Microalgae as an Attractive Source for Biofuel Production - Environmental Sustainability: Role of Green Technologies - page 131

accumulates higher lipids, it is not economically
feasible when the carbon sources have to be pur-
chased, as this directly increases production costs.
Carbon sources used for culturing algae include
glycerol, glucose and sweet sorghum, corn powder
hydrolyzate, and Jerusalem artichokes. This glu-
cose has been widely used as a carbon source for
microalgal cultivation. The most suitable glucose
concentration for a high accumulation of lipids is
2 %. For example, with a glucose concentration of
0.5-8 %, the carbon source showed 44.48 % lipid
content (Wu et al. 2005 ), whereas the glucose con-
centration in the normal medium does not exceed
2.4 % and accumulates lipid content as high as
57 % (Xiong et al. 2008 ). Sweet sorghum is
another carbon source used for culturing algae.
The addition of 50 % sweet sorghum juice into the
culture medium instead of glucose achieved a lipid
content as high as 73.4 % (Liang et al. 2010 ).
Heterotrophic species are one of the best for the
production of biodiesel (Xu et al . 2006 ; Martek
2008 ; Xiong et al . 2008 ). For example, Chlorella
sp. grown in heterotrophic conditions showed a
high level of biomass (7.4 kg/m 3 /day) and lipid
content 50-58 % (g lipid/g dry weight) (Xiong
et al. 2008 ).

with both light and glucose sources showed
higher biomass productivity (254 mg/L/day).
When grown in the dark under CO 2 instead of
glucose, the biomass productivity (151 mg/L/
day) was low (Liang et al. 2009 ; Chen et al.
2011 ). Among all the mixotrophic culture condi-
tions, algae accumulate high lipid contents even
though they are rarely used in microalgal oil pro-
duction. According to Cheirsilp and Torpee
( 2012 ), when freshwater Chlorella sp., marine
Chlorella sp., Nannochloropsis sp., and
Cheatoceros sp. were grown in mixotrophic cul-
tures, a much higher biomass and lipid level was
produced than the photoautotrophic and hetero-
trophic cultures.
The presence of different metabolisms can be
distinguished according to pH fl exibilities, which
depends on the growth stoichiometry of microal-
gae as part of the metabolism involved. In the
autotrophic metabolism, due to the consumption
of CO 2 , the growth medium becomes alkalized,
whereas, in heterotrophy, where CO 2 is produced
from an organic carbon source, the growth
medium becomes acidulated. In a mixotrophic
culture, the pH value depends on the dominating
constituent metabolism, but in most cases
remains constant. Some strains also have the abil-
ity to grow under photoautotrophic, heterotrophic
and mixotrophic conditions (e.g. Ch. vulgaris ,
Haematococcus pluvialis , Arthrospira ( Spirulina )
platensis ). Other strains, such as Selenastrum
capricornutum and Scenedesmus acutus , can
grow either photoautotrophically, heterotrophi-
cally, or photoheterotrophically (Chojnacka and
Marquez-Rocha 2004 ).

5.3 Photoheterotrophic

Microalgae

Photoheterotrophs are also referred to as photo-
organitrophs, photoassimilates, and photometab-
olism. Photoheterotrophic microalgae use light,
but obtain their carbon in organic form. For
example, microalgae under stress conditions pro-
duce glycerol; these glycerols are used as a car-
bon source and it utilizes a light intensity of 35
ʼ

6 Screening of Lipid

Production Under Different
Environmental Conditions

E/m −2 /s −1 . Yang et al . ( 2011 ) recorded an
increase in the biomass concentration of Chlorella
minutissima (UTEX2341) from 1.2 to 8.2 g/L.

Screening of microalgae producing high levels of
oil is one of the most important criteria in the
optimization of biodiesel production. Oil content
in microalgae can be stored as high lipid contents
up to a maximum of 75 % (g lipids/g dry weight)
biomass, but this is associated with low produc-
tivity. Some cultures accumulate a moderate

5.4 Mixotrophic Microalgae

Mixotrophic microalgae combine both an auto-
trophic and a heterotrophic metabolism, simulta-
neously assimilating CO 2 . Ch. vulgaris grown