The absence of nitrogen concentration on population growth rate of Lemna minor
Kelly MacDonald
201001873
Biology 203 Laboratory, Tuesday afternoon
13 November 2012
Submitted to Mr. John Bueglas
Abstract
Nitrogen is essential for proper plant growth (Njambuya et al., 2011). Extreme
concentrations of nitrogen can be detrimental to aquatic plants, especially macrophytes
two regulate the recycling of nutrients in shallow, freshwater ecosystems (Xie et al., 2004).
Lemna minor, or duckweed is a macrophyte which is dependent on nutrient availability.
Our experiment was designed to determine if an absence of nitrogen in a duckweed
population would inhibit growth rate. We had 5 replicates of 25 duckweed thalli in a
control medium containing nitrogen. We also had 5 replicates of 25 duckweed thalli in a
treatment medium without nitrogen, replacing the lost ions with 295 mg/L of calcium
carbonate. Growth was monitored and recorded for two weeks and a t-test was used to
compare population growth at this time. The treatment population had a growth rate of
0.091 and the control population had a growth rate of 0.127. These growth rates are
significantly different, thus the absence of nitrogen does inhibit growth rate of duckweed
populations. Qualitative observations determined that a majority of the treatment medium
thalli were yellow or pale green in colour. Chlorophyll reduction in duckweed is a simple
way to measure toxicity (Taraldsen, Nober-King, 1989). This suggests that even though the
thalli are growing at a positive rate, the thalli are not healthy when compared to the bright
green thalli of the control group, and could possibly have a shorter life-span due to this
absence.
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Introduction
Decomposition of aquatic macrophytes substantially regulates the recycling of
nutrients in shallow freshwater ecosystems. Therefore decomposition is a critical
component in a healthy habitat. High nutrient concentrations in water habitats is an
important factor in controlling decomposition rate (Xie et al., 2004). Having extremely high
concentrations of specific nutrients such as phosphorus or nitrogen can be detrimental in
habitats (Njambuya et al., 2011). For example, nitrogen is essential for proper plant
growth, but having high resource availability can make a habitat toxic to aquatic organisms,
can cause eutrophication, and can be viewed as a pollutant.
Lemna minor or duckweed, is a widespread, free-floating, vascular macrophyte
(Taraldsen, Nober-King, 1989). Duckweed growth is depended on abiotic factors such as
nutrient availability (Njambuya et al., 2011). Our experiment was designed to test whether
an absence of nitrogen in a treatment medium would have an effect on the population
growth of Lemna minor. Previous knowledge learned in the Ecology course at Saint Francis
Xavier University
led us to hypothesize that absent nitrogen concentrations inhibits
normal duckweed population growth.
Materials and Methods
Duckweed was removed from Gasperaux pond in Antigonish county and kept in the
laboratory at a temperature at around 24°C prior and throughout the experiment. The
duckweed was kept under 430-W, high-pressure sodium lamps that were raised by chains.
This provided a light level of approximately 700 mol. photons m-2s-1. We prepared two
growth cultures in ten 100mL beakers. A penny was placed in each beaker to release
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copper to control the competition of the ducked against cyanobacteria (Taylor 2012, p. 23)
5 beakers each contained 25 duckweed thalli, and 90mL of the control medium, as
described in Table 1: (Taylor, 2012, p. 23)
Table 1. Composition of Lemna minor control medium. The ingredients listed are added to
dechlorinated, filtered Antigonish tap water.
Chemical Name
Formula
Concentration (mg/L)
Potassium Nitrate
Calcium Nitrate
Potassium Phosphate
Magnesium Sulfate
Calcium Carbonate
Ferric Chloride
Zinc Sulfate
Manganous Chloride
Boric Acid
Ammonium Molybdate
KNO3
Ca(NO3)·4H2O
KH2PO4
MgSO4·7H2O
CaCO3
FeCl3·6H2O
ZnSO4·7H2O
MnCl2·4H2O
H3BO3
(NH4)6Mo7O24·H2O
350
295
100
100
30
0.76
0.18
0.18
0.12
0.04
5 beakers contained 25 thalli, and the treatment medium (Table 2), which had an absence
of nitrogen. 295 mg/L of calcium carbonate was added to the medium to replace the ion
count.
Table 2. Composition of Lemna minor treatment medium. The ingredients listed are added
to dechlorinated, filtered Antigonish tap water.
Chemical Name
Potassium Phosphate
Magnesium Sulfate
Calcium Carbonate
Ferric Chloride
Zinc Sulfate
Manganous Chloride
Boric Acid
Ammonium Molybdate
Formula
KH2PO4
MgSO4·7H2O
CaCO3
FeCl3·6H2O
ZnSO4·7H2O
MnCl2·4H2O
H3BO3
(NH4)6Mo7O24·H2O
Concentration (mg/L)
100
100
295
0.76
0.18
0.18
0.12
0.04
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The ten beakers were then placed on a tray under the high-pressure sodium lamps.
Beakers were examined every day for the duration of two weeks, and the number of thalli
in each beaker was recorded. As the growth medium evaporated, distilled water was added
to maintain the volume at 90mL and the contents of each beaker were stirred with a small
paint brush. After each recording, either my colleagues or I would change the position of
the trays under a different light. This was done to vary the amount of light the duckweed
thalli received. When all the data was collected, we used Microsoft Excel to carry out
calculations. We used a t-test to determine if there was a significant difference between the
growth rates in the treatment medium without nitrogen, and the growth rate of the control
medium.
My colleagues and I then performed a regression on the natural logistic
transformed data to determine the intrinsic growth rates for each set of data.(Taylor, 2012)
Results
In this experiment, both the control and treatment medium showed positive growth rates (Figure
1). The intrinsic growth rate for the control group is 0.127, as compared to the intrinsic growth rate
for the treatment medium at 0.091. The critical t-value for the experimental group is 26.78 with a
standard error of 0.0034 and a P-value of 9.31x10-13. Therefore there is a significant difference in
the growth rate between the two Lemna minor populations. This data supports our hypothesis of
the absence of nitrogen inhibiting growth of the duckweed population.
4
160
Mean Number of Thalli
140
120
100
80
60
40
20
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
TIme (Days)
Control
Experimental
Figure 1. Effect of absence of nitrogen on growth of Lemna minor in the laboratory. Cultures were
maintained under 430-W, high-pressure sodium lamps for 14 days at 24˚C.
Discussion
Our hypothesis was that the growth of Lemna minor would be inhibited by the absence of nitrogen,
our results support this hypothesis. Although the growth rate of the treatment group (0.091) was
positive, the growth rate was considerably slow when compared to the control group (0.127). This
could indicate that nitrogen may not have detrimental effects if missing in a duckweed habitat
(Körner, Vermaat, 1998). This also suggests that nitrogen should be seen as a valuable nutrient
(Zimmo et al., 2003), which aids in the healthy growth of Lemna minor. Qualitative observations
made showed that the treatment group’s thalli were mostly yellow or a pale green, as opposed to
the bright green thalli of the control medium. Chlorophyll reduction in duckweed is recognized as a
simple way to measure toxicity (Taraldsen, Norber-King, 1989). Therefore these observations may
suggest that even though the treatment population has a positive growth rate, the thalli are not
healthy when compared to the control population, and may be expected to have a shorter life-span
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than the control group. Further experiments and elaboration to validate this observation is
required. Also experiments comparing high, medium, low and zero concentrations of nitrogen
would provide stronger evidence on the effects of nitrogen on a duckweed population.
Literature Cited
Körner, S., Vermaat, J.E. 1998. The relative importance of Lemna gibba L., bacteria and algae
for the nitrogen and phosphorus removal in duckweed-covered domestic wastewater.
32(12): 3651-3660.
Njambuya, J., Stiers, I., Triest, L. 2011. Competition between Lemna minuta and Lemna
minor at different nutrient concentrations. Aquat. Bot. 94: 158-164.
Taraldsen, J.E., Norber-King, T.J. 1989. New Method for determining effluent toxicity using
duckweed (Lemna minor). 9: 761-767.
Taylor, B.R. 2012. Introductory ecology: Laboratory Manual 2012. St. Francis Xavier
University, Antigonish, NS, Canada.
Xie, Y., Yu, D., Ren, B. 2004. Effects of nitrogen and phosphorus availability on the
decomposition of aquatic plants. Aquat. Bot. 80: 29-37.
Zimmo, O.R., van der Steen, N.P., Gijzen H.J. 2004. Nitrogen mass balance across pilot-scale
algae and duckweed-based wastewater stabilization ponds. Water Research. 38: 913-920.
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