Grantee Research Project Results
Final Report: Physiological Mechanisms of Estuarine Sediment Oxidation by Spartina Cordgrasses
EPA Grant Number: R829406Title: Physiological Mechanisms of Estuarine Sediment Oxidation by Spartina Cordgrasses
Investigators: Lee, Raymond W.
Institution: Washington State University
EPA Project Officer: Aja, Hayley
Project Period: November 1, 2001 through October 31, 2004
Project Amount: $110,307
RFA: Phytoremediation (2001) RFA Text | Recipients Lists
Research Category: Hazardous Waste/Remediation , Land and Waste Management
Objective:
In waterlogged soils, rates of microbial degradation of organic pollutants are limited by the availability of oxygen. Extreme and chronic absence of oxygen and highly negative redox potentials are a characteristic of estuarine sediments. Very few vascular plant species can tolerate these conditions, but cordgrasses of the genus Spartina are one of the most successful species that inhabit estuarine marshes. Spartina are thought to oxidize estuarine sediments and transport atmospheric oxygen to belowground tissues and the rhizosphere. Consequently, the basic understanding of how this genus tolerates anoxic conditions and the mechanisms and rates of oxygen transport are of potential application to phytoremediation of wetlands and estuaries.
The original objectives of this project were to: (1) determine rates of oxygen transport and release by Spartina grasses; (2) identify species and strains of Spartina that have enhanced oxygen release capabilities; and (3) determine the mechanisms that facilitate oxygen transport and how transport is induced by environmental and hormonal signals.
Summary/Accomplishments (Outputs/Outcomes):
Rates of Oxygen Transport and Release by Spartina Grasses
Rates of oxygen release and transport to the rhizosphere were determined using static chamber and flow through respirometry. From these experiments we were able to determine rates of oxygen respiration by root tissues as well as fluxes of oxygen from above-ground tissues to the roots and the surrounding medium. In the course of these investigations, measurements have been conducted on over 350 individual plants. Additional experiments were conducted to test for oxygen release by roots when they are not immersed in water (i.e., simulated dry soil conditions). Also experiments are in progress investigating the dependence of respiration and transport on external oxygen concentrations.
Identify Species and Strains of Spartina That Have Enhanced Oxygen Release Capabilities
Oxygen transport rates have been measured in Spartina alterniflora, S. anglica, S. patens, S. densiflora, S. foliosa, S. alterniflora x S. foliosa hybrid, S. pectinata, Distichlis spicata, and oat (Avena sativa). We are presently propagating S. argentinensis, S. cynosuroides, S. bakeri, S. maritime, S. gracilis, S. spartinae, corn (Zea mays), and rice (Oryza sativa) for further investigation. In oxygen release trials conditions including with or without light, salinity, and removal and blockage of above-ground tissues were tested.
Role of Environmental and Hormonal Signals
Environmental conditions were tested for possible roles in enhancing oxygen transport. As stated above, light conditions were tested as well as removal and blockage of above ground tissues.
Salinity. Intensive investigations of the effects of salinity on parameters related to oxygen transport have been tested. Saline conditions have been thought to reduce performance of wetland plants including Spartina grasses. Consequently, the effects of salinity are of importance in growth and survival of plants used in phytoremediation efforts as well as possible aeration of polluted sediments. Effects of salinity on photosynthesis (oxygen source) and stomatal conductance (diffusion pathway) were determined.
Induction of Oxygen Transport by Flooding Conditions (Hypoxia). Comparisons were made between plants grown under conditions of constant flooding versus dry (aerated) conditions. These studies investigated the potential of oxygen transport and other physiological parameters to be induced by oxygen stress on the rhizome.
Hormone Cetyl CoA Carboxylase (ACC). Ethylene has been postulated to play a role in the formation of gas conduction aerenchyma structures. Exposure of plants to ACC (a precursor for ethylene) was investigated as a means of simulating upregulation of hormonal pathways that could result in enhancement of gas conduction. However, experiments performed as part of a student independent research project indicated that exposure of plants to ACC resulted in rapid plant senescence. Consequently, possible confounding effects of senescence made this avenue of investigation unlikely to succeed.
Differences in Aerobic and Anaerobic Respiration in Spartina Species
Rates and potential for aerobic and anaerobic respiration are important for survival of plants in estuarine environments. The characteristics of plants that are well adapted to anoxic soils, and hence candidate species for phytoremediation, were determined. In addition, the potential oxygen sink caused by aerobic respiration may affect radial oxygen release into the environment. Rates of aerobic respiration were determined by respirometric measurements of oxygen consumption. The enzymatic potential for aerobic respiration was measured in root tissues by assaying for activity of the mitochondrial enzymes citrate synthase and cytochrome c oxidase. Potential for anaerobic respiration was determined by measuring activities of alcohol dehydrogenase, which catalyzes the terminal step in the anaerobic fermentation pathway.
Metabolism of Pollutants Using 13C Labeled Tracer Additions
Preliminary investigations were conducted to investigate whether Spartina plants accelerate rates of organic pollutant degradation. 13C labeled toluene was added to potted plants, then the interstitial water was analyzed for residual 13C toluene (to determine toluene removal rates) or 13C carbon dioxide (to determine toluene degradation rates). 13C toluene was found to be too volatile to be readily analyzed. Toluene was extracted from the interstitial water onto C18 resin, and the resin was then analyzed by continuous flow mass spectrometry. Analyses showed that toluene rapidly volatilized from the resin when it was exposed to atmosphere prior to analyses. Further experiments were conducted to test the feasibility of measurement of 13C carbon dioxide rather than 13C toluene in water samples as a measure of toluene degradation. 13C carbon dioxide dissolved in porewater could be analyzed, although there was a moderate degree of variation in 13C levels over time in control plants (no toluene added) that is potentially caused by fractionation of naturally occurring 13C by plants or associated microorganisms. It is likely that high tracer additions of 13C toluene to overcome background 13C fluctuations could be used to determine rates of toluene degradation.
Further activities have been directed at determining stable isotope enrichment in individual compounds by gas chromatography coupled to mass spectrometry. This method development is ongoing and may potentially be used to trace 13C from pollutant tracer additions into compounds such as fatty acids. Fatty acid methyl ester (FAME) analysis provides a fingerprint of processes in soil samples. Certain FAME compounds are specific to groups such as bacteria, fungi, and plants. Incorporation of 13C into FAME compounds may potentially be used to assay for metabolism of 13C pollutants and also provide information as to which organisms in the rhizosphere are involved.
Rates of Oxygen Transport and Differences Among Species
In the field, it has been observed that S. anglica grows lower in the intertidal range than any other emergent plant. This suggests that it is the most tolerant Spartina species with respect to anoxia and sulfide exposure in mudflat conditions. This tolerance may have a physiological basis because of oxygen transport, as oxygen transport rates in S. anglica were higher than in any other species we have tested. Although oxygen transport has widely been assumed to regulate differences in estuarine zonation, we found that high intertidal marsh plants like S. patens and S. densiflora may transport as much oxygen internally as low intertidal marsh species like S. alterniflora. Therefore, other factors may play an important role in the tolerance of estuarine mudflat conditions.
Effects of Environmental Factors on Oxygen Transport
Flooded soil conditions resulted in increased oxygen transport rates compared to drained conditions in S. alterniflora, S. anglica, S. densiflora, and S. patens. Flooding had no effect on oxygen transport in D. spicata. Increasing salinity had no effect on oxygen transport in any species.
Salinity Effects on Photosynthesis, and Stomatal Conductance
Increasing salinity above 20 percent significantly reduced stomatal conductance in S. alterniflora, S. anglica, S. densiflora, S. patens, and D. spicata. There were no significant differences in gross photosynthetic rates between any species, treatment, or their interactions. Net rates of photosynthesis were lower in 30 percent salt compared to 0 percent salt in S. patens, S. alterniflora, and D. spicata. There were no significant differences in net photosynthetic rate between salinity treatment in S. anglica or S. densiflora.
Aerobic and Anaerobic Respiration in Spartina Species
Respiration rates appeared to be important in determining sensitivity to estuarine mudflat conditions. The high marsh species S. patens, S. densiflora, and D. spicata appeared to have high aerobic respiration rates and aerobic enzyme activity. This aerobic oxygen demand may be too high to allow survival in anoxic low marsh conditions, where plants had lower aerobic demand. Anaerobic pathways (root alcohol dehydrogenase [ADH] activities) increased after flooding in all three high marsh species suggesting a high sensitivity to soil waterlogging. In contrast, plants inhabiting low marsh regions must be able to tolerate highly reducing and sulfidic sediments. The low marsh species S. alterniflora, and S. anglica exhibited low aerobic respiration rates and CytOx activity, which when coupled to high rates of internal oxygen transport, may pose a significant advantage for survival under anoxia. Low marsh species must also possess an ability to respire anaerobically because demand for oxygen from highly reduced sediments may overwhelm transport processes. ADH activities measured in S. alterniflora roots indicated a well-developed capacity for fermentation. However, increases in root ADH were not observed in S. anglica. High rates of oxygen transport in S. anglica may be adequate to supply oxygen to roots and external sinks, as suggested by low root ADH activities.
Conclusions:
The original objectives of the proposal were met during the award period. We have documented the oxygen transport capabilities of several Spartina species and have characterized environmental effects on oxygen transport and other aspects of physiology. Hormonal regulation aspects of the project were not intensively investigated after the experimental protocol proved to result in plant senescence. The project was expanded to include biochemical investigations of aerobic and anaerobic metabolism as well as regulation of gas conductance via stomatal resistance. Overall, the physiological and biochemical information acquired from this project should contribute significantly to the understanding of sediment aeration by Spartina species and the physiology of anoxia tolerant plants. Future phytoremediation work in wetlands will require identification of such traits in plants.
Further investigations are underway to characterize oxygen transport and anoxia adaptations in additional Spartina species. In collaboration with phylogenticists, who are working on a molecular phylogeny of the genus Spartina, we anticipate that the combination of these data sets may shed light on the evolution of oxygen transport and anoxia tolerance in Spartina.
Additional future investigations will involve the development of compound specific isotope measurements to assess the role of different microbial and plant groups in metabolism of C and N in soils. These studies will have potential in determining the rates, processes, and fates of C and N from organic pollutants.
Journal Articles on this Report : 2 Displayed | Download in RIS Format
Other project views: | All 18 publications | 3 publications in selected types | All 2 journal articles |
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Type | Citation | ||
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Lee RW. Physiological adaptations of the invasive cordgrass Spartina anglica to reducing sediments: rhizome metabolic gas fluxes and enhanced O2 and H2S transport. Marine Biology 2003;143(1):9-15. |
R829406 (2002) R829406 (Final) |
not available |
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Maricle BR, Lee RW. Aerenchyma development and oxygen transport in the estuarine cordgrasses Spartina alterniflora and S. anglica. Aquatic Botany 2002;74(2):109-120. |
R829406 (2002) R829406 (Final) |
not available |
Supplemental Keywords:
aquatic, marine science, biology, bioremediation, biodegradation, estuarine sediments, organic pollutants, phytoremediation, plant-based remediation,, Scientific Discipline, Waste, Water, Contaminated Sediments, Remediation, Environmental Chemistry, Chemistry, Microbiology, Environmental Microbiology, Bioremediation, Biology, aquatic ecosystem, plant-based remediation, degradation, degradation of organic pollutants, organic pollutants, microbial degradation, biodegradation, estuarine sediments, bioremediation of soils, aquatic ecosystems, phytoremediation, plant-microbe systemProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.