Vital underwater seagrass meadows are under threat from human activity and the climate crisis - but a new study has found a crystallised version of human waste could be the key to revitalising these important marine habitats.

The marine flowering plants, located in shallow, sheltered coastal areas across the world, are critical to marine ecosystems and the wider environment. Sometimes referred to as the lungs of the sea, seagrasses absorb carbon from the water, generate oxygen, and store the carbon in their leaves and roots. In addition seagrass meadows provide shelter, food and protection for an immense diversity of marine life.

Over a quarter of global seagrass has been lost in the past century because of damage from boats and fishing, decreasing light and water quality from pollution, and water temperature increases due to climate change. To add to the challenges, current methods of seagrass restoration are relatively expensive and can be unsuccessful.

In a bid to help protect these precious plants, scientists from the University of Florida (UF) have turned to a rather unexpected source - wastewater.

The first thing to note is that some chemicals found in wastewater are also found in plant fertilisers: namely phosphorus and nitrogen. Many wastewater treatment facilities around the world already process these resources to create byproducts that would otherwise be sent to the landfill. In its crystallised form, this byproduct is called struvite.

A build-up of struvite scale in a wastewater treatment system can clog pipes, pumps and equipment. However, the scientists from UF found there may be another use for this byproduct as applying the struvite to sickly seagrass was shown to have a positive effect.

“Struvite occurs during the wastewater treatment process because magnesium, ammonia and phosphate are all readily available to form the crystal byproduct,” said Conor MacDonnell, who carried out the study as a PhD. student in the UF department of soil, water, and ecosystem sciences. “The result is a relatively insoluble, sustainable compound found in wastewater treatment plants.”

MacDonnell decided to team up with Gdańsk University of Technology student Franciszek Bydalek and UF faculty members Patrick Inglett and Todd Osborne to investigate whether struvite could be used to fertilise seagrass.

In the study, scientists grew three types of plots of seagrass in a simulated setting at the UF Whitney Laboratory for Marine Bioscience. One type received an application of struvite, another received a common controlled-release fertiliser, and the last received no fertiliser. Two experiments were conducted, testing different dosages of fertiliser.

“From the two experiments, we found struvite performed better than the controlled-release fertiliser in seagrass growth,” said MacDonnell. “Struvite seems to provide a slower, more consistent release of nutrients to the seagrass.”

The advantages of using struvite in these efforts, extend into environmental sustainability, researchers say.

Recovering the struvite from wastewater treatment plants could be a way to harness these useful nutrients for plant growth, rather than allowing them to enter the landfill and results in a more useful biosolids product, which in turn makes wastewater treatment process more sustainable.

Sourcing struvite from the wastewater treatment process also adds to its sustainability credentials over more traditional fertilisers. Mining phosphorus, for example, depletes that finite natural resource and degrades the land.