Microalgae as an Attractive Source for Biofuel Production - Environmental Sustainability: Role of Green Technologies - page 130

biomass within 24 h. In fact, the biomass doubling
time for microalgae during exponential growth
can be as short as 3.5 h (Chisti 2007 ), which is
signifi cantly quicker than the doubling time of oil
crops. They have the ability to reproduce them-
selves using simple photosynthesis to convert
solar energy into chemical energy and accumu-
late their lipids in the form of oil, carbohydrate,
and protein etc. (Schenk et al. 2008 ). Microalgae
can grow in simple nutrients and sunlight,
although the growth rate will vary with the
addition of specifi c nutrients and aeration
(Renaud et al. 1999 ; Pratoomyot et al. 2005 ;
Aslan and Kapdan 2006 ). Many microalgae
accumulate high levels of oil, which can be con-
verted into biodiesel. Microalgae can accumulate
more than 80 % of lipids on the dry weight of
biomass (Chisti 2007 ). According to the US
Department of Energy, the oil yield of algae is
10-100 times higher than conventional oilseed
crops (soy, rapeseed, or tree-borne oil plantations
such as Jatropha and palm). Microalgae
have a high theoretical production yield of
47,000-3,08,000 L ha −1 annum −1 ; whereas the oil
palm has the ability to produce 5,950 L of
biodiesel per year (Ahmad et al. 2011 ).
Microalgae biomass, either terrestrial or aquatic,
is considered one of the best alternative renewable
energy sources (Chisti 2008 ; Raja et al. 2008 ).
Microalgae biomass production may be combined
with direct bio-fi xation of waste CO 2 (1 kg of dry
algal biomass requiring about 1.8 kg of CO 2 ).
Depending on the species of microalgae, other
compounds that may also be extracted include a
wide range of fi ne chemicals and bulk products
such as fats, PUFAs, oil, natural dyes, sugars, pig-
ments, antioxidants, high-value bioactive com-
pounds, and algal biomass, which are valuable
applications in the industrial sector (Li et al. 2008a ,
b ; Raja et al. 2008 ; Sathasivam et al. 2012 ). These
high-value biological derivatives with many possi-
ble commercial applications mean that microalgae
can potentially be used in a large number of bio-
technology areas, including biofuels, cosmetics,
pharmaceuticals, nutrition and food additives,
aquaculture, and pollution prevention (Raven and
Gregersen 2007 ; Rosenberg et al. 2008 ; Jacob-
Lopes and Teixeira Franco 2010 ).

5 Classifi cation of Microalgae

Based on Metabolism

Microalgae can be divided into four major groups
of algal metabolism: photoautotrophic, heterotro-
phic, mixotrophic, and photoheterotrophic (Chen
et al. 2011 ). A short explanation follows.

5.1 Photoautotrophic Microalgae

Photoautotrophic microalgae build up CO 2 and
water into organic cell materials using energy from
sunlight. In this condition, microalgae absorbs
energy from light and stores it in the form of ade-
nosine triphosphate (ATP) and nicotinamide ade-
nine dinucleotide phosphate (NADPH) to produce
glucose in a Calvin cycle. However, inadequate
light and CO 2 supply are always an issue for photo-
autotrophic cultures. It has been reported that the
addition of excess CO 2 into the culture system may
increase the microalgae lipid and biomass produc-
tivity, but care should be taken because an excess
amount of CO 2 is released in the atmosphere. Light
also plays an important role because the uneven
distribution of light intensity will directly affect the
growth of microalgae. For example, biomass pro-
ductivity is lower in photoautotrophic species than
heterotrophic microalgae in both photobioreactor
and open ponds—between 0.117 and 1.54 kg/m 3 /day,
respectively (Chisti 2007 ).

5.2 Heterotrophic Microalgae

Heterotrophic microalgae require at least one
organic nutrient from organisms or their by-
products, as a carbon source for producing their
own organic compounds. Heterotrophic cultures
utilize organic wastes containing carbons as an
energy source instead of CO 2 and are independent
of the light source for reproduction. Thus, the high
production of lipids is enabled compared with
phototrophic microalgal cultivation. Heterotrophic
microalgae have the ability to accumulate a higher
level of lipids than photoautotrophic microalgae
(Xiong et al. 2008 ; Liang et al. 2009 ). Although it