Collaborative Research: Life in the Dead Zone: Microbial respiration, production, diversity and gene expression in seasonally anoxic estuarine waters

每年夏天，在许多河口和海岸边缘，富营养化提高了浮游植物的产量，促使细菌快速呼吸，造成缺氧和缺氧的底部水域。这些所谓的“死亡区”排除了鱼类，杀死了底栖生物，并消除了栖息地。尽管它们的名字很流行，但缺氧区并不是真正的死亡，而是充满了活跃的微生物群落。事实上，缺氧水体中的细菌产量可能超过上覆的含氧水体，部分原因是放牧减少，细胞大小和丰度增加。一旦氧气耗尽，微生物呼吸就会经历一系列氧化还原反应，随着终端电子受体（如O2、NO3-、Mn （IV）、Fe （III）和SO42-）的耗尽，能量产量下降。这种高产量和低生长效率的结合创造了呼吸可能非常高的条件，使缺氧区成为有机物的重要汇和营养循环的关键场所。以前的研究基于氧化还原化学测量记录了切萨皮克湾底部水域的呼吸演替。在这个演替的某些阶段，异养细菌的产量非常高，表明呼吸作用的增加。此外，缺氧水体中浮游细菌群落的系统发育组成在近半个夏季与缺氧地表水相似，只有在H2S出现后才发生变化。这表明典型的需氧河口细菌能够转变为无氧代谢并继续占主导地位。大多数关于缺氧水中微生物呼吸和群落组成的知识来自对黑海和卡里亚科盆地等永久缺氧系统的研究。相比之下，人们对更常见、更有活力的海洋环境——季节性缺氧的河口水域知之甚少。该项目将进行为期3年的综合研究，以促进对美国最大的季节性河口缺氧带之一的生物地球化学循环的定量和机制理解。PIs假设：1)占优势的次斜呼吸过程经历了从有氧呼吸到硝酸盐呼吸和金属还原到硫酸盐还原的演替过程；2)细菌生长效率随着呼吸演替而降低，但细菌产量仍然很高，导致碳呼吸速率非常高；3)在呼吸演替过程中，细菌群落组成变化不大，直到硫酸盐呼吸占主导地位（即硫化物阈值），但基因表达密切跟踪氧化还原条件的变化，以支持最具活力的呼吸过程。pi将通过使用几种技术量化碳呼吸速率来解决这些假设，包括碳呼吸速率；定量细菌产量、生物量和生长效率；并对呼吸演替各阶段水柱和沉积物中微生物群落组成和呼吸基因表达模式的演替特征进行了分析。该项目将整合生物地球化学、生物学和基因组数据，以解释生物地球化学如何影响微生物呼吸、生产、多样性和基因表达，以及受微生物呼吸、生产、多样性和基因表达的影响。更广泛的影响。该项目将提供(1)缺氧/低氧水域生产和呼吸的可靠测量，(2)适用于其他生态系统的技术，以及(3)预测缺氧影响河口正在进行的恢复工作的未来变化的生态洞察力。这些测量将有助于校准生物地球化学模型和估计碳收支。两名研究生和一名博士后科学家将接受几项最先进的地球化学和分子生物学技术的培训。两名教师将参与该项目，并参加UMCES霍恩角实验室为期七周的环境科学教育伙伴关系（ESEP）教师研究奖学金计划（www.esep.umces.edu）。新的发现将被纳入研究生水平的课程：水生微生物生态学、生物海洋学和环境地球化学。核酸序列将存入在线存储库，包括GenBank。

Every summer in many estuaries and coastal margins, eutrophication elevated phytoplankton production drives rapid bacterial respiration creating hypoxic and anoxic bottom waters. These so-called "dead zones" exclude fish, kill benthic organisms, and eliminate habitat. Despite their popular name, anoxic/hypoxic zones are not really dead, but rather are populated with living and very active microbial communities. In fact, bacterial production in anoxic waters can exceed that in overlying oxic waters due, in part, to reduced grazing and increased cell size and abundance. Once oxygen is depleted, microbial respiration undergoes a succession of redox reactions with decreasing energy yield as terminal electron acceptors are depleted (e.g., O2, NO3-, Mn (IV), Fe (III), and SO42-). This combination of high production and reduced growth efficiency creates a condition in which respiration may be very high, making anoxic zones significant sinks for organic matter and key sites for nutrient cycling. Previous research documented respiratory succession in Chesapeake Bay bottom waters based on redox chemistry measurements. Heterotrophic bacterial production was very high at some stages of this succession, suggesting elevated respiration. Also, the phylogenetic composition of bacterioplankton communities in anoxic waters was similar to oxic surface waters for nearly half the summer, only changing after the appearance of H2S. This suggests that typical aerobic estuarine bacteria are able to shift to anaerobic metabolisms and continue to dominate. Most of what is known about microbial respiration and community composition in anoxic water comes from studies of permanently anoxic systems like the Black Sea and Cariaco Basin. By comparison, very little is known about what is a much more common and more dynamic marine environment - seasonally anoxic estuarine waters. This project will conduct a 3-year integrated study to advance the quantitative and mechanistic understanding of biogeochemical cycling in one of the largest seasonal estuarine anoxic zones in the USA. The PIs hypothesize that: 1) Dominant sub-pycnocline respiratory processes undergo a succession from aerobic respiration to nitrate respiration and metal reduction to sulfate reduction; 2) Bacterial growth efficiency decreases with this respiratory succession, but bacterial production remains high, resulting in very high carbon respiration rates; 3) Bacterial community composition changes little during respiratory succession until sulfate respiration dominates (i.e., the sulfide threshold), but gene expression closely tracks changes in redox conditions in order to support the most energetic respiratory processes. The PIs will address these hypotheses by quantifying carbon respiration rates using several techniques including carbon respiration rate; quantifying bacterial production, biomass and growth efficiency; and characterizing succession in the composition and respiratory gene expression patterns of microbial communities in water column and sediments during each stage of respiratory succession. This project will integrate biogeochemical, biological, and genomic data to explain how biogeochemistry influences, and is influenced by, microbial respiration, production, diversity, and gene expression.Broader Impacts. The project will provide (1) reliable measurements of production and respiration in anoxic/hypoxic waters, (2) techniques applicable to other ecosystems, and (3) ecological insight for predicting future changes with ongoing restoration efforts in anoxia-impacted estuaries. These measurements will be useful for calibrating biogeochemical models and for estimating carbon budgets. Two graduate students and one postdoctoral scientist will be trained in several state-of-the-art geochemical and molecular biology techniques. Two teachers will be engaged to work on this project and to participate in the seven-week Environmental Science Education Partnership (ESEP) Teacher Research Fellowship Program (www.esep.umces.edu) at UMCES Horn Point Laboratory. New discoveries will be incorporated into graduate-level courses entitled Aquatic Microbial Ecology, Biological Oceanography, and Environmental Geochemistry. Nucleic acid sequences will be deposited in online repositories including GenBank.