Trichodesmium: Fertilizer bacteria from the oceans work together to adapt to light levels

Trichodesmium filaments can fuse together to form an aggregate called a puff

Florida Fish and Wildlife Conservation Commission

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

The bacteria, called trichodesmiumthey can actively join together to form large aggregates in response to changing environmental conditions, or break apart, found Ulrike Pfreundt of ETH Zurich in Switzerland and her colleagues.

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

trichodesmium 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 its name to the Red Sea.

These bacteria not only provide food for other organisms, but they also turn atmospheric nitrogen 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 very large part of the nitrogen that is fixed in the ocean, and many other organisms that sequester CO2 depend on that nitrogen.”

trichodesmium 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 aggregates, each containing up to several hundred filaments.

These aggregates can be 1 or 2 millimeters in diameter, making them visible to the naked eye. In some aggregates, called puffs, the filaments radiate out 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 trichodesmium get the iron you need from dust particles. But how the aggregates form has been a mystery, says Pfreundt. One idea is that the filaments only stick together if they bump into each other, but that doesn’t explain their neat appearance. Another is that they grow up that way.

while growing up trichodesmium in the lab to study their genomes, Pfreundt noticed that the appearance of the aggregates could change completely during the day, making her suspect that an active process was involved. She and her colleagues have already done 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 start to slide along each other, like two trains using each other as a track. If this process continues indefinitely, the filaments completely slide over each other, says Pfreundt. So when bacteria want to stay in clusters, they keep reversing directions.

To make the aggregates more compact, inversions happen more often, keeping the filaments overlapping more, she found. To loosen them, reversals happen less often.

This loosening or tightening of aggregates can happen within just a few minutes in response to changing light levels, the team found. Very bright light can damage the photosynthetic machinery, and more compact aggregates reduce the light levels to which each filament is exposed.

In the ocean it might help trichodesmium deal with the sun coming out or going behind the clouds.

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

“The reversal mechanism of trichodesmium — causing the aggregates to loosen or tighten to affect their density, buoyancy, and light acquisition — may well have contributed to the species’ success,” says Richard Kirby, an independent scientist and author who studies plankton.

Pfreundt and his colleagues also found that, rather than comprising different strains as previously thought, puffs form from the fusion of tufts. But many questions remain unanswered, such as how the filaments slide and how they know when to reverse.


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