Optimizing Growth Conditions of Azolla pinnata R Brown for Phytoremediation

Abstract

Poor agricultural practices involving synthetic fertilizers have caused many environmental issues which leads to the pollution of all biospheres of the earth. In order to meet the ever-increasing demand of fertilizer it has been proposed to turn to eco-friendly biological nitrogen fixation methods. The aquatic fern Azolla pinnata has a symbiotic relationship with the nitrogen fixing bacteria Anabaena azollae. This relationship allows the plant to live in low nitrogen environments due to the bacteria providing for the plant’s nitrogen requirements. Optimally growing the plant would lead to increased use of biological nitrogen fixation, thus decreasing the need for synthetic fertilizers. It is believed that identifying an optimal combination of different growth conditions would be the best way to improve the plants growth and thus it’s biological nitrogen fixation capabilities. An investigation was undertaken to find the optimal growth conditions of the aquatic fern A. pinnata. The growth conditions that were investigated were: light intensity, nitrogen presence, pH control and humidity. The light intensity had three settings, i.e. low light (5 000 lx), medium light (10 000 lx) and high light (20 000 lx), nitrogen was either added to the system in the form of potassium nitrate or omitted, pH control to a pH of 6.5 was either done by daily manual dosing or the system was unaltered, and finally there were three humidity settings – low humidity (60 %), medium humidity (75 %) and high humidity at 90 %. A walk-in greenhouse was constructed so that each growth condition could easily be adjusted to the different settings to facilitate a variety of growth condition combinations. Using a 15 % strength Hoagland’s growth medium, it was found that a high light intensity of 20 000 lx, pH control and 90 % humidity yielded the highest growth rate of 0.321 d−1. It was found that the pH control must be used in conjunction with the higher humidity values or else algal infection would occur and would negatively affect the growth of the plant. The nitrogen presence did not have a significant effect on the growth rate, this is likely due to the symbiosis between the A. pinnata and the A. azollae, proving that the diazotroph fixates enough nitrogen to satisfy the plants nitrogen requirements. A. pinnata has a variety of uses and since the optimal growth conditions study demonstrated that A. pinnata can grow in low nitrogen environments, it was then decided to investigate the plant’s phytoremediation properties of phosphorus under low nitrogen environments and to observe if there was a pH response when the phosphorus was depleted. i Using the same set-up as in the optimal growth conditions study, the phosphate amounts were varied between 0 ppm to 3.1 ppm in the Hoagland’s solution and different pH control schemes were implemented to assess the feasibility of optimising phosphorus uptake in a nitrogen absent environment. It was found that there was no significant difference in growth or phosphorus uptake when the different pH control schemes were used. The pH 5 control scheme caused the A. pinnata to uptake phosphorus more readily than the pH 7. This is due to the fact that phosphorus uptake is improved in acidic environments. The A. pinnata did gain substantial mass when placed under low and no phosphate levels. There was no significant pH response when the phosphate was depleted. In the natural pH runs it was found that the pH behaved similarly, no matter the amount of phosphorus added to the system. There was only a visual difference in the A. pinnata grown in higher concentrations of phosphorus compared to the lower ones. The plants turned a red colour and the fronds were much smaller in size for lower phosphorus levels. Since the A. pinnata was grown optimally and the corresponding growth conditions found and that the phytoremediation study proved the plants resilience to varying amounts of phosphorus it is concluded that A. pinnata could be used for phytoremediation purposes in nutrient-polluted systems.