Oyster biomineralisation in acidifying oceans : from genes to shells

Abstract

Biomineralisation is the process of biologically controlled shell fabrication in marine calcifiers including edible oysters where shell matrix proteins and organic molecules secreted by mantle tissue controls calcium carbonate nucleation, crystallisation, growth, and mechanical properties. It is also one of the key processes that is notably affected in marine calcifiers under human induced environmental stressor, ocean acidification (OA). Understanding molecular changes in the biomineralisation process under OA, therefore, is key to developing conservation strategies for protecting ecologically and economically important oyster species. In this PhD thesis, I have presented hierarchical analyses of biomineralisation mechanisms of Crassostrea hongkongensis (Hong Kong oysters) under OA. The hierarchical analyses include study of changes in DNA methylation and gene expression of mantle tissue of juvenile Hong Kong oysters under OA. On top of studying molecular changes, this study also has incorporated shell mechanical properties in terms of micro-structure, shell crystal orientation and micro-hardness. In addition to juveniles, larvae which are known to be sensitive to OA than juveniles and adults, were also studied for understanding their shell fabrication capacity under OA. This study is also the first to attempt characterisation of shell proteome changes in an oyster species under OA.

The results indicate moderate resilience of Hong Kong oyster biomineralisation to OA. Specifically, calcium binding or signalling related genes were subtly differentially expressed in mantle under OA, with no correlation between gene expression and DNA methylation changes. Hong Kong oysters were able to make unimpaired shells in terms of micro-structure and nanostructure (crystal orientation) in both larval and juvenile stages. We conclude that OA would be still a dissolution problem for resilient species such as Hong Kong oysters despite the organism’s ability to make error free shells under OA. We also define the concept directional dissolution – where shell dissolution is directional from hinge to shell edge; and from outer periostracum to inner layers. Ecologists can adapt the directional dissolution concept for accurate use of shell dissolution as a parameter for OA biomonitoring. This thesis will be of interest not only to marine molecular biologists and ecologists but also to material scientists who are interested in biomimetic material designing.