Lemna Gibba Research Articles

Nitrogen mass balance across pilot-scale algae and duckweed-based wastewater stabilisation ponds

Nitrogen removal processes and nitrogen mass balances in algae-based ponds (ABPs) and duckweed ( Lemna gibba)-based ponds (DBPs) were assessed during periods of 4 months, each under different operational conditions. During periods 1 and 2, the effect of cold and warm temperature was studied. During periods 2 and 3, the effect of low- and high-system organic loading (OL) was studied in warm seasons operation. The pilot-scale systems consisted of four similar ponds in series fed with domestic sewage with hydraulic retention time of 7 days in each pond. Overall nitrogen removal was higher during warm temperature in both ABPs and DBPs, but similar during periods 2 and 3. Nitrogen removal in DBPs was lower than in ABPs by 20%, 12% and 8% during cold temperature, warm temperature and high-OL periods, respectively. Depending on temperature and OL rate, ABPs showed higher nitrogen removal via sedimentation (46–245% higher) compared to DBPs. Also, ABPs also showed higher nitrogen removal via denitrification (7–37% higher) compared to DBPs. Ammonia volatilisation in both systems did not exceed 1.1% of influent total nitrogen during the entire experimental period. N uptake by duckweed corresponds to 30% of the influent nitrogen during warm/low OL period and decreased to 10% and 19% during the cold and warm/high OL period, respectively. Predictive models for nitrogen removal presented a good reflection of nitrogen fluxes on overall nitrogen balance under the prevailing experimental conditions.

Similar stress responses are elicited by copper and ultraviolet radiation in the aquatic plant Lemna gibba: implication of reactive oxygen species as common signals.

Metals and ultraviolet (UV) radiation are two environmental stressors that can cause damage to plants. These two types of stressors often impact simultaneously on plants and both are known to promote reactive oxygen species (ROS) production. However, little information is available on the potential parallel stress responses elicited by metals and UV radiation. Using the aquatic plant Lemna gibba, we found that copper and simulated solar radiation (SSR, a light source containing photosynthetically active radiation (PAR) and UV radiation) induced similar responses in the plants. Both copper and SSR caused ROS formation. The ROS levels were higher when copper was combined with SSR than when applied with PAR. Higher concentrations of copper plus PAR caused toxicity as monitored by diminished growth and chlorophyll content. This toxicity was more pronounced when copper was combined with SSR. Because the generation of ROS was also higher when copper was combined with SSR, we attributed this enhanced toxicity to elevated levels of ROS. In comparison to PAR-grown plants, SSR treated plants exhibited elevated levels of superoxide dismutase (SOD) and glutathione reductase (GR). These enzyme levels were further elevated under both PAR and SSR when copper was added at concentrations that generated ROS. Interestingly, copper treatment in the absence of SSR (i.e. copper plus PAR) induced synthesis of the same flavonoids as those observed in SSR without copper. Finally, addition of either dimethyl thiourea or GSH (two common ROS scavengers) lowered in vivo ROS production, alleviated toxicity and diminished induction of GR as well as accumulation of UV absorbing compounds. Thus, the potential of ROS being a common signal for acclimation to stress by both copper and UV can be considered.

Callus induction and regeneration in Spirodela and Lemna.

The development of tissue culture systems in duckweeds has, to date, been limited to species of the genus Lemna. We report here the establishment of an efficient tissue culture cycle (callus induction, callus growth and plant regeneration) for Spirodela oligorrhiza Hegelm SP, Spirodela punctata 8717 and Lemna gibba var. Hurfeish. Significant differences were found among the three duckweed species pertaining to carbohydrate and phytohormone requirements for callus induction, callus growth and frond regeneration. In vitro incubation with poorly assimilated carbohydrates such as galactose ( S. oligorrhiza SP and L. gibba var. Hurfeish) and sorbitol ( S. punctata 8717) as sole carbon source yielded high levels of callus induction on phytohormone-supplemented medium. Sorbitol is required for optimal callus growth of S. oligorrhiza SP and S. punctata 8717, while sucrose is required for callus growth of L. gibba var. Hurfeish. Sucrose either alone ( S. oligorrhiza SP, L. gibba var. Hurfeish) or in addition to sorbitol ( S. punctata 8717) is required for frond regeneration.

Response of the zooplankton community and environmental fate of perfluorooctane sulfonic acid in aquatic microcosms.

Little is known regarding perfluorooctane sulfonic acid (PFOS) toxicity to freshwater organisms. This field evaluation aims to assess the toxicological risk associated with exposure to PFOS across levels of biological organization. The analysis of variance study was conducted in replicate (n = 3) 12,000 L outdoor microcosms. Multivariate techniques were used to assess the response of zooplankton community structure and dynamics, as well as a floating macrophyte, Lemna gibba. The zooplankton community was significantly affected (p < 0.05) by the treatment regime given by the Monte Carlo permutations for all sampling times. A community-level no-observable-effect concentration ([NOEC]community) of 3.0 mg/L was determined for the 35-day study, however, longer term studies are recommended. The most sensitive taxonomic groups, Cladocera and Copepoda, were virtually eliminated in 30 mg/L treatments after 7 d. The 42-d 50% inhibition concentration (IC50) for L. gibba frond number was 19.1 mg/L and the NOEC was 0.2 mg/L. Furthermore, we investigated the persistence of PFOS over 285 d in microcosms under natural conditions. Perfluorooctane sulfonic acid concentration showed no drastic reduction in any treatment microcosm over the entire study period, confirming that this compound undergoes little degradation in aquatic systems. Presently, there appears to be little hazard to these freshwater organisms at reported environmental concentrations.

Comparison of ammonia volatilisation rates in algae and duckweed-based waste stabilisation ponds treating domestic wastewater

Quantification of ammonia volatilisation from wastewater stabilisation ponds is important in order to understand its significance for overall nitrogen removal in these widely applied low-cost treatment systems. Ammonia volatilisation rates were measured in pilot plant facilities consisting of one line of four algae-based ponds in series and a parallel line of four ponds with a floating mat of duckweed ( Lemna gibba). Ammonia volatilisation was assessed during a period of one and a half years. The method applied is accurate, convenient and is proposed for analysis of a wide range of gasses emitted from stabilisation ponds and possibly other aquatic systems. The ammonia volatilisation rates in algae-based ponds (ABPs) were higher than in duckweed-based ponds (DBPs). This can be explained by the lower values of NH 3 in DBPs due to shading and lower pH values, since the volatilisation rate highly correlated with free ammonia concentration (NH 3) in pond water. The duckweed cover appeared not to provide a physical barrier for volatilisation of unionised ammonia, because whenever NH 3 concentrations were equal in ABP and DBP also the volatilisation rates were equal. Volatilisation was in the range of 7.2–37.4 mg-N m −2 d −1 and 6.4 –31.5 mg-N m −2 d −1 in the ABPs and DBPs, respectively. Average influent and effluent ammonium nitrogen measurements showed that the ammonia volatilisation during the study period in any system did not exceed 1.5% of total ammonium nitrogen removal. Therefore this study confirmed results from simultaneous experimental work in our laboratory indicating that nitrification/denitrification, rather than ammonia volatilisation, is the most important mechanism for N removal in ABPs and DBPs.

Chronic ammonia toxicity to duckweed-fed tilapia ( Oreochromis niloticus)

The effects of prolonged exposure to sub-lethal un-ionised ammonia nitrogen (UIA-N) concentrations on the growth performance of juvenile Nile tilapia ( Oreochromis niloticus) fed on fresh duckweed ( Lemna gibba) grown on pretreated domestic sewage have been investigated. The experiment was conducted over 75 days using juveniles with a mean body weight of 20 g. Five nominal, total ammonia nitrogen concentrations (control, 2.5, 5, 7.5 and 10 mg N l −1) were established as treatment groups. Statistical analysis of the specific growth rate (SGR) showed no significant ( p>0.05) differences between the SGR (0.71) of tilapia in the control (0.004 mg UIA-N l −1) and the SGR (0.67) of those exposed to 0.068 mg UIA-N l −1. The SGR of tilapia exposed to un-ionised ammonia nitrogen over 0.068 mg UIA-N l −1 (0.144, 0.262 and 0.434 mg UIA-N l −1) was significantly reduced ( p<0.01). The no-observable effect concentration was 0.068 mg UIA-N l −1, while the lowest observable effect concentration was 0.144 mg UIA-N l −1. Increasing the un-ionised ammonia concentration increased the feed conversion ratio (FCR). At 0.144 mg UIA-N l −1, the FCR increased by a factor 1.6 of the value observed in the control, while at 0.262 mg UIA-N l −1 the FCR increased by a factor of 2.7. At 0.434 mg UIA-N l −1, the FCR increased by a factor of 4.3. Protein efficiency ratio (PER) was also negatively correlated with the un-ionised ammonia concentration above 0.068 mg UIA-N l −1. This study concluded that, for raising Nile tilapia in fishponds fed on fresh duckweed or other feed, the toxic level of UIA-N and its negative effect on the growth performance lies between 0.07 and 0.14 mg UIA-N l −1. It is recommended that the UIA-N concentration be maintained below 0.1 mg l −1.

Laboratory evaluation of the toxicity of Perfluorooctane Sulfonate (PFOS) on Selenastrum capricornutum, Chlorella vulgaris, Lemna gibba, Daphnia magna, and Daphnia pulicaria.

Perfluorooctane sulfonate (PFOS) is an anthropogenic compound found in trace amounts in many environmental compartments far from areas of production. This, along with the highly persistent nature of PFOS, presents a concern for possible effects in aquatic ecosystems. The objective of this study was to determine the toxicity of PFOS in representative freshwater organisms. Toxicity testing using standard laboratory protocols was performed on the green algae Selenastrum capricornutum and Chlorella vulgaris, the floating macrophyte Lemna gibba, and the invertebrates Daphnia magna and Daphnia pulicaria. No observable effect concentration (NOEC) values were generated from the most sensitive endpoints for all organisms. Autotroph inhibition of growth NOEC values were 5.3, 8.2, and 6.6 mg/L for S. capricornutum, C. vulgaris, and L. gibba, respectively. The 48-h immobility NOEC values for D. magna and D. pulicaria were 0.8 and 13.6 mg/L, respectively. In comparison to immobility, the 21-day lethality NOEC for D. magna was 5.3 mg/L. Based on effect (immobility) values, the most sensitive of all test organisms was D. magna. The most sensitive organism based on 50% inhibition of growth (IC(50)) was L. gibba, with an IC(50) value of 31.1 mg/L determined from wet weight. This is 4.3 times less than the LC(50) for D. pulicaria, which was 134 mg/L. Significant adverse effects (p < or = 0.05) were observed for all organisms in concentrations >134 mg/L. The results indicate that under laboratory conditions PFOS is acutely toxic to freshwater organisms at concentrations at or near 100 mg/L. Based on known environmental concentrations of PFOS, which occur in the low ng/L to low microg/L range, there is no apparent risk to freshwater systems. However, further work is required to investigate long-term effects in these and other freshwater organisms.

Field level evaluation and risk assessment of the toxicity of dichloroacetic acid to the aquatic macrophytes Lemna gibba, Myriophyllum spicatum, and Myriophyllum sibiricum

Dichloroacetic acid (DCA), a haloacetic acid, is a common contaminant of aquatic ecosystems. A study to investigate potential phytotoxic effects on rooted and floating macrophytes ( Myriophyllum spicatum, M. sibiricum, and Lemna gibba) was conducted. Replicate 12,000 L outdoor microcosms ( n=3) were treated with 3, 10, 30, and 100 mg/L of DCA that had been neutralized to the sodium salt, plus controls. Plants were sampled regularly over 21 days and assessed for a variety of endpoints including plant growth, root growth, number of nodes, wet and dry mass, chlorophyll- a, chlorophyll- b, carotenoids, and citrate levels. EC 10, EC 25, and EC 50 values were calculated for each endpoint that exhibited a concentration–response. Overall, M. sibiricum was slightly more sensitive than M. spicatum to DCA exposure. The most sensitive plant endpoints were wet mass and plant length. Pigments showed no response with exposure to DCA. The probability of current concentrations of DCA in Canadian lake water and Swiss river waters exceeding thresholds of toxicity derived from single species effect measure distributions (EC 10s) is ⪡0.01%. The use of effect measure distributions holds promise as a new risk assessment technique for aquatic plants. Currently, environmental levels of DCA do not pose a risk to these plants.

Use of chlorophyll fluorescence of Closterium ehrenbergii and Lemna gibba for toxic effect evaluation of sewage treatment plant effluent and its hydrophobic components

In this study fluorometric methods using an alga, Closterium ehrenbergii, and a higher plant, Lemna gibba, were employed to evaluate the toxicity of sewage treatment plant (STP) effluent and its hydrophobic components. Fluorescence parameters such as the operational photosystem II quantum yield at steady state of electron transport, the nonphotochemical quenching, and the complementary area were modified in the presence of hydrophobic components, particularly with C. ehrenbergii. It was found that C. ehrenbergii was a suitable species to be used in a hydrophobic components bioassay, since this alga was 400 times more sensitive than L. gibba to hydrophobic components. Results indicate that hydrophobic STP effluent components are less toxic as a constituent of the STP effluent than when they are extracted from the effluent. We also demonstrated here that in addition to the growth inhibition test, fluorometric methods can be usefully employed in bioassays for the toxic effect evaluation of pollutants present in municipal wastewater treatment plant effluents.

Mechanisms of EDDHA effects on the promotion of floral induction in the long-day plant Lemna minor (L.)

EDDHA added in an optimal concentration (20.5 mumol.L-1) to a modified Pirson-Seidel nutrient solution induces flowering in some clones of the species Lemna minor, Lemna gibba and Spirodela polyrrhiza, which in the absence of EDDHA in the same nutrient solution do not flower. By adding EDDHA (20.5 mumol.L-1), floral induction under LD conditions is optimally promoted in the long-day (LD) species Lemna minor. After adding EDDHA to the nutrient solution, before floral induction and during flowering, Zn, Mn and Cu content is significantly increased in plants. Zn-EDDHA (0.86 mumol.L-1), Mn-EDDHA (1.51 mumol.L-1) and Cu-EDDHA (0.12 mumol.L-1), when used individually, greatly promote flowering under LD conditions as compared to flowering in the same nutrient solution with an equivalent quantity of Zn, Mn or Cu in the nonchelate form. If, on the other hand, Zn-EDDHA and Mn-EDDHA are added to the nutrient solution together (instead of Zn and Mn in nonchelate form), their effect on the promotion of flowering is less than in the case of their individual use. This shows that there is antagonism between Zn-EDDHA and Mn-EDDHA that is eliminated by adding EDDHA to the nutrient solution. We obtained the highest percentage of flowering plants (i.e. 74%) if we added EDDHA (20.5 mumol.L-1) to the nutrient solution containing Mn, Zn and Cu in chelate form. 74% of flowering plants actually means that flowering was achieved in all physiologically mature plants. Our results show that EDDHA promotes floral induction in Lemna minor under LD conditions, especially through chelating Zn, Mn and Cu, and, in addition, through eliminating the antagonism between Mn and Zn chelates EDDHA. Zn-EDDHA (0.86 mumol.L-1) also promote floral differentiation, especially cell division of microspore mother cells into dyads and those into microspore tetrads, which can be seen in microphotographs. When investigating possible pathways through which Mn-EDDHA, Zn-EDDHA and Cu-EDDHA promote flowering, we studied the effects of various concentrations of IAA and sucrose added to the nutrient solution as well. The results support the hypothesis that one of the possible pathways in which Mn-EDDHA promotes floral induction is through auxin oxidase, whereas Zn-EDDHA and Cu-EDDHA probably promote it through the enhancement of the photosynthesis and synthesis of sucrose.

Quantitative structure‐activity relationship for the photoinduced toxicity of polycyclic aromatic hydrocarbons to the luminescent bacteria Vibrio fischeri

Sunlight can greatly enhance the toxicity of polycyclic aromatic hydrocarbons (PAHs). Photosensitization reactions (e.g., generation of singlet-state oxygen) and photomodification reactions (e.g., photooxidation of PAHs to more toxic species) are both pathways of photoinduced toxicity of PAHs. Previously, a quantitative structure-activity relationship (QSAR) was developed for PAHs showing that a photosensitization factor (PSF) and photomodification factor (PMF) can be additively combined to describe photoinduced toxicity. That QSAR model was developed for the photoinduced toxicity of 16 PAHs to the higher plant Lemna gibba. The objective of this study was to apply the QSAR model developed for L. gibba to another organism. The organism chosen was the luminescent marine bacteria Vibriofischeri. Toxicity data used for the QSAR model were inhibition of luminescence and inhibition of growth of V. fischeri. Both short-term (15 min) and long-term (18 h) assays of toxicity were used. Light did not impact on PAH toxicity in the short-term assay, and thus the QSAR model did not correlate well with these data. Conversely, light greatly enhanced toxicity when the long-term assay was employed. The PMFs for the PAHs from the L. gibba QSAR showed a moderate correlation to bacterial toxicity in the long-term assay, whereas the PSFs showed only a weak correlation to toxicity. As was the case for L gibba, summing the PMF and the PSF resulted in a strong correlation to toxicity that had predictive value. Thus, a QSAR model derived for plants accurately described the toxicity of PAHs to a bacterial species. This indicates that the bipartite mechanism of PAH-photoinduced toxicity may be applicable to other organisms.

Evaluation of monochloroacetic acid (MCA) degradation and toxicity to Lemna gibba, Myriophyllum spicatum, and Myriophyllum sibiricum in aquatic microcosms

The fate of monochloroacetic acid (MCA), a common phytotoxic aquatic contaminant, and its toxicity to the aquatic macrophytes Lemna gibba ( L. gibba), Myriophyllum spicatum ( M. spicatum), and Myriophyllum sibiricum ( M. sibiricum) under semi-natural field conditions was studied. Replicate 12,000 l enclosures were treated with 0, 3, 10, 30 and 100 mg/l of MCA. Each microcosm was stocked with eight individual apical shoots of M. spicatum and M. sibiricum 1 day prior to initiation of exposure. Plants were sampled after 4, 7, 14 and 28 days of exposure and their response assessed using numerous somatic and biochemical endpoints. L. gibba was introduced into the microcosms the day of MCA treatment and monitored regularly for 21 days. The half-life of MCA in the water column ranged between 86 and 523 h. The most sensitive plant species was M. spicatum, followed by M. sibiricum and L. gibba. All species demonstrated toxicity within a threefold range of each other. Endpoint sensitivity varied depending on the duration of exposure and the level of effect chosen. Most species endpoint EC x values were less than an order of magnitude different. Citrate levels in Myriophyllum spp. were not influenced by exposure to MCA. The toxicity of MCA to M. spicatum and M. sibiricum was very similar and thus highly predictive of toxicity observed for each other. The EC 10 was a more conservative estimate of toxicity than the statistically derived no observed effect concentration. Current concentrations of MCA are not likely to pose a risk to these aquatic plants in surface waters.

Nodular somatic embryogenesis and frond regeneration in duckweed,Lemna gibba G3

Duckweed(Lemna gibba) is a useful model system for elucidating plant development, but the techniques needed for regenerating fronds from calli are not yet well established. This study examined the effects of auxin, sucrose, and gelling agents on callus and frond formation inL. gibba G3. After three weeks of culturing on a solid medium, two types of calli were observed: watery, pale-green, and undifferentiated; or white, compact calli that were organized into nodules and which resembled somatic embryogenie calli. Homogeneous callus lines were produced through selective subculture. To induce nodular calli, auxin (2,4-D) was absolutely required, with an effective concentration of 5 to 20 μM; induction was found to be possible with up to a maximum concentration of 4.4%. The calli were then maintained on a medium with a reduced 2,4-D concentration (1 μM), and were transferred every three weeks. Optimal callus induction and growth were obtained by using 3% sucrose with a combination of 0.15% Gelrite and 0.4% agar. Fronds, however, could be regenerated only on distilled water solidified with a combination of 0.4% agar and 0.15% Gelrite. On this medium, 87% of the callus expiants regenerated into fronds after four weeks of culture. These new fronds were morphologically normal but small, approximately 15 to 20% of the size of stock fronds. Continued culture of these fronds in an SH medium produced normal duckweeds, and histological examination of the cultures revealed several distinct types of callus nodules. Nonetheless, because zygotic embryogenesis inL. gibba does not produce distinct bipolar structures, the developmental pathway of frond regeneration from these nodular cultures remains unknown.

New technique for estimating thresholds of toxicity in ecological risk assessment.

The use and utility of the no observed effect concentration (NOEC) in ecological risk assessment is a contentious issue. One concern is that the NOEC is not representative of a concentration at which no biologically significant effect is occurring. A new method has been developed to estimate the threshold of toxicity, or a true NOEC, for aquatic plants. The method involves determining the effective concentration (ECx) of a number of endpoints from one species. These ECx values are plotted on a log-probability scale. The x-intercept, or a low centile, of the distribution can be interpreted as the threshold of toxicity for that plant at that response level. This threshold is the concentration at which no effects should be observed for any endpoint above that response level. It is based on the assumptions that multiple effect measures from a single species will be log-normally distributed and thatthe distribution contains all possible endpoints for that species. The thresholds and the distributions can then be used as a substitute for the NOEC or ECx in risk assessment techniques, such as hazard quotients and probabilistic ecological risk assessment. This new method of estimating toxicitythresholds is more realistic than the use of arbitrary uncertainty factors, is more conservative than current probabilistic risk assessment methods, allows for simple comparison between species and exposure duration to a toxicant, and may be useful for assessing mixture toxicity. This technique was applied to field derived data with Lemna gibba, Myriophyllum spicatum, and M. sibiricumto assess potential risks from monochloroacetic acid (MCA). Using this new risk assessment method, we conclude that MCA does not appear to pose a risk to aquatic macrophytes under field conditions at current environmental concentrations.

Giardia and Cryptosporidium removal from waste-water by a duckweed (Lemna gibba L.) covered pond.

To determine the ability of duckweed ponds used to treat domestic waste-water to remove Giardia and Cryptosporidium. The influent and effluent of a pond covered with duckweed with a 6 day retention time was tested for Giardia cysts, Cryptosporidium oocysts, faecal coliforms and coliphage. Giardia cysts and Cryptosporidium oocysts were reduced by 98 and 89%, respectively, total coliforms by 61%, faecal coliforms by 62% and coliphage by 40%. There was a significant correlation between the removal of Giardia cysts and Cryptospordium oocysts by the pond (P < 0.001). Influent turbidity and parasite removal were also significantly correlated (Cryptosporidium and turbidity, P=0.05; Giardia and turbidity, P=0.01). The larger organisms (parasites) probably settled to the bottom of the pond, while removal of smaller bacteria and coliphages in the pond was not as effective. Duckweed ponds may play an important role in wetland systems for reduction of Giardia and Cryptosporidium.

Performance of duckweed-covered sewage lagoons in Sana'a, Yemen, depending on sewage strength

The performance of a duckweed ( Lemna gibba ) sewage lagoon (DSL) was investigated in non-continuous batch system reactors using high strength sewage under natural environmental conditions in Sana9a. Wastewater effluent from the anaerobic ponds of the Sana9a waste stabilization ponds (WSPs) was used with dilution factors (DF) of 0, 2, 3 and 4. The initial COD concentration range applied was 254–600 mg COD l −1 (150–250 mg BOD l −1 ) and NH 4 + of 25–100 mg N l −1 , while the duckweed stock density used was 500 g wet weight m −2 . The duration of the experiments was 10 days with a harvesting frequency of 5 days. NH 4 + in this very concentrated Sana9a sewage was possibly the most important limiting factor for growth of L. gibba . High pH near the end of the reaction time and lower temperatures at night-time probably also contributed to slower growth. Relative growth rate (RGR) decreased from 0.17±0.04 d −1 at an NH 4 + concentration of 23–40 mg N l −1 to around 0.00 d −1 at a concentration of 100 mg N l −1 . Fresh wastewater helped to grow duckweed, especially at NH 4 + −1 , while after 5 days, algae proliferation and probably the exhaustion of other essential nutrients started to inhibit duckweed growth. COD removal correlated strongly with the applied initial surface loading. At a higher initial COD loading ( λ s ) of 869 kg COD ha −1 , the removal loading ( λ r ) was 710 kg COD ha −1 10 day −1 , while at a lower initial COD loading of 344 kg ha −1 , the removal loading was 210 kg COD ha −1 10 day −1 . Total Nitrogen removal ( λ r,N ) increased with initial NH 4 + concentration and with initial surface loading ( λ N ). At initial nitrogen loading ( λ N ) of 28 and 164 kgN/ha, the removal loading ( λ r,N ) was 25 kgN/ha.10 d and 148 kgN/ha.10 d, respectively. At the same time, the first order COD kinetic removal rate constant increased from 0.10 to 0.16 d −1 at initial COD concentration of 254 and 621 mg/l, respectively. The total nitrogen kinetic removal constant increased from 0.16 day −1 at NH 4 + concentration 93 mg N l −1 to 0.26 day −1 at NH 4 + 34 mg N l −1 . The high DO and pH encountered under outdoor environmental conditions are probably the main cause of the high N removal compared with removal under laboratory conditions. Therefore, total nitrogen removal was taking place through nitrification/denitrification and probably NH 3 stripping.

Indole-3-acetic acid metabolism in Lemna gibba undergoes dynamic changes in response to growth temperature.

Auxin is the mobile signal controlling the rate of growth and specific aspects of the development of plants. It has been known for over a century that auxins act as the messenger linking plant development to specific environmental changes. An often overlooked aspect of how this is accomplished is the effect of the environment on metabolism of the major plant auxin, indole-3-acetic acid (IAA). We have studied the metabolism of IAA in relation to one environmental variable, growth temperature. The model system used was an inbred line of the aquatic monocot Lemna gibba G-3, 3F7-11 grown at temperatures ranging from 5 degrees C to 35 degrees C. IAA levels, the rate of IAA turnover, and the patterns of label incorporation from IAA precursors were measured using stable isotope-mass spectrometric techniques and were evaluated relative to growth at the experimental temperatures. IAA levels exhibited unusually high variability in plants grown at 15 degrees C and 20 degrees C. Turnover rates were quite rapid throughout the range of experimental temperatures except at 25 degrees C, where IAA turnover was notably slower. These results suggest that a transition occurred over these temperatures for some aspect of IAA metabolism. Analysis of [(15)N]anthranilate and [(2)H(5)]tryptophan (Trp) incorporation into IAA showed that Trp-dependent biosynthesis predominated at 15 degrees C; however, Trp-independent biosynthesis of IAA was the major route to IAA at 30 degrees C. The effects of growth temperature on auxin levels have been reported previously, but no prior studies correlated these effects with which pathway becomes the primary one for IAA production.

Nutrient Reduction and Productivity Studies in Low Strength Freshwater Aquaculture Wastewater Using Lemna Gibba (Duckweed) System

The use of macrophytes plants for treatment of wastewater for nutrient removable is addressed A stuoy was conducted to ascertain the nutrient reduction potential in freshwater aquaculture using Lemna gibba (Duckweed) Experiments were conducted with plastic trays of dimensions 34 27 y 6 cm Wastewater from freshwater aquaculture was collected and experimented under different dilutions. Nutrient removal and productivity of the Lemna gibba system were studied for a period of ten days. the experiments were conducted over a period of SO days The nutrient removal patterns were obtained for ammonia-nitrogen, nitrite, nitrate and phosphate Reductions in nutrient in the Lemna gibba system are higher in raw wastewater compared to dilutions. Ammonia nitrogen had maximum removal efficiency followed by nitrate, nitrite and phosphate. Biomass productivity was also higher in the raw wastewater series than with dilutions. The studies indicated the survival of Lemna gibba in low nutrient loading conditions and also as a potential biomass resource&#x0D;

Process performance assessment of algae-based and duckweed-based wastewater treatment systems

A pilot plant experiment was carried out to assess differences in environmental conditions and treatment performance in two systems for wastewater treatment: algae-based ponds (ABP) and duckweed-based (Lemna gibba) ponds (DBP). Each system consisted of a sequence of 4 equal ponds in series and was fed with a constant flow rate of partially treated wastewater from Birzeit University. Physico-chemical parameters and the removal of organic matter, nutrients and faecal coliforms were monitored within each treatment system over a period of 12 months. The results show clear differences in the environmental conditions. In ABP significantly (P>0.05) higher pH and DO values were observed than in DBP. DBP were more efficient in removal of organic matter (BOD and TSS) than ABP. The faecal coliform reduction was higher in ABP. However, the quality of the effluent from the third and fourth duckweed pond (total retention time of 21 and 28 days) did not exceed the WHO-criteria for unrestricted irrigation during both the summer and winter period, respectively. During the summer period, the average total nitrogen was reduced more in ABP (80%) than in DBP (55%). Lower values were measured during the winter period. Seasonal nitrogen reductions of the two systems were significantly different (P>0.05). In DBP, 33% and 15% of the total nitrogen was recovered into plant biomass and removed from the system via duckweed harvesting during the summer and winter period, respectively. This study showed that there were differences in the environmental conditions and treatment efficiencies between the two systems.

Sites of toxicity of specific photooxidation products of anthracene to higher plants: inhibition of photosynthetic activity and electron transport in Lemna gibba L. G-3 (duckweed).

Sites of toxicity of polycyclic aromatic hydrocarbons (PAHs) were examined to determine if inhibition of photosynthetic activity could be correlated to whole-organism toxicity. The inhibition of photosynthesis was observed by detecting the induction kinetics of endogenous chlorophyll a (Chl a) fluorescence. Anthracene (ANT) photooxidation products were applied to the aquatic higher plant Lemna gibba L. G-3 at concentrations ranging from 0.01 to 10 ppm. The impact on Chl a fluorescence was found to correlate with whole-organism toxicity for the 13 PAH compounds tested in this in vivo study. The mechanism of toxic action starts with inhibition of photosystem I (PSI) or the cytochrome-b6/f complex, followed by photooxidative damage to photosystem II (PSII). To study the effects of oxygenated ANTs on photosynthesis in vivo, the IC(50)s for F(V)/F(M) (PSII activity) and F(Q)/F(M) (activity downstream from PSII) were determined. The IC(50)s for a decrease of F(Q)/F(M) for all 13 chemicals were on average twofold lower than those for F(V)/F(M). F(V)/F(M) was found to be a measure of acute toxicity, whereas F(Q)/F(M) was found to be a measure of chronic toxicity. Thus, Chl a fluorescence by use of the whole organism was able to detect the impacts of photomodified ANT products and indicate a site of action for the chemicals.