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).