Morphological variations of calcareous dinoflagellate cysts and its ecological significance in the hypoxic zone

Abstract

Phytoplankton in coastal regions is generally known to display tolerance to ocean acidification. However, we present evidence here that its survival is being threatened by acidic environments in hypoxic zones. In natural sediments of hypoxic zones, morphological changes in populations of calcareous cysts were observed. Two morphotypes of Scrippsiella trochoidea cysts were identified: a typical type with short calcareous spines and a transparent type without calcareous spines. These changes were caused by acidic conditions in sediments, as proven by experiments that involved hypoxia-induced acidification. The calcareous spines of cysts could endure quite low levels of pH (pH 7.39), which suggested that the seed populations of these cysts have already been exposed to extremely acidic environments. After such exposure, cysts without calcareous spines could germinate but the morphological changes induced by acidification might be linked to mortality due to aerobic and anaerobic decay or mechanical damage during ingestion and digestion by predators. Consequently, we suggest that changes in the community structure of phytoplankton and a reduction in biodiversity will result in hypoxic zones.