Development, morphological characteristics and viability of temporary cysts of Pyrodinium bahamense var. compressum (Dinophyceae) in vitro

Abstract

Pellicle or temporary cysts of Pyrodinium bahamense var. compressum (Böhm) Steidinger, Tester & F.J.R. Taylor and their role in bloom dynamics have not yet been adequately characterized and understood. We investigated the role of temperature- and nutrient-mediated stress as factors that could induce pellicle formation in batch cultures. Cellular features and their implications for temporary cyst viability were examined using confocal laser scanning microscopy (CLSM). Our data suggest that temperature change is one of the key factors influencing pellicle formation, preserving viability at low temperature (i.e. 13°C). Hypnocysts (resting cysts) were not observed. During pellicle formation, motile cells generally undergo ecdysis, extrusion of cytoplasmic materials and bacteria, compaction of the nucleus and non-motility. The outermost covering of the temporary cysts shows red autofluorescence and it contains lower concentrations of chlorophyll (chl) a and no detectable chl c. The nuclear region is surrounded by transitional red bodies and other unidentified cellular structures. Temporary cysts can immediately revert back to the motile state upon exposure to optimum conditions. This is accompanied by the expansion of the nuclear region, regeneration of the chloroplasts and enlargement of the cell. Developmental changes during reversal of temporary cysts to motile forms were also observed to cause breaks in the cell covering that could serve as sites for bacterial entry. Though observed in vitro, such behaviour may also be occurring in nature especially as a response to drastic short-lived environmental changes. This is the first detailed report on the characteristics of temporary cysts of P. bahamense var. compressum.

Key words:

• confocal laser scanning microscopy
• intracellular bacteria
• pellicle formation
• Pyrodinium bahamense var. compressum
• temperature/nutrient stress
• temporary cyst

Acknowledgements

This study is part of the research programme ‘Ecology and Oceanography of Harmful Algal Blooms in the Philippines (PhilHABs), Project 1: Biodiversity/Genetic Diversity of selected HAB-forming species in the Philippines and their associated bacterial communities’, funded and supported by the Department of Science and Technology (DOST) through the Philippine Council for Aquatic and Marine Research and Development (PCAMRD now PCAARRD) and a national project of the UNESCO IOC-SCOR programme entitled ‘Global Ecology and Oceanography of Harmful Algal Blooms’ (GEOHAB). We would also like to thank the Marine Toxinology Laboratory of the MSI headed by Dr Lourdes J. Cruz for the use of the sonicator and the spectrophotometer, Emelita Eugenio for her help in maintaining the algal cultures, and Garry Benico, Johanna Munar and Powell Marquez for some technical assistance. Also, we are grateful to the comments of some anonymous reviewers and to Dr Mary Anne Gonzales-Santos for her insights into Pyrodinium biology and ecology.