Since the compositions and thicknesses of cell walls for marigold flowers and for microalgae are similar, their disruption energy may be of the same order of magnitude. However, the very small microalgae cells may significantly reduce the efficiency of applied disruption energy to break the cell walls. For instance, the shear force and fluid vortex yielded by bead mills should be effective down to a length scale of a few tens lm, so can easily tear up the cells of marigold flowers. This mechanism should be very inefficient on microalgae cells since most cells will flow along the streamline of a vortex without shear rate produced by cell shape elongation under stress. As Section 3.2 stated, energy consumption for microalgae cell disruption ranged 33–530 MJ/kg, which is 5 orders of magnitude higher than the disruption energy demand by direct measurement using atomic force microscopy (AFM) (Lee et al., 2012a,b, 2013). Most cell disruption methods should be inefficient in microalgae pretreatments. Efficient mechanical disruption technique that can handle lm size cells is needed. Otherwise, chemical and biological disruption methods may be superior to mechanical means.