The partitioning of electrons photosynthetically fixed in carbon by the cyanobacterium Synechocystis sp. PCC 6803 was evaluated for up to 24-day batch cultures continuously exposed to incident light intensity (LI) from 111 to 598 mu Em(-2) s(-1). The fate of fixed electrons was assayed by Chemical Oxygen Demand (COD) in biomass, generic soluble microbial products (SMP), and excreted laurate (for a modified strain). Because SMP is not a valuable product, light energy embedded in SMP is wasted. Normalized to the total COD (TCOD), wild-type Synechocystis partitioned 75-84% of its TCOD in particulate biomass and 16-25% in generic SMP. A strain modified to excrete lauric acid partitioned 6.6%-10% of its TCOD in laurate, 21%-30% in generic SMP, and 64%-69% in particulate biomass. The greatest electron partitioning in SMP occurred with the highest LI tested, which suggests that moderating the LI in the photobioreactor is a promising mean to accentuate the production of valuable forms of photosynthetically fixed electrons. (C) 2016 Elsevier Ltd. All rights reserved.