Using Cyanobacteria as chassis strains can harness sunlight as an energy source for biotechnology. Their phototrophic lifestyle requires balancing energy assimilation (source) with nutrient assimilation and biomass formation (sink). It is hypothesized that extra carbon and/or electron sinks can increase photosynthetic efficiency, allowing redox biotransformations to tap into otherwise unused photosynthetic potential. We investigated physiological responses of the cyanobacterium Synechocystis sp. PCC 6803 to the onset of an electron consuming biotransformation under three different sink scenarios. A heterologously expressed two-step cascade converting cyclohexanone to 6-hydroxyhexanoic acid served as a proxy for growth-decoupled consumption of photosynthetically generated electrons. Specific growth rates, specific quantum absorption, and effective quantum yield at photosystem II were assessed under steady state conditions. Balancing absorbed photons and generated electrons revealed an increased alternative electron flux upon heterologous gene expression, while the biotransformation (partially) rerouted this flux toward product formation. Despite the capacity of Synechocystis sp. PCC 6803 to react to different sink conditions, high biotransformation rates could not be maintained, likely due to metabolic or enzymatic instabilities. Low-carbon conditions were most suitable to distribute reductants between growth and biotransformation, whereas high-carbon conditions enabled the highest quantum efficiencies. The physiological analysis of an oxygenation-active cyanobacterium represents an important cornerstone to optimize phototrophic metabolism for photo-biotechnological applications.