Abstract
The effects of some phytohormones [indole acetic acid (IAA), indole butyric acid (IBA), gibelleric acid (GA3), and kinetin] on cell dry weight, cell number, cell size, protein and chlorophyll contents of Chlorella sorokiniana IAM-C212 were investigated under photoautotrophic conditions. Treatment with IAA (15 mg/l) and IBA (15 mg/l) resulted in significantly higher dry cell weight and cell number than the control (P < 0.05) but there were no significant effects of GA3 and kinetin on cell growth as expressed by dry cell concentration (g/l) and cell population (cells/ml). Treatment with IAA at a concentration of 10 or 15 mg/l gave the highest cell dry concentration of 4.68 g/l after eight days of cultivation, which is more than 9 times higher than the value obtained in the control culture (without phytohormone). The optimum concentration of each of the phytohormones for C. sorokiniana cell enlargement was 20 mg/l. At this concentration, the average cell sizes were 81.07, 78.67, 78.07, 66.90 and 68.1 µm for GA3, kinetin, IAA, IBA and control, respectively. Addition of 15 mg/l of IAA or GA3 to the culture resulted in significantly higher extractable chlorophyll contents than the control (P < 0.05) but the effects of IBA and kinetin were not significant (P > 0.05). The protein contents of the cells cultivated with 20 or 10 mg/l of GA3, 15 mg/l of kinetin, and 15 mg/l of IBA or IAA were 46.64, 45.83 and 45.81%, respectively. In the control experiment, the protein content was 43.38% after eight days of cultivation, showing that treatment with these phytohormones had no significant effect on the protein contents of the cells (P > 0.05). Combination of IBA and GA3 exhibited synergistic effect on growth and productivity of C. sorokiniana but there was no synergistic effect when IAA was combined with either GA3 or kinetin. Key words: Phytohormones, Chlorella sorokiniana, cell growth, cell size, protein content, chlorophyll content.  
Highlights
The effects of some phytohormones [indole acetic acid (IAA), indole butyric acid (IBA), gibelleric acid (GA3), and kinetin] on cell dry weight, cell number, cell size, protein and chlorophyll contents of Chlorella sorokiniana IAM-C212 were investigated under photoautotrophic conditions
Treatment with Indole acetic acid (IAA), IBA and kinetin resulted in significantly higher biomass concentration than the control (P < 0.05) while treatment with GA3 resulted in lower biomass concentration
Using Least Significant Difference, the effectiveness of the phytohormones in increasing the dry cell concentration of C. sorokiniana can be ranked as IAA (4.685 g/l) > IBA (1.664 g/l) > Kinetin (0.621 g/l) > GA3 (0.471 g/l)
Summary
Colourants, sources of high-value functional ingredients such as polyunsaturated fatty acids, ω-3 fatty acids and pigments (Griffiths and Harrison, 2009; Heydarizadeh et al, 2013; Salama et al, 2014). Many of these applications have commercial values but their production costs are still high due to low growth rates of many microalgae species. Many of these applications of microalgae demand the use of monocultures and controlled cultivation systems. Efforts to develop strains with high growth rates and productivity through genetic engineering approach have not yielded the desired results (Potvin and Zhang, 2010; Brennan et al, 2012)
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