Temperature and daylength regulate encystment in calcareous cyst-forming dinoflagellates

We tested the effect of temperature (15, 20, and 25 degreesC), 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 specie s that form calcareous resting stages (Scrippsiella trochoidea var. aciculi fera, Pentapharsodinium tyrrhenicum, Calciodinellum operosum and S. rotunda ). The 4 species showed different encystment patterns at the temperature co nditions tested; cyst production was inversely related to daylength conditi ons, and higher cyst yields were obtained with the less concentrated growth medium. Experiments with semi-continuous cultures of S, rotunda, in which nutrient concentration was kept within values comparable to in situ concent rations, were carried out with the aim of decoupling the effect of nutrient depletion on cyst production from that of daylength. Cyst production in th is species, kept at a constant growth rate in non-depleted nutrient conditi ons, was only obtained at the shortest daylengths, thus supporting the role of short daylength in inducing the production of calcareous cysts. Our dat a suggest that encystment in the 4 species is regulated by a complex interp lay of at least 3 factors: daylength, temperature and nutrient concentratio n. These results contribute to explain the timing of resting-stage producti on at sea, and point to the possible role of daylength as an environmental signal for the regulation of life cycles in dinoflagellates.