Elsevier

Bioresource Technology

Volume 245, Part A, December 2017, Pages 250-257
Bioresource Technology

Modeling shear-sensitive dinoflagellate microalgae growth in bubble column photobioreactors

https://doi.org/10.1016/j.biortech.2017.08.161Get rights and content

Highlights

  • CO2 mass transfer limits the growth of K. veneficum at low gas-flow rates.

  • Bubble rupture is more harmful for cells than bubble formation.

  • Bubble-related stress disturbs cell physiology, inducing an overproduction of ROS.

  • Overproduction of ROS alters membrane fluidity and photosynthetic machinery.

  • A model that represents cell growth under different conditions is proposed.

Abstract

The shear-sensitive dinoflagellate microalga Karlodinium veneficum was grown in a sparged bubble column photobioreactor. The influence of mass transfer and shear stress on cell growth and physiology (concentration of reactive oxygen species, membrane fluidity and photosynthetic efficiency) was studied, and a model describing cell growth in term of mass transfer and culture parameters (nozzle sparger diameter, air flow rate, and culture height) was developed. The results show that mass transfer limits cell growth at low air-flow rates, whereas the shear stress produced by the presence of bubbles is critically detrimental for air flow rates above 0.1 vvm. The model developed in this paper adequately represents the growth of K. veneficum. Moreover, the parameters of the model indicate that bubble rupture is much more harmful for cells than bubble formation.

Introduction

Dinoflagellate microalgae have been known to play a crucial role in oceanic phytoplankton communities for many years. These organisms produce toxins and bioactive compounds as secondary metabolites that have recently been attracting increasing attention due their promising medical applications (García-Camacho et al., 2007, Gallardo-Rodríguez et al., 2012a). Dinoflagellates are also promising candidates for biofuel production (Fuentes-Grünewald et al., 2015).

The large-scale culture of dinoflagellates to produce these compounds will require the development of suitable photobioreactors that provide the appropriate aeration and agitation necessary for mass- and light-transfer and heat elimination. However, dinoflagellates have been found to be inordinately sensitive to shear stress (Berdalet et al., 2007, Gallardo-Rodríguez et al., 2012a). In general, the shear stress levels that dinoflagellates can withstand are one or two order of magnitude lower than those reported for freely suspended animal cells. Specific energy dissipation rate values in the range of 0.011–10 cm2s−3 have generally inhibited dinoflagellate growth (Gallardo-Rodríguez et al., 2012a). As such, the CO2 demand and shear sensitivity of these microalgae may require special consideration during large-scale photobioreactor design. Low aeration will not provide sufficient carbon dioxide transfer to the culture or oxygen removal, thereby limiting healthy cell growth, whereas excessive aeration and agitation may lead to impaired metabolism (triggering a massive production of reactive oxygen species, altering the fluidity of cell membrane, and producing photoinhibition), decreased cell growth, cell damage, and, eventually, apoptosis and cell death (Sullivan et al., 2003, Gallardo-Rodríguez et al., 2007).

Enclosed sparged photobioreactors, such as bubble columns, airlift, and flat panels, have been widely used to culture different microalgae for research purposes since they provide larger surface-to-volume ratios than other culture systems, thereby reducing self-shading and increasing the light energy available (Contreras et al., 1998). However, as these systems are not yet used on a large scale, there is a need for further development before they can be used as industrial photobioreactors.

It has recently been shown that a stochastic search strategy can be used to optimize the culture of dinoflagellates in bubble column photobioreactors, thereby providing useful guidance as to how shear stress can be controlled to ensure healthy growth (López-Rosales et al., 2015). This paper deal with the study of how mass transfer and hydrodynamic stress influence cell growth and cell physiology of the dinoflagellate K. veneficum and the subsequent development of a model describing cell growth in terms of mass transfer and hydrodynamic stress.

Section snippets

Database

Critical factors influencing mass transfer and cell stress in bubble column photobioreactors include system design (including column diameter and height, and sparger) and operational parameters (mainly gas flow rate). These variables influence system features such as gas velocity at the sparger, bubble diameter and rise velocity, gas hold-up and interfacial area, microeddy length scale, stress fields, etc. All of these variables dictate the conditions to which the cells are subjected, govern

Result and discussion

Carbon represents nearly 50% of the whole microalgal biomass and is the major nutrient for cell growth (Carvalho et al., 2006), with CO2 being the preferred carbon source when microalgae grow photo-autotrophically (Silva et al., 1987). Fixation of CO2 within the cell is catalyzed by the enzyme ribulose-1,5-biphosphate carboxylase oxygenase (Rubisco). Provided the CO2 concentration remains below the saturation limit for Rubisco, the rate of CO2 consumption during photosynthesis will be

Conclusions

Each microalga species has its own optimal conditions that have to be considered when designing production systems. As such, gas flow rate alone should not be used to predict cell growth and damage in sparged photobioreactors as other parameters, including column volume, sparger geometry details, and mass transfer, should also be taken into consideration. The results presented in this paper show that these parameters exert a critical impact on growth and physiology of K. veneficum. A model that

Acknowledgements

This research was funded by the Spanish Ministry of Economy and Competitiveness (Grants SAF2011-28883-C03-02 and CTQ2014-55888-C3-02) and the European Regional Development Fund Program.

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