Environmental gradients can influence morpho-physiological and
life-history differences in natural populations. It is unclear, however,
to what extent such gradients can also modulate phenotypic differences
in other organismal characteristics such as the structure and function
of host-associated microbial communities. In this work, we addressed
this question by assessing intra-specific variation in the diversity,
structure and function of environmental-associated (sediment and water)
and animal-associated (skin and gut) microbiota along an environmental
gradient of pollution in one of the most urbanized coastal areas in the
world. Using the tropical sea cucumber Holothuria leucospilota, we
tested the interplay between deterministic (e.g., environmental/host
filtering) and stochastic (e.g., random microbial dispersal) processes
underpinning host-microbiome interactions and microbial assemblages.
Overall, our results indicate that microbial communities are complex and
vary in structure and function between the environment and the animal
hosts. However, these differences are modulated by the level of
pollution across the gradient with marked clines in alpha and beta
diversity. Yet, such clines and overall differences showed opposite
directions when comparing environmental- and animal-associated microbial
communities. In the sea cucumbers, intrinsic characteristics (e.g., body
compartments, biochemistry composition, immune systems), may underpin
the observed intra-individual differences in the associated microbiomes,
and their divergence from the environmental source. 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. These findings suggest that the
interplay between both, environmental and host filtering underpins
microbial community assembly in H. leucospilota along the pollution
gradient in Hong Kong.