Carotenoids from Dunaliella salina are produced on a commercial
scale in open ponds in e.g. Australia and Israel [1]. Hejazi
et al. [2] developed an in situ extraction process for production
and extraction of carotenoids, the so-called milking process. In
this process Dunaliella is cultured in closed photobioreactors for
the simultaneous production and extraction of carotenoids. After
a growth phase the algae are stressed to produce carotenoids. At
the same time an organic phase is added to the culture. In this
two-phase system the carotenoids are extracted to the organic
phase (dodecane). It was shown that with this process a continuous
carotenoid production and extraction was obtained for
more than six weeks. During this process no biomass growth was
observed. The hypothesis was that carotenoids were extracted from
the cells and cells kept reproducing carotenoids [3]. However, we
recently found that cell-dodecane contact resulted in cell death and
carotenoids were only extracted from dead cells [4]. Dead cells fall
apart and consequently lose their carotenoids. The lipid globules
containing the carotenoids dissolve easily in the lipophilic dodecane.
The fact that thus far phase toxicity of dodecane on Dunaliella cells was not found might be due to the used methods, namely by
measuring e.g. oxygen evolution rates for a total culture and not for
single cells with direct contact.
Apparently part of the cells died due to cell solvent contact and
cell death was compensated by cell growth. Consequently, in a continuous
process constant biomass levels can only be reached if cell
death is compensated by cell growth. The objective of this research
is to demonstrate that cell growth takes place during the milking
process and that the extraction rate increases if the cell death rate
is increased. For this we used turbidostat cultivations combined
with the in situ extraction process. A turbidostat is a continuous
culture with controlled turbidity. Lamers et al. [5,6] showed that
this approach is very suitable to study the effect of light stress on D.
salina. In the turbidostat concentration of stressed biomass is constant.
In this system the net growth rate is equal to the dilution rate.
The sparging rate of dodecane and the light intensity were equal to
those used by Hejazi et al. [3]. As a reference we compared the
volumetric productivity of this in situ extraction experiment with
a continuous turbidostat experiment without extraction, solely
based on the production of carotenoid-rich biomass.