Microalgae species including Chlamydomonas mexicana, Micractinium reisseri, Scenedesmus obliquus and Tribonema aequale were cultivated in batch cultures, and their biochemical composition was determined. C. mexicana showed the highest carbohydrate content of 52.6% and was selected for further study. Sonication pretreatment under optimum conditions (at 40 kHz, 2.2 Kw, 50 °C for 15 min) released 74 ± 2.7 mg g(-1) of total reducing sugars (TRS) of dry cell weight, while the combined sonication and enzymatic hydrolysis treatment enhanced the TRS yield by fourfold (280.5 ± 4.9 mg g(-1)). The optimal ratio of enzyme [E]:substrate [S] for maximum TRS yield was [1]:[5] at 50 °C and pH 5. Combined sonication and hydrolysis treatment released 7.3% (27.1 ± 0.9 mg g(-1)) soluble protein of dry cell weight, and further fermentation of the dissolved carbohydrate fraction enhanced the soluble protein content up to 56% (228.4 mg g(-1)) of total protein content. Scanning and transmission electron microscopic analyses indicated that microalgae cells were significantly disrupted by the combined sonication and enzyme hydrolysis treatment. This study indicates that pretreatment and subsequent fermentation of the microalgal biomass enhance the recovery of carbohydrates and proteins which can be used as feedstocks for generation of biofuels.
Keywords: Biofuel; Chlamydomonas mexicana; Enzymatic hydrolysis; Fermentation; Sonication; Total reducing sugars.