Diel variation in motility of prymnesiophyte Isochrysis galbana under different irradiance

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We investigated the diel variation in motility under different irradiance of the prymnesiophyte Isochrysis galbana Parke. The effectiveness of nutrient uptake by motility relative to molecular diffusion in seawater is quantified by the Sherwood number Sh, which is a function of cell size (µm) and motility rate (µm s⁻¹). We calculated Sh for the prymnesiophyte I. galbana by determining its cellular equivalent spherical diameter (ESD, µm) and motility rate (numbers of ESD s⁻¹). Nutrient-enriched batch cultures were grown at 25°C in 12:12h light:dark cycles at light-limiting or light-saturated irradiances for 48 h. Observed motilities and Sh values were within the range of previously reported values and indicated a significant relationship between motility and Sh with ESD. Diel variations in motility and Sh showed maxima at the middle of the light period and minima at the middle of the dark period under both light conditions. Diel patterns of motility and Sh might be related to those of photosynthesis due to the supply of energy of motility.