Fluorescence microscopy reveals heterogeneity in cyanobacterial survival of photodamage

Jian Wei Tay and Jeffrey C. Cameron

Abstract

In cyanobacteria, the photosynthetic process is driven by reaction center proteins in photosystems I and II within the internal thylakoid membrane system1,2. However, these reaction centers become damaged upon exposure to light, with higher light intensities inducing greater rates of photodamage3. Thus, cyanobacteria have evolved a repair cycle to continuously replace the damaged proteins4. The steady-state level of active photosystems within a cell is therefore a result of the relative rates of synthesis, degradation, damage, and repair5. However, understanding how each of these processes are integrated into the photosynthetic system this has been difficult using traditional ensemble approaches as cultures are exposed to a dynamic light environment that is a function of mixing, cell density, and cell state6–8. Here we demonstrate an automated imaging approach using time-lapse fluorescence microscopy and computational image analysis. By growing cells in a two-dimensional layer, we avoid shading effects, thereby generating uniform and reproducible growth conditions. Using this platform, we analyzed the growth and physiology of multiple strains simultaneously under defined illumination conditions, including those that induce photodamage. Our results revealed an asymmetric cellular response to photodamage among genetically identical siblings which could indicate the presence of a previously unknown photoprotective mechanism through a bet-hedging strategy. We anticipate these results to be a starting point for further studies to better understand photodamage and repair at the single-cell level.