OsHV-1 countermeasures to the Pacific oyster’s anti-viral response

The host-pathogen interactions between the Pacific oyster (Crassostrea gigas) and Ostreid herpesvirus type 1 (OsHV-1) are poorly characterised. Herpesviruses are a group of large, DNA viruses that are known to encode gene products that subvert their host’s antiviral response. It is likely that OsHV-1 has also evolved similar strategies as its genome encodes genes with high homology to C. gigas inhibitors of apoptosis (IAPs) and an interferon-stimulated gene (termed CH25H). The first objective of this study was to simultaneously investigate the expression of C. gigas and OsHV-1 genes that share high sequence homology during an acute infection. Comparison of apoptosis-related genes revealed that components of the extrinsic apoptosis pathway (TNF) were induced in response to OsHV-1 infection, but we failed to observe evidence of apoptosis using a combination of biochemical and molecular assays. IAPs encoded by OsHV-1 were highly expressed during the acute stage of infection and may explain why we didn’t observe evidence of apoptosis. However, C. gigas must have an alternative mechanism to apoptosis for clearing OsHV-1 from infected gill cells as we observed a reduction in viral DNA between 27 and 54 h post-infection. The reduction of viral DNA in C. gigas gill cells occurred after the up-regulation of interferon-stimulated genes (viperin, PKR, ADAR). In a second objective, we manipulated the host’s anti-viral response by injecting C. gigas with a small dose of poly I:C at the time of OsHV-1 infection. This small dose of poly I:C was unable to induce transcription of known antiviral effectors (ISGs), but these oysters were still capable of inhibiting OsHV-1 replication. This result suggests dsRNA induces an anti-viral response that is additional to the IFN-like pathway.

Light and dark carbon uptake by Dinophysis species in comparison to other photosynthetic and heterotrophic dinoflagellates

The marine dinoflagellate genus Dinophysis includes species that are the causative agents of diarrhetic shellfish poisoning (DSP). Recent findings indicate that some Dinophysis species are mixotrophic, i.e. capable of both autotrophic and heterotrophic nutrition. We investigated inorganic (and organic) carbon uptake by several species of Dinophysis in the Light and dark using the 'single-cell C-14 method', and compared uptake rates with those of photosynthetic Ceratium species and heterotrophic dinoflagellates in the genus Protoperidinium. Experiments were conducted with water from the Gullmar Fjord and from the Koster Strait (Swedish west coast). Nutrient-enriched phytoplankton from surface water samples were concentrated (20 to 70 mu m) and incubated at in situ temperature under artificial light conditions with high concentrations of inorganic C-14 (1 mu Ci ml(-1)). Individual cells of each desired species were manually isolated under a microscope and transferred to scintillation vials. C. tripes showed net C-14 uptake only during light periods, whereas both C. lineatum and C. furca showed C-14 uptake in the Light as well as uptake (and sometimes losses) in the dark. Dinophysis species had similar carbon fixation rates in Light compared to Ceratium species. For D. acuminata and D. norvegica, net carbon uptake occurred in both Light and dark periods. D. acuta showed a loss of carbon in the dark in one experiment, but in another, dark C uptake was significantly higher than uptake in Light. When exposed to Light, C. furca, D. norvegica and D. acuta had high specific carbon uptake rates. Growth rates for the different species were calculated from C-14 uptake by the cells during the first hours of incubation in light. D. acuminata and D. norvegica had similar maximum growth rates, 0.59 and 0.63 d(-1) (mu); the maximum growth rate of D. acuta was lower (0.41 d(-1)). The positive dark carbon uptake by Dinophysis may suggest a mixotrophic mode of nutrition. In one experiment, both D. norvegica and D. acuta showed a significantly higher carbon uptake in a dark bottle than in a Light bottle, which would be consistent with uptake of C-14-labeled organic matter by D. norvegica and D. acuta. Demonstration of direct uptake of dissolved and particulate organic matter would provide conclusive evidence of mixotrophy and this will require the development of new protocols for measuring organic matter uptake applicable to Dinophysis in the natural assemblages.