A Principal Coordinates Analysis (PCoA) (Fig. 4) performed using unweighted UniFrac distances allowed to further analyse the factors responsible for significant differences between groups of samples. Fig 4 shows a clear separation along the x-axis (PC1 = 23.40% of variability) between control samples (Ace_, Con_) from the remaining atrazine treated samples. Conversely, along the y-axis (PC2 = 10.97%) all post 30-day recovery period atrazine treated groups form a small cluster and are well separated from all the remaining groups, indicating bacterial composition similarities. Finally, the trend of α-diversity seems to mainly be related to exposure concentration, higher microbial richness was found in oysters that came from treatment groups which had lower concentrations compared to those which had higher concentrations (Fig. 3).