Research Description:

The Relationship between Periplasmic Nitrate Reductase and the Length of Diauxic Lag in Denitrifying Bacteria Switching from Oxic to Anoxic Conditions

Nitrogen removal is an integral part of wastewater treatment due to the detrimental environmental and health effects associated with excess nitrogen. In an effort to remove nitrogen efficiently and economically, biological removal processes have been employed in many wastewater treatment facilities. These processes involve cycling bacteria between oxic and anoxic conditions in order to first oxidize ammonia to nitrate with nitrifying bacteria and then reduce nitrate to dinitrogen with denitrifying bacteria. Under aerobic conditions, synthesis of denitrifying enzymes is repressed, so their intracellular levels drop due to decay and production of new biomass. When the denitrifying bacteria are switched from oxic to anoxic conditions they experience a phenomenon known as diauxic lag, in which their growth rate approaches zero or even becomes negative until they can re-synthesize denitrification enzymes. This lag can last several hours and results in economic inefficiency.

Observations by our research group have shown that bacteria containing both membrane-bound nitrate reductase (Nar) and periplasmic nitrate reductase (Nap) display shorter diauxic lags than bacteria containing only Nar. Nap is thought to be responsible for shortening diauxic lag.

The purpose of my project is to demonstrate the role of Nap in shortening diauxic lag. I am working with a graduate student to test the prediction that a mutant denitrifying bacterium (Paracoccus pantotrophus) in which the gene encoding for the periplasmic nitrate reductase has been deleted will exhibit a longer diauxic lag than the wild type. My role in the research is to characterize both the putatively Nap-deficient mutant and the Nap-containing wild type and compare their diauxic lags.

The results from this project will help to characterize the role of Nap in biological denitrification processes and improve mathematical models of biological nitrate reduction to dinitrogen, which could lead to better design and operation of nitrogen-removing wastewater treatment systems.