Microbes help to clean up polluted soils

Root from cadmium hyper-accumulating plant, taken by Jen WiltshireRobyn Williams, presenter: 

In places where exploitation has gone too far, vast stretches of land are already poisoned. Two weeks ago, in a cover story, The Economist reported that too much of China’s soils are already unusable for agriculture. So here’s this week’s PhD, Jen Wiltshire from La Trobe University in Bundoora, Victoria, with a bright idea. She uses plants and bacteria to gobble up the toxins.

Jen Wiltshire:

The research I'm doing considers the problem of Heavy Metals as environmental pollutants. These include toxic metals such as: Arsenic, Cadmium, Lead and Mercury as well as trace elements like Zinc, Copper, or Nickel which are required for biological systems but become toxic at high levels.

Heavy metals are problematic for two key reasons; the first is that once heavy metals are in the soil, there is no chemical or biological process that can degrade them. So, current remediation strategies require their physical removal through the excavation and washing of the soil. A method that is both expensive and environmentally destructive.

The second issue is that heavy metals can be highly mobile within the soil. This enables them to readily spread in the water table and move into biological systems where they can then propagate up the food chain and accumulate over time.

Because heavy metals do not degrade, once ingested they build up to toxic levels. In humans this can mean diseases of the kidney, liver, blood, brain, miscarriages and even cancer.

Now, since the industrial revolution anthropological activities, such as agriculture and mining, have enabled our population to expand rapidly. An unfortunate byproduct of these processes has been the accelerated addition of toxic heavy metals to our soil.

Today, we find ourselves in a situation where arable land is fast becoming a limited resource; so we need to rethink how we use our land if we are to sustain population growth into the future. We need to maximize the productivity of the land that we have and begin cleaning up the land that we have polluted.

Without change, heavy metals will continue being added to our soils and unpolluted land will become a thing of the past.

To remove heavy metals from the environment, I am looking at ways to exploit one of the characteristics that make heavy metals so problematic: their ability to move into biological systems.

The process I'm looking at is called ‘phytoremediation' and it utilizes special plants known as Heavy metal hyper-accumulators. What makes these plants special is that, not only can they withstand the toxic effects of heavy metals, but they actively remove metals from the soil and store them in their leaves at concentrations that would be deadly to a normal plant.

No one knows why they do this, but we can take advantage of the process to remove the heavy metals from the soil. All we have to do is harvest the plants, which have extracted the heavy metals, to remove the metals from the environment. We can even recover valuable metals like gold and recycle them – a process known as phytomining.

Presently, the main hurdle to phytoremediation becoming an economically viable technology is that these plants are not efficient enough at extracting the metals from the soil: This is the central problem that my research aims to address.

In my lab we deal in environmental and applied microbial ecology; so rather than focus on how the plant is transporting and storing the heavy metals, I’m investigating how microbes living on and around the roots of these plants could be used to improve metal accumulation by the plant.

Now, root associated microbes function for a plant in much the same way our own gut microbe’s work for us: both root microbes and our gut microbes form an interface between their host and its food source. They govern how molecules are processed before entering the host as well as determining which minerals, nutrients and heavy metals are absorbed. 

I have been exploring the root zone an Australian native hyper-accumulator, from which I have isolated twelve microbes with increased resistance to heavy metals.  Interestingly, as well as the heavy metal resistance, I have found that these microbes exhibit a range of plant growth promoting characteristics: some microbes produce plant growth hormones, like auxin; others block the stress response of the plant so that it grows in spite of the presence of toxic heavy metals; other still solubilize key plant nutrients like phosphate, making them available to the plant.

I'm investigating to what extent microbial plant growth promotion could assist in improving the metal accumulation done by the hyper-accumulator. I have been establishing methods to detect these microbial plant growth promoting activities so I can then determine the contribution of each process to the plants metal uptake ability. Essentially, I aim to identify microbial processes that could significantly improve phytoremediation as a technology.

Ultimately the question I wish to address is: ‘Could the right combination of microbes make phytoremediation an economically viable strategy for saving our soils?’

This is the transcript of an interview on The Science Show, ABC Radio National on 31 August 2013.

Jen Wiltshire is a Doctorial Research Scientist in the Department of Microbiology at La Trobe University.

Image: Detail of root from cadmium hyper-accumulating plant; the root surface and the protruding root hairs are hot spots for plant-microbe interactions that can influence heavy metal uptake from the environment. Image credit: Jen Wiltshire.