The microbes clinging to each grain of sand explain why sand plays such a key role in the ocean’s nitrogen cycle. As more and more nitrogen pours into the world’s oceans, understanding how microbes remove it - and how much they can process - is crucial, says a researcher.
Nitrogen runoff from agriculture is a major environmental challenge. When too much nitrogen is released, algae bloom rapidly. When the algae die, bacteria consume large quantities of oxygen to break them down – which can lead to oxygen depletion.
Nitrogen pollution therefore leads to oxygen depletion in bodies of water including lakes and marine areas.
Now, researchers have pinpointed how microbial life on the surface of seabed sand grains helps to strip nitrogen from the oceans.
This discovery could be key to predicting how marine life will cope with rising pollution – and what this means for the fish on our plates, biodiversity and the health of the oceans.
“As more and more nitrogen pours into the world’s oceans, it harms nature — and if we want to gauge the scale of the problem, we need to know not just how much goes in but how and how much is removed,” says a researcher behind the study, Soeren Ahmerkamp from the Leibniz Institute for Baltic Sea Research Warnemünde, Rostock, Germany.
The research and associated method development, conducted with Klaus Koren of Aarhus University, Denmark, Lars Behrendt of the Technical University of Denmark, Kongens Lyngby, and Farooq Jalaluddin of the Max Planck Institute for Marine Biology, Bremen, Germany, has been published in Scientific Reports.
Discovery solves mystery of life in sand
It has long been known that microbes can remove nitrogen from aquatic environments.
They do this through denitrification – turning nitrates into nitrogen gas that escapes into the atmosphere.
Previous research suggested that denitrification occurs in oxygenated sandy seabeds – a finding that has puzzled researchers because denitrification normally happens in oxygen-poor environments.
There has to be some mechanism enabling denitrification in sandy seabeds – and this is exactly what the researchers have now identified.
New method reveals hidden life on the surface of sand grains
Before this study, the researchers developed a method to study microbes and their oxygen use at the microscopic scale.
The method enables scientists to study oxygen turnover on the surface of individual sand grains – which was not possible previously.
The method measures oxygen and shows the microenvironment that microbes experience. The researchers used nanoparticles to detect oxygen and a specialised camera to track changes right on the sand grain’s surface.
The researchers used the method to study the oxygen conditions in the environment surrounding individual microbes, and they can now also study the variation in the distribution of oxygen and microbial life on the surface of individual grains of sand.
“This all stems from the aim of better studying and understanding microscopic environments. Oxygen availability often plays a major role here,” says Soeren Ahmerkamp.
Sand grains contain two worlds – with and without oxygen
Using this method, the researchers showed that oxygen levels vary greatly across the surface of individual sand grains – and so does microbial life.
Between 10,000 and 100,000 bacteria can live on a single grain of sand – and they are far from identical.
As individual bacteria consume the oxygen around them, they create tiny oxygen-poor pockets – for example, in depressions and cracks in the grains.
Other bacteria thrive in these crevices – including denitrifiers, which use nitrates in the absence of oxygen and convert it into nitrogen gas.
Sand grains account for one third of nitrogen removal in the ocean
Soeren Ahmerkamp says the team’s calculations show that denitrifying bacteria on sand grains may account for about one third of all denitrification in the seabed.
The seabed spans an enormous area, much of it covered by sand – meaning that these microscopic processes on sand grains play a major role in the planet’s nitrogen cycle.
About half of the shallow seabed is sandy – precisely where marine life is very active and biodiversity is rich.
“The microbes living on sand grains are doing us a favour. In earlier studies, we and other researchers showed that most nitrogen removal happens around the continental shelves – and the process we describe here makes up about one third of that,” says Soeren Ahmerkamp.
He adds that the findings could deepen understanding of the global nitrogen cycle as a whole.
The results can also feed into mathematical models that predict how the nitrogen balance may change in the future.
“As more nitrogen flows into the world’s oceans, we need to know not just where it ends up but whether we are nearing a tipping point at which the ocean can no longer cleanse itself – putting marine life at lasting risk,” says Soeren Ahmerkamp.
