Quantitative X-ray microanalysis as a method for measuring phosphorus in dinoflagellate resting cysts

Energy dispersive X-ray microanalysis (XRMA) in a scanning electron microsc ope (SEM) was used as a method to measure elemental silicon (Si) and phosph orus (P) in dinoflagellate cysts. Cysts were prepared by quick jeep freezin g and then freeze-drying, thereby avoiding the addition of preservatives. C ysts of Ceratium hirundinella collected from Lake Erken, Sweden and Esthwai te Water, UK, and Ceratium furcoides collected from Esthwaite Water, were a nalysed and compared. The hypothesis that cysts are able to assimilate P du ring dormancy was tested in the laboratory by incubating newly collected cy sts of C. hirundinella in medium with and without phosphate. The analyses s howed that there was no difference in P content between C. hirundinella and C. furcoides, suggesting that P content reflects differences in physiologi cal status rather than species. C. hirundinella had a significantly higher Si content than C. furcoides, which agrees with earlier studies. Comparison of cysts of C. hirundinella from different years - 1995 (stored for 1 year ), 1996 and 1997 - showed that the P content in cysts from 1995 was higher than that in cysts from the following 2 years, which indicates either that P was higher during encystment in 1995 or that cysts accumulated P during d ormancy. The P uptake experiment showed a very slightly, but significantly, higher, P content in cysts incubated in P-rich medium.