The Impact of Eukaryotic Microalgal Growth on the Lettuce Rhizosphere

Intensive agricultural practices around the world have led to both a depletion of soil organic carbon (SOC) in cropped soils and runoff of nitrogen fertilizer into the environment, which can lead to limited crop growth and environmental degradation. Because of the increasing demand for food and the decreasing vitality of soils, there is a great need for less intensive alternatives to traditional synthetic fertilizers and agricultural practices. In recent years, microalgae have been increasingly seen as a potential alternative to traditional fertilizers, with prokaryotic cyanobacteria being able to fix nitrogen for plant growth. While there is potential for eukaryotic microalgae to offer similar benefits to agricultural plants, their overall impacts are not widely known. To study the impacts that eukaryotic microalgae have on the plant rhizosphere, sixteen heads of Arianna lettuce were grown, with eight treated with Chlorella sp. algae amended to the irrigation water. The rhizosphere of the plant was sampled and the microbial community was analyzed using quantitative PCR (qPCR) in order to determine changes in rhizosphere bacterial composition, nitrogen-fixing population abundances, and Chlorella sp. abundances. It was found that the treated plants had a greater mass and a significantly greater presence of nitrogen-fixing bacteria. On the contrary, plant rhizospheres that were untreated were found to have a significantly greater overall abundance of the bacterial community. Lastly, the rhizosphere of amended plants harbored significantly more Chlorella than the untreated plants, indicating that the Chlorella was retained and possibly recruited to the plant rhizosphere throughout the treatment.