To improve photosynthetic efficiency, a synthetic formate assimilation (FA) pathway is designed in this study to function as an additional carbon-fixing route, which possibly also reduce the photorespiratory loss of CO2 via the inhibition of glycine decarboxylation. To establish FA, genes encoding the proteins 10-formyl-THF ligase (FTL), 5,10-methylene-THF dehydrogenase (MtdA), and 5,10-methenyl-THF cyclohydrolase (FchA) from M. extorquens AM1 were transferred into the model cyanobacterium Synechocystis sp. PCC 6803.