Small‐scale fluid motion mediates growth and nutrient uptake of Selenastrum capricornutum

Summary1. A fluid‐flow reactor using submersible speakers was constructed to generate small‐scale fluid motion similar to conditions measured in open water environments; flow was quantified by particle image velocimetry. Additionally a Couette‐type rotating cylinder was used to generate shear flows; flow was quantified using an optical hotwire probe and torque measurements. Growth rates of the green alga Selenastrum capricornutum were determined from changes in cell counts and viability was tested using the fluorogenic probe fluoresceine diacetate.2. Evidence that fluid motion directly affects growth rates was obtained as a significant difference between growth in a moving versus non‐moving fluid. A near 2‐fold increase in growth rate was achieved for an energy dissipation rate of ɛ = 10−7 m2 s−3; a rate common in lakes and oceans. The onset of the viability equilibrium, identified as the day of the test period when the number of active cells equalled non‐active cells, was delayed by 2 days for moving fluid conditions as compared with a non‐moving fluid.3. Nutrient uptake was determined by a decrease in the bulk fluid concentration and cellular phosphorus concentration was also estimated. The thickness of the diffusive sublayer surrounding a cell, a zone dominated by molecular diffusion, was estimated. Increasing fluid motion was found to decrease the thickness of this layer. The Sherwood number (ratio of total mass flux to molecular mass flux) showed that advective flux surrounding cells dominated molecular diffusion flux with regard to Péclet numbers (ratio of advective transport to molecular diffusion transport). Fluid motion facilitated uptake rates and resulted in increased growth rates, compared with no‐flow conditions. The rate‐of‐rotation and the rate‐of‐strain in a moving fluid equally mediated the diffusive sublayer thickness surrounding the cells. Our study demonstrates that small‐scale fluid motion mediates algal growth kinetics and therefore should be included in predictive models for algal blooms.