Cyanobacteria’s built-in sunscreen offers clues for resilient crops

Cyanobacteria are some of the oldest and toughest lifeforms on Earth. They’ve been around for over 2 billion years and live just about everywhere – inside Arctic ice, near underwater volcanoes, and even in health food powders.

These photosynthetic bacteria may be tiny, but they play a huge role in sustaining life on Earth by producing much of the planet’s oxygen.

But how do these microscopic organisms manage to survive in such wildly different environments, especially when the sunlight they rely on can also damage them?

The answer lies in a microscopic structure inside them called a phycobilisome. It works kind of like a solar panel – pulling in light and turning it into energy.

Unlike the solar panels we use on rooftops, phycobilisomes can protect themselves when the sunlight gets too intense.

Cyanobacteria don’t burn in the sun

Phycobilisomes aren’t just light collectors. They also include what’s essentially a built-in sunscreen.

When the sun’s rays get too strong, the structure changes to block some of the energy. A tiny protein steps in to help with this protection – an accessory that senses the change and switches on defense mode.

This protein, known as the orange carotenoid protein, kicks in when light levels spike. It attaches to the phycobilisome and helps “quench” the energy. That means it diffuses or bleeds off the extra sunlight before it can damage the cell.

Study co-author Allison Squires is an assistant professor at the University of Chicago Pritzker School of Molecular Engineering.

“Too much energy can damage the photosynthetic machinery, so having this protein provides a quick way to protect the cyanobacteria from a sudden change in light,” explained Professor Squires.

What wasn’t clear until now was exactly how this protein pulls off that trick. Where does it bind? Why that specific spot? And does it work the same in different versions of the phycobilisome?

The mystery of the binding site

Researchers in the Kerfeld Lab at Michigan State University had reported a surprisingly specific binding site for this protein – but that puzzled Professor Squires.

“There are tons of places where it could bind that look just like the site that our collaborators identified, and phycobilisomes have many different architectures,” she said.

“So why did it bind at this one site and not other sites? And what happens in other architectures where this specific site is blocked?”

An adaptable molecular mechanism

To investigate, Professor Squires and her team used a special technique called single-particle spectroscopy. This allowed them to study individual molecules up close, without having them float away or get lost in noise.

The experts suspended the tiny phycobilisomes in a liquid and used a device called an Anti-Brownian ELectrokinetic (ABEL) trap to hold them steady with electrical fields. That let them track how energy moved around in real time.

They studied two types of phycobilisomes: one with a three-barrel structure and another with five barrels. The orange carotenoid protein did bind in different spots – but still worked just as well at blocking excess energy.

“It’s a really lovely example of an adaptable molecular mechanism, where the protein can easily evolve to do its job under conditions that require different phycobilisome structures,” said Professor Squires.

“Maybe it started out at one binding site, but then as the architecture changed, it could still do its job at a new site.”

Flexible parts, same results

To double-check, the researchers ran computer simulations showing how light particles, or photons, move through the system.

The simulations backed up what the team saw in the lab. Even with different phycobilisome shapes, the orange carotenoid protein kept the energy levels in check. According to Professor Squires, the system balances modularity with site specificity.

That means it can swap out parts and still run just as well – an idea that shows up often in nature. But here, it’s a good example of how living systems can be both flexible and precise at the same time.

“It was very gratifying to see how the precise data obtained from the ABEL trap can be used to gain structural insights into this quenching mechanism,” said Ayesha Ejaz, a recent PhD graduate from UChicago and the study’s lead author.

“I am excited to see what new patterns will emerge once we combine these results with future experiments comparing intrinsic photoprotective mechanisms among phycobilisomes with different structures.”

Cyanobacteria and the future of energy

The orange carotenoid protein isn’t the only part of the phycobilisome with a smart defense system.

Professor Squires and her team suspect there are other built-in mechanisms – like molecular switches or fuses – that also jump in to protect the cell.

Some of these may break or change shape at just the right time to steer energy in new directions, keeping the bacteria safe under changing light conditions. Understanding how these systems work could pay off far beyond basic biology.

If scientists can figure out how cyanobacteria manage light so efficiently, it might inspire hardier crops that thrive under intense sun – or even solar panels that automatically adjust to changing light conditions.

The full study was published in the journal Proceedings of the National Academy of Sciences.

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