Dietary Exposure of Pacific Oyster (Crassostrea gigas) Larvae to Compromised Microalgae Results in Impaired Fitness and Microbiome Shift

Understanding the Dynamic of POMS Infection and the Role of Microbiota Composition in the Survival of Pacific Oysters, Crassostrea gigas

For over a decade, Pacific oyster mortality syndrome (POMS), a polymicrobial disease, induced recurring episodes of massive mortality affecting Crassostrea gigas oysters worldwide. Recent studies evidenced a combined infection of the ostreid herpesvirus (OsHV-1 μVar) and opportunistic bacteria in affected oysters. However, the role of the oyster microbiota in POMS is not fully understood. While some bacteria can protect hosts from infection, even minor changes to the microbial communities may also facilitate infection and worsen disease severity. Using a laboratory-based experimental infection model, we challenged juveniles from 10 biparental oyster families with previously established contrasted genetically based ability to survive POMS in the field. Combining molecular analyses and 16S rRNA gene sequencing with histopathological observations, we described the temporal kinetics of POMS and characterized the changes in microbiota during infection. By associating the microbiota composition with oyster mortality rate, viral load, and viral gene expression, we were able to identify both potentially harmful and beneficial bacterial amplicon sequence variants (ASVs). We also observed a delay in viral infection resulting in a later onset of mortality in oysters compared to previous observations and a lack of evidence of fatal dysbiosis in infected oysters. Overall, these results provide new insights into how the oyster microbiome may influence POMS disease outcomes and open new perspectives on the use of microbiome composition as a complementary screening tool to determine shellfish health and potentially predict oyster vulnerability to POMS. IMPORTANCE For more than a decade, Pacific oyster mortality syndrome (POMS) has severely impacted the Crassostrea gigas aquaculture industry, at times killing up to 100% of young farmed Pacific oysters, a key commercial species that is cultivated globally. These disease outbreaks have caused major financial losses for the oyster aquaculture industry. Selective breeding has improved disease resistance in oysters, but some levels of mortality persist, and additional knowledge of the disease progression and pathogenicity is needed to develop complementary mitigation strategies. In this holistic study, we identified some potentially harmful and beneficial bacteria that can influence the outcome of the disease. These results will contribute to advance disease management and aquaculture practices by improving our understanding of the mechanisms behind genetic resistance to POMS and assisting in predicting oyster vulnerability to POMS.

Does plastic type matter? Insights into non-indigenous marine larvae recruitment under controlled conditions

Marine plastic debris (MPD) are a global threat to marine ecosystems. Among countless ecosystem impacts, MPD can serve as a vector for marine 'hitchhikers' by facilitating transport and subsequent spread of unwanted pests and pathogens. The transport and spread of these non-indigenous species (NIS) can have substantial impacts on native biodiversity, ecosystem services/functions and hence, important economic consequences. Over the past decade, increasing research interest has been directed towards the characterization of biological communities colonizing plastic debris, the so called Plastisphere. Despite remarkable advances in this field, little is known regarding the recruitment patterns of NIS larvae and propagules on MPD, and the factors influencing these patterns. To address this knowledge gap, we used custom-made bioassay chambers and ran four consecutive bioassays to compare the settlement patterns of four distinct model biofouling organisms' larvae, including the three notorious invaders Crassostrea gigas, Ciona savignyi and Mytilus galloprovincialis, along with one sessile macro-invertebrate Spirobranchus cariniferus, on three different types of polymers, namely Low-Linear Density Polyethylene (LLDPE), Polylactic Acid (PLA), Nylon-6, and a glass control. Control bioassay chambers were included to investigate the microbial community composition colonizing the different substrates using 16S rRNA metabarcoding. We observed species-specific settlement patterns, with larvae aggregating on different locations on the substrates. Furthermore, our results revealed that C. savignyi and S. cariniferus generally favoured Nylon and PLA, whereas no specific preferences were observed for C. gigas and M. galloprovincialis. We did not detect significant differences in bacterial community composition between the tested substrates. Taken together, our results highlight the complexity of interactions between NIS larvae and plastic polymers. We conclude that several factors and their potential interactions influenced the results of this investigation, including: (i) species-specific larval biological traits and ecology; (ii) physical and chemical composition of the substrates; and (iii) biological cues emitted by bacterial biofilm and the level of chemosensitivity of the different NIS larvae. To mitigate the biosecurity risks associated with drifting plastic debris, additional research effort is critical to effectively decipher the mechanisms involved in the recruitment of NIS on MPD.