Producing safer drinking water using plant xylem

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.

Figure 1. A) Flow of water through xylem vessels of conifers (gymnosperms) and flowering plants (angiosperms). Conifer vessel cells are shorter in length so water passes through more pits (filtration points). B) Cross section of pine stem. D) Scanning electron microscopy (SEM) image of xylem pits (pores). Scale bar is 20 uM. Figure taken from Figure 1 of Ref (1) via PLOS ONE CC BY 4.0

Figure 1. a) Flow of water through xylem vessels of conifers (gymnosperms) and flowering plants (angiosperms). Conifer vessel cells are shorter in length so water passes through more pits (filtration points). b) Cross section of pine stem. d) Scanning electron microscopy (SEM) image of xylem pits (pores). Scale bar is 20 uM. Figure taken from Figure 1 of Ref (1) via PLOS ONE CC BY 4.0

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).

Figure 2. Concentration of fluorescently labelled E. coli in feed water (unfiltered) and filtrate. The inset images show fluorescence images of the feed (before) and filtrate solutions (after). Scale bar is 200 µm.

Figure 2. Concentration of fluorescently labelled E. coli in feed (unfiltered) and filtered solutions. The inset graphs show bacteria concentrations for the filtered solutions on a smaller scale. The inset images show fluorescence images of the feed (before) and filtered solutions (after). Scale bar is 200 µm.

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.

References:

1) Boutilier MSH et al. (2014) Water Filtration Using Plant Xylem. PLoS ONE

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One thought on “Producing safer drinking water using plant xylem

  1. Pingback: Morsels For The Mind – 11/04/2014 › Six Incredible Things Before Breakfast

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