The purpose of this study is to elucidate the isomeric conversion of glucose to fructose using defatted microalga Dunaliella tertiolecta and Botryococcus braunii. Optimal temperature for immobilized-GI (Imm-GI) activity was determined to be 60 degrees C. Under optimal conditions, glucose liberated from the defatted microalgal biomasses was ketoisomerically converted to fructose in time dependent manner, identified by high performance anion-exchange chromatography equipped with a pulsed electrochemical detector (HPAEC-PED). The specific activity of Imm-GI toward sugars of microalgal biomasses was determined to be 0.69 mmol/mg for B. braunii and 0.83 mmol/mg for D. tertiolecta, respectively. The calculated value for Imm-GI for the apparent equilibrium constant, K'(eq), = 0.96 +/- 0.03, at pH 6.0 and at optimum temperature 60 degrees C. Collectively, our proposed approach first demonstrates the contributions and potential applications of Imm-GI for the isomeric conversion of glucose liberated from defatted microalgal biomasses as a major monosaccharide to fructose. (C) 2015 Elsevier Ltd. All rights reserved.