Dr Erika Tóth, of Eötvös Loránd University in Budapest, was approached by the staff of a Hungarian power plant with an unusual request. The ultrapure water (UPW) purification system that produces the water used in the power plant’s cooling system was no longer supplying UPW in adequate quantities. Upon closer investigation, the water turned out to be free of physical and chemical contaminants, the usual corrosion culprits, so a biological component was suspected.
The UPW that cools the power plant contains very little organic or inorganic matter – the equivalent of one grain of sugar in an Olympic swimming pool. It therefore came as a surprise that even oligotrophic bacteria, known for their ability to thrive in resource-poor environments, could proliferate here. However, Dr Tóth and her Hungarian-German team found hundreds of different bacteria in the filtration systems, among them three new strains of bacteria of the class Alphaproteobacteria. Phylogenetic analysis and a study of biochemical and physiological characteristics, recently published in the journal IJSEM, revealed that these three strains belong to a new genus and species, known as Phreatobacter oligotrophus.
True to the old adage, it was important to understand the enemy in order to defeat it. The team used various molecular studies to pinpoint where in the system the microbial contamination was highest. Having identified the bacteria and their location, the team attempted to culture them in order to study them in the laboratory, which proved a difficult task. Oligotrophic bacteria cannot survive in an environment even slightly too nutrient-rich, so the environment of the filtering system had to be approximated as closely as possible.
The production of UPW is a complex and expensive process, involving an array of mechanisms called ion-exchange resin filters. These filters consist of tiny beads of resin which are porous in order to maximise their surface area. As water flows over these beads, they trap contaminant ions and thus help remove any impurities. In essence, the team had to create a miniature replica of the power plant’s water purification system.
The results were well worth the effort. Tóth and colleagues found that the three newly identified strains, along with many of the other bacteria they detected, formed biofilms of considerable size on the resin filter surfaces, as can be seen in the above image. These slimy microbial films were sometimes large enough to be visible to the naked eye, highlighting that even the UPW in a nuclear power plant harbours significant quantities of life. The biofilms, besides being contaminants themselves, also “clogged up” the UPW purification mechanism, preventing the system from working properly.
Another danger of biofilm formation is that if they grow large enough, anaerobic (oxygen-free) conditions may arise within them. Anaerobic bacteria create acidic compounds that can degrade the rubber lining of the water purification system’s pipes and cause corrosion. Indeed, the team found several anaerobic species capable of degrading rubber in the water purification system. While corrosion had not yet occurred, this meant that the biofilms had to be eliminated as effectively as possible.
It turns out that no single bacterium is responsible for the problems arising in UPW purification systems. It is the community of bacteria as a whole – and the biofilms that they form – which cause problems and can eventually cause corrosion. In restoring the power plant’s purification system to full functionality, therefore, it was key for Tóth and colleagues to identify the bacteria that acted as catalysts for biofilm formation, clogging the resin filters and allowing anaerobic conditions to arise.
Partly in response to studies such as Dr Tóth’s, which highlight the problematic nature of biological contamination in UPW systems, next-generation filtration systems include microbiological filtration systems.
Tóth EM, Vengring A, Homonnay ZG, Kéki Z, Spröer C, Borsodi AK, Márialigeti K, & Schumann P (2014). Phreatobacter oligotrophus gen. nov., sp. nov., an alphaproteobacterium isolated from ultrapure water of the water purification system of a power plant. International Journal of Systematic and Evolutionary Microbiology, 64 (Pt 3), 839-45 PMID: 24277862