The effects of hydrodynamic shear stress on the growth rate of cyanobacteria Synechocystis sp. and Chlamydomonas reinhardtii microalgae cells were studied in agitated photobioreactors, since they have different motility rates and sizes. An experimental setup was designed and constructed to monitor the growth rate of the micro-organisms versus the shear rate; experiments were carried out in a well controlled environment, under constant atmospheric pressure and 20 °C temperature. Digitally controlled magnetic agitator-photobioreactors were placed inside a closed chamber with air flow for 4 weeks, under a uniform full-time light intensity provided by two 6-watt white fluorescent light sources. To study the effects of shear stress produced by mechanical agitation on the growth rate of a micro-organism, different agitation frequencies were tested. All reactors were filled with 150 ml of culture medium and micro-organism suspension, with initial dilution factors (mlsuspenion/mltotal volume) of 1/30 and 1/300 for Synechocystis and C. reinhardtii respectively. The vessels were placed on different agitating systems at the desired agitator rotation speed, and were sealed with a cotton membrane from the top in order to permit air exchange with the external environment. The micro-organisms’ growth was monitored daily by measuring the optical density of the suspensions using a spectrophotometer and was then correlated with the cellular concentration, which was measured in turn using a microscopic cell counter. Throughout the experiments pH levels and temperature were measured regularly and adjusted to 7 and 20 °C respectively in order to maintain the photosynthetic activity of the species. In addition, to measure the shear stress inside the agitated reactors, a mathematical model was derived to determine the global shear stress magnitude. To determine the local shear stress distribution, the velocity field in the reactor was measured for different agitation frequencies using PIV. Different zones of high and low shear stress were identified. The results showed that the growth rate is independent of the shear stress magnitude for Synechocystis; Synechocystis showed strong resistance, unlike C. reinhardtii, which showed linear dependence of growth rate and shear stress.

Under the current global energy crisis the interests in developing a third generation of biofuels produced from non-food feedstock such as microalgae and cyanobacteria have clearly increased. Hydrodynamic stress, always present in cultivation process of these microorganisms, is an essential factor to ensure mixing inside bioreactors; however the importance of its intensity is usually ignored by applying a random agitation (energy consumption) which is unnecessarily overestimated. In this work, two types of agitation, stirring in agitated photobioreactors (APBR) and bubbling air in draft tube airlift photobioreactors (DPBR), are applied to study the effects of hydrodynamic stress on the growth and pigment content evolution of the cyanobacteria Synechocystis sp. PCC 6803, a self-propelled microorganism. The range of applied shear stress was between 0 and 400 mPa. Similar results are obtained for both agitation mechanisms, indicating that the effects of shear stress are limited to the breakdown of the cell colonies; once they are broken down any further increase in shear stress has no significant effect on their growth rate. Moreover, the variation in pigments concentration appeared to be linear with the cellular concentration and independent from shear intensity.

In this work, we focus on the motility behavior of two model microorganisms widely used in the study of active fluids: Chlamydomonas reinhardtii microalga and Synechocystis sp. cyanobacterium. Understanding the physiological responses of microorganisms under variable environmental conditions is essential for bioreactor engineering. Yet, most of the previous studies focused on the observation of cellular motility regardless of the growth process. Here, we measure the motility of Chlamydomonas reinhardtii and Synechocystis sp. during their growth when subjected to different intensities of hydrodynamic shear stress. The results demonstrate a significant difference in the motility response of the two species against the applied hydrodynamic shear stress. Mechanical agitation appears to affect the motility of Chlamydomonas reinhardtii microalgae by stimulating the growth process and increasing the magnitude of the cellular swimming velocity. This effect is described using an empirical model for the time variation of the motility. Synechocystis cells show a high endurance to the applied shear such that the global effect of agitation intensity on their motility is insignificant. However, it seems that the peak of the swimming velocity always occurs in the middle of exponential phase of growth.

No abstract