Effect of microalgae cell composition and size on responsiveness to ultrasonic harvesting

Ultrasonic harvesting could reduce the energy consumption and costs associated with separating microalgae from growth media. The responsiveness of microalgae cells to an ultrasonic standing wave depends on the cell radius and acoustic contrast factor (ACF). The ACF can vary as cell composition (e.g.lipid, protein, carbohydrate content) varies depending on the algae strain, cultivation conditions, and growth stage. Two independent experimental methods were used to characterize the ACF of three algae ;strainsNannochloropsis salina, Chlamydomonas reinhardtii, and Tetraselmis chuiias a function of dynamic cellular composition over 9- to 14-day growth periods. For N. salina, lipid content increased from 25 +/- 1% to 33 +/- 1% ash-free dry weight (AFDW) and ACF decreased by 46% (from 0.041 +/- 0.002 to 0.022 +/- 0.002) between growth days 3 and 10. For C. reinhardtii, lipid content increased from 26 +/- 1% to 40 +/- 1% AFDW and ACF decreased by 33% (from 0.051 +/- 0.013 to 0.034 +/- 0.006) between growth days 3 and 9. For T. chuii, lipid content and ACF remained stable (10% AFDW and 0.3) over the growth period. ACF decreased as lipid content increased because lipids have a negative ACF in the growth media; however, cell size had a greater impact on cell responsiveness because the ratio of the acoustic radiation force to the drag force is proportional to cell radius squared.