Morphological variations of calcareous seed populations and its ecological significance in the hypoxic zone

Abstract

Phytoplanktons in coastal regions are generally known to have a tolerance to ocean acidification. However, we identified that their survival is being threatened by acidic environments in the hypoxic zone, and present the evidence here. In natural sediments of hypoxic zone, morphological changes of calcareous cyst populations were observed, which were caused by acidic conditions in sediments and have been proven by hypoxia-induced acidification experiments. The calcareous spines of cysts could endure quite low pH levels (ca. pH 7.4), suggesting that the seed populations are already experiencing extremely acidic environments. After experiencing the intense acidic environments, the seeds without calcareous spines have germinability however they can be linked to mortality, due to loss of surface coat. Consequently, we suggest that the changes in the community structure of phytoplankton and the reduction in biodiversity will result in the hypoxic zones of coastal regions.