Diel growth and activity of Prochloroccocus in an Oxygen Deficient Zone

海洋缺氧区（ODZs）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。氮是浮游植物的关键限制性营养素，因此海洋中氮的损失会影响海洋光合作用的总量，而海洋光合作用会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。该项目的重点是一种微生物，它位于海洋臭氧消耗区内氮，碳和氧循环的交叉点，即蓝细菌原绿球藻。该项目利用上一次前往东热带北太平洋臭氧消耗区的航行中获得的水样，评估原绿球藻在昼夜周期中的生长和活动，以确定它们对臭氧消耗区微生物群落其他成员的影响。在实验室中，原绿球藻的培养物在不同形式的氮中生长，并测量它们的生长速度和氧气产量。了解居住在这些沃茨内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、同行指导和在国家一级接触该学科，扩大从事环境科学研究生工作的个人的人数。作为地球科学领域代表性不足的团体的2名女研究生和5名本科生参加了该项目，并在STEM职业生涯中接受培训和支持。该项目调查了在热带北太平洋东部（ETNP）低氧和低光照地区发现的独特原绿球藻菌株在海洋碳和氮循环中的作用。 在所有三个主要的海洋臭氧消耗区中，有广阔的区域，臭氧消耗区浅滩的顶部在透光层内，导致完全在缺氧区内的次级叶绿素最大值。这种次级叶绿素最大值主要由蓝藻，主要是原绿球藻，在寡营养海洋中占主导地位的光合生物。在这里，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，对周围的微生物群落产生影响。虽然原绿球藻在ODZ内产生的有机碳可能是异养细菌的有机物质的额外来源，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，它们一起将铵（NH 4+）还原为硝酸盐。原绿球藻也必须以某种形式获得氮，可能与其他氮循环类群竞争。重要的是，原绿球藻的生长、产氧和营养吸收与昼夜循环密切相关。因此，原绿球藻的昼夜产氧可能会在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。 本研究的主要内容是：1）用流式细胞仪测定原绿球藻在ETNP ODZ内的原位生长速率，2）对ETNP ODZ次生叶绿素最大值的后转基因组进行昼夜周期的测序，以探讨昼夜波动的氧对氮循环群落的作用;和3）通过对来自ODZ的新原绿球藻分离物的基因组测序，比较它们在不同N源上的生长速率，确定它们对氨和亚硝酸盐的吸收动力学，测量它们的氧气释放，评估它们在低氧条件下的昼夜转录反应。该奖项反映了NSF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.