Effects of sediment acidification on germinability of Scrippsiella acuminata cysts in hypoxic zones

Abstract

We investigated the calcareous cysts of Scrippsiella acuminata, with a focus on morphological changes from spiny to naked types in the surface sediments of hypoxic zones. The cyst-type abundance and bottom environmental conditions at two stations, representing hypoxic and normoxic conditions, were compared. Germination tests simulating in situ pH conditions were conducted to elucidate differences in germinability between spiny and naked cysts. The pH values at the hypoxic station reached a minimum of 7.2 in September, coinciding with high bottom-water temperatures and low dissolved oxygen levels. Significant differences in cyst abundance were observed; naked and intermediate cysts dominated the hypoxic station, whereas spiny cysts were more abundant at the normoxic station. Both cyst types exhibited a similar negative effect of germinability decrease (62–25 % for spiny cysts and 75–32 % for naked cysts) in acidic conditions (7.2) compared to normal pH conditions (7.7). Morphological changes in ungerminated cysts, such as cytoplasmic degradation and wall thickening, occurred under acidified conditions.