Ocean acidification caused by increasing emission of carbon dioxide (CO 2 ) is expected to have profound impacts on marine ecological processes, including the formation and evolution of harmful algal blooms (HABs). We designed a set of experiments in the laboratory to examine the effects of increasing CO 2 on the growth and toxicity of a toxic dinoflagellate Alexandrium minutum producing paralytic shellfish toxins (PSTs). It was found that high levels of CO 2 (800 and 1200 ppm) significantly promoted the growth of A. minutum compared to the group (400 ppm) representing the current CO 2 level. The total yields of PSTs by A. minutum , including both intracellular and extracellular toxins, were significantly enhanced, probably due to the induction of core enzyme activity and key amino acids synthesis for PST production. More interestingly, high level of CO 2 promoted the transformation from gonyautoxin2&3 to gonyautoxin1&4 and depressed the release of PSTs from inside to outside of the cells. All these processes collectively led to an apparent increase of A. minutum toxicity. Our study demonstrated that rising CO 2 would increase the risk of toxic A. minutum based on the comprehensive analyses of different processes including algal growth and toxin synthesis, transformation and release.
Toxic harmful algal blooms (HABs) have received increasing attention owing to their threat to the health of aquatic life and seafood consumers. This study evaluated the impacts of elevated atmospheric partial pressure of CO 2 ( p CO 2 ) on the production of paralytic shellfish toxins (PSTs) in different Alexandrium spp. strains, together with its further effects on the bioaccumulation/elimination dynamics of PSTs in bivalves contaminated with PSTs from toxic dinoflagellates. Our results showed that elevated p CO 2 stimulated the growth of the two Alexandrium spp. ( A. catenella and A. pacificum ) isolated from the northern and southern coastal areas of China, respectively, and affected PST production including content and toxicity of the two strains differently. Further PSTs bioaccumulation/elimination in PSTs-contaminated Manila clam, Ruditapes philippinarum under high p CO 2 also occurred. It is worth noting the biotransformation of neosaxitoxin (NEO) with high toxicity through trophic transfer with effect of elevated p CO 2 . When in microalgae cultured under the control (410 ppm) and elevated p CO 2 conditions (495 and 850 ppm), the proportion of NEO in the PST content produced by A. catenella was reduced from 11.1 to 6.4 and 2.6 %, while the proportion of NEO in A. pacificum was increased from 3.1 to 3.6 and 4.7 %, respectively. NEO accounted for >50 % of total PST contents in clams, which were biotransformed via transfer from dinoflagellates and higher p CO 2 enhanced this biotransformation leading to increased NEO accumulation. The negatively affected elimination of PSTs, especially NEO, in clams fed with A. catenella or A. pacificum , indicates that the detoxification of PSTs-contaminated clams may be more difficult under elevated p CO 2 . This study provides reference for developing models to assess the safety of bivalves under the co-stress of environmental change and toxic HABs, suggesting that ocean acidification may lead to the higher safety risk of Manila clams exposed to toxic HAB dinoflagellates.
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