Bacteria may help keep water cleaner

The researchers have identified the presence of these microorganisms in agricultural soils but do not yet know what effects they have on phosphorus mobility in this context.

In the lab, the researchers have designed controlled experiments where they are inoculating soil columns with Shewanella oneidensis— a type of DIRB — and PAO obtained from a local wastewater treatment plant.

“We are monitoring water and soil chemistry in addition to changes in microbial communities so we can better understand the contributions of these two groups of bacteria to phosphorus cycling in a controlled system,” said postdoctoral researcher Claudia Rojas.

These two types of bacteria are important to phosphorus mobility for a number of reasons.

The first kind of bacteria, DIRBs, use iron to “breathe,” which can then cause a release of phosphorus.

“When the soil is saturated with water, the oxygen concentration in the water goes down,” said Regan. “Some bacteria are able to respire using iron in the soil, and when they do that, they free up phosphorus that was stuck to that iron, making it mobile.” The researchers are hoping to define the conditions that promote this process.

The second kind of bacteria, PAOs, can store phosphorus inside their cells in high concentrations. They can also break up these phosphorus stores and release phosphate.

“They have a metabolism that cycles between storing and releasing phosphorus,” said Regan. “They basically store phosphorus when the soil water has oxygen and release phosphorus when the water lacks oxygen.”

In streams that are impacted by phosphorus overflow, a slippery microbial growth called a biofilm forms on the stream bottom. Within that biofilm, bacteria are subjected daily to aerobic and anaerobic states — gaining and lacking oxygen.

“We are looking to see if there are microbes in the biofilm that can take up and release phosphorus under these cyclic conditions,” said Regan. “So far, we’ve found that there are lots of PAOs there.”

Regan said there has been a lot of research that models and monitors phosphorus mobility in the soil, but it has not included the role that bacteria have played in that movement.

“We know these bacteria are there, we know they influence phosphorus mobility, and that’s where our knowledge stops,” Regan said.

“No one has studied how abundant they are, what are the rates of reactions, or how much the phosphorus concentration changes when these bacteria are able to grow, so we’re really shining some light into that black box to figure out what the bacteria are doing.”

Once the researchers find out exactly how, and to what extent, bacteria influence the release of phosphorus, they can include these microbial processes in their models and develop predictive tools that can map them across watersheds based on their relationships with the native hydrology.

“The engineer in me wants to reduce this problem,” Regan said. “But before we can develop strategies to do this, we need to understand the role and mechanisms of these microbial reactions.”