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.
Scientific Merit. Every summer in many estuaries and coastal margins, eutrophication-elevated phytoplankton production drives rapid bacterial growth, which consumes oxygen and creates 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 active microbial communities. Once oxygen is depleted, microbes switch to anaerobic metabolisms to respire organic matter, and they follow a succession based on the energetics and availability of the compound they use in place of oxygen (e.g., O2, NO3-, Mn(IV), Fe(III), and SO42-). Anaerobic respiration and anaerobic bacterial production had never been measured simultaneously in the water column of seasonally anoxic estuarine waters, and thus the impact of these zones on estuarine carbon cycles are either unknown or approximated from indirect measurements (e.g., sulfate respiration rate). Our detailed surveys of anoxic bottom waters applied new techniques for measuring bacterial growth, respiration, diversity, and gene expression to better understand the Life in the Dead Zone. We mapped patterns in redox chemistry and demonstrated parallel space-for-time development in which changes associated with aging bottom waters as they moved up-estuary matched changes at a single station over time. From April to October, redox conditions shifted from oxic to hypoxic to sub-oxic to sulfidic, and then shifted back to oxic either gradually (2010) or quickly (2011) following hurricane-associated mixing of the Bay. These changes in redox chemistry paralleled changes in the phylogenetic diversity of bacterial communities, in microbial gene expression patterns, and in heterotrophic activity. Spatially, bottom waters moving up-estuary changed from oxic to hypoxic to sub-oxic to sulfidic, with comparable shifts in phylogenetic diversity and heterotrophic activity. A third formation of this coordinated shift in redox chemistry and biology existed at the pycnocline/oxycline overlying anoxic bottom waters. This gradient varied in thickness with stratification strength, and was often thin and difficult to sample. Our results suggest that this redox gradient hosts a gradient in microbial diversity, heterotrophic activity, and thin layers of intense chemoautotrophic metabolism. Thus, we identified spatial and temporal transitions between oxic and sulfidic waters each of which hosted coordinated shifts in microbial activity, diversity and gene expression. Biological communities were surprisingly active under all redox states, including sulfidic waters where somewhat reduced rates of heterotrophic production were paired with elevated rates of chemoautotrophic production. Broader Impacts. Estuaries are centers of human population, and they provide society with innumerable recreational and economic benefits. Recently, anoxic and hypoxic zones in estuaries came to the public’s attention as indicators of anthropogenic eutrophication. Anoxia/hypoxia is now considered a bellwether of eutrophication in coastal waters, and consequently, most anoxia-related estuarine research is aimed at managing the problem. In contrast, relatively little effort has gone into studying the basic biogeochemistry, microbial ecology and diversity of these waters and the impact of anaerobic respiration on estuarine ecosystems. This research advanced estuarine science by providing (1) reliable measurements of production and respiration in anoxic/hypoxic waters, (2) techniques and protocols to make these measurements in other systems, (3) ecological interpretation for predicting future changes with ongoing restoration efforts in anoxia-impacted estuaries, and (4) novel insight into the diversity and genomics of microorganisms in seasonal anoxic zones using the most current molecular techniques. Mitigation of anoxia/hypoxia is a major goal of many estuary restoration efforts; an improved understanding of the mechanisms involved in coastal anoxia/hypoxia will help resolve the nature of low oxygen zones.
