926 cells are probably more shear sensitive. Step responses to shear stress have previously been reported for enzymes [36] and starch molecules [37, 38], suggesting a more general role of shear stress in deactivation and breakage processes. The only deviation from the general interpretation of our results is that a shear stress of 1.8 Pa could be applied to the algae without LBG at the highest shear rate without any detrimental effect on the viability. Flow instabilities like Taylor vortices could have occurred in that experiment, so that the results could have been inﬂuenced in a way that they are not reliable or comparable. These results imply that a PBR for cultivating Chae- toceros muelleri should be designed in such a way that the maximum exhibited shear stress is lower than the threshold value. The question that now remains is whether this shear stress can occur in a PBR. Considering the study of Contreras et al. [17], shear rates up to 14,000 s-1 can occur, which implies a local shear stress of about 20 Pa. This value clearly exceeds the value reported above. In other words, shear stress is likely to negatively inﬂuence the algae in a PBR. However, reducing the ﬂuid velocity too much can lead to other limitations, such as mass transfer, which also negatively inﬂuences the growth rate. Time dependence of shear stress effect Chaetoceros muelleri in 0.3% LBG was exposed to a constant shear stress of 5.89 Pa at a shear rate of 215.7 s-1 in the shear cylinders to elucidate the time dependence of the shear stress effects. The viability already decreased after 1 min to 82% and kept on decreasing at a reduced rate for the following 7 min. Longer exposure times to Fig. 6 The effect of time dependency of shear stress on the viability of Chaetoceros muelleri. Error bars represent 95% conﬁdence intervals 123 Bioprocess Biosyst Eng (2010) 33:921–927 shear stress did not reduce the viability any further. The viability was found to vary between 65 and 75% for all exposure times longer than 8 min (Fig. 6). This would suggest that the effect of shear stress is almost time- independent. Compared to normal cultivation times, the effect can be considered as almost instantaneous. The fact that the viability does not decrease completely indicates that only the more sensitive cells are susceptible to shear stress, while the more resistant cells are not affected by shear stress even over a longer period. This result is in contrast with a study done with hybridoma cells [39]. The viability of these mammalian cells, which were also sheared for time periods of less than 2 h, decreased over time. However, the shear stress levels applied were much higher and varied between 5 and 100 Pa. The time dependence of shear stress was also investigated with plant cells [40]. Although the main conclusion was that the effect of shear stress on plant cells was time dependent, it was clear that the ﬁrst minutes of exposure to a certain shear stress were the most detrimental. The study of Dunlop et al. [40] also showed that similar shear stress levels and exposure times as in this study gave almost the same viability results. Complete cell death was only reached when signiﬁcantly higher shear stresses (50–100 Pa) were applied to the cells. These high shear stress values are expected to be unrealistic in PBRs given the low viscosity of the medium used. Conclusions Shear stress had an adverse effect on the viability of Chaetoceros muelleri with a threshold value between 1 and 1.3 Pa. Shear stress higher than the threshold value caused a sudden decrease in viability to levels between 52 and 66%. It appears that only a certain fraction of the cells are susceptible to shear stress up to 19.4 Pa. No external damage was observed, meaning that internal damage must have taken place in the affected cells. The effect of shear stress was almost instantaneous as compared to regular cultivation times. The detrimental effect already took place after 1 min of exposure to shear stress. The viability did not decrease any further with shear stress exposure times longer than 8 min. This would suggest that the sensitive cells were affected by shear stress instantaneously and that high shear stress over a long period did not have a negative effect on the more resistant cells. Acknowledgments This research is supported by the Zeeuwse Tong project (Zeeland Sole project), co-funded by the European Fisheries Fund. Furthermore, the authors thank Lieke van Riemsdijk for all her effort in the development of the shear cylinders and Katarzyna Grabowska for her assistance.