After learning how fish are dealing with high salt concentrations (and especially with changing salt concentrations), I would like to continue with a similar topic this week – but in the plant kingdom. How do plants filter salt out of the water? In particular, I will focus on mangroves – plants which are actually growing in sea water. They have remarkable capabilities to survive in such harsh conditions and researchers implemented their ‚formula for success‘ already into technology. But let’s start at the beginning:
What are mangroves?
Mangroves are woody plants which are growing at the land-sea-interface in the tropics and subtropics . This means their habitat is salty or highly brackish water which is often also subject to tidal changes . As the term ‚mangroves‘ has been used to describe the plants themselves as well as the ecosystem they form, the term ‚mangal‘ is suggested alternatively to describe the ecosystem of a mangrove forest community and avoid ambiguity . Mangroves compromise many different plant species with diverse morphology. Important genera of the mangroves are for example Avicennia and Rhizophora. Both exhibit extraordinary root morphologies, demonstrating adaptions to major challenges mangroves are facing. They live on muddy and anaerobic substrate, which is often also very unstable. That is why they are in need for specialized roots for gas exchange and support. Avicennia for example has pneumatophores (roots which are directed upwards) for passive oxygen diffusion and Rizophora possesses stilt roots (aerial roots, often growing in lateral direction) . Nevertheless, besides the gas exchange and structural support, it is the filtration of salt from the surrounding water, which I will elaborate further now.
How do mangroves manage salt?
The salt balance in mangroves has been of interest for botanists since way back. Salt accumulation on the leaves of mangroves has been detected in several species long ago, leading to the early assumption that mangroves can handle taking up water with high osmotic potential and excrete salt via specialized glands . Scholander et al. studied this topic extensively in the 60ths of last century. They concluded that the separation of freshwater from seawater in mangroves cannot be ’simply‘ due to ultrafiltration processes because the sap pressure (sap is the fluid transported in xylem and phloem) is supposedly not low enough for that . That would mean that the separation of salt ions from water cannot be happening via hydraulic pressure and osmotic forces along a membrane and Scholander et al. suggested that the separation most likely involves active transport .
Nowadays, with modern technology like cutting-edge visualization and measuring equipment, the secret of the salt management is getting lot clearer. Scientists from Korea have found out that Na+ ions are filtered at the tip of mangrove roots, which has three layers . The actual filtration however is mainly happening at the first – most outer – layer and is mainly due to the high surface zeta potential of its membrane . Here is how it works: through surface charge effects, Cl- ions are repelled from the first layer (because it is also highly negatively charged) . Na+ ions on the other hand accumulate here. They have an opposite charge and therefore, the membrane ‚attracts‘ them. Their accumulation at the outermost layer was even visualized via a Na+ specific fluorescent dye and can be nicely observed in a microscope . Via these processes, salt is retained to enter the root tip and water can be ’sucked‘ in via the hydraulic pressure gradient . Still, the researchers could not completely exclude the involvement of active transporters – as hypothesized by Scholander. Maybe, there are still some secrets behind the mangrove desalination process, we are not yet understanding!
A bio-inspired desalination membrane
What I personally like the most in the study of Kim et al. is that they recognized the biomimetic potential of their finding and conducted a feasibility study under in vitro conditions right away . With the aim to determine the actual desalination rate of the outermost root layer, it was dissected from the plant and inserted as a membrane in the experimental setup . A sodium solution was pumped through the membrane with a syringe and the amount of sodium in the filtrate was determined . Via this simple experiment, the researchers could nicely confirm their findings via showing that the isolated membrane was capable to filtrate 62% of the sodium ions from the supplied water . In a follow-up study, published in the same year, the first author already presents the biomimetic implementation of the mangrove desalination procedure . An artificial – bio-inspired – membrane, with a negatively charged potential, which is able to filter sodium ions from a salty solution via repelling co-ions and ‚holding back‘ the counter ions due to electroneutrality .
An amazingly effective desalination process – inspired by mangroves!
- Kathiresan K, Bingham BL (2001) Biology of Mangroves and Mangrove Ecosystems. Advances in Marine Biology 40:81-251
- Scholander PF, Hammel HT, Hemmingsen E, Garey W (1962) Salt Balance in Mangroves. Plant Physiology 37(6): 722-729
- Kim K, Seo E, Chang SK, Park TJ, Lee SJ (2016) Novel water filtration of saline water in the outermost layer of mangrove roots. Scientific Reports 6:20426. doi: 10.1038/srep20426
- Kim K, Kim H, Lim JH, Lee SJ (2016) Development of a desalinations membrane bioinspired by mangrove roots for spontaneous filtration of sodium ions. ACS Nano 10,11428-11433. doi: 10.1021/ascnano.6b07001