Features and implications of nitrogen assimilation trait variability in populations of Prochlorococcus

原绿球藻种群氮同化性状变异的特征和意义

• 批准号: 2048470
• 负责人: Paul Berube
• 金额: $ 59.59万
• 依托单位: Massachusetts Institute of Technology
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2021
• 资助国家: 美国
• 起止时间: 2021-03-01 至 2025-02-28
• 项目状态: 未结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=2048470&HistoricalAwards=false
• 关键词: Features; implications; nitrogen; assimilation; trait

The marine bacterium, Prochlorococcus, is a central part of the food web in the subtropical open ocean, one of the largest biomes on the planet. Like plants on land, Prochlorococcus and other phytoplankton are capable of photosynthesis, harnessing light to convert carbon dioxide into sugars and other organic matter. This matter feeds all the life in the sea. Akin to terrestrial plants, Prochlorococcus requires additional nutrients or “fertilizer” to grow and photosynthesize. Among these nutrients, nitrogen is often scarce across much of the sunlit ocean. Thus, understanding the means through which nitrogen is obtained and used by Prochlorococcus has important consequences for understanding how the nitrogen and carbon cycles are coupled in the ocean. Not all Prochlorococcus cells have the genetic capacity to use all sources of inorganic nitrogen, i.e. nitrate, nitrite, and ammonium. Some can use all three, some the last two, and some only ammonium. Cells that can use nitrate must sequentially transform it to nitrite and then to ammonium before they can make the building blocks for proteins. The genesis of this project derives from the observation that some Prochlorococcus cells release nitrite into the seawater during growth on nitrate. This project examines this feature of the physiology of these cell lines, and asks whether cells that release nitrite can support the growth of other cells than cannot use nitrate, in effect creating a cross-feeding situation that could make the system more robust. Understanding the drivers behind the coexistence of cells with different ways of obtaining nitrogen, a key currency in the ocean, will provide important insights on the flow of nitrogen in marine ecosystems. This project also sheds light on the structure of interactions between microbes and provide the broader scientific community (for instance, those studying diverse microbiomes related to human health and disease or agriculture) a new perspective on how microbes form beneficial partnerships. This project supports immersive laboratory-based research experiences for undergraduate students, who design and execute experiments directly related to the overall project goals. The project further supports the work of the investigators to engage with the general public on topics related to phytoplankton, photosynthesis, and the ecosystem services provided by these marine organisms.In the low-light adapted LLI clade of Prochlorococcus, the focus of this project, nearly all cells possess the downstream half of the nitrate assimilation pathway (for the assimilation of nitrite). Only a fraction of LLI cells, however, have the complete nitrate assimilation pathway. Incomplete assimilatory nitrate reduction, with concomitant nitrite release, has been observed for LLI cells during growth on nitrate as the sole nitrogen source. Further, the nitrite released by cells growing on nitrate can support the growth of Prochlorococcus that can use nitrite but not the more oxidized nitrate. Overall, within a group of closely-related Prochlorococcus, there is genotypic and phenotypic diversity related to the production and consumption of nitrite, a central intermediate in the nitrogen cycle. The investigators propose to further develop Prochlorococcus as a model system to explore nitrite cycling within populations and provide new insights on how trait variability and the selection of complementary functions facilitates robustness and/or resiliency in microbial populations. The overarching hypothesis is that the population level assembly of distinct functional types of Prochlorococcus emerges through interactions that are mediated, in part, by cross-feeding of nitrite. To address this broad hypothesis, the investigators are focusing on the following objectives: 1) assessing the physiological underpinnings of incomplete assimilatory nitrate reduction and nitrite release through in-vitro biochemical characterization of nitrite reductase enzymes and transcription profiling of cells subjected to light and temperature stress, 2) examining the nitrite production and consumption rates of Prochlorococcus strains across environmental gradients such as light, temperature, and nutrient availability in order to constrain the environmental parameters that modulate nitrite cycling, and 3) determining the frequencies and activities of nitrogen assimilation genotypes within laboratory and field populations, under varying environmental conditions and perturbations. Outcomes from objectives 1 and 2 help to constrain the tradeoffs associated with incomplete nitrate reduction and the release of nitrite (a valuable commodity to nitrogen limited cells) to facilitate modelling and interpretation of how partnerships between nitrogen assimilation genotypes are structured. These insights help to direct experiments in Objective 3, which aims to examine controlled laboratory co-cultures and field populations in order to produce quantitative data on the emergent features of Prochlorococcus populations where interactions are mediated by the cross-feeding of nitrite. These data are being used to develop an improved understanding of how interactions mediated by a common public good might give rise to emergent properties of populations, including resilience to perturbation and greater population-wide efficiency in nitrogen assimilation.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.

海洋细菌，原绿球藻，是亚热带公海食物网的中心部分，是地球上最大的生物群落之一。像陆地上的植物一样，原绿球藻和其他浮游植物能够进行光合作用，利用光将二氧化碳转化为糖和其他有机物质。这些物质供养着海洋中的所有生物。类似于陆生植物，原绿球藻需要额外的营养或“肥料”来生长和光合作用。在这些营养物质中，氮在阳光照射的海洋中往往很稀缺。因此，了解原绿球藻获得和使用氮的方式对于了解海洋中氮和碳循环如何耦合具有重要意义。并非所有的原绿球藻细胞都具有利用所有无机氮来源的遗传能力，即硝酸盐，亚硝酸盐和铵。有些人可以使用所有三种，有些人使用最后两种，有些人只使用铵。能够利用硝酸盐的细胞必须依次将其转化为亚硝酸盐，然后再转化为铵，然后才能制造蛋白质的构建模块。该项目的起源来自于观察到一些原绿球藻细胞在硝酸盐上生长期间向海水中释放亚硝酸盐。该项目研究了这些细胞系的生理学特征，并询问释放亚硝酸盐的细胞是否可以支持其他细胞的生长，而不是不能使用硝酸盐，实际上创造了一种交叉喂养的情况，可以使系统更加强大。 了解具有不同获得氮（海洋中的关键货币）方式的细胞共存背后的驱动因素，将为海洋生态系统中氮的流动提供重要见解。该项目还揭示了微生物之间相互作用的结构，并为更广泛的科学界（例如，研究与人类健康和疾病或农业相关的各种微生物组的科学家）提供了一个关于微生物如何形成有益伙伴关系的新视角。该项目为本科生提供沉浸式实验室研究体验，他们设计和执行与整体项目目标直接相关的实验。该项目进一步支持研究人员与公众就浮游植物、光合作用以及这些海洋生物提供的生态系统服务等主题进行交流。在本项目的重点原绿球藻的低光适应LLI分支中，几乎所有细胞都拥有硝酸盐同化途径的下游一半（用于同化亚硝酸盐）。然而，只有一小部分LLI细胞具有完整的硝酸盐同化途径。在硝酸盐作为唯一氮源的生长过程中，已观察到LLI细胞的不完全同化硝酸盐还原，伴随着亚硝酸盐的释放。此外，在硝酸盐上生长的细胞释放的亚硝酸盐可以支持原绿球藻的生长，原绿球藻可以使用亚硝酸盐，但不能使用氧化的硝酸盐。总的来说，在一组密切相关的原绿球藻，有相关的生产和消费亚硝酸盐，在氮循环的中央中间的基因型和表型多样性。研究人员建议进一步开发原绿球藻作为模型系统，以探索种群内的亚硝酸盐循环，并提供关于性状变异性和互补功能的选择如何促进微生物种群的鲁棒性和/或弹性的新见解。总体假设是，不同功能类型的原绿球藻的群体水平组装出现通过介导的相互作用，在一定程度上，通过交叉喂养的亚硝酸盐。为了解决这个广泛的假设，研究人员正在关注以下目标：1）通过亚硝酸盐还原酶的体外生物化学表征和经历光和温度胁迫的细胞的转录谱分析来评估不完全同化硝酸盐还原和亚硝酸盐释放的生理基础，2）检测原绿球藻菌株在不同环境梯度如光照、温度和养分可用性，以限制调节亚硝酸盐循环的环境参数，以及3）在不同的环境条件和扰动下，确定实验室和田间种群内氮同化基因型的频率和活动。目标1和2的结果有助于限制与不完全硝酸盐还原和亚硝酸盐释放（氮有限细胞的宝贵商品）相关的权衡，以促进氮同化基因型之间的伙伴关系是如何构建的建模和解释。这些见解有助于指导目标3中的实验，该目标旨在检查受控的实验室共培养物和田间种群，以产生关于原绿球藻种群的紧急特征的定量数据，其中相互作用是由亚硝酸盐的交叉喂养介导的。这些数据被用来发展一个共同的公共产品介导的相互作用如何可能会引起人口的新兴属性，包括扰动的弹性和更大的人口范围内的氮同化效率的更好的理解。这个奖项反映了NSF的法定使命，并已被认为是值得通过使用基金会的智力价值和更广泛的影响审查标准进行评估的支持。