Environmental Engineering Reference
In-Depth Information
Fig. 2
Variation of biomass productivity (
) and astaxanthin productivity (
◦
) in continuous
Em
-2
s
-1
)
cultures of
H. pluvialis
as a function of specific nitrate input (
I
o
=
1,000
μ
conditions. Optimal
SNI
value can be set in the range of 0.5-0.9 mmol NO
3
-
(g dry
biomass)
-1
d
-1
, where the combination of biomass and astaxanthin productivities
results more adequate.
For evaluating the influence of light availability on the behavior of the system,
average irradiance,
I
av
, is the parameter of choice.
I
av
takes into account not only
incident irradiance, but also biomass concentration and the geometry of the reac-
tor [47]. Under conditions of nitrogen sufficiency, the growth rate is a hyperbolic
function of average irradiance, and the cultures perform green, astaxanthin being
present at basal levels only [48]. At
SNI
values lower than 2.7 mmol g
-1
d
-1
(nitro-
gen limitation), however, the cells accumulate astaxanthin, reaching 1.1% of the dry
biomass at an
SNI
value of 0.8 mmol g
-1
d
-1
. For an optimal combination of
SNI
(0.8 mmol g
-1
d
-1
) and
I
av
(100
Em
-2
s
-1
), a daily productivity close to 20 mg
astaxanthin l
-1
d
-1
is achieved. The increase in
I
av
results in enhancement of astax-
anthin accumulation, but under nitrogen limiting conditions only. Astaxanthin does
not accumulate at
SNI
values higher than 2.7 mmol g
-1
d
-1
(nitrogen sufficiency),
even at high irradiance. Thus, once nitrogen limitation (the primary determin-
ing factor) triggers astaxanthin accumulation, the latter would behave as a linear
function of light availability. Notwithstanding, contrary to the generalized belief
[10, 13, 19, 21, 49], light itself does not suffice to promote astaxanthin accumulation
when nitrogen and other nutrients are fully available. Nitrogen limitation is, there-
fore, the factor that triggers the accumulation of astaxanthin. Once this condition is
established, pigment accumulation becomes a linear function of light availability.
Statistical analysis (ANOVA) of a very significant collection of data confirms that
SNI
, the specific nitrate input, and
I
av
, the average irradiance, are decisive parame-
ters in determining astaxanthin content of the biomass, as well as the productivity
μ
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