This is also reflected in gill associated microbial communities of other oyster species that differ more strongly from the surrounding sea water than PFT�� mouse for example gut communities . The numerical abundance of α-proteobacteria in open water could however partly been attributed to PCR bias by preferential amplification of sequences from this taxonomic group . The dominant genus detected, was Sphingomonas which contains opportunistic species  and can also commonly be found in gill tissue of European plaice Pleuronectes platessa from the same region . It was also abundant on freshly
prepared cod in Iceland , indicating that this genus can reach high numbers on living hosts but is quickly outcompeted after the host’s death. Dominance of a few closely related OTUs has been reported for other species of oysters. Zurel et al.  for example found that between 59 – 79% of OTUs in Chama spp. oysters in the Red Sea and the Mediterranean
belonged to OTUs from the class Oceanospirialles closely related to the genera Spongiobacter or Endozoicomonas (Hahellaceae), which is known for symbiotic associations. While we also observed 47 OTUs from the Oceanospirialles, these were relatively rare (99 reads in total) and only a single OTU was affiliated to the family Hahellaceae. Similarly, we only found very few OTUs classified as Arcobacter spp. (13 OTUs, 16 reads), which represent a major and common component of Chilean oysters Tiostrea chilensis. find more This suggests that oyster microbiomes can have similar structures in terms of abundances but dominant taxa differ strongly between species, habitats and sampled tissues. Under certain environmental conditions gut communities of other Crassostrea species were found to be dominated by Mycoplasma, which also became dominant in some oysters after disturbance in our experiments (Figure 5A).
The natural dominance of Mycoplasma in oysters from much warmer habitats  may thus suggest that Mycoplasma represents a temperature sensitive part of oyster microbiota and may proliferate preferentially at higher temperatures. Host stress and abiotic disturbance both could have contributed to the major shift in microbial Glutamate dehydrogenase community structure (Figure 3). The direction and magnitude of the shift was dependent on the initial community composition, and although no significant differences were observed between oyster beds in ambient conditions there was some indication for oyster bed specific shifts (Figures 3 and 4). The strongest shifts occurred in the beds with initially high microbial diversity (OW and PK), manifested in a sharp decrease in microbial diversity. In the oyster bed with low diversity on the other hand we observed no significant change in bacterial diversity (Figure 2).