Environmental Engineering Reference
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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 =
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
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