Biomedical Engineering Reference
In-Depth Information
13.4 VARIATIONS IN ALGAL PRODUCTION:
CRUCIAL BUT IGNORED
Wide variations exist in units of measurement, and standardization is required
with regard to the growth conditions of algae to permit comparison of outputs
(Coronet, 2010). On a volume basis, biomass in several species of autotrophic
algae varied considerably between 0.002 and 4 g L
−1
d
−1
and 1.7 to 7.4 g L
−1
d
−1
in
the heterotrophic algae (Table 13.1); on an areal basis, values ranged from 0.57 to
150 g m
−2
d
−1
(Table 13.1). The highest production of algal biomass (120 to
150 g m
−2
d
−1
) has been reported in PBRs under artificial light (Tsoglin and Gabel,
2000).
The success of microalgal biotechnology entrepreneurship depends on the opti-
mization of biomass and production yields. It is necessary to establish to what extent
these variations are intra-specific or inter-specific, whether or not these yields are
based on optimal growth conditions, and how to prime the algal production. Between
several species of
Dunaliella,
cell division rates ranged from 0.12 to 3.0 div d
−1
(Subba Rao, 2009). Within the one species,
Chlorella sorokiniana,
biomass produc-
tion rates (div d
−1
) varied between 0.32 and 4.0 div d
−1
; and in
Dunaliella teriolecta,
rates varied between 0.15 and 3.0 div d
−1
(Subba Rao, 2009). Such variations could
be due to differences in strains of isolates and/or culture conditions. Even in the
most commonly used strain,
Neochloris oleoabundans
UTCC 1185, biomass varied
between 0.03 and 1.50 g L
−1
d
−1
(Table 13.2).
In
Dunaliella tertiolecta
, a green alga often used in biotechnology, Duarte and
Subba Rao (2009) discussed the relationship between biomass (B determined as Chl-
a
),
photosynthesis (P), and light energy I (μmol m
−2
s
−1
):
P
B
= {P
B
s
[1 − exp(−α
B
I
/P
B
s
)]exp(−β
B
I
/P
B
s
)} + P
B
d
where P
B
s
is the maximum potential photosynthesis in the absence of photo-
inhibition, and P
B
d
is the intercept of the P-I
curve on the
y
-axis and has the same
units as P
B
m
. In
D. teriolecta,
P
B
m
varied between 3.3 and 7.43 mg C
mg Chl-
a
h
−1
(Duarte and Subba Rao, 2009). They showed that the photosynthesis and respira-
tion activities were dependent on the light energy and the cell density; that is, over a
21-day period, gross production and respiration decreased by sevenfold and fourfold,
respectively, at 42 μmol m
−2
s
−1
. The optimal light energy for photosynthesis ranged
between 627 and 1,356 μmol m
−2
s
−1
. Also, the gross primary production:respiration
ratio decreased with higher cell densities. It will be crucial in biotechnology opera-
tions to optimize the relationships among high biomass yields, photosynthetic effi-
ciencies, and yield of bioactive compounds. These criteria are crucial and could
greatly improve commercial algal harvest.
Grobbelaar (2010), while discussing the light energy relationships in algae, sug-
gested that by optimizing light, photosynthetic yield could be doubled from 1.79 g
(DW) m
−2
d
−1
and pointed out that several factors determine volumetric yields of
mass algal cultures. Furthermore, Grobbelaar pointed out that many biotechnology
start-up companies make the mistake of simple extrapolation of controlled labora-
tory rates to large-scale outdoor production systems.
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