Diel growth and activity of Prochloroccocus in an Oxygen Deficient Zone

海洋缺氧区（odz）占海洋体积的不到1%，但每年从海洋转移到大气中的氮的一半来自海洋缺氧区。氮是浮游植物的关键限制养分，因此海洋氮的损失对海洋光合作用的总量产生了影响，而海洋光合作用可以从大气中去除二氧化碳，并为海洋食物网提供基础。随着海洋温度的升高，缺氧区的面积预计会增加。这个项目关注的是一种微生物，它位于海洋odz中氮、碳和氧循环的交叉点，即蓝藻原绿球藻。该项目利用之前前往东热带北太平洋ODZ的巡航中获得的水样，评估了原绿球藻在昼夜循环中的生长和活动，以确定它们对ODZ微生物群落其他成员的影响。在实验室里，原绿球藻的培养物在不同形式的氮上生长，并测量了它们的生长速度和产氧量。了解居住在这些水域的生物群落的功能对于预测全球元素循环非常重要。该项目的一个中心目标是通过为来自代表性不足的少数群体的本科生提供早期研究经验、持续的财政支持、同伴指导和在国家层面接触该学科，扩大攻读环境科学研究生工作的个人数量。两名女研究生和五名本科生是地球科学领域代表性不足的群体的成员，他们正在参与该项目，并在STEM职业生涯中接受培训和支持。本项目研究了在热带北太平洋东部低氧低光地区发现的独特原绿球藻菌株在海洋碳氮循环中的作用。在所有三种主要的海洋ODZ中，ODZ的顶部在光带内有广阔的区域，导致次级叶绿素的最大值完全在缺氧区。这个次级叶绿素最大值由蓝藻主导，主要是原绿球藻，在少营养海洋中在数值上占主导地位的光养菌。这里原绿球藻在非常低的光照水平下进行光合作用，固定碳并产生氧气，这对周围的微生物群落有影响。虽然原绿球藻在ODZ内产生的有机碳可能是异养反硝化菌的额外有机物来源，为氮气（N2）的产生提供燃料，但伴随的氧气产生可能为硝化古菌和细菌创造有利的栖息地，它们共同将铵（NH4+）转化为硝酸盐。原绿球藻也必须以某种形式获得氮，可能会与其他氮循环类群竞争。重要的是，原绿球藻的生长、产氧和营养吸收与diel循环密切相关。因此，原绿球藻产生的氧气可能在一个昼夜循环过程中改变硝化和反硝化之间的平衡，最终影响净氮损失的程度。本项目是：1)利用流式细胞术建立ETNP ODZ内原绿球藻种群的原位生长速率；2)对ETNP ODZ次级叶绿素最大值在一个日循环中的后叶组进行测序，探索日波动氧对N循环群落的作用；3)通过测序原绿球藻的基因组，比较其在不同氮源下的生长速度，确定其对氨和亚硝酸盐的吸收动力学，测量其氧演化和评估其在低氧条件下的双转录反应，来表征新的原绿球藻。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。

Marine oxygen deficient zones (ODZs) occupy less than 1% of the volume of the ocean, but are responsible for up to half of the nitrogen moved from the ocean to the atmosphere each year. Nitrogen is a key limiting nutrient for phytoplankton, thus this loss of nitrogen from the ocean has impacts on the total amount of marine photosynthesis, which removes carbon dioxide from the atmosphere and provides the base for marine food webs. As ocean temperatures increase, the size of oxygen deficient zones is predicted to increase. This project focuses on a microorganism that sits at the intersection of the nitrogen, carbon and oxygen cycles within marine ODZs, the cyanobacterium Prochlorococcus. Using water samples acquired on a previous cruise to the Eastern Tropical North Pacific ODZ, the project assesses the growth and activity of Prochlorococcus over the day/night cycle to determine their impacts on other members of the ODZ microbial community. In the lab, cultures of Prochlorococcus are being grown on different forms of nitrogen and their growth rates and oxygen production are being measured. Understanding the functioning of biological communities that reside within these waters is important for predicting global elemental cycles. A central goal of this project is to broaden the pool of individuals who pursue graduate work in environmental sciences by providing undergraduate students from underrepresented minority groups early research experience, sustained financial support, peer mentorship and exposure to the discipline at a national level. Two female graduate students and five undergraduate students that are members of groups underrepresented in earth sciences are participating in the project and are being trained and supported in their STEM careers.This project investigates the roles of unique strains of Prochlorococcus found in low oxygen and low light regions of the Eastern Tropical North Pacific (ETNP) in the oceanic carbon and nitrogen cycles. In all three major marine ODZs, there are broad regions where the top of the ODZ shoals to within the photic zone, resulting in a secondary chlorophyll maximum entirely within the anoxic zone. This secondary chlorophyll maximum is dominated by cyanobacteria, largely Prochlorococcus, the numerically dominant phototroph in the oligotrophic oceans. Here Prochlorococcus is photosynthesizing under very low light levels, fixing carbon and producing oxygen, with implications for the surrounding microbial community. Although the organic carbon produced by Prochlorococcus within the ODZ may be an additional source of organic matter for heterotrophic denitrifiers, fueling nitrogen gas (N2) production, the concomitant oxygen production may create a favorable habitat for nitrifying archaea and bacteria which together return ammonium (NH4+) to nitrate. Prochlorococcus must also acquire nitrogen in some form, potentially competing with other nitrogen cycling taxa to do so. Importantly, the growth, oxygen production and nutrient uptake in Prochlorococcus are tightly linked to the diel cycle. Thus, diel production of oxygen by Prochlorococcus may shift the balance between nitrification and denitrification over the course of a diurnal cycle, ultimately influencing the degree of net nitrogen loss. This project is 1) establishing the in situ growth rate of Prochlorococcus populations within the ETNP ODZ using flow cytometry; 2) sequencing metatransciptomes from ETNP ODZ secondary chlorophyll maximum over a diel cycle to explore the role of diurnally fluctuating oxygen on the N cycling community; and 3) characterizing novel Prochlorococcus isolates from the ODZ by sequencing their genomes, comparing their growth rates on different N sources, determining their uptake kinetics for ammonia and nitrite, measuring their oxygen evolution and assessing their diel transcriptional response under low oxygen.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.