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有多达一半是由海洋缺氧区造成的。氮是浮游植物的关键限制营养物质，因此海洋中氮的损失对海洋光合作用的总量产生了影响，海洋光合作用将二氧化碳从大气中移除，并为海洋食物网提供基础。随着海洋温度的升高，预计缺氧区的大小将会增加。这个项目的重点是一种微生物，它位于海洋ODZ中氮、碳和氧循环的交叉点，即蓝藻原氯球菌。该项目利用前一次前往东热带北太平洋ODZ邮轮时采集的水样，评估原氯球菌在昼夜循环中的生长和活性，以确定它们对ODZ微生物群落其他成员的影响。在实验室里，原氯球菌正在不同形态的氮气中生长，并测量它们的生长速度和产氧量。了解居住在这些水域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，通过在国家一级为代表不足的少数群体的本科生提供早期研究经验、持续的财政支助、同行指导和接触该学科，扩大环境科学研究生的人才库。两名女研究生和五名本科生是地球科学中代表性不足的群体的成员，他们参加了该项目，并在STEM职业生涯中接受培训和支持。该项目调查在东热带北太平洋(ETNP)的低氧和低光照地区发现的独特的原氯球菌菌株在海洋碳和氮循环中的作用。在所有三个主要海洋ODZ中，ODZ浅滩的顶部都有宽阔的区域，位于光学带内，导致次生叶绿素最大值完全在缺氧带内。这种次生的叶绿素最大值主要由蓝藻主导，主要是原氯球菌，这是寡养海洋中数量上占优势的光生生物。在这里，原氯球菌在非常弱的光照下进行光合作用，固定碳并产生氧气，这对周围的微生物群落有影响。尽管ODZ内原氯球菌产生的有机碳可能是异养反硝化细菌的额外有机物来源，促进了氮气(N2)的产生，但伴随的氧气产生可能为硝化古菌和细菌创造了一个有利的栖息地，它们共同将铵(NH4+)转化为硝酸盐。原氯球菌还必须以某种形式获得氮，这可能会与其他氮循环分类群竞争这样做。重要的是，原氯球菌体的生长、产氧和营养吸收与二次循环密切相关。因此，原氯球菌产氧可能会在一天的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。该项目是1)使用流式细胞术建立ETNP ODZ内原氯球菌种群的原位增长率；2)对ETNP ODZ次生叶绿素最大值中的偏受试者进行测序，以探索日变化的氧气对N循环群落的作用；以及3)通过对来自ODZ的新型原氯球菌分离株的基因组测序，比较它们在不同氮源上的生长速度，确定它们对氨和亚硝酸盐的吸收动力学，测量它们的放氧量，并评估它们在低氧条件下的转录反应，来表征它们的特征。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的审查标准进行评估，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.