Process Biochemistry 47 (2012) 1603–1611 Contents lists available at ScienceDirect Process Biochemistry jo u rn al hom epage: www.elsevier.com/locate/procbio Mechanical stress tolerance of two microalgae M. Scarsella a,∗ , G. Torzillo b , A. Cicci a , G. Belotti a , P. De Filippis a , M. Bravi a a Dip. Ing. Chimica Materiali Ambiente, Sapienza Università di Roma, Roma, Italy b CNR Istituto per lo Studio degli Ecosistemi, Via Madonna del Piano, 10, Sesto Fiorentino, Italy a r t i c l e i n f o Article history: Received 10 February 2011 Received in revised form 7 June 2011 Accepted 1 July 2011 Available online 12 July 2011 Keywords: Mechanical stress Chlorella vulgaris Scenedesmus dimorphus Centrifugal pump Air-lift pump Nozzle Photosynthetic activity Chlorophyll fluorescence Turbulence microscale a b s t r a c t Aim of the present work is quantifying the mechanical stress generated by some major process equipment used in massive microalgae culturing plants (centrifugal and air-lift pumps, and nozzles) and highlight- ing its effects on the microalgal population. Two microalgal species were used as test cases: Chlorella vulgaris (unicellular) and Scenedesmus dimorphus 1237 (colonial). The evaluation of the shear effect on algal growth was carried out through measurement of absorbance, photosynthetic activity (oxygen evo- lution) and variable chlorophyll fluorescence. Cell aggregate development/breakage was effectuated by visual inspection and light scattering. The use of centrifugal pumps for culture recycling strongly affected the growth of C. vulgaris, while nozzles effects were confined to aggregate breakage of S. dimorphus. The analysis of experimental data is supported by the consideration of hydrodynamic stress calculated by: shear rate, shear stress, stress volumes/times, energy dissipation rates, and turbulence microscale size. © 2011 Elsevier Ltd. All rights reserved. 1. Introduction Mixing of microalgae culture at micro- and macroscopic level is necessary to ensure that all the cells are exposed to light and that nutrient and substrate supply from the one hand and prod- uct elimination from the other is promptly carried out. A positive relationship between increasing mixing rates and culture produc- tivity has been demonstrated [1–5]. Culture circulation, either by mechanical or pneumatic power input establishes a shear field whose effect may range from beneficial to deleterious. Stress may be beneficial, e.g. because a stimulatory effect on the metabolism (e.g. secretory) is produced or because the end point of the pro- cess itself is an intracellular fraction, which requires shattering to be recovered (e.g. S. costatum cell chain size reduction [6]); it may be neutral, when shear is large enough to support mass transfer and avoid CO 2 limitation, yet not so large as to cause significant cell damage [7]; it may be deleterious when it proves inhibitory to the desired metabolic response, diminutive of the observed culture growth rate, impeding biomass recovery (e.g. membrane fouling [8]), or providing feed to protozoa. Shear sensitivity is strain-, shear regimen- and environment- dependent [9,10]. Shear sensitivity varies greatly among the microalgae and cyanobacteria [11]. In some cases shear sensitiv- ∗ Corresponding author. E-mail address: marco.scarsella@uniroma1.it (M. Scarsella). ity changes during the cell cycle, as in the case of Haematococcus. For each type of culture, an optimal power supply exists [2]. Different process circumstances require the adoption of devices capable of generating different heads and ensuring different flow rate and rangeability: e.g. circulating the microalgal suspension against small or large head losses such as a large diameter photo- bioreactor pipework or a cross-flow microfiltration unit; however, devices with such different process performance features also cre- ate different shear, specific power dissipation, and flow conditions (laminar or, more often, turbulent) depending on the local geome- try and on the culture rheology. Hydrodynamic shear in turbulent flow is explained (Kolmogorov) by differently sized eddies whose energy content scales downward with their size. Eddies whose size is equal or slightly smaller than particles or aggregates are able to damage them, while larger eddies would simply entrain them convectively [12]. The length scale of micro-eddies depends on kinematic viscosity and local energy dissipation rate and is in the order of 1 mm in lakes and rivers [13], but only a few microns in shaken flasks [7] and low-capacity centrifugal pumps [2]. Aim of this paper is studying the response to mechanical stress of Chlorella vulgaris and Scenedesmus dimorphus 1237. While nei- ther of these two species is particularly robust or particularly weak, they were chosen because of their importance in the food/feed production, wastewater treatment and microbial lipid produc- tion areas [14,15], and for their suitability as model organisms (single-celled and colony-forming microalga respectively). Previ- ous research work was carried out on C. vulgaris by Mitsuhashi 1359-5113/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.procbio.2011.07.002