Temperature and daylength regulate encystment in calcareous cyst-forming dinoflagellates

We tested the effect of temperature (15, 20, and 25°C), daylength (8:16, 12:12 and 16:8 h light:dark cycles), and culture medium (K/5 and K/50) conditions on cyst production in batch cultures of 4 dinoflagellate species that form calcareous resting stages (Scrippsiella trochoideavar.aciculifera,Pentapharsodinium tyrrhenicum,Calciodinellum operosumandS. rotunda). The 4 species showed different encystment patterns at the temperature conditions tested; cyst production was inversely related to daylength conditions, and higher cyst yields were obtained with the less concentrated growth medium. Experiments with semi-continuous cultures ofS. rotunda, in which nutrient concentration was kept within values comparable toin situconcentrations, were carried out with the aim of decoupling the effect of nutrient depletion on cyst production from that of daylength. Cyst production in this species, kept at a constant growth rate in non-depleted nutrient conditions, was only obtained at the shortest daylengths, thus supporting the role of short daylength in inducing the production of calcareous cysts. Our data suggest that encystment in the 4 species is regulated by a complex interplay of at least 3 factors: daylength, temperature and nutrient concentration. These results contribute to explain the timing of resting-stage production at sea, and point to the possible role of daylength as an environmental signal for the regulation of life cycles in dinoflagellates.