Bioremediation of industrial emissions and dairy wastewater using the marine microalga, Heterosigma akashiwo, coupled to biofuel production
Date
2016
Authors
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Publisher
University of Delaware
Abstract
Photosynthetic microalgae have the ability to convert solar energy and carbon dioxide into algal biomass that can used as a feedstock for biofuel. The carbon dioxide and nitric oxide in industrial emissions, and the ammonium and phosphate in dairy waste, can be used as sustainable nutrient sources for micro-algal biomass production. Previous studies have shown that the marine microalga, Heterosigma akashiwo, is well suited for biofuel production due to high lipid content and productivity. H. akashiwo may be able to metabolize the nitric oxide in flue gas due to a novel nitrate reductase enzyme, NR2-2/2HbN, however, nitrate reductase (NR) activity may be inhibited by the ammonium present in dairy waste. The goal of this study was to determine if H. akashiwo is suitable for the simultaneous bioremediation of industrial emissions and dairy waste during the production of quality biodiesel. First, the effects of nitrogen limitation on growth and lipid content of H. akashiwo was evaluated at varying nitrogen to phosphate ratios (16:1, 10:1, and 7:1) and nitrogen sources (nitrate and ammonium). Biofuel production was highest under N-replete conditions. The nitrogen source had no effect on the growth rate, biochemical profile, or fatty acid profile of H. akashiwo, indicating that the quality of the biodiesel will not be affected by the nitrogen source. Then, the effects of varying nitrate to ammonium ratios (1:0, 1:1, 1:10, 1:40) on NR activity in H. akashiwo was evaluated. NR activity was detected in cultures with 10x and 40x more ammonium than nitrate, although NR activity was significantly reduced compared to cultures supplemented with only nitrate. This reduction in activity is most likely due to the lower concentrations of nitrate and not the presence of ammonium. NR activity was detected in all cultures, indicating that ammonium does not completely inhibit NR activity. These results suggest that bioremediation of the ammonium in the dairy wastewater and the nitric oxide in the flue gas may be able to occur simultaneously. Finally, the potential for biodiesel production coupled with bioremediation of flue gas when H. akashiwo was grown on dairy waste was evaluated. Here, H. akashiwo was cultured on a factorial combination of CO2 or flue gas containing CO2 and nitric oxide, with either ammonium or dairy waste. It was found that H. akashiwo could sustain growth on dairy waste and industrial flue gas, but was unable to bio-remediate the CO2 and nitric oxide in the flue gas under these experimental conditions. Additionally, low lipid contents indicated that the treatments used here are not suitable for biofuel production from H. akashiwo. However, high protein content supports the use of this alga for animal feed and/or bioplastics.