The growth-promoting effect and mechanism of microalgae on plants in PAHs contaminated soil

Microalgae can effectively degrade polycyclic aromatic hydrocarbons (PAHs) in water. However, the remediation mechanism of microalgae in PAH-contaminated soil remains unclear. In this study, the growth-promoting effects of wheat by Chlorella vulgaris in PAH-contaminated soil were studied. Structural changes in the rhizosphere bacterial community and the bacterial metabolism were further explored. It revealed that the addition of C. vulgaris promoted wheat's dry weight and height by 10.22 % and 122.15 %, respectively. One explanation was the degradation and transformation of PAHs by C. vulgaris, which relieved the inhibitory effect on wheat growth. Compared with the blank control group, C. vulgaris addition enhanced the degradation efficiencies of phenanthrene (Phe) and pyrene (Pry) by 4.81 % and 8.34 %, respectively (with the initial concentrations in soil of 1.03 × 10⁴ and 2.21 × 10⁴ µg/g, respectively). The binding state of Phe and Pyr changed to a free state, which facilitated microbial degradation. The Phe and Pyr contents in wheat decreased by 22.23 % and 18.54 %, respectively. The presence of C. vulgaris increased the abundance of Sphingosinomonas bacteria capable of degrading PAHs by 95.24 %. Enzyme activities related to the transport, oxidation, and dehydrogenation of PAHs in the bacterial community also increased. This study demonstrated C. vulgaris' multiple pathways for remediating PAH-polluted soil, including PAH degradation, nutrient and hormone release, and bacterial community adjustment. In conclusion, C. vulgaris addition enhanced the algae-bacteria symbiosis, which was of great significance for the removal of PAHs from the soil and the promotion of plant growth.