Source: Tkavc R, Matrosova VY, Grichenko OE, Gostincˇ ar C, Volpe RP, Klimenkova P, Gaidamakova EK, Zhou CE, Stewart BJ, Lyman MG, Malfatti SA, Rubinfeld B, Courtot M, Singh J, Dalgard CL, Hamilton T, Frey KG, Gunde-Cimerman N, Dugan L and Daly MJ. (2018). Prospects for Fungal Bioremediation of Acidic Radioactive Waste Sites: Characterization and Genome Sequence of Rhodotorula taiwanensis MD1149. Front. Microbiol. 8:2528. doi: 10.3389/fmicb.2017.02528
Radioactive waste is still a problem
Between 1945 and 1986, radioactive waste was generated from the production of 46,000 nuclear weapons in the U.S. Most of this waste was disposed of by puttingit directly into the ground or in underground storage tanks. Unfortunately, underground storage tanks are prone to leaking, whether it is oil, nuclear waste, or septic systems. For the past six decades the waste sites have been contaminating soils and groundwater with radionuclides, heavy metals, and strong acids; potentially resulting in public health threats! Because the amount of waste is so vast, cleanup options are dangerous and expensive, and therefore limited. One option is bioremediation: using natural organisms to consume or break down the waste. The problem is these waste sites have extremely harsh conditions including a low pH (acidic) and high concentrations of heavy metals. This led to a search for microorganisms that could survive in this extreme environment.
The search for a cleanup helper begins
Even though these extreme environments make it seem like it would be impossible to support life, back in 2000 a group of researchers led by James Fredrickson from the Pacific Northwest National Laboratory found 110 different bacteria (one type of microorganism) living below a radioactive waste storage tank in the state of Washington known to be leaking. The researchers discovered that the bacteria were not ideal candidates for bioremediation because they could not survive in the acidic environments found inside the waste tanks. However, this year, a new group of researchers found another type of microorganism that might be suitable: fungi. Fungi have mostly been ignored as an option for bioremediation due to the lack of knowledge on their tolerance to radiation.
Exploring fungi as an option
In this study, the researchers screened 60 different environmental samples for fungi that could grow at the pH found in radioactive waste tanks. To find these fungi, scientists looked in extreme environments including desert sands, water and sediment from abandoned mines, hot springs from an acidic river, and radioactive waste storage tanks. The samples were collected from all over the world and grown in conditions that mimicked those of a radioactive waste tank including exposure to radiation, heavy metals and low pH.
One fungus stands out
The researchers determined that one fungus MD1149-Rhodotorula taiwanesis (Figure 2) was most suitable for bioremediation. This fungus was found in a sample from an abandoned acid mine drainage facility in Maryland, USA. It was the only strain capable of growing under the radiation level and low pH present in radioactive waste tanks. The fungus performed well when exposed to both short term and long-term radiation and was even more resistant to radiation than bacteria found in the 2000 study. In addition, the researchers found that this fungus could form biofilms, which can help absorb toxins and heavy metals in the waste and remove them from the soil.
Since R. taiwanesis was such a promising option for bioremediation, the researchers sequenced its genome, or in other words mapped its DNA. This “map” allowed the scientists to examine what genes give this fungus the unique ability to survive in the radioactive waste. Knowing this, we can look for other organisms with similar genes and test them for use in bioremediation, and hopefully tackle some of the challenges we face in remediating soil contaminated with radioactive waste.
Fredrickson, J. K., Zachara, J. M., Balkwill, D. L., Kennedy, D., Li, S. M., Kostandarithes, H. M., et al. (2004). Geomicrobiology of highlevel nuclear waste-contaminated vadose sediments at the Hanford site, Washington state. Appl. Environ. Microbiol. 70, 4230–4241. doi: 10.1128/AEM.70.7.4230-4241.2004