1
The influence of coins on the growth of duckweed
by
Charles G.D. Roberts (1860-1943)
200001000
Submitted to: Mr. William Wordsworth
Biology 203: Introductory Ecology
Wednesday laboratory section
Department of Biology
St. Francis Xavier University
27 November 20xx
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ABSTRACT
The objective of this study was to determine the effect of coins on duckweed growth. Lemna
minor (duckweed) was the subject of study. This experiment was performed in conditions similar
to that of the natural L. minor environment over a period of 13 days. To introduce the
manipulated variable into the experiment, three copper pennies were placed in half of the thalli
test beakers; no coins were placed in the beakers of the control group. It was hypothesized that
increased metal content in the growth medium provided by the coins would affect the growth
rate of Lemna minor. This experiment did not produce expected results. After 13 days, the
growth rates for thalli colonies with coins and for thalli colonies without coins were 0.0598 and
0.0518, respectively. The Student’s t-test showed no significant difference between the growth
rates of thalli colonies with and without the presence of coins.the presence of the coins inhibited
the growth of algae in the beakers. This study showed that copper does not affect the growth of
Lemna minor.
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INTRODUCTION
Duckweed is a small, aquatic, flowering plant. The plant is popular as a subject for research in
biology. The attractiveness of duckweed (genus Lemna) for research stems from its small size,
simple structure and ability to reproduce quickly. Lemna is small enough to have a rapid
reproductive rate yet large enough to be easily seen and counted (Debur 1991). The most
common species of duckweed is Lemna minor, the subject of our research. Each individual
duckweed plant (thallus) consists of a small leaf with a single root hanging beneath (U. of
Florida, 2001). Previous research has used Lemna minor as a key subject when studying growth
rates and toxicity (Wang 1990, Wang and Williams 1988, Thomas 1986).
The initial phase of duckweed growth in optimal conditions shows unlimited population
growth, increasing geometrically. Growth rate is dependent on temperature, nutrient supply,
light and crowding (competition) (Taylor 20xx). In attempting to create nearly optimal
conditions for growth, our experiment was performed to study the initial growth phase of Lemna
minor colonies with the addition of coins in the growth medium.
MATERIALS AND METHODS
Portions of our procedure were modifications of a population growth experiment outlined in
(Taylor 20xx). In the experiment, 10 beakers of a 400-mL size were used as containers for the L.
minor cultures. Each beaker was filled with the same amount of growth medium. Growth
medium levels were marked on the side of each beaker. The growth medium was composed of
varying levels of nutrients to produce optimum growth conditions for Lemna minor. (Table 1).
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For the duration of the experiment, the beakers were refilled with distilled water when the
growth medium levels dropped below the original.
Using forceps to remove them from their temporary buckets, groups of 20 duckweed thalli were
transferred into each of the 10 beakers. Three Canadian pennies were placed in 5 test beakers
labeled number 1 through 5; no coins were placed in control beakers 6 through 10. The 10
beakers were then placed under a fluorescent light source in random order, each obtaining
approximately equal light intensity.
The light source for this experiment was “grow lights”. These were fluorescent tubes that
produce light in wavelengths mimicking the spectrum of natural sunlight (Taylor, 20xx). From
these fluorescent tubes, a light level of approximately 50 µmol photons m-2s-2 is provided.
The number of thalli growing in each beaker was counted every two days for thirteen days.
Thalli needed to be a minimum of 1 mm in diameter to be counted. Each new thallus was
counted regardless of whether it was detached from the parent thalli. After the thalli were
counted, the beakers were placed under the lights in a new random order. Any changes to water
levels were noted, as well as growth of blue-green algae. Algae were visible as a light green
mass growing in the Lemna growth medium. The temperature of the laboratory where the
duckweed colonies were kept was maintained at 18°C.
Table1. Composition of the Lemna minor culture medium. The ingredients listed
were added to surface water from Lochaber Lake (NS, Canada)
_____________________________________________________________
Ingredient
Concentration (mg/L)
======================================================
Potassium phosphate monobasic
100
5
Potassium nitrate
Magnesium sulfate
Calcium nitrate
Iron (ferric) chloride
Zinc sulphate (heptahydrate)
Manganous (II) chloride
Boric acid
Ammonium molybdate
Calcium carbonate
350
100
295
0.76
0.18
0.18
0.12
0.04
34
The raw data of this experiment were analyzed using Microsoft Excel to plot the graphs and
produce the linear equation of each line. The “r” (intrinsic growth rate) of the thalli was handcalculated from the linear equation of the best-fit line on the 1n(Nt) vs. Time graph for each
growth condition. The t-statistic was hand-calculated using the information given in Table 2.
RESULTS
our experiment showed that the presence of coins in growth medium has no significant effect on
Lemna minor growth. L. minor growth data collected for this experiment is summarized in the
following table and graphs.
Complete results of this experiment, the average thalli growth and standard deviation for each
condition for each recorded day can be found in Appendix 1. In this experiment, population
growth for both colonies was very similar. The growth of both colonies exhibited a
growth rate that doubled the thalli population in thirteen days (Figure 1).
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45
40
Mean N of Thalli
35
30
25
20
Added Copper
15
Control (No Copper)
10
5
0
0
5
10
15
Time (days)
Fig. 1 The population growth of L. minor thalli for duration of experiment (13 days).
The intrinsic growth rate (r) for the colonies with added coins was 0.0598; the r-value for the
control colonies was 0.0518. The r-value is obtained from the linear equation of the best-fit line
for each colony ( Fig. 2), where the slope of the best-fit line is equal to ‘r’.
algae growth began to appear in the control beakers (no added copper) by day 5. Algal growth
rapidly increased in these beakers for the duration of the experiment until by day 13 the entire
surface of the control group beakers was covered in algae. There was no visible algae growth in
the beakers containing coins.
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4
3.5
3
Added Copper
ln(Nt)
2.5
Control (No Copper)
2
1.5
Linear (Added Copper)
1
Linear (Control (No
Copper))
0.5
0
0
5
10
15
Time (days)
Fig. 2. Plot of 1n(Nt) against time for L. minor colonies with and without added coins, over 13
days. The intrinsic growth rate (r) for both colony types can be calculated using the slope of the
line of best fit from this graph, (where r = slope of line).
DISCUSSION
We showed that there is no significant difference between the growth rates of L.minor colonies
with coins added and those without. All beakers showed a similar increase in thalli growth each
day (Table 2). This suggests that L. minor is not affected by an increase in metal content (coinsadded) to their growth medium.
The findings from our experiment were consistent with the findings of similar research
experiments on L. minor and toxicity. There are indications that duckweed is tolerant to
environmental toxicity (Wang 1990). Gabrielson et al. (l980, cited in Wang 1990) found
duckweed could grow under a wide range of nutrient conditions, including high metal
concentrations. An explanation for duckweed’s tolerance to environmental toxicity is proposed
by Wang (1990). Wang (1990) suggests that “At sublethal range, the duckweed plants may adapt
and/or develop resistance quickly due to their fast growth rate”.
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In contrast to our findings, there are other studies that suggest “Duckweed was much more
tolerant … when the plant was cultured in a higher-nutrient solution than in a low-nutrient
solution.” (Seto, et al. cited in Wang 1990).
Although the presence of coins had no significant effect on the growth of L. minor, algal growth
was drastically reduced. Clearly, the increased metal content of the growth medium in the coinadded colonies produced toxic effects on algae growth. This phenomenon is verified by similar
tests performed by Thomas et al. (1986, cited in Wang and Williams 1990) that indicate “algae
and higher plants [such as L.minor] did not respond to toxicity alike”. The varying affects of
coins on the growth of L. minor and algae may be explained by an important point raised by
Kline (1985, cited in Wang, 1990) that, “Since the alga is distributed throughout the water
column and the duckweed only inhibits the water surface, the alga maintains a more constant
exposure to the toxicant”.
Error in experimental procedure and uncertainties regarding the collection of data may account
for a small portion of error in our population growth results. Although a data collection method
was clearly outlined, the alternation of group members is collecting the data every two days may
have affected results. The time of day that students counted the thalli may have also varied and
small errors in counting may have occurred.
In conclusion, the results of this study show the introduction of coins to the growth medium of L.
minor did not affect Lemna minor growth rates, but did inhibit the growth of algae. this
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experiment confirms the results of others in similar studies (Wang 1990, Thomas et al 1986,
Kline 1985). Further studies on Lemna growth that may be relevant could include studying how
the method of metal distribution affects growth rates or studying how varying concentrations
(small amounts and extreme values) of metal affect Lemna minor growth rates.
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Appendix 1
The number of thalli present in L.minor colonies with added coins (beakers 1-5) and control
L. minor colonies (beaker 6-10) over a 13 day period. Mean and standard deviation are found at
the bottom of coin and control group results.
Beaker Number
Day1
Day 3 Day 5 Day 7 Day 9
Day 11
Day 13
Added Copper
______________________________________________________________________
1
20
23
29
30
32
39
40.8
2
20
21
32
34
37
38
42
3
20
22
28
30
32
36
42
4
20
22
29
31
32
37
40
5
20
22
29
31
33
40
41
_______________________________________________________________________
Mean
20
22
29.4
31.2
33.2
38
39
_______________________________________________________________________
Standard Deviation
0 0.70711 1.51658 1.64317 2.16795 1.58114 0.85323
===============================================================
6
20
17
26
26
27
28
30
7
20
23
28
29
32
33
37
8
20
21
31
32
32
34
35
9
20
24
24
35
39
43
42
10
20
28
30
34
36
40
45
_______________________________________________________________________
Mean
Standard Deviation
0 5.03733 3.03315 3,70135 4.54973 5.94138 5.89067
===============================================================
LITERATURE CITED
Taylor, B.R. 20xx. Introductory ecology: Laboratory manual 20xx. St. Francis Xavier
University, Antigonish, N.S. Canada.
Wang, W. 1990. Literature review on duckweed toxicity testing. Environmental Research, 52:
7-22.
Wang, W., and Williams, J.M. 1990. The use of phytotoxicity tests (common duckweed,
cabbage, and millet) for determining effluent toxicity. Environmental Monitoring and
Assessment 14: 45-58.