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Detoxification of Dissolved SO2 (Bisulfite) by Terricolous Mosses
Group 1De Juan, Michelle Ligaya E.
Gamboa, Domina Flor L.Manalaysay, Jessica Alba G.Matundan, Celine Marie C.
*Sulfur dioxide exists in these forms: SO2, SO3-2
and HSO3- (bisulfite)
*Terricolous: Land-dwelling
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INTRODUCTION
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Pleurozium schreberi- calcifuge moss- moderately tolerant of SO2
Rhytidiadelphus triquetrus
- calcareous or calcicole moss- strongly affected by SO2 pollution in the 20th century
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Pleurozium schreberi
Source: http://commons.wikimedia.org/wiki/File:Rhytidiadelphus_triquetrus.JPG
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Rhytidiadelphus triquetrus
Source: http://commons.wikimedia.org/wiki/File:Pleurozium.schreberi.jpg
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⇨ Tolerance of SO2 is seen on the metabolic
detoxification of dissolved bisulfite by these mosses
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Bryophytes (as well as Lichens) are sensitive to atmospheric pollution, particularly SO2
- Limited cuticle- High surface area- Low metabolic activities- Modest innate growth rates
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* Detoxification ⇨ Resistant bryophytes: relatively high growth rates: which means that they have the ability to detoxify
* In higher vascular plants: tolerance on SO2 is by detoxification mechanisms (excluding tolerance by cuticle and stomata)
oxidized to sulfate ion (SO4-) ORreduced to sulfide
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*SO2 Phytotoxicity
Intracellular O2-
production as cause of SO2 phytotoxicity
*Photo-oxidation: oxidation in the presence of radiant energy (light)
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*SO2 Phytotoxicity
Intracellular O2-
production as cause of SO2 phytotoxicity
Superoxide dismutase- Active in SO2 tolerant plants
- Catalyses decomposition of O2-
- Inhibits photo-oxidation of SO2
*Photo-oxidation: oxidation in the presence of radiant energy (light)
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*SO2 Phytotoxicity
Intracellular O2-
production as cause of SO2 phytotoxicity
Superoxide dismutase- Active in SO2 tolerant plants
- Catalyses decomposition of O2-
- Inhibits photo-oxidation of SO2
Diethyl dithiocarbamate (DETC)- Controls activity of superoxide
dismutase
*Photo-oxidation: oxidation in the presence of radiant energy (light)
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* Detoxification like higher vascular plants in bryophytes: Sphagnum- Higher tolerance for plants in more polluted areas- Oxidation of bisulfite: brought about by metal cations (Fe3+, Mn2+ and Cu2+)
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*Focus of the studyPleurozium schreberiRhytidiadelphus triquetrus
⇨ In dilute bisulfite solutions: Photosynthesis in these mosses was strongly inhibited by short (2-hour) incubations with bisulfite ⇨ Longer incubations: no effect → Shoots have a high capacity to detoxify dissolved SO2
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*Hypotheses(a) tolerance of bisulfite by P. schreberi and R.
triquetrus depends primarily on detoxification (oxidation) of the pollutant
(b) bisulfite detoxification involves metabolic energy
(c) Ca2+ and Fe3+ stimulate the detoxification process
(d) SOD is involved in bisulfite detoxification
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MATERIALS AND METHODS
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(1) Pleurozium schreberi: collected from an acid, sandy loam soil under grassland and scrub
(2) Rhytidiaelphus triquetrus: was collected from chalk grassland on a rendzina soil
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(1) Incubation treatmentsShort-term incubation experimentLong-term incubation experiment
* Reagent used: NaHSO3
(2) Bisulfite disappearance in relation to initial concentration
(3) DCMU experiment (3-( 30,40-dichlorophenyl)-1,1-dimethylurea)- Inhibits photosynthetic electron transport and oxygen evolution
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(4) DETC experiment (diethyldithiocarbamate)- Inhibitor of superoxide dismutase- To see if the enzyme plays a role in bisulfite oxidation
(5) Bisulfite oxidation: influence of Ca2+ and Fe3+
(6) Bisulfite and sulfate determinations- Spectrophotometric methods
(7) Statistical analyses- One-way ANOVA- Duncan’s multiple range test
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RESULTS
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* Disappearance of bisulfite during short-term incubations with moss shoots
- Decay of bisulfite greater in the presence of light for both species
- Rhytidiadelphus triquetrus: has a greater capacity in catalyzing disappearance of bisulfite
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*Disappearance of bisulfite during long-term incubations
- Absence of mosses: 28% remaining at the end of the fifth day of incubation
- Presence of mosses: ≈ 95% of bisulfite had disappeared after a three day incubation
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*Disappearance of bisulfite in relation to initial concentration
- Disappearance of bisulfite: dependent on initial concentration and presence of light
(1) Pleurozium schreberi: initial concentration of bisulfite is roughly proportional to the final; no significant effect of light
(2) Rhytidiadelphus triquetrus: total bisulfite lost increased in the presence of light
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*Disappearance of bisulfite in relation to acidity
- pH 3 to 5: small reduction in final volume of bisulfite for both species* Much greater reduction at pH 6
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*Effects of DCMU on disappearance of bisulfite
- Presence of DCMU in both light and dark: great bisulfite persistence
- Twice as much bisulfite remained in the incubation solutions of Pleurozium schreberi as in those of Rhytidiadelphus triquetrus
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*Effects of DETC on disappearance of bisulfite
- inhibited bisulfite loss from the incubation medium in both species
- Pleurozium schreberi: doubling of the concentration of bisulfite remaining
- Rhytidiadelphus triquetrus: even greater increase in concentration of bisulfite remaining
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*Evidence for extracellular oxidation of bisulfite to sulfate: influence of Ca and Fe
- Pleurozium schreberiFe: no differenceCa: more sulfate was detectedEDTA: no change or small increase in sulfate
- Rhytidiadelphus triquetrusFe: small but significant increase in sulfateCa: reduction in sulfate productionEDTA: reduction in sulfate
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DISCUSSION
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- Addition of bisulfiteRapid cessation of photosynthesisIncreasing incubation periods: photosynthesis restored→ Due to oxidation of bisulfite
- Presence of the two mossesGreatly accelerates decrease in amount of bisulfite in incubation solutions
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- Rate of bisulfite loss depends on:(1) Presence or absence of light(2) Application of metabolic inhibitors(3) Acidity(4) Nature and concentrations of adsorbed metal cations(5) Species of moss
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- Light significantly stimulated bisulfite loss from the external solution
- Differences in the degree of photoprotection between the two mosses might also explain their different abilities to detoxify bisulfite solutions
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- DCMU inhibits photosynthetic electron transport and oxygen evolution. It caused a substantially reduced rate of bisulfite loss from the incubation solution, especially in R. triquetrus
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- Experiment with DETC, an inhibitor of SOD, led to a very significant reduction in the rate of bisulfite loss from the incubation solution with both mosses
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- Fe(III) catalysed extracellular oxidation of bisulfite as the pretreatment was accompanied by an increase in the sulfate concentration of the external solution.
- The reduced extracellular sulfate observed could indicate that Ca(II) enhances cellular uptake of bisulfite (indirect effect). - Ca(II) functions for stabilizing cell membranes or
embedded portein channels against loss of permeability control
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- EDTA pretreatement did not cause lowered bisulfite disappearance
- Possibly, EDTA was relatively ineffective in chelating metals such as Fe(III) from the moss shoots.
- Alternatively, the EDTA may have disrupted normal membrane function so that bisulfite uptake or retention rates were modified
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CONCLUSION
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*Loss of bisulfite(1) External oxidation of bisulfite using metabolic (including photooxidative) energy
(2) ‘passive’ external oxidation of bisulfite catalysed by adsorbed Fe3+ ions
(3) cellular uptake and metabolic detoxification of bisulfite
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AuthorsBhagawan Bharali
Jeffrey Bates
Department of Crop Physiology, Assam Agricultural University, Jorhat-785013, Assam, India and 2Division of Biology, Imperial College
London, Silwood Park Campus, Ascot, Berkshire SL5 7PY, UK