Effects of small-scale turbulence on microalgae

Abstract

Turbulence flows are characterized by their viscous dissipation rates ɛ and the kinematic viscosity of the fluid ν, but the effects of turbulence on organisms such as microalgae smaller than the Kolmogorov inertial-viscous length scale L_K ≡ (ν³/ε)^/14 depend on the stress τ ≡ µγ, where µ = ϱν is the dynamic viscosity, ρ is the density, and the rate-of-strain γ ≡ (ε/ν)^/12. While various workers have shown qualitatively that turbulence affects several microalgal physiological processes, these effects have not been quantified in terms of ε, τ or γ. Various microalgal groups seem to have different sensitivities to inhibition by turbulence. The relative sensitivities aregreen algae < blue-green algae < diatoms < dinoflagellates with dinoflagellates being most sensitive. We have quantified the growth sensitivity to turbulence for a red tide dinoflagellate,Gonyaulax polyedra Stein, by imposing constant γ values on cultures placed within a gap between rotating outer and fixed inner concentric cylinders. Threshold turbulence values for growth inhibition are consistent with turbulence parameters near the sea surface with light winds, suggesting turbulence may be the reason that high winds inhibit red tides. For ɛ > 0.18 cm²s⁻³, τ > 0.04 dynes cm⁻² (0.002 N M⁻² or Pa), γ > 4.4 rad s⁻¹, cell numbers and chlorophyll fluorescence declined, and cells lost their longitudinal flagella and the ability to swim forward. At lower ε, τ and γ values growth rates and cell morphology were the same as in unsheared control cultures. High turbulence may affect other algae, such asSpirulina, which are commonly mass cultured.