How mangroves filter heavy metals

Indonesian researchers have revealed how grey mangrove trees, Avicennia marina, filter heavy metals out of the surrounding soil and water. Their study has been published in the Pertanika Journal of Tropical Agricultural Science.

Researchers from Universitas Diponegoro analysed copper concentrations in a mangrove forest in Tapak Tuguerjo, an area along the northern coast of Java. The forest is downstream from a river polluted by a nearby factory. Copper concentrations in seawater samples from the study area ranged from 0.02 to 0.05 mg/L — as much as six times the 0.008 mg/L maximum permissible level for marine biota set by the Indonesian Ministry of Environment.

Over the span of 12 weeks, the team collected samples of water, soil, roots, young leaves and leaf litter. After drying and grinding the plant material, they analysed its copper content using atomic absorption spectroscopy.

The researchers found that copper concentrations in the plant material were up to 10 times more than the water samples. Leaf litter carried the highest concentration, followed by live leaves and then roots.

“The relatively high concentration of copper in the A. marina litter is a product of adaptation whereby the plant defends itself against contaminated environments by excreting copper through the leaves, which will then be discarded through defoliation,” the study authors explained. Mangroves are able to do this better than many other plant species due in part to their adaptation to living in coastal zones, where they absorb and eliminate salt in a similar way.

As the leaf litter breaks down, copper can be reintroduced back to the soil and water, the researchers found. They do however suspect that the impact is minimal, as the estimated amount released is less than 3.5% of the total absorbed and is spread over a large area.