Immediate and delayed effects of a heatwave and Prorocentrum lima ((Ehrenberg) Stein 1878) bloom on the toxin accumulation, physiology, and survival of the oyster Magallana gigas (Thunberg, 1793)

Duplicate CgCREBL2β involved in the response of oyster upon high-temperature stress through the induction of glycolysis and haemocyte apoptosis

The cAMP response element-binding protein-like 2 (CREBL2) is involved in the regulation of response to environmental stress. A CREBL2 homologue, CgCREBL2β, was identified in the Pacific oyster Crassostrea gigas and considered a paralog derived from CREBL2 duplication. In the present study, its evolutionary characteristics and involvement in the regulation of glucose metabolism and cell apoptosis after 6 h and 60 h of high-temperature stress were investigated. At 6 h after CgCREBL2β dsRNA injection and high-temperature stress, the mRNA expressions of CgENO1 (enolase 1) and CgPGK1L (phosphoglycerate kinase 1-like), the activities of HK (hexokinase) and PK (pyruvate kinase), and the contents of glucose and GLY (glycogen) were 0.55-fold (p < 0.01), 0.44-fold (p < 0.05), 0.60-fold (p < 0.05), 1.35-fold (p < 0.05), 1.29-fold (p < 0.05) and 0.60-fold (p < 0.05) of that in the control group, respectively. CgCREBL2β was suggested to be involved in the regulation of glucose metabolism through glycolysis at very early stage of high-temperature stress. The mRNA expressions of apoptosis-related genes CgBcl-2, CgBax and CgCaspase3 were 1.80-fold (p < 0.05), 0.53-fold (p < 0.05) and 0.62-fold (p < 0.05) of that in the control group at 6 h after high-temperature stress, respectively, and were 1.60-fold (p < 0.05), 0.57-fold (p < 0.05) and 1.00-fold (p > 0.05) of that in the control group at 60 h after high-temperature stress, respectively. The apoptosis rate in the CgCREBL2β-RNAi group was 16.70 % (p < 0.05) and 20.31 % (p > 0.05) at 6 h and 60 h after high-temperature stress, respectively, which was lower than that in the control group. It is indicated that CgCREBL2β transcript was involved in the upregulation of the mRNA expressions of pro-apoptotic genes and the downregulation of the mRNA expressions of anti-apoptotic genes, thereby promoting haemocyte apoptosis. These results collectively demonstrated that the duplicate CgCREBL2β was involved in the response to high-temperature stress through the induction of glycolysis and haemocyte apoptosis.

Hot and toxic: Accumulation dynamics and ecotoxicological responses of mussel Mytilus galloprovincialis exposed to marine biotoxins during a marine heatwave

Climate change is increasing marine heatwaves (MHWs) frequency and severity worldwide. These extreme events often cause bivalves' mass mortality and facilitate the growth, proliferation and dispersion of toxin-producing microalgae blooms associated with threats to seafood safety. Yet, the interactive effects between MHW and uptake of marine biotoxins by biota are a novel topic still lacking thorough research, from both the ecotoxicological and seafood safety standpoints. This study assessed the effects of a MHW event on the accumulation/elimination dynamics of diarrhetic shellfish toxins in Mytilus galloprovincialis exposed to Prorocentrum lima and the ecotoxicological responses of mussels co-exposed to these two stressors. Results showed that acute exposure to +4 °C reduced toxins accumulation (-49 %) and elimination (-77 %) compared to control temperature. Moreover, exposure to MHW and toxins affected mussels' antioxidant activity, lipid and protein damage, and metabolism in a tissue-specific manner. These findings highlight that M. galloprovincialis can face higher vulnerability to toxins when MHW events strike.

Multi-stressor dynamics: Effects of marine heatwave stress and harmful algal blooms on juvenile mussel (Perna canaliculus) survival and physiology

In New Zealand, the frequency and intensity of marine heatwaves (MHWs) and blooms of the harmful algal species, Alexandrium pacificum, are increasing in areas where there are natural reefs and commercial farms of the mussel, Perna canaliculus. In this study, we assessed the whole organism, tissue and molecular-level response of juvenile (spat) P. canaliculus exposed to these abiotic and biotic stressors, alone and together. Spat of P. canaliculus were held at a control temperature of 17 °C or a MHW temperature of 22 °C for the duration of the experiment and exposed to two environmentally relevant concentrations of the dinoflagellate A. pacificum for 4 days, followed by a recovery period of 4 days. The MHW temperature alone had no detrimental effect on mussel survival; however, exposure to A. pacificum at both temperatures led to reduced spat survival, especially at higher microalgal concentrations (< 90 % survival vs 100 % at the control temperature, no A. pacificum). The combination of both MHW temperature and A. pacificum exposure resulted in higher mortality than either stressor alone (as low as 59 % survival compared to 99 % at the MHW temperature alone). Spat exposed to A. pacificum alone showed up to a 4-fold reduction in byssal plaque production, crucial for spat attachment and therefore survival. Growth rate was >7.5 times slower, and there were increased histological alterations as compared to mussels in the control treatment. Similarly, spat exposed to A. pacificum showed significant changes in gene expression, particularly in pathways related to lipid metabolism and detoxification. Spat exposed to MHW temperatures alone had fewer differentially expressed genes, most being heat shock proteins. These findings emphasise the importance of understanding multi-stressor impacts in marine environments, particularly in the context of climate change and harmful algal blooms (HABs).

Marine Algal Toxins and Public Health: Insights from Shellfish and Fish, the Main Biological Vectors

Exposure to toxigenic harmful algal blooms (HABs) can result in widely recognized acute poisoning in humans. The five most commonly recognized HAB-related illnesses are diarrhetic shellfish poisoning (DSP), paralytic shellfish poisoning (PSP), amnesic shellfish poisoning (ASP), neurotoxic shellfish poisoning (NSP), and ciguatera poisoning (CP). Despite being caused by exposure to various toxins or toxin analogs, these clinical syndromes share numerous similarities. Humans are exposed to these toxins mainly through the consumption of fish and shellfish, which serve as the main biological vectors. However, the risk of human diseases linked to toxigenic HABs is on the rise, corresponding to a dramatic increase in the occurrence, frequency, and intensity of toxigenic HABs in coastal regions worldwide. Although a growing body of studies have focused on the toxicological assessment of HAB-related species and their toxins on aquatic organisms, the organization of this information is lacking. Consequently, a comprehensive review of the adverse effects of HAB-associated species and their toxins on those organisms could deepen our understanding of the mechanisms behind their toxic effects, which is crucial to minimizing the risks of toxigenic HABs to human and public health. To this end, this paper summarizes the effects of the five most common HAB toxins on fish, shellfish, and humans and discusses the possible mechanisms.

Marine thermal fluctuation induced gluconeogenesis by the transcriptional regulation of CgCREBL2 in Pacific oysters

Marine thermal fluctuation profoundly influences energy metabolism, physiology, and survival of marine life. In the present study, short-term and long-term high-temperature stresses were found to affect gluconeogenesis by inhibiting PEPCK activity in the Pacific oyster (Crassostrea gigas), which is a globally distributed species that encounters significant marine thermal fluctuations in intertidal zones worldwide. CgCREBL2, a key molecule in the regulation of gluconeogenesis, plays a critical role in the transcriptional regulation of PEPCK in gluconeogenesis against high-temperature stress. CgCREBL2 was able to increase the transcription of CgPEPCK by either binding the promoter of CgPEPCK gene or activating CgPGC-1α and CgHNF-4α after short-term (6 h) high-temperature stress, while only by binding CgPEPCK after long-term (60 h) high-temperature stress. These findings will further our understanding of the effect of marine thermal fluctuation on energy metabolism on marine organisms.

Resistance of an intertidal oyster(Saccostrea mordax)to marine heatwaves and the implication for reef building

Marine heatwaves (MHWs) have a significant impact on intertidal bivalves and the ecosystems they sustain, causing the destruction of organisms' original habitats. Saccostrea mordax mainly inhabits the intertidal zone around the equator, exhibiting potential tolerance to high temperatures and maybe a species suitable for habitat restoration. However, an understanding about the tolerance mechanism of S. mordax to high temperatures is unclear. It is also unknown the extent to which S. mordax can tolerate repeated heatwaves of increasing intensity and frequency. Here, we simulated the effects of two scenarios of MHWs and measured the physiological and biochemical responses and gene expression spectrum of S. mordax. The predicted responses varied greatly across heatwaves, and no heatwave had a significant impact on the survival of S. mordax. Specifically, there were no statistically significant changes apparent in the standard metabolic rate and the activities of enzymes of the oyster during repeated heatwaves. S. mordax exposed to high-intensity heatwaves enhanced their standard metabolic rate to fuel essential physiological maintenance and increasing activity of SOD and expression of HSP70/90. These strategies are presumably at the expense of functions related to immunity and growth, as best exemplified by significant depressions in activities of enzymes (NaK, CaMg, T-ATP, and AKP) and expression levels of genes (Rab, eEF-2, HMGR, Rac1, SGK, Rab8, etc.). The performance status of S. mordax tends to improve by implementing a suite of less energy-costly compensatory mechanisms at various levels of biological organization when re-exposed to heatwaves. The adaptive abilities shown by S. mordax indicate that they can play a crucial role in the restoration of oyster reefs in tropical seas.