The Mercury-Tolerant Microbiota of the Zooplankton Aids in Host Survival and Maintains Fecundity under Mercury Stress.

Many aquatic organisms can thrive in polluted environments by having the genetic capability to withstand suboptimal conditions. However, the contributions of microbiomes under these stressful environments are poorly understood. We investigated whether a mercury-tolerant microbiota can extend its phenotype to its host by ameliorating host survival and fecundity under mercury-stress. We isolated microbiota members from various clones of , screened for the mercury-biotransforming gene, and determined their mercury tolerance levels. We then introduced the mercury-tolerant microbiota, -10, to axenic and quantified its gene expression, mercury reduction capability, and measured its impact on host survival and fecundity. The expression of the gene was up-regulated in -10, both in isolation and in host-association with mercury exposure. -10 is also capable of significantly reducing mercury concentration in the medium. Notably, mercury-exposed daphnids containing only -10 exhibited higher survival and fecundity than mercury-exposed daphnids supplemented with parental microbiome. Our study showed that zooplankton, such as , naturally harbor microbiome members that are eco-responsive and tolerant to mercury exposure and can aid in host survival and maintain host fecundity in a mercury-contaminated environment. This study further demonstrates that under stressful environmental conditions, the fitness of the host can depend on the genotype and the phenotype of its microbiome.