Effects of soluble organic materials and toxicity of copper on Lemna minor population
growth
Dryden Tanner
201102406
Biology 203: Introductory Ecology
Wednesday p.m. Lab Section
Submitted to Kris Hunter
St. Francis Xavier University
19 November 2014
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Abstract
The purpose of the experiment was to test whether the soluble organic compound
would have an effect on the growth rate of Lemna minor. We had two colonies of
duckweed with five replicates of each were compared. Our control colonies nutrition was
strictly the pond water medium and the treatment was the pond water medium and 20 mL
of the soluble organic compound, each replicate colony also contained a penny. We
found that the treatment population grew at a higher linear rate than the control
population. Soluble organic compounds were found to aid the growth more than the pond
water medium alone. Further studies, including more replicates of the treatment are
needed to determine how more available soluble organic compounds would affect
growth.
Introduction
Lemna Minor also referred to as duckweed, is a small plant that is found in slow
moving or still freshwater environments. Duckweed has been found to grow at rapid
rates, which makes it great to culture and study. Duckweed can acquire large quantities of
toxic metals from its environment, which cause growth rate to be hindered. (Taraldsen et
al. 1989).
Copper is a metal that has structural components essential to plant growth,
however too high a concentration can be toxic (Dirilgen et al. 1994). Previous research
has shown that high copper concentrations found in the environment, oxygen exchange
and net photosynthesis rates were significantly reduced (Prasad et al. 2001). However, if
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copper is present with other compounds, such as cobalt, it will not show such negative
effects on duckweed growth (Cvjetko et al. 2010).
Beside competing toxicity in the environment of the copper penny, previous
research has shown that organic molecules provide nutrients for duckweed, aiding their
growth (Körner et al. 1998). With that being said there is little research on the effects of
additional soluble organic compounds in the medium of a colony of duckweed on growth
of that population.
The present experiment was designed to test whether the additional organic
soluble compounds would have an effect on duckweed population growth rate. We
hypothesized that the duckweed population in the treatment group containing soluble
organic compounds would grow at a higher rate than duckweed in the control group
growing in a pond water medium without additional soluble organic matter.
Methods
To test the effectiveness of using soluble organic compounds to counteract the
toxicity of the penny, we prepared five treatment beakers each containing 70ml of the
regular pond water medium, a copper penny and 20 ml of a soluble organic material
mixture potting soil tea which was made by steeping potting soil for 48 hours in regular
pond water medium, before straining out the solids, leaving the remaining liquid, which
presumably contained soluble organic substances, this mixture is referred to as “potting
soil tea”. We also prepared five control beakers each containing 90 ml of a regular pond
water medium with copper penny. Both the treatment and control populations had initial
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counts of twenty duckweeds in each beaker. The duckweed populations were placed
under a 430-W, high-pressure sodium lamp that produces light with wavelengths that are
similar to the spectrum of natural sunlight, and the temperature of the room was set at
approximately 24 degrees Celsius. Once each day around the same time, for fourteen
days, the number of duckweed in each beaker was counted, while any disease, bugs or
other growing organisms found were noted. Medium levels in each beaker were
maintained at 90mL, this level was replenished as needed with distilled water.
Microsoft Excel was used for data analysis. Mean number of duckweed thalli vs.
Time in days was plotted for both the control and treatment populations and compared
the slopes. The natural logarithm of the daily population means of duckweed thalli were
also plotted against time in days for both control and treatment groups, and then
compared the slopes and R2 values. A regression analysis of this data was performed; we
then compared our t-statistic against the t-critical value at a 95% confidence interval.
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Results
Our experiment displayed that duckweed grows better with additional soluble
organic compounds added to a general pond water medium.
100
Number of duckweeds
90
80
70
60
50
Control
40
Treatment
30
20
10
0
0
5
10
15
Time (Days)
Fig. 1: Mean number of the control and the soluble organic compound treated
populations.
In figure 1, we have the mean number of duckweed thalli plotted for each day for
the control and treatment populations. Growth was found to be relatively linear in both
populations but more so in the control group.
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5
y = 0.1026x + 3.2939
R² = 0.885
4.5
4
y = 0.0781x + 3.0184
R² = 0.9757
3.5
Ln(Nt)
3
Control
2.5
Treatment
2
Linear (Control)
1.5
Linear (Treatment)
1
0.5
.
0
0
5
10
Time (Days)
15
Figure 2: Ln of the daily means of the control and pot soil tea treated populations. A line
of regression is included for each population.
In figure 2, the natural logarithm of the mean population of the control group
along with the natural logarithm of the mean population for the treatment group for each
day are plotted. A line of regression was included for both the control and treatment
groups, which displays that both have quite linear equations. R2 values for each both
groups were also calculated. The R2 for the control group is 0.9757 for the control
population, and 0.88503 for the treatment population.
A regression was run for the natural logarithm of both the treatment and control
mean populations against time in days to calculate a t-statistic value of 4.316. Our tstatistic was greater than our t-critical value of 1.646 at a 95% confidence limit.
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Discussion
We found that duckweed populations in the treatment group grew at a higher rate
than in the control. We expect that in a population that has no limiting resources, the
growth rate would be exponential until it reaching carrying capacity. The mean growth
rate of our control population was found to be linear, which we attributed to having one
or several limiting factors. The mean growth rate of our treatment population was found
to be linear, but slightly less than our control group. This indicated that our treatment
group still had limiting factors, but not to the extent of control. Our R2 values for both the
control and treatment groups were found to be close to 1 meaning that our data lies close
to the line of regression; displaying that there was little variance. Our t-statistic of 4.316,
was greater than our t-critical value of 1.646, we can confirm that there is a significant
difference between the population growth rates of the control and treatments groups and
reject our null hypothesis. Possible sources of error include bugs growing in beakers, lack
of water throughout the day. In future studies I would suggest looking at several
treatment groups containing varying amounts of soluble organic compounds.
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References
Dirilgen, N., Inel, Y. 1994. Effects of Zinc and Copper on Growth and Metal
Accumulation in Duckweed, Lemna minor. Bull. Enviorn. Contam. Toxicol.
53:442-449. Available from
http://link.springer.com/article/10.1007%2FBF00197238? LI=true#page-1
[accessed 15 November 2014].
Taraldsen, J.E., and Norberg-King, T.J. 1989. New method for determining effluent
toxicity using duckweed (Lemna minor). Available from http://onlinelibrary.wiley.
com/store/10.1002/etc.5620090610/asset/5620090610_ftp.pdf?v=1&t=i2j8epqe&s
=79a7d3602d348db927915abc0acf6e085a756896 [accessed 15 November 2014].
Cvjetko, P., Tolić, S., Sikić, S., Balen, B., Tkalec, M., Vidaković-Cifrek, Z., Pavlica, M.
2010. Effect of copper on the toxicity and genotoxicity of cadmium in duckweed
(lemna minor L.). Available from http://www.ncbi.nlm.nih.gov/pubmed/20860969
[accessed 14 November 2014].
Körner, S., Lyatuu, G.B., Vermaat, J.E. 1998. The influence of Lemna gibba L. on the
degradation of organic material in duckweed-covered domestic wastewater.
Available from http://www.sciencedirect.com/science/article/pii/S0043135
498000542 [accessed 14 November 2014].
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Prasad, M.N.V., Malec, P., Waloszek, A., Bojko, M., Strzalka, K. 2001. Physiological
responses of Lemna trisulca L. (duckweed) to cadmium and copper
bioaccumulation. Available from http://www.sciencedirect.com/science/article/
pii/S0168945201004782 [accessed 15 November 2014].