Trichodesmium: the bacteria that fertilize the oceans work together to adapt to light levels

One of the most abundant and important types of photosynthetic bacteria in the oceans owes its success to teamwork.

The bacterium, called tricodesmiumthey can actively join together to form large aggregates in response to changing environmental conditions, or split apart, Ulrike Pfreundt at ETH Zurich in Switzerland and her colleagues have discovered.

“This behavior is possibly the key to why tricodesmium it is so plentiful and so successful,” says Pfreundt.

tricodesmium is a group of several species of cyanobacteria. Its members are sometimes called sea sawdust, as they often form reddish-brown flowers, which may have given the Red Sea its name.

These bacteria not only provide food for other organisms, but also convert nitrogen in the atmosphere into chemicals that other photosynthetic organisms can use. They fertilize vast areas of the ocean that would otherwise be too nutrient-poor for anything to grow, Pfreundt says.

“It’s the living fertilizer for the oceans, essentially,” she says. “They provide a large part of the nitrogen that is fixed in the ocean, and many other organisms that sequester CO2 depend on this nitrogen.”

tricodesmium it grows in hair-like filaments up to several hundred cells long. The filaments can be found floating singly, but they also often occur in colonies or aggregations, each containing up to several hundred filaments.

These aggregates can be 1 or 2 millimeters wide, making them visible to the naked eye. In some aggregates, called puffs, the filaments radiate from the center like a pompom. In others, called tufts, the filaments are parallel like a lock of hair.

Aggregates have been shown to help tricodesmium get the iron you need from the dust particles. But how the aggregates form has long been a mystery, Pfreundt says. One idea is that the filaments simply stick together if they bump into each other, but this doesn’t explain their organized appearance. Another is that they grow this way.

while growing tricodesmium In the lab to study their genomes, Pfreundt noticed that the appearance of the aggregates could change completely during the day, leading her to suspect that it was an active process. She and her colleagues have now run a series of experiments to confirm this and show how it happens.

Filaments can slide along surfaces, and when two filaments come into contact, they can begin to slide past each other, like two trains using each other as tracks. If this process continues indefinitely, the filaments completely separate from each other, Pfreundt says. So when the bacteria want to stay in aggregates, they keep reversing directions.

He found that to make the aggregates stack tighter, inversions occur more frequently, which maintains greater overlaps of the filaments. To loosen them, reversals occur less frequently.

This loosening or hardening of the aggregates can occur in just minutes in response to changes in light levels, the team discovered. Very bright light can damage the photosynthetic machinery, and more compact aggregations reduce the light levels each filament is exposed to.

In the ocean, this can help tricodesmium face the sunrise or go behind the clouds.

Pfreundt believes that this loosening or tightening also helps aggregates control their buoyancy, allowing them to move up or down as needed. tricodesmium it is known to move deeper for phosphate when this nutrient is depleted at the surface.

“The investment mechanism of tricodesmium – causing the aggregates to loosen or tighten to affect their density, buoyancy and light acquisition – may well have contributed to the success of the species,” says Richard Kirby, independent scientist and author studying plankton.

Pfreundt and colleagues also found that instead of comprising different strains as previously thought, puffs form from the fusion of tufts. But many unanswered questions remain, such as how the filaments slip and how they know when to back off.