Collaborative Research: MSB: The Role of Sulfur Oxidizing Bacteria in Salt Marsh C and N Cycling

盐沼是在世界各地的河口发现的非常多产的生态系统。盐沼位于沿海海洋和沿海流域之间，经常受到人类活动的严重影响。许多地区从陆地获得大量硝酸盐，使水质退化，在某些情况下导致有害的藻类大量繁殖和对鱼类有害的低氧区。先前的研究表明，盐沼可以作为清洁场所，污染物硝酸盐可以转化为无害的氮气并释放到大气中。这是通过异养反硝化作用发生的，这是一种微生物介导的过程，利用有机碳将植物可用的硝酸盐转化为氮气（N2）。然而，这种向无害氮气的转化并不总是盐沼硝酸盐的命运。最近的研究表明，沼泽沉积物中的硫和碳化合物的形式直接影响微生物的类型和它们的活动决定硝酸盐？的命运，即硫，氮和碳的转化都是通过微生物活动联系在一起的。例如，硝酸盐可以通过微生物控制的过程异化硝酸盐还原（DNRA）转化为铵，而不是转化为氮气。硝酸盐的这种不同命运在几个方面对环境具有重要意义。如果硝酸盐转化为氮气，它会从生态系统中流失到大气中，而如果DNRA主导硝酸盐还原，污染物氮会以铵的形式留在系统中。此外，根据微生物的类型控制硝酸盐？如果生物利用硝酸盐来帮助降解有机物，那么生态系统中储存的碳就会减少，这可能会影响沼泽地跟上海平面上升的能力。为了研究环境和微生物控制对盐沼中硝酸盐去向的影响，将在梅岛河口进行实验室和现场试验。该项目的重点是硫氧化细菌，这是一组特别重要的化学合成微生物，它们利用沉积物中硫化合物中捕获的能量来生产能源，从而有助于碳储存。本研究的主要目的是（1）鉴定盐生湿地草互花米草（Spartina alterniflora）密集分布的沉积物中存在的硫氧化剂，并检测它们在变化的环境条件下与硫和硝酸盐处理相关的基因表达;和（2）将这些分子信息与沉积物中发生的生物地球化学反应的速率和特征的测量相结合，以检测是否与硫氧化剂相关的DNRA在特定的环境条件下，氮的释放（从而使氮在生态系统中得以保留）或反硝化作用（从而使氮从生态系统中流失）占主导地位。这项多学科研究将生物地球化学过程测量与分子分析相结合，并将与正在进行的研究在梅岛生态系统长期生态研究（PIE-LTER）网站协同。盐沼为人类提供了各种生态系统服务，包括去除营养物和风暴防护，但它们正面临着沿海开发和海平面上升的压力。对沼泽微生物功能的详细了解可能有助于恢复工作，特别是如果沼泽中存在的硫的形式允许预测污染物氮是否最有可能随着时间的推移而丢失（作为氮气）或保留。项目人员将作为研究主管或教师参加MBL？每年秋季的环境科学学期，每年吸引大多数来自本科文科院校的女性进入严格的课堂学习和个人生态系统研究。一名博士后也将在这种跨学科的氛围中接受培训，PIE-LTER和马萨诸塞州奥杜邦协会之间的合作为调查人员提供了一个渠道，向初中和高中学生讲授陆地上人类活动的氮负荷及其对当地河口的影响。

Salt marshes are extraordinarily productive ecosystems found in estuaries worldwide. Located between the coastal ocean and coastal watersheds, salt marshes are often heavily influenced by human activities. Many receive high nitrate input from land, degrading water quality and leading in some cases to harmful algal blooms and low oxygen zones harmful to fish. Previous research has shown that salt marshes can act as cleansing sites where pollutant nitrate can be transformed to harmless nitrogen gas and released to the atmosphere. This occurs through heterotrophic denitrification, a microbe-mediated process that transforms plant-available nitrate to nitrogen gas (N2) using organic carbon. However, this transformation to harmless nitrogen gas is not always the fate of salt marsh nitrate. More recent research suggests that the forms of sulfur and carbon compounds in the marsh sediment directly affect the types of microbes and their activities determining nitrate?s fate, i.e. sulfur, nitrogen, and carbon transformations are all linked via microbial activities. For example, instead of being converted to nitrogen gas, nitrate can be converted to ammonium via the microbially controlled process dissimilatory nitrate reduction (DNRA). This different fate of nitrate is environmentally important in several ways. If nitrate is converted to nitrogen gas, it is lost from the ecosystem to the atmosphere, whereas if DNRA dominates nitrate reduction, pollutant nitrogen remains in the system as ammonium. Also, depending on the type of microbial process governing nitrate?s fate, if organisms are using nitrate to help degrade organic matter, less carbon is stored in the ecosystem, potentially influencing the ability of marshes to keep up with sea level rise. To investigate the environmental and microbial controls affecting the fate of nitrate in salt marshes, lab and field experiments will be carried out at Plum Island Estuary. The project focuses on sulfur-oxidizing bacteria, a group of particularly important chemosynthetic microbes that use energy trapped in sulfur compounds in sediment to for energy production, and thus contribute to carbon storage. The proposed studies aim (1) to identify sulfur-oxidizers present in sediment densely populated with the salt marsh grass, Spartina alterniflora, and to examine their gene expression linked to sulfur and nitrate processing under shifting environmental conditions; and (2) to combine this molecular information with measurements of rates and characteristics of biogeochemical reactions occurring in the sediment to detect whether sulfur-oxidizer-linked DNRA (thus retention of nitrogen in the ecosystem) or denitrification (thus loss of nitrogen gas from the ecosystem) dominates under specific environmental conditions.Broader Impacts. This multidisciplinary research integrates biogeochemical process measurements with molecular analyses, and will be synergistic with ongoing studies at the Plum Island Ecosystem Long-Term Ecological Research (PIE-LTER) site. Salt marshes provide a variety of ecosystem services to humanity, including nutrient removal and storm protection, but they are under pressure from increasing coastal development and rising sea level. A detailed understanding of marsh microbial function will likely contribute to restoration efforts, particularly if the form of sulfur present in marshes allows prediction of whether pollutant nitrogen will most likely be lost (as nitrogen gas) or retained over time. Project personnel will participate as research supervisors or teachers in MBL?s annual fall Semester in Environmental Science, which each year draws mostly women from undergraduate liberal arts colleges into rigorous classroom learning and individual ecosystems research. One post-doc will also be trained in this interdisciplinary atmosphere, and collaboration between PIE-LTER and Massachusetts Audubon Society provides a conduit for the investigators to teach middle and high school students about nitrogen loading from human activities on land, and its effects in local estuaries.