In northern California, rivers feed people, wildlife, and culture. But what fuels these river waters? Scientists found that a partnership between microscopic algae and specialized bacteria feeds the food web from the ground up. It is not fertilizer, not runoff, but life itself that is keeping rivers alive.
Along the Eel river, this unexpected algae partnership benefits the health of the ecosystem and supports the salmon populations.
Salmon are at the center of it all – they are food, heritage and a sign of healthy waters. When salmon run strong, rivers thrive. When they struggle, entire communities feel the loss.
Algae and bacteria work together
The study shows how tiny algae and bacteria work as one. Together they create a clean nitrogen source that acts as a natural engine, powering the food chain without pollution.
The key player is a diatom named Epithemia, that carries inside it bacterial partners called diazoplasts.
“This is nature’s version of a clean nutrient pipeline, from sunlight to fish, without the runoff that creates harmful algal blooms,” said Jane Marks, biology professor at Northern Arizona University, and lead author of the study.
How Epithemia works
The diatom captures sunlight and turns it into sugar. The diazoplast eats that sugar and uses the energy to pull nitrogen from the air. In return, it hands nitrogen back to the diatom.
The diatom then keeps making more sugar. The loop is tight, efficient, and endlessly productive.
“It’s like a handshake deal: both sides benefit, and the entire river thrives,” said Mike Zampini, a postdoctoral researcher at NAU and isotope tracing lead.
Salmon thrive on Epithemia
In spring, Cladophora algae grow long and green, using runoff nutrients. By summer, nutrients run low. Epithemia takes over.
It covers Cladophora until the mats turn rusty red. At this stage, almost 90 percent of new nitrogen in the food web comes from Epithemia.
“Healthy rivers don’t just happen – they’re maintained by ecological interactions, like this partnership,” said Mary Power, co-author of the study and faculty director of UC Berkeley’s Angelo Coast Range Reserve.
Insects such as caddisflies feed on the mats. Young salmon eat the insects. A chain begins with invisible microbes and ends with one of the most important fish in the region.
Algae feed river insects
Caddisflies grazing on algae thick with Epithemia gain far more energy than those feeding on plain green algae. They take in ten times more carbon. Nitrogen transfer reaches over half of what the diatoms fix.
Insects also prefer diatoms because they are easier to digest. Their thin silica shells crack open more easily than the walls of green algae.
Diatoms are richer too. They carry fatty acids, carotenoids, and even thiamine. This last nutrient matters for salmon.
Without enough thiamine, hatchlings often fail. By supplying insects with thiamine-rich cells, Epithemia helps salmon indirectly, but crucially.
Changing roles in summer
Cladophora begins the season as the main producer. But as Epithemia spreads, the green algae fade. They stop making much food themselves. Instead, they serve as scaffolding, a support structure for diatom growth.
Even in this weakened state, Cladophora matters. Its filaments stretch across rocks, providing space for billions of microbes to attach.
This switch shows how ecosystems adapt. One species starts the chain. Another takes over when conditions change. Together they keep the food web running, even during lean months.
Epithemia dominate healthy rivers
Not all microbes fix nitrogen equally. Many free-living bacteria can do it, but diazoplasts dominate in the Eel River. By late summer, they fix nearly 90 percent of all new nitrogen.
Their close tie with Epithemia gives diazoplasts the edge. While free-living bacteria must split their effort between different jobs, diazoplasts specialize in one: fixing nitrogen.
That specialization makes them fast, efficient, and critical to the ecosystem. Their size also matters. Even when they are few, they make up most of the biomass of nitrogen fixers.
Fuel for nitrogen production
This is not a small-scale effect. Field surveys show that mats of red Cladophora cover nearly a quarter of some streambeds. Despite covering less ground than other algae, they produce over half the nitrogen in the system.
When scaled across the river network, the contribution is huge. Scientists estimate that about 90 percent of midsummer nitrogen comes from Epithemia and its diazoplasts.
That rivals the productivity of massive cyanobacteria blooms seen elsewhere. Yet here, the same force is carried by a diatom the size of a hair’s width.
Algae partnerships are found worldwide
Diatom-bacteria partnerships like this are not unique to the Eel River. They appear in lakes, rivers, and oceans around the world, especially in places where nitrogen is scarce. They may quietly boost food webs far more than scientists realized.
This has human implications too. If crops or biofuels could mimic the same process, farmers might cut costs and reduce pollution. Instead of spreading chemical fertilizers, plants could feed themselves using microbial help.
“When nature engineers solutions this elegant, it reminds us what’s possible when people, places and discovery come together,” said Professor Marks.
Epithemia and future salmon survival
The lesson is clear. Salmon survival depends on far more than the obvious. It rests on unseen alliances between algae and bacteria, locked in a trade of sugar and nitrogen.
By following these links, we see how even the smallest Epithemia cells can shape an entire river ecosystem.
Using stable isotope tracing and advanced imaging, the researchers followed carbon and nitrogen from single cells to entire watersheds.
They showed how invisible interactions scale up into forces that sustain fish, rivers, and even human culture.
The study is published in the journal Proceedings of the National Academy of Sciences.
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