Light is a key factor for inducing rapidly the astaxanthin biosynthesis.
Differed with suspended system, thick algal film in the
attached system may cause insufficient illumination for the innermost
cells. Thus initial cell amount was set to a high level of
40 g m2 to study the relationships of light intensity with cell
growth and astaxanthin accumulation. As the light intensity
increased until 120 lmol m2 s1, the astaxanthin content of cells
in attached induction increased (Fig 4). And the astaxanthin
content reached the maximum under light intensity of
120 lmol m2 s1, and decreased slightly with the increasing light
intensity. The biomass productivities were similar under light
intensities below 90 lmol m2 s1. In the range of light intensities
from 90 to 160 lmol m2 s1, the biomass increased with light
intensity. However, the increase rate of biomass slowed down
when the light intensity exceeded 160 lmol m2 s1. These results
indicated that the 160 lmol m2 s1 of light density could be considered
as the light saturation point (LSP) for this attached induction
system in our investigated conditions. This result was
different than those in suspended induction that the astaxanthin
content was reached the maximum at the light intensity
300 lmol m2 s1 (Li et al., 2010; Liang, 2009). Therefore, the
attached induction of H. pluvialis, even at initial cell amount of
40 g m2, can work well under lower light intensity than suspended
approach. Furthermore, the attached induction is easy to
avoid harmful high light intensity by tilting attached bioreactor
toward the light direction, which means more induction bioreactors
in the same illumination area can be operated.