kinetics microalgae-3BT

1 23
3 Biotech
ISSN 2190-572X
3 Biotech
DOI 10.1007/s13205-014-0264-3
Kinetic modeling of microalgal growth and
lipid synthesis for biodiesel production
D. Surendhiran, M. Vijay,
B. Sivaprakash & A. Sirajunnisa

1 23
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ORIGINAL ARTICLE
Kinetic modeling of microalgal growth and lipid synthesis
for biodiesel production
D. Surendhiran • M. Vijay • B. Sivaprakash •
A. Sirajunnisa
Received: 27 August 2014 / Accepted: 21 October 2014
Ó The Author(s) 2014. This article is published with open access at Springerlink.com
Abstract A mathematical modeling of microalgae bio-
mass is an essential step to optimize the biomass and lipid
production rate and to reduce the cost of microalgal bio-
diesel production system. In the present study, kinetic
studies were carried out to describe the growth and neutral
lipid production of two marine microalgae Chlorella salina
and Nannochloropsis oculata under the nitrogen-repleted
and -depleted conditions using logistic and Luedeking–
Piret equations. This research paper provides the infor-
mation on mathematically efﬁcient procedure to predict
suitable environment condition for biomass and lipid pro-
duction. The predicted results were compared with exper-
imental data, which showed that this model closely agreed
with simulated results. From this investigation, it was
found that nitrogen was an essential nutrient for algal
growth, which increased under nitrogen-rich condition,
whereas during nitrogen-limited condition some loss in
growth was observed but with increased lipid content.
Since metabolic changes occurred under nitrogen- depleted
state, the protein and carbohydrate pathways were shifted
to lipid biosynthesis.
Keywords Chlorella salina Á Nannochloropsis oculata Á
Biodiesel Á Logistic model Á Luedeking–Piret model Á
Nitrogen stress
Introduction
Rapid hike in oil prices and depletion in fossil fuels have
created a serious issue of energy shortage globally. Con-
tinuous use of fossil fuels, resulting in greenhouse gases
accumulation, leads to global warming (Xin et al. 2011).
Biodiesel, produced generally from food and oil crops
using conventional methods (Tran et al. 2013), is an
alternative biofuel which is renewable, biodegradable,
nontoxic with no net carbon emissions and free from
sulfur (Li and Yan 2010; Kim et al. 2011; Kirrolia et al.
2013). Plant sources cannot realistically fulﬁll the vast
usage of diesel fuel due to increasing population that leads
to serious land shortage and rise in the issue of food
security (Surendhiran and Vijay 2012). Microalgae, the
photosynthetic microorganisms, thus have become the
recent attraction because of their high oil content, with
lipids being the intracellular storage matter (Xiao et al.
2010). Microalgae can be grown in wastewater as they do
not compete with food crops for arable land and water and
give 20 times more biomass productivity rate than ter-
restrial crops (Chisti 2007; Mutanda et al. 2011; Lai et al.
2012).
Nevertheless, biodiesel production process from micro-
algae is not being commercialized yet, because of the slow
culturing process and high cost of feedstock than diesel
from fossil fuels (Yang et al. 2011). Moreover, biodiesel
production from microalgae mainly depends on the avail-
ability of biomass and scale-up process that needs opti-
mization, to reduce cost (Decostere et al. 2013). The
kinetics deals with how the rate of the reaction is dependent
on the concentration of the reactants. For the cells to
multiply and grow, a certain amount of substrate and other
required components from the production medium is con-
sumed (Rao 2005). Kinetic models serves in designing a
D. Surendhiran (&) Á M. Vijay Á B. Sivaprakash Á
A. Sirajunnisa
Bioelectrochemical Laboratory, Department of Chemical
Engineering, Annamalai University, Annamalai Nagar 608002,
Tamilnadu, India
e-mail: suren_micro@yahoo.co.in
123
3 Biotech
DOI 10.1007/s13205-014-0264-3

bioreactor, controlling the microbial processes and pre-
dicting the behavior of the processes more easily than
laboratory experiments (Bailley and Ollis 1986). To learn
the dynamics of biomass growth and lipid production by
microalgae, suitable kinetic modeling has to be developed
for predicting the performance and optimization of photo-
bioreactor operating conditions (Galvao et al. 2013). Many
research studies have been conducted and many mathe-
matical models proposed for microalgal growth and lipid
production. Recent mathematical studies showed that under
photoheterotrophic conditions, microalgae accumulate
more amount of intracellular oil (triglyceride-TAG) than
during photoautotrophic cultivation. Extensive literature
survey shows that most of the modeling papers only focus
on heterotrophic growth of microalgae and very few
reports are available on kinetic modeling of algae growth
and lipid production under nitrogen-depleted conditions.
In this study, marine microalgae Chlorella salina and
Nannochloropsis oculata were selected and their growth
and product formation kinetics were modeled for the cul-
ture under nitrogen-repleted and -depleted growth using
logistic and Luedeking–Piret models, respectively.
Materials and methods
Culture for study
Chlorella salina and Nannochloropsis oculata were obtained
from Central Marine and Fisheries Research Institute
(CMFRI), Tuticorin, Tamil Nadu, India, and was cultivated in
25 L photobioreactor using sterile Walne’s medium. The fil-
tered sterilized seawater was enriched with the required quan-
tity of Walne’s medium (Manikandan et al. 2011) containing
(g L-1
): NaNO3, 100; NaH2PO4Á2H2O, 20.0; Na2EDTA, 4.0;
H3BO3, 33.6; MnCl2Á4H2O, 0.36; FeCl3Á6H2O, 13.0; vitamin
B12, 0.001 and vitamin B1, 0.02. The trace metal solution
contained (g L-1
): ZnSO4Á7H2O, 4.4; CoCl2Á6H2O, 2.0;
(NH4)6Mo7O24 H2O, 0.9; and CuSO4Á5H2O, 2.0. The medium
was adjusted to pH 8 and autoclaved at 121 °C for 20 min. The
filter-sterilized vitamins were added after cooling and the cul-
ture maintained under 5,000 lux illuminated at 12:12 h light
and dark condition for 15 days at room temperature. One
photobioreactor was filled with nitrogen-rich Walne’s medium
and another with the same medium with nitrogen for the first
4 days, after which the nutrients were added to the reactor
without nitrogen for scaling up.
Effect of illumination time and pH on microalgal
growth
The effect of illumination time was investigated in the
ranges of 10:14, 12:12 and 14:10 light and dark cycle for
microalgae growth. For the effect of pH on microalgae
growth, the culture medium was adjusted to three different
pH levels 7, 8 and 9, using 1 M HCl and 1 M NaOH. The
pH adjustment was carried before autoclaving the culture
medium.
Development of kinetic modeling
Growth kinetics
The effects of pH and illumination time on microalgae
growth were studied at OD680 nm before conducting the
kinetic modeling. Many growth kinetic models are avail-
able; among these, the logistic model uses simple calcu-
lation for studying microbial growth, because it is
independent of substrate consumption and is highly suit-
able for autotrophic culture of microalgae (Khavarpour
et al. 2011; Yang et al. 2011). The rate of growth of cells is
directly proportional to the cell mass concentration at the
given time. When the cell mass reaches the stationary
phase, the growth rate ceases. A gradual decrease is
observed in the late exponential phase or when the cells are
near the stationary phase. The advantage of this model is
that it facilitates the exponential phase and endogenous
metabolic phase. Hence, the logistic equation (Bailley and
Ollis 1986) was selected for this study and expressed as
follows.
Assuming that inhibition is proportional to x2
, they used
dx
dt
¼ kx 1 À
x
xs

ð1Þ
where dx
dt is the rate of microalgal growth, x is the biomass
concentration at a given time, t, and xs is the biomass
concentration at stationary phase. The logistic curve is
sigmoidal.
The logistic model leads to a lag phase, an initial
exponential growth rate and stationary cell concentration,
which is described as,
x ¼
x0ekt
1 À x0=xSð1 À ekt
Þ
ð2Þ
Lipid production kinetics
A typical, widely used and unstructured kinetic model for
product formation is the Leudeking-Piret model (1959),
contributed to both growth and nongrowth-associated
phenomena for product formation and the equation is given
as,
rfp ¼ a rfx þ bx ð3Þ
where rfp is the rate of product formation, rfx is the rate of
biomass formation and a and b are the kinetic constants of
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the Luedeking–Piret model. This two-parameter expression
has proven extremely useful and versatile in fitting product
formation data from different fermentations. This is an
expected kinetic form when the product is the result of
energy-yielding metabolism. According to this model, the
product formation rate depends linearly on the growth rate
and the cell concentration,
dp
dt
¼ a
dx
dt
þ bx ð4Þ
where dp
dt
À Á
is the lipid production rate, a the lipid formation
coefficient and b a nongrowth correlation coefficient. The
Luedeking–Piret kinetic parameters, a and b, depend on
and vary with the fermentation dynamics. The kinetic
constant b can be evaluated from the stationary phase of
batch culture, which implies
b ¼
dp
dt
À Á
stationary phase
xs
ð5Þ
where xs is the cell concentration at the stationary phase.
Gaden (2000) classified the mode of product formation
in terms of its relationship with microorganism growth as
follows: Class I, product formation was connected to
microbial growth; Class II, product formation was par-
tially connected to microbial growth; and Class III,
product formation was unrelated to microbial growth.
Consider Eq.(5): for a = 0 and b = 0, the relationship
between product formation and microbial growth was that
of Class III. For a = 0 and b = 0, the relationship
between product formation and microbial growth was
partial and thus of Class II. For a = 0 and b = 0, there
was a linear relationship between product formation and
microalgal growth, and so was of Class I. Integrating
equation (4) gives
PðtÞ À Pð0Þ À b
xS
K

1 À
x0
xS
ð1 À ekt
Þ
!
¼ aðxt À x0Þ
ð6Þ
The logistic constant for C. salina grown under
nitrogen-repleted and -depleted experiments were
evaluated using the cftool kit in MATLAB (Sivaprakash
et al. 2011a). The balance for cell growth and product
formation is represented by two differential equations
which can be solved by numerical integration and is
available in MATLAB software as ‘ode solver’. In the
present simulation work, ode23 solver is employed for
simulation purpose (Sivaprakash et al. 2011b). The error
percentage was calculated for cell growth and product
formation using the equation
% error ¼
ðexperimental À predictedÞ
experimental
Â 100 ð7Þ
Estimation of dry biomass and lipid content
The dry biomass concentration was measured by transfer-
ring a culture to a pre-weighed centrifuge tube and cen-
trifuging at 4,000 rpm for 10 min to get a thick microalgal
paste every 24 h. The paste, rinsed with distilled water to
remove residual salts, was dried in a hot air oven at 60 °C
for 8 h. The lipid was extracted from dry biomass using
Bligh and Dyer’s (1959) method with slight modification.
The amount of oil extracted (% w/w) was calculated
according to Suganya and Renganathan (2012).
Estimation of chlorophyll a content
The chlorophyll a content (mg/L) was estimated according
to Su et al. (2008). Two milliliter of culture broth was taken
in a centrifuge tube and ultrasonicated for 10 min in an ice
bath with 2 ml of 90 % methanol overnight. The homog-
enate was then centrifuged at 3,000 rpm for 5 min. The
supernatant was separated and absorbance was read at
665 nm and the amount of chlorophyll was calculated
using the following formula:
Chlorophyll a mg LÀ1
À Á
¼ 13:43 Â OD665
Estimation of protein and total carbohydrates
The total carbohydrate content was determined by the DNS
method using glucose as reference and the total protein
content was estimated according to Lowry’s method (1951)
using bovine serum albumin as standard.
Results and discussion
Optimization of microalgal growth
To investigate the effect of illumination time, marine
microalgae C. salina and N. oculata were cultivated at
three different light and dark cycle periods and pH values.
Among these, 12:12 cycle was found to be the most suit-
able condition to attain maximum growth (Fig. 1). C. sal-
ina and N. oculata were cultivated at varying pH values
such as 7, 8 and 9 at 12:12 light and dark cycle. Among
these, pH 8 showed the maximum growth (Fig. 2), since
marine microalgae do not grow well in acidic or neutral
conditions and they are generally alkaliphiles.
Kinetic studies
The kinetics of cell growth and lipid production by mic-
roalgae C. salina and N. oculata, grown in normal condi-
tions and nitrogen-depleted conditions, were analyzed and
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simulated with respect to the obtained experimental values.
Logistic equation and Luedeking–Piret model were found
to fit the growth and product formation kinetics, respec-
tively. The kinetic modeling comprises two steps, namely
evaluation of parameters of logistic (k) and Luedeking–
Piret models (a, b) and simulation of theoretical biomass
and product concentration from the kinetic parameters and
initial conditions. The evaluated constants are reported in
Table 1.
The high R2
and prediction values denote that the
logistic model is highly suitable for microalgal growth with
a minimal error of 5.29 % from C. salina at normal con-
ditions and 4.92 % from C. salina under nitrogen-depleted
conditions. The equation also proved to fit the experiment
with a minimal error of 4.45 % from N. oculata in normal
conditions and 3.56 % from N. oculata under nitrogen-
depleted conditions. Figures 3, 4 illustrate the experimental
and predicted values for each variable—cell mass and
product formed at a given time. Luedeking–Piret model for
product formation kinetics was also found to fit the
experiment of lipid production of each organism. The
model suited the production well with a minimal error of
4.59 and 4.06 % for C. salina at normal conditions and
under nitrogen-depleted conditions, respectively, and 5.33
and 4.66 % for N. oculata at normal conditions and under
nitrogen-depleted conditions, respectively (Figs. 3, 4).
Various stress factors such as nutrient depletion on the
algae trigger neutral lipid accumulation. This leads to a
slower growing or even contracting overall biomass, but an
increase in oil production (James and Boriah 2010). The
maximum lipid production by C. salina was found to be
28.26 and 37.53 % dry weight for nitrogen-repleted and -
depleted conditions, respectively, whereas for N. oculata
the maximum lipid production was found to be 33.18 and
0
0.2
0.4
0.6
0.8
1
1.2
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
C.salina 10:14
C.salina 12:12
C.salina 14:10
N.oculata 10:14
N.oculata 12:12
N.oculata 14:10
Incubation Time (Days)
OpticalDensityat680nm
Fig. 1 Effect of illumination
time on the growth of C. salina
and N. oculata
0
0.2
0.4
0.6
0.8
1
1.2
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
C.salina-pH7
C.salina-pH8
C.salina-pH9
N.oculata-pH7
N.oculata-pH8
N.oculata-pH9
OpticalDensityat680nm
Incubation Time (Days)
Fig. 2 Effect of pH on the
growth of C. salina and N.
oculata
Table 1 Parameters of logistic and Luedeking–Piret models
Organism Logistic model Luedeking–Piret model
Cell growth Product formation
K (h-1
) R2
a b Error %
C. salina, N?
0.4441 0.992 0.125 0.002 4.59
C. salina, N-
0.3786 0.9843 0.164 0.0024 2.58
N. oculata, N?
0.447 0.9878 0.151 0.0006 5.33
N. oculata, N-
0.4053 0.9961 0.211 0.001 4.66
N?
nitrogen-repleted growth, N-
nitrogen-depleted growth
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54.26 % dry weight nitrogen-repleted and -depleted con-
ditions, respectively.
Packer et al. (2011) state that microalgal cultures sus-
pended in growth media with low nitrogen (low-N) con-
centration yield biomass with significantly higher lipid
content than those suspended in high-N media. Regulation
of lipid synthesis in microalgae may be a means by which
energy can be spent during stressed conditions, helping to
maintain a safe turnover rate of the ATP and reductant
pools sustained by the light reactions. Fatty acid produc-
tion is expensive in terms of ATP and reductant require-
ments (Xiong et al. 2010). Intracellular lipid storage in
microalgae produce significantly more energy than car-
bohydrates: 37 and 17 kJ/g, respectively; and, on a per-
mass basis, lipid synthesis requires twice the reducing
energy (NADPH) than carbohydrate or protein synthesis
(Hu et al. 2008). Lipid synthesis is an effective energy
sink. It may be due to the fact that certain species
maintain a relatively high rate of photosynthesis during
nitrogen stress, but compensate by synthesizing lipids. It
has been suggested that newly fixed carbon is used for
lipid synthesis (Scott et al. 2010), particularly in instances
when the lipid dry weight of a suspension exceeds its
initial dry weight (Rodolfi et al. 2009). Oleaginous species
of algae use excess carbon and energy to synthesize
storage lipids under nitrogen stress. For supporting this
statement, chlorophyll a, protein and total carbohydrate
were analyzed for C. salina and N. oculata grown under
nitrogen-repleted and -depleted conditions and the results
tabulated in Table 2.
Ahlgren and Hyenstrand (2003) and Hoffmann et al.
(2010) reported that under nitrogen-deficient conditions,
algal cells often accumulate a surplus of carbon metabolites
as neutral lipids more than polar lipids. It was also reported
that microalgae respond to the nitrogen starvation condi-
tion by degrading nitrogen-containing macromolecules and
-0.2
0.3
0.8
1.3
1.8
2.3
2.8
0
0.5
1
1.5
2
2.5
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
C.salina-experimental,N+
C.salina-predicted,N+
C.salina-experimental,N-
C.salina-predicted,N-
N.oculata-experimental,N+
N.oculata-predicted,N+
N.oculata-experimental,N-
N.oculata-predicted,N-
Cellmass(g/L)
Cellmass(g/L)
Time (days)
Fig. 3 Experimental and
predicted values of cell
concentrations of C. salina and
N. oculata under nitrogen-
repleted and -depleted
conditions (N?
nitrogen-
repleted growth, N-
nitrogen-
depleted growth)
0
0.1
0.2
0.3
0.4
0.5
0.6
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0.5
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
C.salina-experimental N+
C.salina-predicted N+
C.salina-experimental N-
C.salina-predicted N-
N.oculata-experimental N+
N.oculata-predicted N+
N.oculata-experimental N-
Time (days)
Product(g/L)
Product(g/L)
Fig. 4 Experimental and
predicted values of lipid
concentrations of C. salina and
N. oculata under nitrogen-
repleted and -depleted
conditions (N?
nitrogen-
repleted growth, N-
nitrogen-
depleted growth)
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accumulating carbon reserve compounds for the mainte-
nance of cells, such as lipids.
Conclusion
Novelty of the present work is the use of kinetic models for
better construction of the experiment for large scale operations
and the complete agreement of simulated data with the
experimental values prediction with the correlation coefficient
R2
andpredictionvalues0.992and0.9843forC.salina,0.9878
and 0.9961 for N. oculata were its growth and lipid produc-
tion.The maximum lipid production of C. salina was found to
be 28.26 % dry weight for nitrogen-repleted condition and
37.53 % dry weight for nitrogen-depleted condition. Maxi-
mum lipid production was found to be 33.18 and 54.26 % dry
weight for nitrogen-repleted and -depleted conditions,
respectively. This work also revealed that lipid was produced
at a greater quantity when cells were grown under stress, i.e.,
nitrogenstarvationcondition,whichrevertedcarbohydrateand
protein metabolism to lipid. Furthermore, the lipid formation
coefficient (a) was greater than nongrowth correlation coeffi-
cient (b), which reveals that the lipid production in C. salina
and N. oculata was growth associated. This kinetic study will
be useful in the analysis of commercialization of microalgal
biodiesel by increasing the scaling up process.
Acknowledgments The authors gratefully acknowledge the Depart-
ment of Chemical Engineering, Annamalai University, for providing
the necessary facilities and UGC-Non SAP, Government of India, for
financial support to successfully complete this research work.
Conflict of interest The authors declare that there is no conflict of
interest.
Open Access This article is distributed under the terms of the
Creative Commons Attribution License which permits any use, dis-
tribution, and reproduction in any medium, provided the original
author(s) and the source are credited.
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Table 2 Chlorophyll a, protein content and carbohydrate content of
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Parameter C. salina N. oculata
N?
N-
N?
N-
Chlorophylla content (lg/ml) 9.24 7.83 7.37 5.96
Protein content (lg/ml) 1.613 0.860 1.95 0.98
Total carbohydrate (lg/ml) 6.34 5.56 4.89 2.79
N?
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nitrogen-depleted medium
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