Impacts of partially hydrolyzed polyacrylamide (HPAM) on microbial mats from a constructed wetland treating oilfield produced water

Constructed wetlands have been successfully used in the treatment of produced water brought to the surface in large quantities during oil extraction activities. However, with the increasing use of partially hydrolyzed polyacrylamide (HPAM) in enhancing oil recovery, the impacts of HPAM on the biological processes of wetlands is still unknown. Microbial mats in wetlands play a key role in hydrocarbon degradation. Here, we compared the bacterial communities of four wetland microbial mats after flooding with different concentrations of HPAM. Two mats (i.e. the HPAM-free and the 500 ppm HPAM pre-exposed mats) were selected to further investigate the effect of HPAM on respiration and biodegradation activities. The field mats exhibited clear differences in their bacterial community structure, where Cyanobacteria and Alphaproteobacteria became dominant in the presence of HPAM. In the laboratory experiments, the generated CO₂ by the HPAM-free and the 500 ppm HPAM pre-exposed mats did not vary significantly when HPAM was added, although CO₂ values were slightly higher in the presence of oil. Both mats were still able to degrade between 15 ± 14.4 to 50 ± 13.0% of C₁₀ to C₃₀ alkanes in 28 days, and this degradation was not affected by HPAM addition. The HPAM concentration decreased by 22–34% of the initial amount after 28 days of incubation in the HPAM-free mat, versus only 7–18.4% decrease in the 500 ppm HPAM pre-exposed mat. We conclude that the wetland microbial mats seem to have become well adapted to HPAM and could maintain their respiration and hydrocarbon degradation activities.