Order-of-Magnitude Tuning of Microbial Growth Rates through Photo-Excitation Growth Spectroscopy

When exposed to light, microbial life shows unique and complex behaviors that are yet to be fully understood. Major questions remain about the effects of light on ATP production (e.g., photobiomodulation in mitochondria) and the near theoretical quantum limits of efficiency of photosynthetic bacteria. To unambiguously quantify such phenomena, it is necessary to develop new techniques that probe microbial growth dynamics in a highly controlled multi-parameter light environment. Here, we describe a novel method, called Photo-Excitation Growth Spectroscopy (PhotoExGS), that combines broadband supercontinuum excitation and real-time laser probes of microbial growth in highly controlled environments. Using this technique, we measured the growth of highly characterized and metabolically diverse bacteria, Rhodobacter sphaeroides and several key mutants. By shining spatially separated white light (supercontinuum laser wavelength from 400 nm to 2300 nm) with a highly characterized intensity distribution on an agarose microbial farm, we are able to generate high resolution movies of spectrally resolved dynamic growth. Strikingly, we find that the growth rate of Rhodobacter sphaeroides can be fine tuned over an order of magnitude using only light intensity and wavelength. Such tuning reveals several surprising features of growth dynamics in Rhodobacter sphaeroides, yet also indicates a powerful control for future population dynamics measurements in microbial systems.