This study aimed to evaluate the effect of sound wave exposure in different photoperiods on Synechococcus HS-9 cell density and lipid content using tubular photobioreactors (PBRs). In this study, nine PBRs were used: three PBRs were exposed to a sine wave of 279.9 Hz for three hours during the day (A), three PBRs were exposed to a sine wave of 279.9 Hz for three hours during the night (B), and three PBRs remained unexposed to any sound wave to serve as a control (K). All PBRs were studied for 18 days. The results showed that the highest average cell densities of Synechococcus HS-9 in PBR A, B, and K respectively were 8.883×105 cells/mL, 7.242×105 cells/mL, and 6.175×105 cells/mL. The highest lipid percentage, which was 17%, was observed in PBR A; the percentage in PBR B was 16%, and in PBR K, 7%. However, Synechococcus HS-9 in PBR B showed a higher growth rate compared to PBR A and PBR K. Sound waves could have increased cell activity and metabolism which led to the increase in cell densities and lipid percentages in Synechococcus HS-9. The photoperiodic differences might have resulted in a lower photosynthetic rate and cell metabolism, but the sound wave could have helped promote the growth of Synechococcus HS-9 despite the lower photosynthetic rate.
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