eNews

#05 2021

The search for a hero plant in the fight against metal toxicity in the estuarine environment

By Marelé Nel, Janine Adams, Gletwyn Rubidge and Lucienne Human, SAEON Elwandle Node

Erosion control, water quality maintenance via assimilation or filtration, carbon sequestration, storm protection and flood control – these are some of the important ecosystem services provided by wetlands such as salt marsh and seagrass beds.

Wetland plants in estuaries occur in environments prone to high enrichment loads. They adapted to these environments by various tolerance strategies such as accumulation in roots, away from sensitive photosynthetic structures.

Metal pollution 

Metal pollution has come to the attention of scientists around the world after people have been releasing pollutants into estuaries for years, expecting these pollutants to be washed out to sea. However, metals stay and accumulate in the sediment. High levels of metal pollution can result in the collapse of entire ecosystems, as the consequences of metal toxicity increase higher up the food chain (from plants to herbivores to carnivores).

The Swartkops Estuary presented the perfect location for Elwandle Node scientists and students to study how three plant species accumulate metals.

From left: The study species Salicornia tegetaria (Glasswort), Spartina maritima (Cordgrass) and Zostera capensis (Eelgrass) as they occur in the intertidal area.

Plants already adapted to high pollution loads can be used to stave off the toxic consequences of metals. The Swartkops Estuary in the Eastern Cape province of South Africa is a heavily developed, predominantly open urban estuary which is home to the third largest salt marsh in the country. It presented the perfect location for Elwandle Node scientists and students to study how three plant species – Glasswort (Salicornia tegetaria), Cordgrass (Spartina maritima) and Eelgrass (Zostera capensis) – accumulate metals.

The bioconcentration and translocation factors (Figure 1) were measured by analysing the metal concentrations in the roots, shoots, leaves and sediment of the three plants via TXRF (total X-ray fluorescence) spectroscopy. The ratio of the sediment and roots indicates the bioconcentration factor (BCF), that is, the concentrations of metals taken up and stored in the roots.

The translocation factor (TF) is a ratio between metal concentrations in two of the plant organs and indicates translocation from one organ to another. Higher values indicate more bioconcentration or translocation respectively.

Figure 1. (a) Relative translocation of Fe from roots to leaves, (b) shoots to leaves, (c) root to shoots and (d) sediment to roots.

Marelé Nel used the total X-ray fluorescence (TXRF) in the Elwandle Node’s laboratory for the analyses of metal concentrations in the roots, shoots, leaves and sediment of the three plants.

Findings 

The researchers found that Glasswort and Cordgrass predominantly accumulated metals in their roots, while Eelgrass accumulated large amounts of metal in its roots and leaves.

Glasswort is not only an excellent accumulator of metals, but it is also the best phytostabiliser of the three plants as it has a healthy root system that is replaced more slowly than that of Cordgrass. This means that the metals are unavailable for other organisms like crabs and fish to take up, and for a longer period of time, “stabilising” the metals in the area.

Due to its large biomass and developed stem structures which are advantageous to metal accumulation, Cordgrass is renowned throughout the world as an accumulator and phytostabiliser. Although Eelgrass is also an excellent accumulator, this plant occurs in the subtidal area and is often washed away or dies off in large sections due to environmental changes, which releases these metals back to the rest of the food chain.

All three of these plants displayed different ways to contribute to phytoremediation, that is, using plants to reduce the concentrations or toxic effects of contaminants in the environment.

A long-term monitoring approach to metal contamination in the Swartkops Estuary is crucial in keeping the system healthy. Conserving the wetlands in the estuary is important to realise this goal.

The Swartkops Estuary in the Eastern Cape province of South Africa is a heavily developed, predominantly open urban estuary which is home to the third largest salt marsh in the country.