3.3 Microbial community composition in environmental and host samples
The taxonomic composition of microbial community abundance on the phylum level of environmental samples showed Pseudomonadota (ClassAlphaproteobacteria, 56.7 %), Actinobacteriota (23.6 %),Bacteroidota (11.4 %), Firmicutes (4.9 %), andGammaproteobacteria (1.3 %), as the most abundant phyla in seawater. For sediments, Actinobacteriota (35.6 %),Alphaproteobacteria (32.9 %), Bacteroidota (10.9 %),Gammaproteobacteria (6.0 %), and Chloroflexi (4.0 %) were the main groups (Table S2). However, the microbial composition varied between sites, with more accentuated differences in the contribution of major phyla (> 0.1 % of the total microbial community) in more polluted sites. These differences in microbial composition between seawater and sediments gradually declined in moderated and less polluted sites, particularly forAlphaproteobacteria and Actinobacteriota (Figure 2 and 3, Table S3), albeit some site-specific and source-specific departures of the trend. In the moderately polluted site, Campilobacterota andChloroflexi increased their overall contribution to the sediment microbiota while remaining invariable in the seawater (Figure 2). In the sea cucumber host, the microbiota was also dominated byAlphaproteobacteria , Bacteroidota ,Actinobacteriota , Firmicutes, andGammaproteobacteria (Table S2). This community composition differed from the local environment and varied along the cline (Figure 2), with a major contribution of the family Rhodobacteraceae(Alphaproteobacteria ; Figure S2) and species from the associated genera Ruegeria , Dinoroseobacter, and Oceanicella(Figure S3).