This disclosure describes modified photosynthetic microorganisms, including Cyanobacteria that produce carotenoids, including zeaxanthin, astaxanthin, and/or canthaxanthin. The modifications include one or more genetic modifications such as gene deletion, up regulation of an endogenous gene, and/or addition of an exogenous gene. In some embodiments the modified photosynthetic microorganisms may be subjected to stress conditions.