MEPS 306:143-152 (2006)  -  doi:10.3354/meps306143

ABSTRACT: An intriguing case of encystment of the dinoflagellate Scrippsiella lachrymosa in culture has recently been presented: cyst concentrations still increased after the abundance of the vegetative cells had decreased to very low numbers. To account for this apparent time lag, and using a model with an encystment rate equation in which vegetative growth and encystment rates were coupled, the calculated growth and encystment rates had to be 8 to 12 d^–1, values which were considered ‘not biologically meaningful’. Here a new model of dinoflagellate growth and encystment is presented in which the mitotic cycle (vegetative growth) is coupled quantitatively to the sexual cycle (cyst formation) by having 4 gametes emanate from 1 vegetative cell, but without directly coupling the rates of vegetative growth and encystment. Calibrated on literature data of S. lachrymosa cultured in f/4 medium, this model satisfactorily describes motile cell (vegetative cells and gametes) and cyst development with correlations between log-transformed model and experimental data of r² = 0.80 (motile cells) and r² = 0.94 (cysts) and with typical maximum rates in the exponential growth phase of μ_cell = 0.55 d^–1 (gross vegetative cell rate), μ_gamete+cell = 0.38 d^–1 (net motile cell growth rate), ε = 0.42 d^–1 (encystment rate). All these rates declined in the stationary growth phase. Sample sonication is suggested as the cause for low motile cell concentrations in the original experiment when cyst production was high. Inorganic carbon limitation due to low inorganic carbon to nitrogen concentrations in the growth media is probably the reason why cysts in f/4 medium stopped making calcite covers in later stages of the experiment and why cyst yield in f/2 medium was not double the yield in f/4. A new method for measuring in situ encystment rates of dinoflagellate populations with a phased sexual cycle is proposed.

KEY WORDS: Dinoflagellate · Vegetative growth · Sexual cycle · Gametes · Cysts · Model