Resting cysts of Fibrocapsa japonica (Raphidophyceae) from coastal sediments of the northern Adriatic Sea (Mediterranean Sea)

Fibrocapsa japonica resting cysts were detected in coastal sediments of the Marche region (northern Adriatic Sea, Italy), where summer blooms regularly occur. Sampling was carried out along 18 transects (2 stations each) during May–June 2007. The identification and counting of raphidophyte cysts were carried out under the inverted microscope and molecular analyses were applied to sediment samples to confirm species-specific identification. Raphidophyte cysts were recorded in almost all stations with maximum abundance of 9 ± 5 and 12 ± 3 cysts g−1 dw for cysts of F. japonica and undetermined Raphidophyceae, respectively. Cyst formation was induced in F. japonica monoclonal cultures under different conditions: shadow-optimal temperature and dark-low temperature treatment; both experiments were performed at two salinity values (30 and 35). Cyst formation (preliminary cysts) was observed in both experiments, although with different encystment rates. However, only a further permanence in the dark at 15 °C in microplates led to the formation of mature cysts, probably supported by the possibility to adhere to solid surfaces. Before pre-cyst formation, vegetative cells showed a cytoplasmatic “brown body”.