Marine sponges filter seawater for both microplastic, live microscopic algae and bacteria. These sponges can therefore probably be used to monitor the marine environment, but researchers needed to determine how the sponges actually work, a task they have now accomplished.
Sponges are among the most ancient animals and have existed for many millions of years. During all this time, they have filtered seawater, extracting algae, bacteria, viruses and dead organic material in the process.
Within the past 50 years, however, microplastic particles have entered the marine environment and have been filtered by sponges in the same way as organic material. This has led researchers to speculate whether these sponges can be used to monitor the level of microplastic in the sea.
New research now shows what happens to the particles filtered by the sponges. As a result, researchers have advanced their knowledge on how the sponges could be used to monitor the sea – and perhaps also biodiversity.
“Sponges have enormous potential in monitoring microplastic and biodiversity, and this study has advanced our knowledge on what happens to the plastic the sponges filter from the marine environment. If we can figure out how to use the sponges in our marine monitoring, we can save much time collecting samples, since the sponges can do this for us, and we just have to analyse what they collect,” explains a researcher behind the study, Peter Funch, Associate Professor, Department of Biology, Aarhus University.
The research has been published in Marine Pollution Bulletin.
Sponges can monitor the marine environment
Sponges can filter the sea around them with incredible efficiency.
For example, sponges can filter water equivalent to six times their body size every minute, and in the process, they can retain up to 100% of the bacteria and 40% of the viruses from the water. The sponges break down these particles and use them as food.
The researchers wanted to discover what happens to microplastic particles that pass through the sponges’ filtering system.
To achieve this, the researchers studied the breadcrumb sponge Halichondria panicea: the sponge that looks like insulating material when it washes up on a beach.
Breadcrumb sponges are very widespread and have a very important role in filtering seawater. They can be up to 60 cm in diameter and 30 cm tall.
“This is why many people have thought about the possibility of using sponges to monitor the marine environment based on the idea that a sample of the microplastic particles in a sponge reflects the volume of microplastic particles in the surrounding sea. Before that is possible, however, we need to know how much microplastic the breadcrumb sponges retain and how much time elapses before the particles passes through them,” says Peter Funch.
Fed microplastic particles to a breadcrumb sponge
The researchers placed breadcrumb sponge cuttings on glass slides to enable the cuttings to grow and filter water and then studied the filtration process under a microscope.
The researchers added both food particles (algae) and two different sizes of microplastic particles (2 and 10 µm) to the water the sponges filtered in the experiments.
Inside the sponges, the smallest microplastic particles were captured by their collar sieves, which the sponges normally use to acquire microscopic food particles. The larger particles were captured in the incurrent canals.
The researchers observed the algae being broken down inside the sponge, while the plastic particles were expelled from the sponge again after some time.
“We were not surprised that the sponges could expel the microplastic particles, but we quantified how long the process takes for different particle sizes,” explains Peter Funch.
Larger particles passed through more slowly
The results show that the breadcrumb sponges take about 30–90 minutes to transport the smallest microplastic particles through their system and out again versus 1–2 hours for the slightly larger particles.
This means that measuring the content of microplastics in a sponge requires considering the difference in transport time and the fact that the concentration of larger microplastic particles will be higher in the sponge than in the seawater, since the sponges retain these particles longer.
“We need to know these things to use the sponges to monitor the sea for the concentration of microplastics. In the future, we also need to investigate how the process works for other microplastic particle sizes and different types of plastic,” says Peter Funch.
He elaborates that knowing more about the process will probably enable the researchers to collect samples of sponges and examine them for the concentration of microplastic particles – thereby enabling them to determine changes in the concentration of microplastic over time.
Since sponges also filter environmental DNA from the surrounding seawater, they can also monitor biodiversity. Analysing the DNA the sponges extract from the sea should enable researchers to determine which organisms have most recently been in the vicinity of the sponges.
“This type of experiment provides more knowledge on how sponges could be useful for biomonitoring pollution and biodiversity,” concludes Peter Funch.