The Phytoremediation Capabilities of Duckweed

The Phytoremediation Capabilities of Duckweed

Introduction

Plants require a large diversity of nutrients to grow, most of which are provided by the soil. However, some plants require application of fertilizers for additional support in growth. Application of fertilizers in fields can cause leaching and run offs of nutrients into surrounding waterways (Trewavas 2004). An excess of nutrients can be harmful for some plants, but species such as duckweed, Lemna Minor, are known for their phytoremediation capability, the ability to restore plant balance by either extracting or recovering nutrients (Parsad 2001).

The objective of this experiment is to determine the effects of different concentration of elemental solutes on the population of duckweed, and to further determine the phytoremediation capability of duckweed. The elements tested in this lab are, iron, copper, potassium and nitrogen, which are all essential in plant growth.

Our hypotheses are that duckweed will increase in population size for all solutions with a 5 M concentration and decrease in population size for the 25 M concentrations. We further believe that the final ionic concentrations for all four elements will be lower.

Methods

Five elemental solutions, each with two molarities, were tested for their effects on populations of duckweed and phytoremediation capability. The solutions included: cupric sulphate (CuSO4), potassium thiosulfate (K2S2O2), ferrous sulphide (FeSO3), ammonium nitrate (NH4NO3) and a control solution of water.

Each group was assigned two assigned two elements and a control. With a pipette, 25 ml of the 5 M elemental solution was added to a labelled petri dish and 25 ml of the 25 M solution was added to another petri dish. An ionic concentration reading for each solution was taken using test strips. These steps were repeated for the second element and the control. Lastly, 20 lobes of duckweed were counted and transferred into each petri dish with an inoculating loop and tweezers. The tops of petri dishes were placed on them and were set under grow lights.

After a period of two weeks, the petri dishes were removed from the light source. The amount of alive duckweed in each petri dish were recorded. Green lobes were counted as alive and white or brown were both disregarded. Final ionic concentration readings were taken for each treatment using the test strips.

For each treatment, the change in population of the duckweed and the change in elemental concentrations was calculated. Class data was pooled together and organised on an excel sheet by elemental solution and molarity of that solution. All comparisons between the change in elemental concentrations and change in lobe count were done by performing two tailed t-tests on excel with XLMiner Analysis Toolpak. A significance value of 0.05 was used to determine whether there was a significant difference between two variables.

Results

For the 5 M solutions, there was an overall decrease in the solute concentration and population size for the copper, iron and potassium solutions. The change in concentration for the nitrate solution remained constant but the lobe count differed greatly (Figure1).

For the 25 M solutions, potassium and iron both decreased in solute concentration and duckweed population size. The copper solution increased at a slow rate in solute concentration and population size. The nitrate solution has no change in concentration but varies in population growth (Figure 2). The control solutions had a zero ionic concentration reading for all elements, except one treatment had a small nitrate reading, but they did vary in population growth (Figure 1 and Figure 2).

A significant difference was found in the mean change in concentration between the 5 M and 25 M potassium thiosulfate. The t-stat value was 2.575 and the t-critical value for two tailed t-test was 2.306. There was no significant difference in change in concentration between the 5 M and 25 M of the other solutions. Moreover, there was no significant difference in the change in population size between the two molarities for each of the solutions.

Figure 1: The change in Lemna minor population size and ionic concentration in water and 5 M solutions of cupric sulphate (CuSO4), potassium