| The objective of this research was to examine the effects of small-scale fluid motion on the kinetic behavior and some key physiological aspects of Dunaliella primolecta Butcher (D. primolecta / Dunaliella). D. primolecta, a fast growing microalga, is a promising organism for alternative energy production because of its capability to accumulate significant amount of "lipids", a major prerequisite for commercial production of microalgal oil-derived biofuel. For kinetic response studies of Dunaliella, flow visualization and quantification techniques such as Particle Image Velocimetry (PIV) and Digital Holographic microscopy were employed. The two-dimensional PIV results showed that Dunaliella were influenced by the fluid flow as soon as the local (or ambient) flow velocities surrounding the cells exceeded the individual (flow subtracted) swimming velocity of Dunaliella. Further inspection of the swimming characteristics of Dunaliella under shear flow in a three-dimensional holography revealed that Dunaliella preferred to swim cross-stream (i.e. also the direction of local vorticity) when the shear flow exceeded a critical value, and this resulted in Dunaliella dispersing in a thin two-dimensional horizontal layer. The cell body rotation was absent during this display in shear flow, although the cell body rotation was evident while swimming in stagnant fluid. A physical model was developed that provided a possible explanation for the cell orienting and swimming in the cross-stream direction in a shear flow while cell body remained irrotational. The experimental swimming data also showed good agreement with the computational results. In order to investigate the biochemical composition and some physiological aspects in Dunaliella under different flow conditions, a laboratory bioreactor equipped with speakers was utilized. The fluid flow velocities in the proximity of the cells generated by the speaker bioreactor are observable in natural water ecosystems. The results showed that the flow condition with the highest turbulence investigated favored the growth and lipid accumulation in Dunaliella. |
