Abstract: Investigation into the diversity of fungal-bacterial symbionts in pearl oysters from the Weizhou Island-Xieyang Island sea area of Beihai, Guangxi, and their natural defense capability against homologous Vibrio.Five fungal isolation media and two bacterial isolation media were used to isolate and purify fungal-bacterial symbionts and Vibrio species from pearl oyster samples collected from six different sampling sites. Fungal host diversity was analyzed by sequencing using fungal ITS primers, and endosymbiotic bacteria were screened using 16S rRNA universal bacterial primers. The symbiotic status of certain endosymbiotic bacteria was verified via fluorescent staining.Homologous pathogenic Vibrio strains were used as indicator bacteria to evaluate the inhibitory capability of fungal-bacterial symbionts against homologous Vibrio using the agar block diffusion method.The results showed that: (1) A total of 63 pairs of fungal-bacterial symbionts were screened from Beihai pearl oysters. Their host fungi belonged to 2 phyla, 4 classes, 11 orders, 18 families, 23 genera, and 44 species, while the endosymbiotic bacteria belonged to 1 phylum, 3 classes, 8 orders, 11 families, 18 genera, and 26 species.(2) Activity screening revealed that 29 pairs of fungal-bacterial symbionts exhibited inhibitory effects against at least one Vibrio strain, accounting for 46.03% of the total symbionts. Among them, several strains demonstrated broad-spectrum and significant inhibitory activities; the symbiont GXIMD22849 produced inhibition zones with diameters reaching 20 mm against four Vibrio strains isolated from pearl oysters.This study elucidates the natural defense of pearl oysters against homologous Vibrio through fungal-bacterial symbionts, providing new insights into understanding the strategies of marine organisms and microbes in synergistically maintaining ecological balance.

Key words: Fungal bacterial symbionts, diversity, anti-Vibrio activity, pearl oyster