A Newly Identified Microorganism Affecting the N cycle: Ammonium Oxidation in Iron Reducing Soils
Ammonium (NH4+) oxidation coupled to iron reduction in the absence of oxygen and nitrate/nitrite (NO3–/NO2–) was first noted in a forested riparian wetland in New Jersey, and was coined Feammox. Feammox is a process that can be described as the oxidation of NH4+ under iron reducing conditions, with iron oxides [Ferric iron, Fe(III)] as the electron acceptor. In this reaction Fe(III) is reduced to ferrous iron Fe(II), while NH4+ is transformed to NO2–, nitrogen gas (N2), or other nitrogen forms. We has first identified and recently isolated the pure Acidimicrobiaceae bacterium A6 strain that is responsible for this Feammox process. Feammox is an important process for nitrogen loss in iron rich, acidic soil environments under oxygen-limited conditions. Our results from operating a membrane reactor with a high Acidimicrobiaceae bacterium A6 content (~ 50%) indicated that it might be possible to develop a novel anaerobic NH4+ removal technology from wastewater based on the Feammox process, which might be more robust at low temperatures than Anammox-based processes.
Furthermore we have been able to show that Acidimicrobiaceae bacteria A6 can reduce other trace metals/radionuclides such as U(VI), showing hat natural sites of active ammonium oxidation under Fe reducing conditions by Acidimicrobiaceae bacteria A6 could be hotspots of U immobilization by bioreduction.
The enzyme responsible for the Feammox process has been identified and is related (~ 90% similarity) to methane monooxygenase. Similar to methane monooxygenase, it was shown that this enzyme could aid in the degradation of compounds such as chlorinated ethenes as well as aromatic organics.
Characterization and Optimization of the Feammox Process for the Development of an Energy Efficient Anaerobic Ammonium Removal System
Autotrophic Actinobacteria Acidimicrobiaceae-bacterium named A6 have been linked to anaerobic ammonium (NH4+) oxidation under iron reducing conditions. These organisms obtain their energy by oxidizing NH4+ and transferring the electrons to a terminal electron acceptor (TEA). Under environmental conditions, the TEAs are iron oxides [Fe(III)], which are reduced to Fe(II), this process is known as Feammox. Our studies indicate that alternative forms of TEAs can be used by A6, e.g. iron rich clays (i.e. nontronite) and electrodes in bioelectrochemical systems such as Microbial Electrolysis Cells (MECs), while sustaining NH4+ removal and biomass production. Due to Acidimicrobiaceae–bacterium A6’s ability to use various TEAs, these bacteria represents an interesting alternative for removal of NH4+, an extremely common water contaminant, and also of other toxic compounds (e.g. Uranium). Current work focuses on the optimization of A6-bacteria biomass production in situ using electrodes and in laboratory conditions using different TEAS. The implementation of Feammox for the removal of some pollutants in wastewater treatment is an attractive option as an energy efficient form of ammonium removal as it does not require aeration or heating of the wastewater in temperate zones.
Microbial Community Composition and Shifts in a Nitric Oxide Denitrification Reactor
Different forms of reactive N are responsible for environmental quality problems such as eutrophication (e.g. NH4+, NO2– and NO3–) or green house effect (e.g. nitrous oxide, N2O). In fact, N2O has green house effect 300 times more potent than carbon dioxide (Clais and Sabine 2013). This brings the need to fully denitrify N gases to its non-reactive form. Biological denitrification is the sequential reduction of NO3– or NO2– to nitric oxide (NO), nitrous oxide (N2O) and finally N2. These reduction steps are catalyzed in sequence by four types of nitrogen reductases: Nar, Nir, Nor and Nos. With this information, a study of the bacterial population composition and its changes over time due to changes in their environment can be detected. With a better understanding of the microbial population activity and composition, we can optimize the engineered system to maintain a healthy population of denitrifiers to achieve the objective of fully reducing reactive N.
Biological Removal of Contaminants from Water and Gas Streams
Over 95% of the nitric oxides (NOx) compounds emitted from off-gas in coal-fired power plants are in the form of nitric oxide (NO). Treatment of NO through conventional biological processes (e.g. biofilters and trickling filters) typically requires long contact time due to the low solubility of NO in aqueous systems.
Here we tested the efficiency of hollow fiber membranes bioreactors to solubilize NO from a gas stream into a liquid phase where it was then denitrified. Optimization, detailed parameterizations and scale up will also be investigated in the near future after obtaining required results from the first phase. Different NO flow rates, effects of alternative dosage of NO3–, impacts of liquid flow rates and kinetics of the bioprocess will also be examined and the microbial characterization for each operating condition will be determined.
Feammox Process in Constructed Wetland Mesocosms
To begin applying the Feammox process to natural systems, we are working on the anaerobic ammonium oxidation coupled to iron reduction in constructed wetland (CW) mesocosms. Reactors were designed for monitoring ammonium, Fe (II) levels and other chemicaland/or biological properties along the longitudinal axis during the experiment. The goal is to understand the performances of Feammox process in constructed wetland. The application of Feammox process in constructed wetland would be promising in ammonium removal because of its low energy consumption compared to the traditional nitrification process.
Biological Reduction of Selenium during the Feammox Process
Reduction of heavy metals in wastewater through biological processes using the recently discovered feammox method is a novel area of pollutant mitigation research. The feammox process, conducted by Acidimicrobiaceae bacterium A6 (ATCC, PTA-122488, hereon referred to as A6), has been shown to reduce heavy metals such as Uranium(VI) and Copper(II) simultaneously with coupled iron reduction and ammonium oxidation in the absence of oxygen. This process thus suggests a novel environment in which to biologically remediate other metal contaminants. As industrial wastewater is high in both ammonium and heavy metals, this environment is predicted to be ideal for the application of the feammox process.
Selenium is a common heavy metal pollutant in industrial wastewater, particularly from power plants, metal mining and smelting practices, landfills, oil refinery and agricultural irrigation. The primary forms in wastewater are selenite and selenate, and both are highly mobile. We are investigating the use of A6 to reduce selenium for potential use in industrial treatment processes.
Biodegradation of TCE and PCE by Feammox process
Trichloroethylene (TCE) and Tetrachloroethylene (PCE) are two of the five most frequently reported organic contaminants in groundwater. This study investigates the correlation between biological degradation of the two components and ammonium oxidation under iron (Fe) reducing conditions. Compared to ferrihydrite, nontronite was determined to be an acceptable Fe(III) source due to lower TCE sorption. Incubations of Feammox bacteria and nontronite with TCE or PCE indicated that these bacteria could reduce both TCE and PCE. The production of Fe(II) suggested that Fe(III) was an electron acceptor during the Feammox process. Incubations of Feammox bacteria and nontronite with TCE at different pH levels showed that the highest TCE removal was obtained at pH 4. The degradation efficiency was higher than 40% for both TCE and PCE after one week. These results implied that natural sites of active ammonium oxidation under Fe reducing conditions by Feammox bacteria could be hotspots of TCE and PCE bioreduction. To the best of our knowledge, this is the first report of biological TCE and PCE reduction that is not coupled to carbon oxidation.
Effects of nutrients and iron loading on arsenic dynamics in wetland sediments
We are investigating the effect of phosphate (PO43-), sulfate (SO42-) and iron (Fe(III)) reduction on As dynamics and its bioaccumulation in wetlands using greenhouse mesocosms. Results indicate that high Fe (50µM ferrihydrite/g solid medium) and SO42- (5mM) treatments are most favorable for As sequestration in the solid medium in the presence of wetland plants (Scirpus actus), because root exudates facilitate the microbial reduction of Fe(III), SO42-, and As(V) to sequester As(III) by incorporation into iron sulfides and/or plant uptake. Whereas, in the Fe(III)-rich solid medium with plants, high PO43- (100 µM) loading strongly enhanced As release into pore water under anoxic conditions, possibly due to the competitive sorption between PO43- and As(V) as well as the reductive dissolution of Fe(III) and As(V).
Evolution of pore water As depth profiles in a variety of experimental conditions. Low SO42− (LS) and high SO42− (HS) treatments contained 0.1 mM and 5 mM SO42−, respectively. Low Fe (LFe) and high Fe (HFe) consisted of no ferrihydrite addition and 50µM ferrihydrite/g solid medium addition, respectively. Planted mesocosms were planted with Scirpus actus. ((a) LS LFe without plants, (b) LS HFe without plants, (c) LS LFe with plants, (d) LS HFe with plants, (e) HS LFe without plants, (f) HS HFe without plants, (g) HS LFe with plants, (h) HS HFe with plants))
Impacts of stormwater green infrastructure on hydrology and nutrient fluxes
Stormwater green infrastructure (SGI), including rain gardens, detention ponds, constructed wetlands, bioswales, green roofs, and tree plantings, is being implemented in cities across the globe to help reduce flooding, decrease combined sewer overflows, and lessen pollutants being transported to urban streams and rivers. Despite the increasing use of urban SGI, little is known about the cumulative effects of multiple SGI projects on hydrologic and water quality at the watershed scale. In recent years we have conducted detailed nutrient balances on urban detention ponds as well as study the effect of green infrastructure on urban watersheds.
Effect of stormwater green infrastructure (SGI) on a) NO3–, b) TN, c) PO43-, and d) TP annual loads. Each point represents average annual nutrient export (for years 2011-2013) for each watershed in this study.
Measuring Hydrogen Gas Regulation of Methane Gas in Wetland Sediments
Wetland soils are amongst the most active soils in nature and are especially effective in sequestering carbon. However, because of the saturated conditions of these soils they are also hotspots for methanogenic activity. Wetland soils are the largest natural sources of methane, a potent greenhouse gas (GWP = 25-34), and account for upwards of 20% of all methane released into the atmosphere. Although, rice paddies are not classified as natural wetlands, they are also responsible for extremely high methane emissions.
A few years ago, the International Rice Research Institute (IRRI) proposed to produce a rice plant that limits gas transport through its stalk, with the hopes of lowering methane emissions to the atmosphere. We hypothesize that these stalks may decrease the volatilization of additional trace gases such as hydrogen (which can act as a methane precursor) which may actually increase methane production in soils. We are examining this hypothesis through field sampling with peepers, small scale soil incubations, and mesocosm experiments in the greenhouse.
Immobilization of Trace Metals in Tidal Wetlands
Both constructed and natural wetlands can be used to reduce contaminant concentrations in water. We have studied two tidal marshes in the New Jersey Meadowlands to compare their ability to remove a variety of trace metal contaminants (Cd, Cr, Cu, Mn, Pb, and Zn) from water.
Recent analysis of sediment cores and surface and pore-water samples indicates that Cr, Mn, and Pb are largely immobilized by precipitation with sulfides. Measurements reveal no net trace metal accumulation in the sediments, indicating a dynamic equilibrium of trace metal flux into and out of the sediments. Different flooding scenarios (sustained and intermittent) did not result in differences in the metal immobilization potential of the constructed marsh. The newly constructed wetland immobilized significantly more Cr, Mn, and Zn than the natural marsh and similar concentrations of Cd, Cu, and Pb, possibly due to increased redox buffering through a larger FeS pool buffering against tidally induced oxygen delivery.
Stability of Immobilized Uranium in Wetland Sediments
Uranium contamination in soil and groundwater is a serious problem due to this heavy metal’s biological toxicity. In-situ microbial reduction of water-soluble uranium(VI) to insoluble uranium(IV) has shown to be a promising remediation technique.
We have conducted laboratory and greenhouse experiments focusing on the dynamics of uranium in wetland systems, studying uranium speciation as well as the iron and microbial dynamics amongst roots. Since wetland sediments cycle iron and plants provide a carbon source via root exudates and plant turnover, wetland sediments seem ideal environments for biological uranium immobilization by iron-reducing bacteria. Furthermore, since droughts and seasonal water level changes might re-mobilize previously bio-immobilized uranium by exposure to oxygen, we have studied the effect of periodic water level fluctuations on the stability of reduced uranium.
Gas Transfer Processes in Wetlands
Wetlands are important ecological tools for water quality protection, and soil-atmosphere exchanges of carbon and nitrogen gases are key components of wetland nutrient budgets. A detailed description of the physical-chemical processes governing the dynamics of dissolved gases in wetland pore waters is essential for linking biochemical production and/or consumption with eventual migration to the surface, yet these transport processes are poorly represented in biogeochemical models.
In this research, we use gas tracer measurements in well-controlled laboratory experiments and natural wetland environments to quantify the kinetics of root-mediated gas exchange between pore water and the roots of wetland plants, which is a critical gas transport pathway in saturated soils. An important research initiative has been to combine dissolved gas tracers with push-pull measurements and simple analytical models to infer the root-mediated gas exchange kinetics in situ and examine spatial and temporal variability in gas exchange rates.
Nitrogen Modeling under Land Use and Climate Changes
To characterize implications of human and climate driven perturbation in the Earth N cycling and its implication for water and air quality, the next-generation of N cycling models need to (1) account for regional and local changes in terrestrial and aquatic ecosystem structure and functioning, (2) represent in a consistent manner emissions and transformation of N to air, rivers, and coasts, and (3) be global in extent and integrated with climate and earth system models.
The new GFDL land model LM3-TAN is capable of addressing these challenges and assessing the combined effects of direct human influences and climate change on Terrestrial and Aquatic Nitrogen (TAN) cycling. LM3-TAN was evaluated in the largest of the watersheds in the northeastern U.S., Susquehanna River Basin (71,220 km2) at the resolution of 1/8 degree. The model simulates well N soil budget and losses, trends and variability in riverine N inputs and exports of nitrate, ammonium, and dissolved organic N.