The impacts of hypoxic environments on calcareous dinoflagellate cysts in sediments

Abstract

To investigate the effect of hypoxic environments on the resting stage of planktonic organisms, a calcareous cyst population (Scrippsiella trochoidea) was monthly monitored in two different sites, hypoxic (G1) and normoxic zone (G2) of Gamak Bay from February 2009 to February 2010. Hypoxia indicating a lower level of dissolved oxygen (< 2.0 mg/L) was developed in the bottom waters of G1 in August however was not occurred at G2. Morphological and excystment characteristics of S. trochoidea cysts isolated from the natural sediment samples in the sites were compared. Two morphotypes of S. trochoidea cysts, one typical type with short calcareous spines (calcareous cyst) and another transparent type without the calcareous outer wall (organic-walled cyst), were simultaneously detected in the surface sediments. Most of the cyst distributed at G2 was the calcareous cyst while organic-walled cysts were abundant at G1. The abundances of organic-walled cysts were relatively high in the winter and spring (January to June) when the vegetative population was not detected or occurred in lower abundances. On the contrary, higher abundances of calcareous cysts were in the summer (July to September), which is coincided with flourishing of vegetative cells in the water column. The excystment capability of S. trochoidea cysts exhibited over the excystment ratio of 35% from March to May and the maximum ratio (> 70%) at G1, however there was no difference in the seasonal germination pattern between two sites.As the outer wall of S. trochoidea cysts is composed of calcium carbonate, the cyst surface can theoretically be solved in the acidic environments produced by the fermentation process in hypoxic bottom waters. We confirmed that the spines of S. trochoidea cyst are dissolved below pH 7.07, using CO2 incubator. This suggests that the hypoxic environments cause acidification in the bottom layer and sediments and are contributed to break out the morphological change of S. trochoidea cy