Antenna size reduction as a strategy to increase biomass productivity: a great potential not yet realized

Abstract

A major limitation in achieving high photosynthetic efficiency in microalgae mass cultures is the fact that the intensity of direct sunlight greatly exceeds the photosynthetic capacity of the cells. Due to the high pigment content of algal cells, the light absorption rate surpasses the much slower conversion rate to biochemical energy. The excess of light energy is predominantly dissipated as heat, decreasing the light use efficiency of the culture. Algae with a truncated antenna system could substantially increase biomass productivity of mass cultures because oversaturation of the photosystems and concomitant dissipation of light energy are minimized. In this study, we measured the areal biomass productivity of wild-type strain cultures and four promising antenna size mutant cultures of Chlamydomonas reinhardtii. This was performed under simulated mass culture conditions. The strains were cultivated in turbidostat controlled lab-scale panel photobioreactors at an incident light intensity of 1500 μmol photons m⁻² s⁻¹. The mutant cultures did not exhibit the expected higher productivity. The greatest mutant culture productivity values were approximate to those of the wild-type productivity of 1.9 g m⁻² h⁻¹. The high sensitivity to abrupt light shifts indicated that the mutant cultures experienced reduced fitness and higher susceptibility to photodamage. This can possibly be explained by impaired photoprotection mechanisms induced by the antenna complex alterations, or by unintended side effects of the genetic modifications. Still, if these effects could be eliminated, the principle of antenna size reduction is a promising strategy to increase productivity. Selection criteria for the future creation of antenna size mutants should, therefore, include tolerance to high light conditions.

Appendices

Appendix 1. Microalgae growth model and photobioreactor productivity

A simple kinetic model was constructed to describe microalgae productivity as a function of the light intensity. The model is based on two compartments. In the first compartment, the chloroplast, there is photosynthetic production of 3-carbon sugars (triose) symbolized by the 1-carbon sugar equivalent CH₂O. This sugar production rate in the chloroplast (q ^c _CH2O, mol_CH2O mol_x ⁻¹ s⁻¹) is dependent upon light intensity I _ph and described by the hyperbolic tangent model of Jassby and Platt (1976):

$$ {q}_{\mathrm{CH}2\mathrm{O}}^{\mathrm{c}}={q}_{\mathrm{CH}2\mathrm{O},\mathrm{m}}^{\mathrm{c}}\cdot \tanh \left(\frac{I_{\mathrm{ph}}\cdot {a}_{\mathrm{x}}\cdot {Y}_{\mathrm{CH}2\mathrm{O}/\mathrm{ph},\mathrm{m}}}{q_{\mathrm{CH}2\mathrm{O},\mathrm{m}}^{\mathrm{c}}}\right) $$

(A1)

Where I _ph is the PAR photon flux density (mol_ph m⁻² s⁻¹), q ^c _CH2O,m is the maximal sugar production rate (mol_CH2O mol_x ⁻¹ s⁻¹) in the chloroplast, Y _CH2O/ph,m is the maximal yield of sugar on light energy (mol_CH2O mol_ph ⁻¹), and a _x is the spectrally averaged optical cross section of the microalgae (m² mol_x ⁻¹). a _x can be calculated according to the following equation:

$$ {a}_{\mathrm{x}}={\displaystyle {\sum}_{\lambda =400}^{\lambda =700}{a}_{\mathrm{x},\uplambda}}\cdot {E}_{\mathrm{n},\uplambda}\cdot \varDelta \uplambda $$

(A2)

In which a _x,λ is the optical cross section at wavelength λ. E _n,λ (nm⁻¹) represents the normalized spectral distribution of the light source (Fig. 2). It is the fraction of photons in the PAR region in a 1-nm interval at specific λ.

The cell minus the chloroplast comprises the second compartment in which the 3-carbon sugar is used to build new biomass at a specific growth rate μ. Another part of the sugar is respired in the mitochondria to provide energy (ATP) to support the growth reactions and to fulfill the maintenance requirements. The consumption of sugar in the chloroplast can be described by Pirt’s Law (Pirt 1965), resulting in the following relation:

$$ \upmu =\left({q}_{\mathrm{CH}2\mathrm{O}}^{\mathrm{c}}-{m}_{\mathrm{CH}2\mathrm{O}}\right)\cdot {Y}_{\mathrm{x}/\mathrm{C}\mathrm{H}2\mathrm{O}} $$

(A3)

and:

$$ {\upmu}_{\mathrm{m}}=\left({q}_{\mathrm{CH}2\mathrm{O},\mathrm{m}}^{\mathrm{c}}-{m}_{\mathrm{CH}2\mathrm{O}}\right)\cdot {Y}_{\mathrm{x}/\mathrm{C}\mathrm{H}2\mathrm{O}} $$

(A4)

In these equations, m _CH2O is the biomass specific maintenance rate (mol_CH2O mol_x ⁻¹ s⁻¹), Y _x/CH2O is the biomass yield on 3-carbon sugar (mol_x mol_CH2O ⁻¹), and μ_m is the biomass specific growth rate (s⁻¹).

The biomass on yield on light energy (mol_x mol_ph ⁻¹) can now be calculated as follows:

$$ {Y}_{\mathrm{x}/\mathrm{ph}}=\frac{\upmu}{q_{\mathrm{ph}}} $$

(A5)

Here, q _ph is the specific light absorption rate (mol_ph mol_x ⁻¹ s⁻¹), which is defined as follows:

$$ {q}_{\mathrm{ph}}={a}_{\mathrm{x}}\cdot {I}_{\mathrm{ph}} $$

(A6)

The light intensity at which μ is equal to zero (i.e., where photosynthetic sugar production is compensated by maintenance-associated sugar consumption) is referred to as the photosynthetic compensation point (I _ph,c). By numerical integration of Y _x/ph from I _ph = I _ph,c to I _ph = 1500 μmol photons m⁻² s⁻¹, the maximal biomass productivity per illuminated surface area (r _x) is obtained in g m⁻² h⁻¹ (Eq. A7).

$$ {\boldsymbol{r}}_{\mathrm{x}}=3600\cdot {10}^{-6}\cdot {\displaystyle {\sum}_{{\boldsymbol{I}}_{\mathrm{ph},\mathrm{c}}}^{{\boldsymbol{I}}_{\mathrm{ph},\mathrm{in}}}{\boldsymbol{Y}}_{\mathrm{x}/\mathrm{ph}}}\cdot \varDelta {\boldsymbol{I}}_{\mathrm{ph}} $$

(A7)

Table 2 Overview of the model parameters

Appendix 2. Light intensity distribution over reactor surface

Table 3 Incident light intensity (μmol photons m⁻² s⁻¹) distribution over the reactor surface. The light was measured at 28 points evenly distributed over the light exposed surface of the front glass panel of the culture chamber. For this measurement, the average light intensity was 1501 μmol photons m⁻² s⁻¹

Appendix 3. Calculation of the extrapolated biomass concentration C _x’ using the attenuation coefficient K _x

For each mutant, the spectrally averaged attenuation coefficient (K _x, m² g⁻¹) of the experiment with the darkest light regime (i.e., lowest I _ph,out) was calculated. This attenuation coefficient K _x was determined by measuring the light transmittance in the photobioreactor by measuring the I _ph,in and I _ph,out using a PAR light meter. This attenuation coefficient K _x, therefore, also includes the effect of light scattering as it occurs in the photobioreactor. The coefficient K _x is fundamentally different from the spectrally averaged optical cross section (a _x) which only reflects true light absorption. K _x was calculated using the equations indicated below in which d (m) is the light path of the photobioreactor.First, K _x is calculated using Eq. C1 which was obtained by rearranging Eq. C2, Lambert-Beer’s law equation.

$$ {K}_{\mathrm{x}}=\frac{ \ln \frac{I_{\mathrm{ph},\mathrm{out}}}{I_{\mathrm{ph},\mathrm{in}}}}{-{C}_{\mathrm{x}}\cdot d} $$

(C1)

$$ {I}_{\mathrm{ph},\mathrm{out}}={I}_{\mathrm{ph},\mathrm{in}}\cdot {e}^{\hbox{-} {\mathrm{C}}_{\mathrm{x}}\times {\mathrm{K}}_{\mathrm{x}}\times \mathrm{d}} $$

(C2)

With K _x known, C _x’ at I _ph,out = 10 μmol photons m⁻² s⁻¹ can be estimated using Eq. C3, which was also obtained by rearranging Eq. C2.

$$ {C}_x^{\prime }=\frac{ \ln \frac{I_{\mathrm{ph},\mathrm{out}}}{I_{\mathrm{ph},\mathrm{in}}}}{-{K}_{\mathrm{x}}\cdot d} $$

(C3)