The photoprotective response in plants and green algae involves three light-absorbing pigment molecules: violaxanthin (V), antheraxanthin (A), and zeaxanthin (Z). In the so-called VAZ cycle, fluctuations in light trigger chemical conversions between these molecules, dissipating bursts of solar energy and helping regulate and protect photosynthetic proteins.
To understand how the VAZ cycle responds to fluctuating light, the researchers worked with an alga known as Nannochloropsis oceanica, a simple model organism that also has applications in the biofuels industry. They exposed N. oceanica to irregular sequences of alternating dark and high intensity light with intervals ranging from one to 15 minutes. Using high-performance liquid chromatography the researchers measured the varying concentrations of the three pigments throughout the light sequences to determine the rates of conversions between them. They also probed the alga's ability to dissipate light energy during the sequences by performing time-correlated single photon counting experiments.
A new model of algae memory
Informed by the experimental results, the team developed a theoretical model of the system capable of predicting its behavior in a greater range of conditions.
"Modeling pulls together our ideas about biochemistry and the quantitative measurements into a cohesive picture of what's going on, filling in the gaps that we can't directly access with experiment," said Thomas Fay, a postdoctoral researcher at UC Berkeley and one of the lead authors of the study. "Using the model, we can learn which components of this VAZ cycle are important in photoprotection and how the system can respond more quickly after previous exposures."
Emerging from these results was the insight that the three-part nature of the VAZ cycle is key to the alga's photoprotective memory. When a bright light turns on, V converts to A which converts to Z, the molecule best able to dissipate the extra light energy. When the light turns off, Z converts back to A, which eventually converts back to V. However, the various conversions happen at different rates. When the bright light turns off, the conversion from A back to V happens much slower than Z to A, so if the light suddenly comes back on, the system can reverse direction and quickly replenish the concentration of Z instead. These differences in conversion rates act as a buffer, slowing down crucial steps and allowing the system to quickly change direction if needed.
"A plant doesn't know what kind of light conditions to expect at any moment, so this memory means that during periods of low light, when they're operating most efficiently, they're still primed and ready for protection if another high light, high stress event occurs," said Audrey Short, a UC Berkeley graduate student in Berkeley Lab's Biosciences Area and co-lead author of the study.
"People imagine plants and algae are like green solar cells passively absorbing the sun but that's absolutely not true," said Fleming. "They are reacting all the time, constantly adjusting to what's going on in their environment."
