The World Health Organisation (WHO) estimates that 1.6 million people die each year from gastrointestinal diseases caused by a lack of clean water (1). The biggest threats from unclean water are infectious diseases caused by bacteria (e.g. cholera), viruses (e.g. hepatitis), protozoa (e.g. giardiasis) or parasites (e.g.schistosomiasis). In developed countries we often take clean water for granted, relying on various technologies including chlorination, boiling, pasteurisation, UV-disinfection or filtration to make our water safe. However, in less developed countries, especially in small rural communities, these technologies are often too expensive or impractical. In a paper published recently in PLOS ONE, the authors explore the use of plant xylem vessels, which are found in the stems of plants, as inexpensive water filters.
Xylem vessels are tubes that carry water (and some nutrients) from the roots of plants to the stems (Figure 1b). The vessels themselves are made up of cells that die as they mature leaving a strengthened cell wall with pores called pits in their sides (Figure 1d). Groups of parallel vessels are arranged together so that water flows between the parallel tubes through the pits (Figure 1a). The size of the pits of xylem vessels vary from a few nanometers to hundreds of nanometers, just the right kind of size to potentially filter out disease-causing microbes.
The xylem vessal cells of conifers are smaller in diameter and shorter than those of flowering plants. Although this makes them less efficient at transporting sap, it also makes conifer stems more ideal for filtration because the water is forced to pass through the pits more often (more filter points) so a shorter length of stem is needed. Also, xylem vessels make up a greater cross-section of a conifer stem, which is useful for a filter material. The authors tested the filtering capability of short lengths of pine stem. They found that several litres of water could flow through 3 cm3 of pine stem per day (1). This is enough to meet the clean drinking water needs of one person.
The xylem filters were efficient at filtering out bacteria. Figure 2 shows the concentration of fluorescently labelled E.coli in the feed (unfiltered) and fitrate solutions. Using a haemocytometer to count the bacteria the authors estimate that at least 99.9% of the bacteria were trapped by the filter (1).
The pits in the xylem of the pine stem are too large to filter out viruses from the water but it is possible that stems from other plant species with smaller xylem pits, such as deciduous trees might be more suitable for this. Since all that is required to make the filter is a tube, a suitably sized plant stem, and a way of sealing the connection, xylem water filters could be cheaply made and maintained by individuals. Therefore, plant stems have the potential to be used as inexpensive, small-scale water filters.