一类细菌，氧化铁细菌(FeOB)，可以直接利用铁作为能源来推动它们的生长，并可能控制海洋其他部分铁的可获得性。虽然这类生物在热液喷口中可能很丰富，但人们对它们在海洋沉积物中的丰度或活动知之甚少。这些细菌在控制从沉积物到水柱的铁通量方面发挥了重要作用吗？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具有技术挑战性的问题。这项工作与我们理解生物控制海水中看似次要的成分--铁--如何对整个海洋的生产力产生影响有关。值得注意的是，气候变化对海洋环境的预测影响是降低海洋中的氧气水平。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，这反过来又可以缓解海洋某些区域的铁限制，从而提高固定二氧化碳的速度，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。公众宣传活动将包括由学生发起的展览，题为“铁与地球上生命的演变”，在哈佛自然历史博物馆举办，为本科生培训和宣传提供了独特的机会。这一建议的中心假设是，FeOB在海洋沉积环境中比以前认识的更常见，并通过限制沉积物中溶解铁(DFE)的释放，在调节从沉积物进入水柱的铁通量方面发挥实质性作用。对缅因湾近岸地区和加利福尼亚州海岸外蒙特利峡谷的一条横断面进行的调查将获得沉积泥浆的岩心，并使用敏感技术检测FeOB和可能的铁还原细菌的垂直分布，以检测它们的存在和相对丰度。沉积物将被用于一种新型的反应堆系统，该系统将允许精确控制氧气水平和铁浓度，以测量不同氧气方案下铁循环的动态。具有不同氧气亲和力的纯FeOB培养物将在与高灵敏度质谱仪耦合的生物反应器中进行测试，以确定在铁上生长的不同FeOB的氧气利用下限，从而提供对它们在低氧环境中的活性和分布的机械洞察。

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.