Environmental Engineering Reference
In-Depth Information
under favorable growth conditions, whereas reddening and differentiation of vege-
tative cells to the resting form (cyst or aplanospore) is observed under suboptimal
growth circumstances. Cysts are bigger in size than vegetative cells and are
characterized by a thick cell wall and the massive accumulation of astaxanthin
(Fig. 1c).
A relatively fast accumulation of astaxanthin occurs upon severe nutritional or
environmental stress. A fair amount of information concerning factors influencing
astaxanthin production and accumulation is available. High irradiance, nitrogen
limitation, phosphate starvation or high temperature [11-14] leads to enhanced
astaxanthin levels. Moreover, the presence of acetate [15-17], ferrous ion [13, 18],
or high salt concentration [11, 19, 20] also seems to stimulate pigment accumula-
tion. Nevertheless, the relative importance of individual factors is far from clear,
since heterogeneity in culture conditions used by different authors (environmental
conditions, length of experience, culture system) hampers the interpretation of
results. This especially applies to conclusions drawn from experiences performed
with batch cultures, an easy and widely used culture strategy.
Accumulation of astaxanthin under the effect of the stress factors mentioned
above is generally assumed to be linked to cessation of growth and subsequent
transformation of vegetative cells into cysts. This has led to the conclusion that ces-
sation of cellular division is a pre-requisite for astaxanthin accumulation [11, 13, 19,
21-23]. Nevertheless, different authors have described astaxanthin accumulation in
growing, flagellated and palmelloid, cells [24-29].
2 Current Methodology for the Production of
Haematococcus
astaxanthin: The Two-Stage Approach
The special characteristics of the physiology of
H. pluvialis
have determined the
design of current production systems. An efficient production of astaxanthin by
Haematococcus
can be achieved in a two-stage process [5, 17, 21, 30, 31], first
producing biomass under optimal growth conditions (“green” stage) and then expos-
ing the alga to adverse environmental conditions as to induce the accumulation of
astaxanthin (“red” stage).
In commercial systems, accumulation of astaxanthin is generally induced by a
combination of nutrient deprivation (mainly nitrate and phosphate) and increase
of irradiance and/or temperature [5]. Astaxanthin accumulation accompanies the
development of red aplanospores, containing about 1.5-3.0% astaxanthin [32].
Harvesting of aplanospores is performed by settling and subsequent centrifugation.
They are then dried and cracked afterwards, to ensure maximum availability of the
astaxanthin [5].
Different systems exist for growth and handling of microalgae on a large scale,
with sunlight as the energy source [33-39]. Within the open systems, the best choice
seems to be the open shallow pond, made of leveled raceways 2-10 m wide and
15-30 cm deep, running as simple loops or as meandering systems. Each unit covers
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