Phytoplanktons in coastal regions are generally known to have a tolerance to ocean acidification. However, we found that their survival is being threatened by acidic environments in the hypoxic zone, and we present the evidence in this study. In natural sediments of the hypoxic zone, morphological changes of the calcareous seed population caused by acidic conditions in sediments have been observed and have been demonstrated by hypoxia-induced acidification experiments. The calcareous spines of cysts can endure quite low pH levels (pH 7.39), suggesting that the seed populations in the hypoxic zone are already experiencing extremely acidic environments. After being exposed to intense acidic environments, cysts without calcareous spines were able to germinate however, they can be easily linked to mortality, due to aerobic and anaerobic decay. Consequently, we suggest that changes in the planktonic community structure and a reduction in biodiversity in the hypoxic zone will result.y. In natural sediments of the hypoxic zone, morphological changes of the calcareous seed population caused by acidic conditions in sediments have been observed and have been demonstrated by hypoxia-induced acidification experiments. The calcareous spines of cysts can endure quite low pH levels (pH 7.39), suggesting that the seed populations in the hypoxic zone are already experiencing extremely acidic environments. After being exposed to intense acidic environments, cysts without calcareous spines were able to germinate however, they can be easily linked to mortality, due to aerobic and anaerobic decay. Consequently, we suggest that changes in the planktonic community structure and a reduction in biodiversity in the hypoxic zone will result.