Conclusion
Marine chemical pollution is an important driver modulating the structure and function of microbial communities. Spatial clines in the intensity and magnitude of this driver can result in different patterns of environmental filtering, even across short geographic scales. However, for microbial communities associated with marine animal hosts (e.g., the sea cucumber H. leucospilota ), there are additional mechanisms influencing their composition and abundance. Such mechanisms are underpinned by intrinsic characteristics of their host (e.g., body compartments, biochemistry composition, immune systems), resulting in intra-individual differences in associated microbiomes, and their divergence from the environmental source. These findings support the hypothesis of an intrinsic capacity of the host to regulate its microbiome. Such regulation favours specific microbial functional pathways that may play an important role in the survival and physiology of the animal host, particularly in high polluted areas. Despite the observed differences in the environment and sea cucumber hosts, there was a small component of the microbial community (core microbiome) that was constant across the pollution cline and the animal body parts, suggesting that other mechanisms are also involved in the control of microbial communities in these animals.