**3. Microbiological synthesis of astaxanthin**

In general, the microbiological synthesis of carotenoids has conservative steps among the different species of carotenogenic microorganisms. The most conserved carotenoid biosynthesis pathways in eukaryotic microorganisms involve three general steps [1]:


Despite the common steps in carotenogenesis, there are particular differences in the synthesis pathways between different microorganisms. In the following sections, the carotenogenic pathways in the microalgae *H. pluvialis* and in the yeast *X. dendrorhous* are described.
