Study discovers that 'marine snow' keeps the ocean alive

It can be challenging to spot the everyday forces that keep our ocean in harmony. We often think of sunlight, waves, and tides as the main players in our seas, but there is more to the story.

A new study shows that marine snow (tiny bits of drifting debris) acts as a hotspot for bacteria that help maintain key processes in the water.

These unexpected helpers turn inert nitrogen gas into a form that organisms can use, a process called nitrogen fixation.

Subhendu Chakraborty led this study at the Leibniz Centre for Tropical Marine Research in Germany.

Hidden pockets of marine life

Ocean waters might appear uniform at a glance. In reality, floating clumps of matter create unique zones where microbes gather.

This flotsam offers heterotrophic bacteria the right mix of low-oxygen centers and enough organic bits to keep them going.

Researchers tapped into mathematical models to find out how these bacteria manage to fix nitrogen across different latitudes.

By simulating conditions from warmer tropical regions to colder areas, they noted that these bacterial communities can function in waters often thought to be beyond their comfort zone.

Why nitrogen matters to bacteria

Every living thing needs nitrogen to build proteins and other vital molecules. In many parts of the ocean, dissolved nitrogen gas is abundant, but most organisms cannot use it in that form.

Some ocean cyanobacteria can work this gas into a usable state in sunlit waters.

Heterotrophic bacteria on particles go further by handling this job in deeper zones. They also cover a range of temperatures and oxygen levels, adding to the ocean’s overall stability.

The scientists estimated that these particle-attached microbes contribute about 10% of total nitrogen fixation in the seas.

Shaping bacterial nitrogen fixation

“It has been almost five years since we started this work when I was a postdoc at the University of Copenhagen. But it was definitely worth the effort, since the results are quite a breakthrough,” explained Chakraborty.

The discovery clears up questions about the reach of these microscopic workers.

“The magnitude of the N2 fixation and the distinct distribution of the particle-associated activity relative to what is known for cyanobacteria are highly interesting,” said Professor Lasse Riemann, co-author of the study.

This finding suggests a shift in how we understand productivity in ocean depths, as these bacteria are a steady support rather than mere bit players.

Temperature plays a key role in how efficiently these bacteria can fix nitrogen. The researchers discovered that fixation rates peak around 63°F (17°C) and slow down when waters get either too cold or too warm.

At lower temperatures, bacterial respiration slows down too much to remove oxygen from inside the particle, blocking nitrogen fixation.

At higher temperatures, the bacteria break down organic materials too quickly, which causes a mismatch. They can’t absorb the nutrients fast enough before they diffuse away. This leads to energy shortages that shut down the fixation process, even when other conditions are favorable.

Paths through shifting waters

Global warming raises concerns about changes in ocean chemistry. Warmer surface waters can affect where nutrients move and may reduce mixing in some areas.

When that happens, deeper parts of the ocean could see a drop in organic matter.

Despite those challenges, results hint that these bacterial fixers might adapt well, even in cooler waters.

That is partly because anoxic zones form in the core of large particles, giving bacteria a safe place to fix nitrogen. Some ocean regions also have extended layers of low oxygen, which can broaden their habitat.

Wider connections to marine systems

Marine biology is all about balance. Phytoplankton often rely on fixed nitrogen, thriving when conditions are right. Fish and other sea life benefit from that growth.

By shoring up nitrogen supplies from deeper waters, these particle-based bacteria bridge two layers of the food web.

These findings may encourage researchers to look more closely at a variety of ocean particles, including those near oxygen minimum zones (OMZs).

Learning more about how these bacterial groups live can offer guidance for long-term predictions and climate models.

How oxygen minimum zones boost bacterial activity

OMZs create ideal conditions for particle-associated bacteria. These deep stretches of water, where oxygen is nearly gone, make it easier for bacteria to carry out nitrogen fixation without spending extra energy to shield their enzymes from oxygen exposure.

The study found that some of the highest nitrogen fixation rates occur within these OMZs, especially in regions like the eastern tropical Pacific and Arabian Sea.

These zones extend over large depths, allowing for more time and space for bacteria to thrive on sinking particles rich in organic material.

Implications for future studies

Ocean dynamics are always in flux. Each year, new data shapes our approach to climate science and marine ecology.

The recent focus on deeper-living bacteria opens fresh angles for exploring how carbon is locked away in ocean depths.

Investigations may also delve into the interplay between these heterotrophic communities and better-known nitrogen fixers.

While cyanobacteria still rule the sunlit zones, the synergy with these newly highlighted partners underscores the intricate relationships we are only beginning to understand.

The study is published in Science Advances.

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