Oyster microbial communities and implications for chalky deposit formation

Although bivalve shell formation is an area of active research, less attention has been paid to mechanisms of precipitation for shell features with unique or unusual crystal morphologies. One example is the chalky calcite deposits found in some bivalves, commonly oysters. The goal of this study was to investigate chalky deposit formation in oysters as a possible case of microbially influenced mineralization by seeking a correlation between the presence of chalky deposits and calcifying bacteria. Extrapallial fluid was collected from specimens of Crassostrea gigas and Ostrea lurida, which do and do not display chalky deposits, respectively. Microbial communities from oyster and seawater samples were characterized and compared using 16S ribosomal RNA gene PCR and sequencing. Results indicate that microbial communities are dominated by the following bacterial phyla: Actinobacteria, Bacteroidetes, Cyanobacteria, Firmicutes, Chloroflexi, and Proteobacteria. In addition, seawater is the major source of the microbiome isolated from oyster calcifying fluid. In general, microbial communities in C. gigas are not skewed towards bacterial taxa known for influencing calcium carbonate precipitation in other systems. Although no evidence was found to support a microbial influence on chalk formation, this work provides additional data and insight into the growing body of research on invertebrate microbiomes.