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The effect of sediment exposure on Lemna minor population growth
Harvey Rabbit
Biology 203
Laboratory Section: Monday
Submitted to: whoever
12 November 20xx
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Abstract
Duckweed, Lemna minor, is a small, aquatic plant frequently manipulated by
ecologists due to its rapid growth rate. The purpose of this experiment was to observe the
effects of the presence or absence of pond sediment on growth of Lemna minor. We grew
duckweed under 430-W grow lamps slightly above room temperature for two weeks. We
placed 25 thalli in each of five, 100-mL beakers containing a nutrient-rich medium
without sediment. We repeated this process in another set of five beakers but added 10
mL of sediment to each beaker. Duckweed in the presence of sediment grew at a rate of
0.155 ± 0.0059 d-1, and in the absence of sediment grew at a rate of 0.131 ± 0.0082 d-1.
After one week’s growth, blue-green algae formed in beakers containing sediment.
Copper absorption by organic matter in the sediment probably caused the faster growth
rate of duckweed and presence of algae. Subsequent studies would be required to explore
whether sediment increases the L. minor growth rate regardless of copper contamination.
Introduction
A duckweed plant (Lemna minor) is a small, free-floating, aquatic plant. It is
found on the surface of any still body of water with the proper nutrients for its growth.
An example environment would be a swamp or pond. These plants are found across
Eurasia and temperate North America. Ecologists use duckweed to measure population
growth because of its easy maintenance and high asexual reproduction rate. Lemna minor
have a simple structure. Each duckweed plant is called a thallus. The thallus is a
miniature leaf floating on the water’s surface with a rootlet extending into the water
below. Duckweed form clumps containing varying numbers of thalli. Through
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reproduction, new thalli emerge around the border of parent plants and eventually break
free. Sometimes they stay attached to the parent plant (Taylor, 2013).
In our experiment, we observed the effects of pond sediment on the growth of
Lemna minor. We extracted the pond sediment from the duckweed’s original habitat. Our
previous experiment confirmed that duckweed grew sufficiently in a mixture of essential
nutrients (Taylor, 2013). Unfortunately, it did not test the effects on the growth of L.
minor in the absence and presence of sediment. One study proved that in the absence of
sediment, teflubenzuron, an insecticide toxic to duckweed, slowed the growth rate of L.
minor, but in the presence of sediment the growth rate was not inhibited (Medeiros et al.,
2013). Therefore the pond sediment reduced the toxicity of teflubenzuron. This proved
that sediment absorbs toxins, promoting duckweed growth. We did not know if the pond
sediment would still have a positive effect on the growth of Lemna minor regardless of
toxins, but we believed that chemicals absorbed or released by the sediment may help
increase L. minor growth. We therefore hypothesized that the addition of sediment to
each individual treatment beaker would increase the population growth rate of L. minor.
Materials and Methods
Lemna minor needs a specific set of environmental conditions to grow. A 430-W,
high-pressure sodium lamp provided light for the duckweed. This lamp was designed for
hydroponic agriculture, meaning that it produced wavelengths similar to the wavelengths
emitted by sunlight. Altering the lamp’s height provided a light level of 400 mol. photons
m-2s-1. The temperature was maintained at 24°C.
We used 10 labeled 100-mL beakers to grow our duckweed. We labeled five of
these beakers as our control beakers. Each control beaker contained 90 mL of duckweed
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medium (Table 1). Duckweed medium mimicked pond water and provided all the
dissolved nutrients in larger amounts to obtain healthy thalli. We placed 25 healthy thalli,
removed from the large tank in the laboratory, into each of the control beakers filled with
duckweed medium. The duckweed in the laboratory tank, as well as its sediment, were
removed from an unnamed pond adjacent to Gaspereaux Lake in Antigonish County. We
labeled the other five beakers as our sediment treatment beakers. Each sediment
treatment beaker contained approximately 10 mL of sediment extracted from the bottom
of the large tank. Following, we added duckweed medium until we reached the 90 mL
mark on each one of the beakers. In each one of the beakers we placed a penny. The
pennies contained trace amounts of copper which controlled the growth of cyanobacteria
in the beakers. We placed all of the beakers in a mesh basket under the sodium lamp to
begin growth (Taylor, 2013).
Unfortunately the pennies did not subdue the growth of blue-green algae in the
beakers containing the sediment treatment. On October 29, 2013, we did a 30-mL
medium change in both the control and treatment beakers to remove some of the algal
growth. This worked efficiently and the algae continued to grow but remained in
manageable amounts for the duration of counting.
We observed and counted our duckweed every day for two weeks within a few
hours of 5:00 pm. Each day we removed the mesh container from under the light and
counted each thallus with the aid of a paintbrush. During our counting we made
observations about the general healthiness of the duckweed and the presence of algae.
After we counted the thalli, we stirred each beaker with the end of the paintbrush to
ensure proper nutrient distribution to the rootlets.
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Table 1. Composition of duckweed medium. All of the ingredients below were added to
distilled water.
Chemical Name
Formula
Concentration (mg/L)
Potassium Nitrate
KNO3
350
Calcium Nitrate
Ca(NO3)  4H2O
295
Potassium Phosphate
KH2PO4
100
Magnesium Sulfate
MgSO4  7H2O
100
Calcium Carbonate
CaCO3
Ferric Chloride
FeCl3  6H2O
0.76
Zinc Sulfate
ZnSO4  7H2O
0.18
Manganous Chloride
MnCl2  4H2O
0.18
Boric Acid
H3BO3
0.12
Ammonium Molybdate
(NH4)6Mo7O24  H2O
0.04
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We replaced the mesh tray in a different area after each counting to alter its angle of light
exposure. We also had to fill each beaker to the 90 mL mark with distilled water because
of water loss through evaporation. This did not affect the concentration of the chemicals
in the duckweed medium (Taylor, 2013).
To analyze our data we plotted the mean number of thalli of both the control and
treatment data sets against time in days. I then used Student’s t-test to compare numbers
of thalli in control and treatment beakers. . Following, I did a regression analysis. I first
plotted the Ln-transformed data to visualize an estimation of both growth rates. I then
calculated the regression equations of the control and sediment treatment beakers on
Excel. With these values, I used a special form of the t-test to compare the two slopes (rvalues) to see if the growth rates of the control and treatment populations were
significantly different.
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Results:
our experiment went very well. We counted our duckweed every day and had a
wide range of data to test. Our only issue was the algal growth in the beakers containing
sediment. Algae began to grow after one week of counting. This algal growth, however,
did not affect duckweed growth rate. The thalli continued to multiply and appeared to
have both longer rootlets, and broader fronds.
The mean number of thalli in the control beakers increased from 25 ± 0 to 155.2 ±
32.1 (SD) over 14 days. The mean number of thalli in the sediment treatment beaker had
a larger increase from 25 ± 0 to 233.6 ± 80.5 (Figure 1). Both curves represented
exponential growth, although the curve representing sediment treatment was more
defined as exponential. This visually represented that the growth rate with sediment was
more rapid, although we cannot conclude that the difference in the mean number of thalli
between either population was significant over the two week counting period (t=1.14,
P>0.05, df=28).
The intrinsic growth rate, r, of the L. minor colony exposed to sediment was
larger than the intrinsic growth rate of the control colony (Figure 2). The control group
grew at rate of 0.131 ± 0.00815 d-1 and the group exposed to sediment grew at a rate of
0.155 ± 0.00594 d-1. there was a significant difference between the growth rates of the
two populations (t=4.57, P<0.05, df=26).
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Control
High Density
350
Number of Thalli
300
250
200
150
100
50
0
0
2
4
6
8
Time (Days)
10
12
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Figure 1. Effect of sediment exposure on growth of Lemna minor under a 430-W light
for two weeks. The control was grown in only duckweed medium and the treatment was
grown in medium exposed to sediment. Each point represents the mean of five replicates.
Error bars are standard deviation.
Discussion
the intrinsic growth rate of the L. minor population when exposed to sediment
was greater than the growth rate of the L. minor control population. The rootlets may
have been longer in the sediment treatment colonies because the thalli had to compete
for nutrients with the blue-green algae. By extending their rootlets they would have been
able to absorb the nutrients in the duckweed medium that the blue-green algae couldn’t
reach. The fronds in the sediment
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Control
Treatment
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Ln (Number of Thalli)
5.5
5
4.5
4
3.5
3
0
2
4
6
8
Time (Days)
10
12
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Figure 2. Ln-transformed data with best lines of fit representing estimations of the
intrinsic growth rates of the control (r=0.131 ± 0.136) and the sediment treatment (r=
0.155 ± 0.0994) populations of Lemna minor. Each point represents the mean of five
replications (P<0.05).
treatment beakers may have been larger to create a larger surface area to absorb light
from the grow lamps. Therefore these plants exhibited overgrowth competition to grow
over the algae to reach the light first (Cain et al. 2011).
Algal growth in the sediment beakers occurred because the organic matter in the
sediment absorbed copper from solution. most copper ions in solution are strongly
attracted and bind to organic matter, and as a result do not travel far after their release
into the water (Sauvé et. al. 1997). According to one experiment on the sorption of
copper by various organic sorbing agents, organic matter can absorb the metal from very
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dilute, 2.5 to 4.5% copper solutions (Kashirtseva, 1960). Our beakers presumably
contained very dilute concentrations of copper from the pennies, and therefore organic
matter in sediment would have absorbed the metal from the medium.
As we learned in preliminary experiments, , the copper in pennies can prevent
algal growth in beakers. There was a 50% inhibition in carbon fixation, nitrogen fixation,
and accumulation of chlorophyll a at 15-20 µg/L of added copper to blue-green algae
samples (Wurtsbaugh & Horne, 1982). Therefore as copper is added the growth rate of
blue-green algae decreases. Since the sediment in our beakers absorbed the copper, bluegreen algae grew rapidly. Also, Phosphorus released into the water by sediment is a
major cause of excessive algae growth (“Nutrients: Phosphorus,” 2008). This could
also explain the excess algae in our treatment beakers.
The copper that was absorbed by the organic matter in the sediment may have
affected the growth rate of the thalli. copper disrupts L. minor growth even in sub-lethal
doses. In this experiment, the lowest duckweed growth rates of 0.03 were found in
beakers containing copper and either Cr or Pb (Uçüncü et al., 2013). The containers
containing only Cr and Pb had higher growth rates ranging from 0.05 to 0.10, and the
control container had a growth rate of 0.06 (Uçüncü et al., 2013). Another experiment
concluded that in copper concentrations of 1000 µg/L there was a 90% growth
inhibition on L. minor (Megateli et al., 2009). Therefore as copper is added to
solution L. minor growth rate falls, supporting our results. In the control beakers,
copper in solution remained in the duckweed medium and the growth rate fell, while the
sediment in the treatment beakers absorbed the copper from solution and the growth rate
rose.
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We do not know the exact chemical contents of the sediment used in our
experiment, but it may have contained certain chemicals needed for optimal growth. We
couldn’t prove this due to the copper contamination and sediment absorption that affected
the growth rate. We are also uncertain of the exact quantity of organic matter in this
particular pond sediment. In the future, the contents of the sediment should be analyzed
prior to experimentation.
Our study adds to our understanding of the importance of plant growth exposed to
their original sediment and sediment’s removal of toxins in solution. A further study to
test our hypothesis would be to test the growth of L. minor in the presence and absence of
sediment without the addition of pennies. Although algae growth will persist, this would
allow researchers to see if the growth rate was still faster when the duckweed were
exposed to sediment, or if the removal of copper by the sediment was the sole variant on
growth rate.
Literature Cited
Cain, M. L., Bowman, W. D., Hacker, S. D. (2011) Ecology (2nd ed.) (pp. 242-244).
Sunderland, MA: Sinauer Associates, Inc.
Kashirtseva, M.F. (1960). Experimental data on sorption of copper by various minerals
and organic sorbing agents. International Geology Review, 1(2).
Medeiros, L. S., Souza, J. P., Winkaler, E. U., Carraschi, S. P., Cruz, C., Souza-Júnior, S.
C., Machado-Neto, J. G. (2013). Acute toxicity and environmental risk of
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teflubenzuron to Daphnia magna, Poecilia reticulata and Lemna minor in the
absence and presence of sediment. Journal of Environmental Science and Health,
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Nutrients: Phosphorus, Nitrogen Sources, Impact on Water Quality, 3(22). 2008
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