Study finds diatoms with surprising appetites


image: Diatoms grow on pebbles in a pond. They can be seen as brownish biofilms. VS
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Credit: Stief et al/University of Southern Denmark

Diatoms – single-celled organisms – have long been known to science, but their overwhelming appetite for nitrate is a new discovery made in a global survey.

Nitrate is a nutrient widely used as a fertilizer in agriculture, and today nitrate concentrations in Earth’s aquatic systems are much higher than in pre-industrial times.

– There is too much nitrate in rivers, fjords and coastal ecosystems today. What’s happening to him ? Will it still accumulate? Where will it be consumed – and by whom? Our new study helps answer these questions, said Peter Stief, associate professor in the Department of Biology at the University of Southern Denmark.

He led the research team behind the new study, which was Posted in Earth & Environment Communications. Researchers from Denmark, Germany, Spain, USA, Sweden, Jordan and Oman contributed.

– Our research reveals that diatoms in all types of aquatic ecosystems have a previously unknown appetite for nitrates. In fact, they’re extremely efficient at storing it in their cells, he explained.

The team studied diatoms collected from different aquatic habitats in different parts of the world. Stief wasn’t surprised to find them in sediment and water column samples, but he was very surprised to find so many of them congregating, side by side, on rocks in rivers and streams. ponds, where they form slimy, greenish/brownish layers called biofilms. .

A cell filled with nitrogen

In fact, he was so surprised that he threw away the first of those samples, believing there had been a mistake.

– After a while, I decided to look at these samples under a microscope, and then I saw the many different diatom cells. So we now know that a very large number of nitrate-storing diatoms also live on the surface of stones in aquatic environments.

Research has also revealed that diatoms living on the surface of stones are extremely efficient at absorbing and storing nitrate inside their cells. When you look at a diatom biofilm on a stone surface, 95% of the nitrate in that biofilm will be inside the diatom cells. For comparison, only 1% of the nitrate in a water sample will be inside diatom cells; the rest circulates freely in the water.

Thus, recently discovered diatom biofilms on stone surfaces are very effective at storing nitrate. But what’s going on with it?

– Nitrate reservoirs make diatoms independent of their environment and also more competitive with microbes that cannot store nitrate. They can save it for bad times and they end up doing mainly two things with it: Either they use it as a nutrient to grow, or – and this is very cool and very unusual in the microalgae world – they use it to replacing the oxygen, explained Peter Stief.

To date, diatoms are the only group of microalgae known to use nitrate for respiration in the absence of oxygen, which is a relatively widespread form of (anaerobic) metabolism in the bacterial world.

Why replace oxygen?

When should a diatom replace oxygen? In deep sediments and at night.

To protect themselves against (tidal) currents, diatoms migrate to deeper layers of sediment which are usually lacking in oxygen. At night, photosynthetic diatoms cannot produce oxygen and their habitat can become anoxic.

Breathing on nitrate allows them to live and function in the depths and at night.

What happens to nitrate?

Do diatoms help remove nitrates from the environment? Yes. And no.

Like many other microbial and algal life forms, they feed on nitrate as a nutrient. But there’s also a byproduct when they use nitrate for respiration, and that byproduct is ammonia.

Ammonia is a nutrient, similar to nitrate, so the short version is that diatoms eat one nutrient and produce another.

The cycles of the major elements on Earth

Nitrate is part of the global nitrogen cycle, one of the great cycles of the earth’s elements – the others are the carbon cycle and the sulfur cycle.

– With the discovery of these new habitats for nitrate-breathing diatoms, we can now add a chapter to understanding the nitrogen cycle. The study of the nitrogen cycle is important to possibly predict the effect of nitrate pollution on aquatic life in general and on diatoms as key players in climate regulation in particular.

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