Continuous Cultivation as a Method to Assess the Maximum Specific Growth Rate of Photosynthetic Organisms.

Elena Barbera,Alessia Grandi,Eleonora Sforza,Lisa Borella,Alberto Bertucco

doi:10.3389/fbioe.2019.00274

Elena Barbera, Alessia Grandi + Show 3 more

Open Access

https://doi.org/10.3389/fbioe.2019.00274

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Abstract

Modeling the growth of photosynthetic organisms is challenging, due to the complex role of light, which can be limiting because of self-shading, or photoinhibiting in the case of high intensities. A case of particular interest is represented by nitrogen-fixing cyanobacteria, whose growth is controlled not only by the light intensity, but also by the availability of atmospheric nitrogen in the liquid medium. The determination of the maximum specific growth rate is often affected by many variables that, in batch growth systems, may change significantly. On the other hand, in a continuous system, once the steady state is reached the values of all the process variables remain constant, including the biomass concentration and the specific light supply rate. In this work, the diazotrophic cyanobacterium Anabaena PCC 7122 was cultivated in continuous photobioreactors, to investigate the role of nitrogen, light and residence time on growth kinetics, and to retrieve the value of the maximum specific growth rate of this organism. In addition, the kinetic parameters for temperature and the half saturation constant for nitrogen (3 mg L−1) were measured by respirometric tests. Based on the results of continuous experiments, the specific maintenance rate was found to depend on the light intensity supplied to the reactor, ranging between 0.5 and 0.8 d−1. All these parameters were used to develop a kinetic model able to describe the biomass growth in autotrophic conditions. The maximum specific growth rate could hence be determined by applying the kinetic model in the material balances of the continuous photobioreactor, and resulted equal to 8.22 ± 0.69 d−1.

Highlights

Photosynthetic microorganisms, such as microalgae and cyanobacteria, have recently gained researchers’ interest due to their great potentialities
We propose a new approach that combines the use of cultivation in continuous systems and respirometric tests to determine the kinetic parameters of photosynthetic microorganisms’ growth (Sforza et al, 2019)
Anabaena was cultivated under a constant light intensity of 650 μmol m−2 s−1 by feeding BG11 medium with 3 g L−1 of sodium nitrate, in order to verify if the concentration of atmospheric nitrogen dissolved in the liquid, determined by its solubility, was limiting the growth of the cyanobacterium

Summary

Introduction

Photosynthetic microorganisms, such as microalgae and cyanobacteria, have recently gained researchers’ interest due to their great potentialities They can double their cells even 100 times faster than terrestrial plants (Lam et al, 2012) and require significantly less area to grow with respect to other crops thanks to their high photosynthetic efficiency per unit surface (Darvehei et al, 2018). An important contribution is given by mathematical models that take into account the effect of each process variable (light, temperature, residence time, etc.) for the estimation of key production parameters (biomass growth rate, productivity, etc.) (Darvehei et al, 2018). They can be used to bridge the existing gap between lab-scale observations, on which prediction of the growth on large scale is based, and the industrial-scale reality (Bernard et al, 2015)

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