Oyster biomineralisation under ocean acidification: from genes to shell.

Biomineralisation is one of the key processes that is notably affected in marine calcifiers such as oysters under ocean acidification (OA). Understanding molecular changes in the biomineralisation process under OA and its heritability, therefore, is key to developing conservation strategies for protecting ecologically and economically important oyster species. To do this, in this study we have explicitly chosen the tissue involved in biomineralisation (mantle) of an estuarine commercial oyster species, Crassostrea hongkongensis. The primary aim of this study is to understand the influence of DNA methylation over gene expression of mantle tissue under decreased ~ pH 7.4, a proxy of OA, and to extrapolate if these molecular changes can be observed in the product of biomineralisation - the shell. We grew early juvenile C. hongkongensis, under decreased ~ pH 7.4 and control ~ pH 8.0 over 4.5 months and studied OA-induced DNA methylation and gene expression patterns along with shell properties such as microstructure, crystal orientation and hardness. The population of oysters used in this study was found to be moderately resilient to OA at the end of the experiment. The expression of key biomineralisation related genes such as carbonic anhydrase and alkaline phosphatase remained unaffected, thus, mechanical properties of the shell (shell growth rate, hardness and crystal orientation) were also maintained without any significant difference between control and OA conditions with signs of severe dissolution. In addition, this study makes three major conclusions: 1) higher expression of Ca2+ binding/signalling related genes in the mantle play a key role in maintaining biomineralisation under OA, 2) DNA methylation changes occur in response to OA, however, these methylation changes do not directly control gene expression, 3) OA would be more of a 'dissolution problem' rather than a 'biomineralisation problem' for resilient species that maintain calcification rate with normal shell growth and mechanical properties.