Sulfadiazine proliferated antibiotic resistance genes in the phycosphere of Chlorella pyrenoidosa: insights from bacterial communities and microalgal metabolites

The phycosphere is an essential ecological niche for the proliferation of antibiotic resistance genes (ARGs). However, how ARGs’ potential hosts change and the driving mechanism of metabolites under antibiotic stress in the phycosphere have seldom been researched. We investigated the response of Chlorella pyrenoidosa and the structure and abundance of free-living (FL) and particle-attached (PA) bacteria, ARGs, and metabolites under sulfadiazine by using real-time quantitative PCR, 16S rRNA high-throughput. The linkage of key bacterial communities, ARGs, and metabolites through correlations was established. Through analysis of physiological indicators, Chlorella pyrenoidosa displayed a pattern of "low-dose promotion and high-dose inhibition" under antibiotic stress. ARGs were enriched in the PA treatment groups by 117%. At the phylum level, Proteobacteria, Bacteroidetes, and Actinobacteria as potential hosts for ARGs. At the genus level, potential hosts included Sphingopyxis, SM1A02, Aquimonas, Vitellibacter, and Proteiniphilum. Middle and high antibiotic concentrations induced the secretion of metabolites closely related to potential hosts by algae, such as phytosphingosine, Lysophosphatidylcholine, and α-Linolenic acid. Therefore, changes in bacterial communities indirectly influenced the distribution of ARGs through alterations in metabolic products. These findings offer essential details about the mechanisms behind the spread and proliferation of ARGs in the phycosphere. Environmental implication Novel pollutants antibiotic resistance genes (ARGs) have become one of the threats to human health on a global level. As the dominant primary producer in aquatic systems, microalgae are essential ecological niches for the proliferation of ARGs and affect the transmission of the ARGs in the food web and the propagation. In this study, metabolites were introduced into the microalgae-bacteria symbiotic systems, combined with the abundance and diffusion of ARGs in the algal circle, the changes in the composition of bacterial community structures of different lifestyles, to explore the intrinsic relationship between algae and bacteria interactions.