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

by The Insights

Trichodesmium filaments can coalesce to form an aggregate called a puff

Florida Fish and Wildlife Conservation Commission

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

The bacteria, called Trichodesmiumcan actively unite to form large aggregates in response to changing environmental conditions, or separate, found Ulrike Pfreundt of ETH Zurich in Switzerland and colleagues.

“This behavior may be the key to why Trichodesmium is so abundant and so successful,” says Pfreundt.

Trichodesmium is a group of several species of cyanobacteria. Its members are sometimes called 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, they also turn nitrogen from the atmosphere into chemicals that other photosynthetic organisms can use. They fertilize large areas of the ocean that would otherwise be too low in nutrients for anything to grow, Pfreundt says.

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

Trichodesmium grows in hair-like filaments up to several hundred cells long. Filaments can be found floating individually, but also often in colonies or aggregates, each containing up to several hundred filaments.

These aggregates can measure 1 or 2 millimeters in diameter, making them visible to the naked eye. In some aggregates, called puffs, the filaments radiate from the center like a tassel. In others, called tufts, the filaments run parallel like a strand of hair.

Aggregates have been shown to help Trichodesmium get the iron it needs from the dust particles. But how the aggregates form is a mystery, says Pfreundt. One idea is that the filaments simply stick together if they collide, but that doesn’t explain their organized appearance. Another is that they grow up that way.

Growing up Trichodesmium in the lab to study his genomes, Pfreundt noticed that the appearance of the aggregates could completely change during the day, which made him suspect that an active process was involved. She and her colleagues have now 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 sliding past each other, like two trains that use each other as a track. If this process continues indefinitely, the filaments slide completely past each other, says Pfreundt. Thus, when the bacteria want to remain in aggregates, they continue to reverse directions.

To make the aggregates cluster more tightly, reversals occur more often, maintaining greater overlaps of the filaments, she found. To loosen them, inversions occur less often.

This loosening or tightening of aggregates can occur within minutes in response to changes in light levels, the team found. Very bright light can damage the photosynthetic machinery, and tighter aggregates reduce the light levels each filament is exposed to.

In the ocean it can help Trichodesmium face the sun coming out or passing behind the clouds.

Pfreundt believes this loosening or tightening also helps aggregates control their buoyancy, allowing them to rise or fall as needed. Trichodesmium is known to move deeper to obtain phosphate when this nutrient is depleted at the surface.

“The reversal mechanism of Trichodesmium — causing 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 discovered that, rather than understanding different strains as previously thought, puffs form from the fusion of clumps. But many questions remain unanswered, like how the filaments slide and how they know when to reverse.


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