Modelling vegetative growth, gamete production and encystment of dinoflagellates in batch culture

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 forma- tion) 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 devel- opment with correlations between log-transformed model and experimental data of r 2 = 0.80 (motile cells) and r 2 = 0.94 (cysts) and with typical maximum rates in the exponential growth phase of μcell = 0.55 d -1 (gross vegetative cell rate), μgamete+cell = 0.38 d -1 (net motile cell growth rate), e = 0.42 d -1 (encystment rate). All these rates declined in the stationary growth phase. Sample sonication is sug- gested as the cause for low motile cell concentrations in the original experiment when cyst produc- tion 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.