l-Lactate treatment by photosynthetic cyanobacteria expressing heterogeneous l-lactate dehydrogenase

l-Lactate is a major waste compound in cultured animal cells. To develop a sustainable animal cell culture system, we aimed to study the consumption of l-lactate using a photosynthetic microorganism. As genes involved in l-lactate utilization were not found in most cyanobacteria and microalgae, we introduced the NAD-independent l-lactate dehydrogenase gene from Escherichia coli (lldD) into Synechococcus sp. PCC 7002. The lldD-expressing strain consumed l-lactate added to basal medium. This consumption was accelerated by expression of a lactate permease gene from E. coli (lldP) and an increase in culture temperature. Intracellular levels of acetyl-CoA, citrate, 2-oxoglutarate, succinate, and malate, and extracellular levels of 2-oxoglutarate, succinate, and malate, increased during l-lactate utilization, suggesting that the metabolic flux from l-lactate was distributed toward the tricarboxylic acid cycle. This study provides a perspective on l-lactate treatment by photosynthetic microorganisms, which would increase the feasibility of animal cell culture industries.