Publications: Research reports and publications

Understanding bacterial communities for informed biosecurity and improved larval survival in pacific oysters

16 February, 2018
Cawthron Report No. 3129 Prepared for Cawthron Institute

Pochon X, Smith K, Banks JC, Mae H, Symonds JE 2018. Understanding bacterial communities for informed biosecurity and improved larval survival in Pacific oysters. Prepared for Cawthron Institute. Cawthron Report No. 3129. 27 pages plus appendices.

EXECUTIVE SUMMARY

Bacteria are ubiquitous in all marine habitats and can either be beneficial or detrimental to the survival and growth of shellfish raised in aquaculture. For larval survival and biosecurity purposes, the Cawthron Aquaculture Park (CAP) treats influent Pacific oyster (Crassostrea gigas) hatchery seawater with ultraviolet light (UV; 100 mJoules/cm2) to reduce the exposure of cultured shellfish to potential pathogens. The overarching goals of this project were to study the effect of seawater UV treatment on microbiome assemblages and develop an appropriate seawater UV disinfection protocol for Pacific oyster larvae rearing.

We used a combination of bacterial cultures and 16S rDNA metabarcoding to measure the effect of low (50 mJoules/cm2) and high (200 mJoules/cm2) doses of seawater UV treatment on the bacterial communities present in hatchery rearing water systems, and in reared Pacific oysters at various developmental stages. A total of 81 samples were collected between 13 and 29 March 2017 for high-throughput DNA sequencing of bacterial communities, including differentially-treated seawater and oyster larvae (fertilised eggs, early veliger larval stage [D-larvae], and pre-settlement stage) samples. Differences in larval mortality between low and high UV treatments were also assessed.

We found that although the two UV treatments had an effect on the overall bacterial communities’ composition in seawater samples, the microbiome of the hatchery influent seawater was primarily driven by temporal changes in the water source. The bacterial microbiome associated with the oyster larvae changed more significantly in response to both UV treatments and sampling dates. For example, marked shifts in dominant bacterial families were observed between the fertilised eggs (Gammaproteobacteriaceae, Rhodospirillaceae, Pelagibacteraceae), the D-larvae (Alteromonadaceae), and the pre-settlement larvae (Flavobacteriaceae and Rhodobacteraceae) which were also characterised by an increasingly complex bacterial community structure at the molecular species level. These bacterial community changes were likely driven by multiple factors, including the microbiome associated with micro-algal cultures, which increased in complexity over time. However, the oyster larval mortality assessment showed no significant differences between low and high UV treatments.

Our findings indicate that it is possible to increase the UV treatment to 200 mJoules/cm2 without adversely influencing larval survival or unfavourably changing the bacterial community during rearing. This research also enabled ‘in-house’ development of applied aquaculture microbiomics and bioinformatics capability, a new and rapidly developing field. Further research is required to better understand the dynamics of the functionally critical bacterial taxa and their successive roles in post-larval development of Pacific oysters.