Characterizing the effects of exogenous reactive oxygen species on marine microbial ecosystem dynamics

构成海洋浮游植物群落的微生物约占地球上所有光合作用的一半，因此对地球的气候和主要营养循环至关重要。海洋表面的绝大部分相对来说已经耗尽了生物生长所必需的营养物质。对于距离海岸数百或数千英里、距离海底数千米的海洋环流尤其如此。在这些地区，不同的浮游植物之间对营养物质的竞争被认为特别激烈。光合作用群落中数量最多的成员，单细胞细菌原绿球藻，也是最小的，长度为半微米。有趣的是，原绿球藻对活性氧（ROS）（如过氧化氢）非常敏感，在没有“辅助”微生物的情况下无法生长，这些微生物可以解毒阳光与海水中的有色有机物质反应时产生的ROS。原绿球藻依赖辅助物的真实程度目前尚不清楚：到目前为止，实验只评估了原绿球藻在ROS胁迫下的生存，而在自然环境中，这种生长条件在其他方面是很罕见的。最近有证据表明原绿球藻和辅助藻可以竞争营养物质，这又增加了一层复杂性。该项目结合实验培养工作、野外测量和生态系统建模来表征活性氧在海洋表层群落动态中的作用。原绿球藻和其他微生物的实验室培养物在一定的营养浓度、温度和光照强度下暴露于活性氧、单独培养和相互共培养时的生长和存活变化正在进行检查。实验室实验的结果随后被用于数学生态系统模型来模拟自然海洋环境。最后，将实验室结果和数学模型与暴露于一系列活性氧浓度的北太平洋自然群落进行比较。通过这种方式，本研究正在对微生物群落组成和死亡率如何依赖于ROS的产生和衰变进行更深入和更具预测性的理解。该项目更广泛的影响包括培训海洋研究方面的本科生和研究生，并向诺克斯维尔当地社区以及现役海军陆战队员的家属宣传微生物学和海洋学。该项目的总体目标是利用实验室实验和现场操作相结合的方法，对生态系统模型进行经验参数化，以探索HOOH和少营养微生物群落的耦合动态。原绿球藻是低营养海洋中最丰富的浮游植物，对全球碳循环有重要贡献。它丰富的关键是它有能力胜过其他微生物获取营养。这种生态优势被认为涉及基因组流线型的进化过程，包括过氧化氢（HOOH）抗性机制的丧失和原绿球藻对微生物群落的依赖，以降解阳光照射的表面混合层中光化学产生的HOOH。然而，当温度偏离最佳时，原绿球藻对HOOH的敏感性就会提高，因此对辅助物的依赖也会增加。同样的道理也适用于光和营养条件。同样地，我们对hooh解毒“助手”微生物的环境敏感性知之甚少，包括浮游植物群落的成员。因此，原绿球藻在多大程度上需要帮助，以及不同的hooh消耗微生物在多大程度上提供这种功能，目前还不清楚。该项目为了解地表混合层微生物群落的聚集规律提供了HOOH和微生物动力学的定量测量。一些原绿球藻菌株、潜在的辅助微生物和竞争微生物在一系列营养限制和HOOH条件下在恒温器中生长，以评估浮游植物的生长、死亡和光合作用。与其他微生物（包括聚球菌和几种光合作用的微真核生物）共同培养，正在被用来测试关于HOOH解毒的假设，以及在最佳和次优条件下竞争对原绿球藻- ROS动力学的影响。这些生态系统模型预测正在与现场操作一起进行解释，这些操作直接评估了在其他损失过程（主要是放牧和病毒溶解）背景下ROS介导的影响。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

The microbes that constitute the phytoplankton community of the ocean account for about half of all photosynthesis on the earth and as a consequence are critically important to the climate and major nutrient cycles of the planet. The vast majority of the ocean surface is relatively depleted in nutrients essential for organisms to grow. This is especially true for the ocean gyres that are hundreds or thousands of miles from the coast and thousands of meters above the ocean floor. In these regions, competition among different phytoplankton for nutrients is thought to be especially strong. The most abundant member of the photosynthetic community, the single-celled bacterium Prochlorococcus, is also its smallest, measuring a half a micrometer in length. Interestingly, Prochlorococcus is very sensitive to reactive oxygen species (ROS) such as hydrogen peroxide and cannot grow in the absence of “helper” microbes which detoxify the ROS generated when sunlight reacts with pigmented organic material in the seawater. The true extent to which Prochlorococcus depends on helpers is currently unknown: thus far, experiments have only assessed Prochlorococcus survival of ROS stress under otherwise optimal growth conditions which are rare in the natural environment. Recent evidence that Prochlorococcus and helpers can compete for nutrients adds another layer of complexity. This project combines experimental culture work, field measurements, and ecosystem modeling to characterize the roles of ROS in surface ocean community dynamics. Laboratory cultures of Prochlorococcus and other microbes are being examined for growth and survival changes when exposed to ROS under a range of nutrient concentrations, temperatures, and light intensities, when grown separately and in co-culture with each other. Outcomes from the laboratory experiments are then being used in mathematical ecosystem models to simulate the natural marine environment. Finally, laboratory results and mathematical models are being compared to natural communities in the North Pacific Ocean exposed to a range of ROS concentrations. In this way, this research is developing a deeper and more predictive understanding of how microbial community composition and mortality depend upon ROS production and decay. Broader impacts of the project include the training of undergraduate and graduate students in oceanographic research and public outreach about microbiology and oceanography to the local Knoxville community, as well as dependents of active duty Marines. The overarching goal of this project is to empirically parameterize ecosystem models using a combination of lab experiments and field manipulations to explore the coupled dynamics of HOOH and oligotrophic microbial communities. Prochlorococcus is the most abundant phytoplankter in the oligotrophic ocean and contributes significantly to global carbon cycling. Key to its abundance is its ability to outcompete other microbes for nutrients. This ecological advantage is thought to involve an evolutionary process of genomic streamlining, including a loss of hydrogen peroxide (HOOH) resistance mechanisms and reliance of Prochlorococcus on the microbial community to degrade photochemically-generated HOOH in the sun-exposed surface mixed layer. When temperature deviates from optimal, however, sensitivity of Prochlorococcus to HOOH – and thus reliance upon helpers - is heightened. The same may hold true for light and nutrient conditions. Similarly, little is known about the environmental sensitivity of HOOH-detoxifying “helper” microbes, including fellow members of the phytoplankton community. Therefore, the extent to which Prochlorococcus requires help, and to which different HOOH-consuming microbes provide this function, is not currently understood. This project is providing the quantitative measurements in HOOH and microbial dynamics to understand the rules of microbial community assembly in the surface mixed layer. Several strains of Prochlorococcus, potential helper, and competitor microbes are being grown under a range of nutrient-limiting and HOOH conditions in chemostats to assess growth, mortality, and - for the phytoplankton – photosynthesis. Co-cultures with the other microbes, including Synechococcus and several photosynthetic picoeukaryotes, are being used to test hypotheses about HOOH detoxification and the impacts of competition on Prochlorococcus–ROS dynamics under optimal and suboptimal conditions These ecosystem model predictions are being interpreted alongside field manipulations which directly assess ROS mediated impacts in the context of other loss processes, primarily grazing and viral lysis.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.