Metabolic engineering of Synechocystis sp. PCC 6803 for astaxanthin production

Astaxanthin (Asx) is the highest value carotenoid produced by microalgae, with a global annual market estimated to exceed $ 1.5 billion for 2020 [1]. It possesses a wide range of applications in the nutraceutical, pharmaceutical, cosmetics and feed industries, due to its well-known antioxidant activity [2]. The richest natural source of Asx is the green alga Haematococcus pluvialis, however the biological process for Asx production from H. pluvialis is limited by slow growth times and low cell density, while the chemical synthesis process is expensive and the synthetic Asx is not approved for human consumption [1]. Several attempts have been made in order to produce Asx from other biological sources [3], but the commercial applications are still poorly developed. Here we present an alternative strategy based on the metabolic engineering of the cyanobacterium Synechocystis sp.PCC6803. Synechocystis sp. PCC 6803 is a model species, commonly employed for biotechnological applications, which is naturally able to accumulate zeaxanthin (Zea), the metabolic precursor of Asx, and is easy to be cultured and harvested. Two genes (a b-carotene ketolase, CrtW, and an hydroxylase, CrtZ) from a naturally Asx accumulating proteobacterium have been cloned under the control of an inducible promoter [4]. Upon induction, the engineered strain (Syn CrtZ/W) is able to accumulate significant amounts of Asx. On the basis of the spectra analysis (400-760 nm) of the total carotenoids and Chlorophyll a (Panel A) and on their subsequent characterization by means of HPLC (Panel B), the mutant strain Syn CrtZ/W has been shown to accumulate Asx at a level comparable to b-carotene. Moreover the growth of the mutant strain is not negatively affected by the production of the exogenous carotenoid Asx. Therefore this engineered strain has a strong potential for further optimization and scale-up.