6.2.4. *In Situ* Extraction

*In situ* extraction of Ά-carotene from *Dunaliella salina* was recently reported by Kleinegris *et al*. [44], using a flat-panel photobioreactor operated as a turbidostat— where the numbers of stressed cells were kept essentially constant via a continuous, well-defined level of irradiation. This two-stage system comprised an organic phase of dodecane, sparged at a rate of 286 Ldodecane Lreactorƺ1 minƺ1 that promoted formation of an emulsion in the aqueous phase; Ά-carotene was then continuously extracted  from the aqueous to the organic phase, at a rate of *ca*. 2.75 mgΆ-carotene Ldodecaneƺ1 dƺ<sup>1</sup> (equivalent to 0.7 mgΆ-carotene Lreactorƺ1 dƺ1). However, this process exhibited a poor efficiency—as the yield of Ά-carotene extracted by the solvent was a mere one-tenth of that removed from the reactor via biomass overflow. 

If the aforementioned carotenoid-rich biomass was extracted as well, then the overall volumetric productivity of the system would reach 8.3 mgΆ-carotene Lreactorƺ1 dƺ1; this is still below the yield attained if downstream rather than *in situ* extraction was promoted (*ca*. 13.5 mgΆ-carotene Lreactorƺ1 dƺ1) [44], so in this system simultaneous biosynthesis and extraction cannot be justified relative to the classical sequential approach. 

## **7. Final Considerations**

Carotenoid production appears to be one of the most successful case studies of blue biotechnology. The rising market demand for pigments from natural sources has promoted large-scale cultivation of microalgae for synthesis of such compounds, so significant decreases in production costs are expected in coming years. 

The recognized therapeutic value of some carotenoids (especially lutein) in prevention and treatment of degenerative diseases has indeed opened new avenues for development of mass production systems. Advances in knowledge of the underlying physiology, biochemistry and molecular genetics of carotenoidproducing microalgae are now urged—which would have a major impact upon development and optimization of this (and alternative) microalga-based technologies. In this regard, the genes encoding enzymes that are directly involved in specific carotenoid syntheses need in particular to be investigated—so that further development of transformation techniques will permit considerable increase of carotenoid cellular contents, and accordingly contribute to increase the volumetric productivities of the associated processes. 
