Effect of kelp waste extracts on the growth and lipid accumulation of microalgae

•Kelp residue was reused by preparation of kelp waste extracts (KWE) with enzymolysis.•KWE dramatically stimulated growth and shortened life cycles of Chlorella strains.•KWE simultaneously enhanced biomass yields and lipid contents of Chlorella strains.•KWE promoted the syntheses of SFA and MUFA in Chlorella strains and P. tricornutum.•KWE increasingly enhanced intracellular soluble sugar contents in microalgae. Kelp waste extracts (KWE) contained massive soluble sugars, amino acids and various mineral elements. To probe the effects of KWE on microalgal physiological and biochemical responses, the cultures were carried out under the different dilutions. The results showed that 8.0% KWE increased the biomass productivities and total lipid contents of Chlorella strains dramatically, which were 1.83–31.86 times and 20.78–25.91% higher than that of the control. Phaeodactylum tricornutum and Spirulina maxima presented a better growth performance in 1.0% and 4.0% treatment respectively, while their lipid accumulation were not enhanced. In Chlorella-Arc, Chlorella sorokiniana and P. tricornutum, the contents of saturated and monounsaturated fatty acids could be increased, and polyunsaturated fatty acids could be decreased under the conditions of high concentration of KWE (6.0–8.0%). Briefly, KWE facilitated to enhance the biomass productivity and lipid content of Chlorella strains, also improved the fatty acid compositions for biodiesel production.