Effects of Shear Stress on the Growth Rate of Micro-Organisms in Agitated Reactors

Abstract

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.

Exopolysaccharides from Cyanobacteria: Strategies for Bioprocess Development

“…The growth rate of cyanobacteria has been observed to increase with shear stress, probably by promoting orientation and increasing the contact area of the microorganisms and nutrients [73,74]. For the Cyano-EPS a certain degree of shear stress will be necessary once the rheology of the medium is altered and the mass and gas transfers become less efficient throughout the time.…”

Evaluation of the effect of agitation speed on the growth and high‐value LC‐PUFA formation of Porphyridium cruentum based on basic rheological analysis

“…48 Because the biofilm built by EPS around the cells is very weak 47 and possesses poor resistance to high hydrodynamic stress, the microalgal cells can be easily damaged under a high agitation speed. On the other, a low agitation speed weakened turbulent mixing, and then decreased the CO 2 or air mass transfer coefficient from gas phase to liquid phase (culture medium) 49 ; it also influenced light distribution 17 and as a result inhibited biomass growth. Thus, biomass growth was optimal under middle agitation speeds.…”

Pilot‐scale open‐channel raceways and flat‐panel photobioreactors maintain well‐mixed conditions under a wide range of mixing energy inputs

“…In other recent research, the effects of fluid shear stress were studied for the cyanobacteria and green algae strains Synechocystis sp. and Chlamydomonas reinhardtii, in 150 ml cultures (Fadlallah et al, 2016). In this study, the growth rate of Detrimental impacts of turbulent mixing have also been demonstrated, with inhibition attributed to cell disruption from shear stress (Chalmers, 2015;Han & Yuan, 2004;Kunnen, Malas, Semeins, Bakker, & Peters, 2017;Richardson, Lannigan, & Macara, 2015;Sieck et al, 2014Sieck et al, , 2013Wang, Lü, Mao, & Long, 2014;Yi et al, 2010).…”

“…Turbulent cultivation environments induce physiological responses in microalgae in raceways and photobioreactors (Fadlallah et al, 2016; Havens, James, East, & Smith, 2003; Kumar, Dasgupta, Nayak, Lindblad, & Das, 2011; Michels, van der Goot, Norsker, & Wijffels, 2010; Mirón et al, 2003; Moisander, Hench, Kononen, & Paerl, 2002; Nguyen & Hoang, 2016; Xiao, Li, Li, Zhang, & Guo, 2016). For example, Xiao et al (2016) studied the effects of turbulence dissipation rates ranging from 0 to 0.08 m 2 ·s −3 in 1‐L cultures (Xiao et al, 2016).…”

“…In other recent research, the effects of fluid shear stress were studied for the cyanobacteria and green algae strains Synechocystis sp. and Chlamydomonas reinhardtii , in 150 ml cultures (Fadlallah et al, 2016). In this study, the growth rate of Synechocystis sp .…”