Bioprocess Biosyst Eng (2010) 33:921–927 The effect of shear rate on the viability is shown in Fig. 4. Without LBG, no effect of shear rate on the via- bility of the algae was observed. No effect on the viability of the algae in 0.3% LBG was seen until a shear rate of 22 s-1. Higher shear rates applied to the algae in 0.3% LBG affected the algae negatively, reducing the viability to 56%. At a shear rate of 8.6 s-1 applied to the algae in 0.5% LBG, the viability dropped to a value of 59%. Increasing the shear rate reduced the percentage of viable cells slightly to 52%. There seems to be a slight difference between the viability levels of the samples with 0.3 and 0.5% LBG. We believe that these differences might be attributed to the fact that the experiments were done on different days. In addition, LBG did not affect the viability of the algae directly. No negative effect on the viability was seen when the algae in 0.3 and 0.5% LBG were not sheared. Furthermore, 100% viability was seen when low shear rates were applied to the algae in both LBG concentrations. From Fig. 4, it can be seen that the shear rate is not the determining factor in explaining the negative effect on cell viability. Obviously, a higher viscosity leads to a lower shear rate at which a negative effect can be observed. When analyzing the effect of shear stress on the viability, the effects of shear rate and viscosity can be combined. The effect of all different levels of shear stress on the viability of Chaetoceros muelleri can be seen in Fig. 5. Shear stress values up to 1 Pa had no effect on the viability. At shear stress levels higher than 1.3 Pa, a sharp drop in viability can be seen with resulting viabilities between 52 and 66%. The threshold value of shear stress for Chaetoceros muelleri is therefore to be found between 1 and 1.3 Pa. A further increase of the shear stress to 19.4 Pa did not reduce the viability signiﬁcantly. The effect of shear stress on the viability of Chaetoceros muelleri can thus be described as a step response with an almost similar effect beyond the threshold value, which is represented by the dotted line in Fig. 5. This would suggest that only a certain percentage of the cells are sensitive to shear stress Fig. 3 Microscopic pictures of Chaetoceros muelleri; with normal light (a) 11 cells can be counted and with ﬂuorescent light (b) 8 viable cells show ﬂuorescence Fig. 4 Effect of shear rate on the viability of Chaetoceros muelleri; without LBG (diamonds), 0.3% LBG (squares) and 0.5% LBG (triangles). Error bars represent 95% conﬁdence intervals Fig. 5 Effect of shear stress on the viability of Chaetoceros muelleri with all treatments (without LBG, 0.3% LBG and 0.5% LBG) taken together. Error bars represent 95% conﬁdence intervals and that higher shear stresses than the threshold value, up to the maximum applied value of 19.4 Pa, only affect the sensitive cells. Although it is unclear which cells are more susceptible to shear stress, other studies have speculated that shear stress disrupts cell division [18, 35]. Dividing 925 123