We present that the rise of inorganic?biological hybrid organisms for solar-to-chemical production has spurred mechanistic investigations into the dynamics of the biotic?abiotic interface to drive the development of next-generation systems. The model system, Moorella thermoacetica?cadmium sulfide (CdS), combines an inorganic semiconductor nanoparticle light harvester with an acetogenic bacterium to drive the photosynthetic reduction of CO<
sub>
2<
/sub>
to acetic acid with high efficiency. In this work, we report insights into this unique electrotrophic behavior and propose a charge-transfer mechanism from CdS to M. thermoacetica. Transient absorption (TA) spectroscopy revealed that photoexcited electron transfer rates increase with increasing hydrogenase (H<
sub>
2<
/sub>
ase) enzyme activity. On the same time scale as the TA spectroscopy, time-resolved infrared (TRIR) spectroscopy showed spectral changes in the 1,700?1,900-cm<
sup>
-1<
/sup>
spectral region. The quantum efficiency of this system for photosynthetic acetic acid generation also increased with increasing H<
sub>
2<
/sub>
ase activity and shorter carrier lifetimes when averaged over the first 24 h of photosynthesis. However, within the initial 3 h of photosynthesis, the rate followed an opposite trend: The bacteria with the lowest H<
sub>
2<
/sub>
ase activity photosynthesized acetic acid the fastest. These results suggest a two-pathway mechanism: a high quantum efficiency charge-transfer pathway to H<
sub>
2<
/sub>
ase generating H<
sub>
2<
/sub>
as a molecular intermediate that dominates at long time scales (24 h), and a direct energy-transducing enzymatic pathway responsible for acetic acid production at short time scales (3 h). Lastly, this work represents a promising platform to utilize conventional spectroscopic methodology to extract insights from more complex biotic?abiotic hybrid systems.