The impact of UV radiation on the health and pathogen development of the Pacific oyster (Crassostrea gigas)

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Kett, Gary
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University College Cork
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Pacific oysters Crassostrea gigas are cultured worldwide and play an important role in global food supply and the sustainable blue economy. Oyster culture sites in Europe, USA, Australasia, and Asia have been experiencing episodic summer mass mortality events. These mortality events can be severely damaging with significant impacts on stock reliability and profitability. Summer mass mortality events are believed to have a multifactorial aetiology driven by high water temperatures and the presence of pathogens, particularly Ostreid herpesvirus-1 and variants (OsHV-1 Var) and bacteria of the genus Vibrio such as V. aestuarianus. UV radiation (UVR) is an intertidal stressor which functions as an ecosystem regulator. UVR has disinfectant properties with the energetic potential to damage nucleic acids of microbes inhabiting surface waters. UVR can also have both positive and negative impacts on animal immune functioning by the activation or inactivation of certain biochemical pathways. Climate model predictions show UV levels changing globally due to changes in cloud cover, aerosols, ozone, and precipitation patterns. This study aimed to investigate the impact of UV radiation (UVR) on oyster health and pathogen performance. Firstly, a desk-based literature review study found that UVR predominantly hinders pathogens, although with varying efficacy, has mixed effects on aquatic invertebrates and has mixed effects on host-pathogen relationships. A clear knowledge gap was identified in that no study could be found which investigated the impact of UVR on bivalve health and survival. Vibrio bacteria are reported to be highly sensitive to UVR while herpesviruses either have high tolerance or can even be activated by solar UVR. UVR can be additive, synergistic, antagonistic, or neutral in outcomes of host-pathogen dynamics. Secondly, novel diagnostic methods for the detection and localisation of Vibrio bacteria within oyster tissues were designed, a generic conventional polymerase chain reaction PCR and a DIG-labelled in situ hybridisation (ISH) assay. These tools were designed to complement existing PCR and qPCR tools and allow for improved understanding of pathogen behaviour inside a C. gigas host exposed to UVR. Primers (VibF3/VibR3) were designed to amplify a 286 bp product from the 16S ribosomal RNA gene common to all Vibrio spp. and to form the ISH probe. ISH was carried out on C. gigas seed sourced from a V. aestuarianus endemic bay (n = 17) and on C. gigas juveniles sourced from a V. aestuarianus naive site (n = 12). Positive ISH signals were observed in PCR and qPCR positive C. gigas while no ISH signal was observed in uninfected samples from the naïve site. Direct Sangar sequencing of PCR products (n = 30), Blastn analysis and Clustal Omega alignments were used to confirm Vibrio sp. detection and assess similarity. Next, to examine the effect of supplemental UV-B on C. gigas seed, a set of laboratory-based experiments were constructed. Various size classes of C. gigas seed were exposed to two conditions: i) a short duration, high intensity UV-B exposure while immersed underwater or ii) a longer duration, low intensity UV-B exposure while emersed out of water. These experimental conditions were chosen to mimic tidal immersion and emersion. The intensity of exposure was lowered in the second trial in order to carry out the treatment over the length of a typical solar peak (midday) during low tide, with the total dose typical of what would be experienced in the south coast of Ireland on a clear summer day. The impact of UV exposure on oyster health was measured by monitoring survival daily, gill tissue DNA samples were used to monitor pathogen prevalence and intensity, and histological tissue cross-sections were examined for pathological damage. Results showed that UV-B exposure negatively impacted oyster survival, most notable in the smallest seed, reducing survival by up to 35%. UV-B also impeded the development of V. aestuarianus, although most effects were transitory and returned to pre-exposure infection levels within 1 - 3 days. Moribund oysters exposed to UV-B had significantly weaker V. aestuarianus infection intensities than moribund oysters in the control group. OsHV-1 Var was not detected in any sample throughout the experiment. These findings indicate that oyster mortality was caused by UV-B exposure rather than by pathogen infection. These data are the first reported impacts of UV-B on C. gigas health and the host-pathogen dynamic with V. aestuarianus. Results from this study suggest that UVR is likely to be a causative factor in C. gigas summer mass mortality episodes. Lastly, to bridge the prior findings to the natural environment, a field trial was designed on a commercial oyster culture cite to investigate the impact of shore grow-out height and the resulting emersion conditions including solar UVR on C. gigas and pathogen performance. Emersion has been shown to have mixed effects on C. gigas performance, though little is known about the impact of UVR in this host-pathogen-environment model. The field experiment in this study was carried out over 5 months, in July C. gigas seed (n = 570) were relayed in 6 replicate mesh bags split across two shore heights equating to a +4-hour emersion time in High Shore (HS) groups compared to the Low Shore (LS) cohort. Mortality (%) was counted in the field and samples (n = 30/shore height) were returned to the lab for pathogen screening for OsHV-1 Var and V. aestuarianus using PCR and qPCR. Increased oyster mortality was associated with emersion, particularly in periods of high UV exposure (>2.4 kJ/m2) and high air temperatures (>21 oC). Pathogen partitioning was observed, OsHV-1 Var was detected more in high shore cohorts while a higher prevalence of V. aestuarianus was detected in low shore C. gigas. Results indicate that environmental conditions impacted spat survival more so than pathogen infection. These findings further demonstrate that oyster mortality and infection levels are influenced by shore height and emersion time. Results from this study can be applied in husbandry practices to reduce losses during summer mass mortality events. Research outcomes are discussed in terms of the wider framework of theoretical knowledge and global development goals, future research questions are posed and recommendations for experimental design are offered. In terms of commercial application, specific husbandry practices are suggested based on the findings of this study, however additional research should be carried out to support or improve upon these recommendations.
Crassostrea gigas , Vibrio aestuarianus , Ostreid herpesvirus-1 microVar , UV radiation , Aquaculture
Kett, G. F. 2022. The impact of UV radiation on the health and pathogen development of the Pacific oyster (Crassostrea gigas). PhD Thesis, University College Cork.