Electronic Thesis and Dissertation Repository

Thesis Format

Monograph

Degree

Master of Science

Program

Biology

Supervisor

Macfie, Sheila M.

Abstract

Due to their antimicrobial properties, silver nanoparticles (AgNPs) have become more popular in consumer and industrial products, leading to increasing agricultural and environmental concentrations. Exposure to AgNPs could be detrimental to plants, microbes, and their symbiotic relationships. When subjected to 10 µg/mL AgNPs in a 96-well plate, growth of Bradyrhizobium japonicum USDA 110 was halted. In hydroponic culture with 2.5 µg/mL AgNPs, biomass of inoculated Glycine max (L.) Merr. was 50% of control. Axenic plants were unaffected by this dose, but growth was inhibited at higher doses, indicating that AgNPs inhibit both nodulation and growth. Nodules treated with 2.5 µg/mL AgNPs were absent of bacteroids, and plants given 0.5-2.5 µg/mL AgNPs had 40-65% decreased nitrogen fixation. In conclusion, I determined AgNPs not only interfere with plant-microbe relations but also with general plant and bacterial growth. As a consequence, we should be mindful of not releasing AgNPs to the environment and agricultural land.

Summary for Lay Audience

It has been known for thousands of years that silver can kill bacteria. Hence, human use of silverware. In recent years, silver in the form of nanoparticles (very small particles about 500 hundred thousand times smaller than a grain of salt) has gained popularity. These tiny particles possess properties different from bulk silver. One of these properties is a greater ability to kill bacteria. This enhanced ability comes from high surface area, small size, and increased release of toxic silver ions. As their popularity increases, we see more use of silver nanoparticles (AgNPs) in consumer products and industry. This includes their incorporation into clothes, soaps, and other materials. Through production, use, washing, and disposal, AgNPs are released into the environment. They first enter wastewater but are not removed at water treatment facilities. They enter agricultural soil when sludges (biosolids) are used as fertilizers. While crop fertilization is important, this addition of antimicrobial silver could lead to problems with plant-microbe relationships. Specifically, the soybean-nitrogen fixing bacteria relationship could be harmed. My goal was to determine the potential toxicity of AgNPs to soybean, its nitrogen-fixing bacterial partner Bradyrhizobium japonicum, and their symbiotic relationship. I found decreased mass of soybeans as the concentration of AgNPs increased. I also found that smaller AgNPs (16 nm average diameter) were more harmful to plants compared to larger AgNPs (30 nm average diameter). High concentrations of AgNPs also inhibited growth of the nitrogen-fixing bacteria. Nodules in the plant root are the ‘house’ in which bacteria fix nitrogen. Nodule mass and the amount of nitrogen fixed decreased with increasing concentrations of AgNPs. All these results point towards AgNPs harming the nitrogen-fixing relationship and reducing soybean crop yield. Using electron microscopy, I showed that AgNPs are taken up and transported throughout the plants; however, I found that AgNPs in soybean tissues could not be detected using Raman spectroscopy. In conclusion, AgNPs inhibited bacterial growth, plant growth, and development of functional nodules. As a consequence we need to be careful in our use of AgNPs and control or prevent their release into wastewater and their subsequent addition to agricultural fields.

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