Carlow team aims to bug the pollutants

A research team in Carlow is involved in an EU project seeking to use bacteria living on plant roots to break down chemical pollution in the soil

For every pollutant there is a matching bacterium somewhere willing and able to gobble it up. The problem for researchers is to find the bacterium and encourage it to snack on the chemical waste.

Dr David Dowling and a team at the Institute of Technology, Carlow, are involved in an EU research effort to use combinations of plants and bacteria as a means of breaking down serious pollutants such as polychlorinated biphenyls (PCBs) in the soil. The £1.29 million project involves Carlow and University College Cork and groups in Denmark, Spain and Germany, with additional collaboration with a United States team.

Those involved meet later this week in Carlow to discuss progress in the research which began last September, Dr Dowling explained. The technology is known as "rhizodegradation". Rhizo means root. It involves choosing targeted bacteria known to degrade certain pollutants and encouraging them to colonise the root systems of specific plants.

Bioremediation - the use of bacteria to break down soil pollution - is not new, Dr Dowling points out, but efforts so far have drawbacks, including excessive costs. Existing systems involve inoculating soils with specific bacteria, but these then need to be fed to encourage growth and sometimes aerated to keep the bacteria colonies active.

But bacteria thrive on or next to root systems. A teaspoon of soil taken from around roots might carry up to 20 billion bacteria, whereas soil sampled away from plants might only have two million bacteria.

Bacteria like living near the roots because they provide food (sugar or glucose) and also oxygen, Dr Dowling said. The trick is to find a bacteria species that will both thrive in and around the roots but also be able to digest the pollution.

"The rhizosphere is a very complex environment," Dr Dowling said. Bacterial colonies are subject to predation by protozoans and can be attacked by fungi and viruses. They must also compete and retain their place against the pressures exerted by other bacterial strains fighting for their place in the soil ecosystem.

Plant/bacteria combinations are already in use as a way to break down sewage, although they are not in widespread use here yet, Dr Dowling said. Sewage is piped into artificial wetlands planted with reeds. The plants' permanent bacteria get to work digesting the sewage, and the reeds thrive in the nutrient-rich environment. The resultant discharges can safely be run into water courses.

The removal of toxic chemicals is more complex. The EU project is focusing on two plant species, alfalfa and willow. They have deep roots that can reach out five metres from the plant base. Willow thrives in wet conditions and alfalfa tolerates poor soils - thanks to another root resident, the rhizobium bacteria strain, which converts nitrogen to ammonia-based fertiliser for the plant in exchange for food from its roots.

Bacteria that break down the key pollutants under study - PCBs, polycyclic aromatic hydrocarbons and the solvent trichloroethylene - are available. The research is an attempt to integrate these populations into the rhizosphere, find ways to ensure their survival and develop methods to inoculate the soil.

The Carlow team is examining bacteria that can break down PCBs, trying to understand how they manage to eliminate the substances and remain viable and active in the complex chemistry of the rhizosphere.

It is also involved in developing the inoculation strategy. One useful system involves encapsulating the bacteria in pellets which could easily be scattered on a contaminated site with alfalfa seeds, or planted with willow saplings, Dr Dowling said. Another approach could involve pelletising the seed itself, coating it in a gel which holds and protects the bacteria and releases them over time as the pellet, a gel derived from brown algae, gradually dissolves.