In photosynthetic organisms, the task of excitation energy transfer has been perfected by

evolution to achieve a quantum efficiency that can approach unity. The idea that quantum

effects, such as delocalisation, play a role in this process has attracted considerable attention

over the past decades, following the advent of specialised spectroscopic techniques for measuring

energy transfer. An attempt to explain this remarkable efficiency has been the theory of vibronic

coupling — the conjecture that the nuclear vibrational modes of the photosynthetic complexes

are fine-tuned to prevent decoherence of the exciton and assist in the excitation transfer. In this

work, we experimentally tested the above hypothesis using the light harvesting 2 complex (LH2)

of the purple non-sulfur bacteria Rhodobacter sphaeroides as a model system. To this end, we

labelled the wild-type LH2 with the isotope carbon-13 — whose mass is heavier than carbon-

12 and therefore vibrates at a lower frequency — by growing the bacteria in media containing

carbon-13 labelled glucose as the major carbon source. The Raman spectra of the LH2 complex

displayed a shift in all peaks of the spectrum for the carbon-13 samples, consistent with the lower

vibrational frequency associated with carbon-13. Subsequently, the effect of isotopic labelling

was tested both in vivo and in vitro. In vivo, we did not observe a statistically significant

difference between the growth rate and maximum population, extracted from the growth curves

of bacteria grown in carbon-12 and carbon-13 labelled media. Accordingly, the in vitro ultrafast

transient absorption decay curves did not exhibit a marked difference between the two conditions,

showcasing the remarkable robustness of this process. Most importantly, we have demonstrated

that there is variation in the decay curves in both the carbon-12 and carbon-13 LH2 sample

replicates, highlighting the need to include multiple samples per condition in future spectroscopic

experiments of photosynthetic complexes, in order to draw accurate conclusions. On the other

hand, two-colour transient absorption measurements indicate modest inter-ring acceleration,

suggesting a supportive role for broad vibronic interactions. Together, these findings endeavour

to bridge the gap between biology and physics in the emerging field of quantum biology.