Effects of Erythromycin on Nereis succinea and the Intestinal Microbiome across Different Salinity Levels

The exposure of aquatic organisms to pollutants often occurs concomitantly with salinity fluctuations. Here, we reported the effects of erythromycin (0.250, 7.21, and 1030 mu g/L) on marine invertebrate N. succinea and its intestinal microbiome under varying salinity levels (5 parts per thousand, 15 parts per thousand, and 30 parts per thousand). The salinity elicited significant effects on the growth and intestinal microbiome of N. succinea. The susceptibility of the intestinal microbiome to erythromycin increased by 8.7- and 6.2-fold at salinities of 15 parts per thousand and 30 parts per thousand, respectively, compared with that at 5 parts per thousand salinity. Erythromycin caused oxidative stress and histological changes in N. succinea intestines, and inhibited N. succinea growth in a concentration-dependent manner under 30 parts per thousand salinity with a maximum inhibition of 25%. At the intestinal microbial level, erythromycin enhanced the total cell counts at 5 parts per thousand salinity but reduced them at 15 parts per thousand salinity. Under all tested salinities, erythromycin diminished the antibiotic susceptibility of the intestinal microbiome. Two-way ANOVA revealed significant interactive effects (p < 0.05) between salinity and erythromycin on various parameters, including antibiotic susceptibility and intestinal microbial diversity. The present findings demonstrated the significant role of salinity in modulating the impacts of erythromycin, emphasizing the necessity to incorporate salinity fluctuations into environmental risk assessments.