1. tSmax = instant when the starch productivity reach the maximum.
Factors influencing starch accumulation in microalga Chlorella sorokiniana
Imma Gifuni1, Giuseppe Olivieri1, Antonino Pollio2, Antonio Marzocchella1
1Dipartimento di Ingegneria Chimica, dei Materiali e della Produzione Industriale, Università degli Studi di Napoli Federico II, Piazzale Tecchio80, Napoli, Italy
2Dipartimento di Biologia Università degli Studi di Napoli Federico II, Via Foria 223, Napoli,Italy
The recent research regarding the microalgal exploitation is focused on the biorefinery of microalgal biomass: all components are valorized in different fields to make the whole process cost-effective.
This contribution is focused on the starch fraction of microalgal biomass. Autotrophic batch cultures of Chlorella sorokiniana have been carried out at different concentration of nitrogen sources and of CO2.
The effects of the concentration of CO2 and of nitrogen source on growth and starch production have been investigated.
Introduction
Fig. 2: Starch applications in food and non-food industries
Nitrogen depletion is known to enhance energy molecules accumulation (starch and lipids).
Microalgal biorefinery is a potential strategy to produce renewable industrial feedstocks for goods and services production[1].
Current biorefinery schemes (e.g. Fig. 1) mainly valorize proteins and value added lipids. The carbohydrate fraction is proposed as feedstock for bioethanol production [2]; [3].
The increase of CO2 concentration can promote carbon rich molecules accumulation.
The largest fraction of microalgal carbohydrates is composed by starch which has a lot of industrial applications more profitable than ethanol production. Moreover only a mechanical extraction is required for starch industrial use.
Literature studies show conflicting results because of the photobioreactor design, irradiance level and strategy [4], [5].
Fig. 1: Microalgal biorefinery scheme from Cellana.
Aim
The effects of the concentration of the nitrogen and carbon source concentration on growth and starch accumulation have been investigated for the microalgal strain Chlorella sorokiniana.
Results and Discussion
Effect of initial Nitrogen concentration
The time to reach the maximum starch fraction increases with the nitrogen concentration in the medium because of the nitrogen reserves accumulation inside the cells.
Fig. 5: Comparison of the maximum starch fraction (ωStarch), starch productivity (PStarch) and biomass productivity (PX) of tests at different nitrogen concentration
Effect of Carbon concentration
The CO2 concentration in the gas stream influence the pH of the culture and cause a deflection from the optimal growth conditions (pH=7–7.5).
[CO2] =0.04% → pH=8.5
[CO2] =0.5% → pH=7.5
[CO2] =2% → pH=7.5
[CO2] =5% → pH=6.5
[CO2] =10% → pH=6
Fig. 6: Comparison of the maximum starch fraction (ωStarch), starch productivity (PStarch) and biomass productivity (PX) of tests at different CO2 concentration
It is interesting that the maximum starch fraction of the biomass remains almost unchanged for the different tests. Therefore CO2 concentration does not influence starch accumulation.
Materials and Methods
MICROORGANISM: Chlorella sorokiniana, Shihiraet Krauss strain ACUF 318
For the tests at different nitrogen concentration:
thickness=8 cm
Fig. 3: C. soronianiana
Fig. 4: Sketch of the inclined square bubble column photobioreactor
Biomass productivity
Starch productivity
CULTURE SYSTEM: inclined squared bubble column photobioreactors reported in the fig. 4. Operating conditions:
Irradiance= 300 μE/m2s 24/24h
Temperature=25°C
pH= 7
Volume= 1,5 L
Gas stream: 20 L/h air + CO2
BBM (NaNO3 as N source)
Biomass concentration (X), nitrate concentration, pH of the medium and biomass starch fraction (ωStarch) were analyzed daily. The following parameters were calculated:
BBM
(Bold Basal Medium)
For the test at different carbon concentration:
2% CO2
Air stream +
N=32mg L-1
tSmax = instant when the starch productivity reach the maximum.
Low increase of Xmax
Photoautotrophic batch growth
Decrease of PX and PStarch +N
Decrease of ωStarch
[N] = 32 mg L-1
OPTIMUM
[N] =19mg L-1
[N] =32mg L-1
[N] =61mg L-1
[N] =122 mg L-1
[CO2] =0.04%
[CO2] =0.5%
[CO2] =2%
[CO2] =5%
[CO2] =10%
pH stress +
2% CO2
OPTIMUM -
less carbon for supporting the growth
Conclusions
At continuous illumination of 300µE/m 2s, the increase of nitrogen concentration up to 32 mg L-1 does not led to an increase in biomass and starch production due to a lack of energy substrate (light).
The optimal CO2 concentration for C. sorokiniana growth is 2%.
The CO2 concentration does not influence starch fraction accumulated by microalgal cells.
OPTIMAL RESULTS PX
(kg m-3 day-1)
Pstarch
ωStarch
% on DW
N= 32 mg L-1
CO2 2%.
443
170 38
References
[1]Soh et al. 2014, Bioresource technology. (151)
[2]Winck et al. 2013, Journal of Proteomics (94)
[3]Zhu and Hiltunen 2016, Renewable and Sustainable Energy Review. (54)
[4]Adestanya et al. 2014, Bioresource Technology. (157)
[5]Takeshita et al. 2013, Bioresource Technology. (158)