Ultrasonication is a cell disruption process of low energy efficiency. This study dosed K+, Ca2+ and Al3+ to Chlorella vulgaris cultured in Bold's Basal Medium at 25 degrees C and measured the degree of cell disruption under ultrasonication. Adding these metal ions yielded less negatively charged surfaces of cells, while with the latter two ions large and compact cell aggregates were formed. The degree of cell disruption followed: control = K+ > Ca2+ > Al3+ samples. Surface charges of cells and microbubbles have minimal effects on the microbubble number in the proximity of the microalgal cells. Conversely, cell aggregates with large size and compact interior resist cell disruption under ultrasonication. Staining tests revealed high diffusional resistance of stains over the aggregate interior. Microbubbles may not be effective generated and collapsed inside the compact aggregates, hence leading to low cell disruption efficiencies. Effective coagulation/flocculation in cell harvesting may lead to adverse effect on subsequent cell disruption efficiency. (C) 2015 Elsevier Ltd. All rights reserved.