Testing eukaryotic routes for heterologous production of ketocarotenoids in cyanobacteria

Pigments and dyes are widely used in multiple industries. Ketocarotenoids are a highly sought-after group of pigments with a dark-red hue and strong antioxidant properties. Currently, chemical synthesis is the major route for ketocarotenoid production. However, the environmental risks and large demand have led to a call for the development of more sustainable sources. Cyanobacteria have promising properties for sustainable ketocarotenoid production: Some strains show fast growth, are genetically tractable, and natively produce ketocarotenoid precursors. Despite a few promising studies, engineering ketocarotenoid production in cyanobacteria is so far underexplored. Here, we expand the biosynthetic routes for ketocarotenoids by testing the astaxanthin synthase CrtS of Xanthophyllomyces dendrorhous and the β-carotene ketolase CrBKT from Chlamydomonas reinhardtii. While CrtS was successfully produced in Synechocystis sp. PCC 6803, no changes in ketocarotenoid profiles were detected. On the other hand, expression of CrBKT in Synechococcus elongatus UTEX 2973 and Synechocystis sp. PCC 6803 led to the production of both canthaxanthin (0.12% and 0.1% of dry cell weight, respectively) and astaxanthin (0.012% and 0.004% of dry cell weight, respectively). Looking forward, expression of CrBKT is a promising route to produce ketocarotenoids in cyanobacteria. But further work is needed to increase strain stability and product yields.