Enhanced Resting Cyst Production in Harmful Dinoflagellate Akashiwo sanguinea Amended with Taebaek Coal Powder

The dinoflagellate Akashiwo sanguinea is a prominent harmful algal bloom (HAB) species responsible for significant mortalities of marine fauna. Its life cycle, which includes a benthic resting cyst stage, is fundamental to its bloom dynamics and geographic dispersal. This study investigates the effects of Taebaek coal powder, a silicate-rich mineral supplement, on the growth and life-stage transitions of A. sanguinea. Cultures were grown in standard f/2 medium (control) and f/2 medium amended with an extract of the coal powder. We monitored culture performance using fluorometry for quantitative biomass assessment and imaging flow cytometry (FlowCam) for qualitative life-stage analysis. The coal powder amendment conferred a distinct advantage, promoting both vegetative proliferation and the formation of resting cysts. Fluorescence-based measurements showed that the coal powder-amended cultures reached a density equivalent of 3851 ± 214 cells mL−1 by day 4, significantly outpacing the control (2963 ± 351 cells mL−1). Peak vegetative abundance in the treated cultures reached 6967 ± 423 cells mL−1 on day 14, compared to 5979 ± 288 cells mL−1 in the control. Critically, resting cyst production was substantially enhanced in the coal powder treatment, with densities reaching 32–37 cysts mL−1 by the end of the experiment, compared to 22–26 cysts mL−1 in the control. These findings demonstrate that mineral supplementation with Taebaek coal powder can significantly augment both vegetative growth and encystment in A. sanguinea, suggesting a potential link to micronutrient availability, though the underlying mechanisms remain to be elucidated. This enhanced cyst production method may prove valuable for harvesting cysts for ecophysiological research and highlights the need to explore how mineral-induced life-cycle shifts could influence bloom dynamics in a context-dependent manner.