Toxicity and removal mechanism of nonylphenol by free and immobilized microalgae
Student thesis: Doctoral Thesis
The presence of nonylphenol (NP) in the environment has become a serious concern due to its ubiquitous distribution, persistence, toxicity and estrogenic effect. Although numerous investigations have been carried out to explore the feasibility of using microalgae to remove a variety of organic pollutants from wastewater, there is no report on the tolerance and removal of NP by microalgae. The present study aims to: 1) compare the NP tolerance and removal ability of different microalgal species, including four commercially available species, namely Chlorella vulgaris (CV), Selenastrum capricornutum (SC), Scenedesmus quadricauda (SQ) and Ankistrodesmus braunii (AB) and three local isolates, Chlorella miniata (WW1), Chlorella sp. (1uoai) and Chlorella sp. (2f5aia); 2) determine the physiological and biochemical mechanisms involved in NP tolerance; 3) investigate factors affecting NP removal to enhance the efficiency and understanding of the NP removal mechanisms; and 4) evaluate the feasibility of using immobilized microalgae to remove NP. The cell count, antioxidant and photosynthetic responses were used as the endpoints to compare NP tolerance among different microalgae. The half maximal effective concentration at 96 hours (96-hour EC50), based on cell count, showed that exposed to NP, and the NP-tolerant species displayed more evident and rapid changes in antioxidant responses than the NP-sensitive ones, particularly under high NP concentration. The 96-hour EC50, based on chlorophyll concentration, further confirmed that CV was more tolerant to NP than SC. Cells of both microalgae acclimated to NP through down-regulating their photosynthetic activities, while the dissipation of energy from reaction centres increased with increase of NP concentrations. The changes of these photosynthetic activities in CV were more rapid and significant than that in SC, but these changes in CV were completely recovered with values comparable to the control (without NP) after a 96-hour exposure. The antioxidant responses, such as SOD, CAT and POD activities in CV also enhanced with increases of NP concentrations after a 24-hour exposure, but such increase was not observed after a 96-hour exposure. In SC, these three enzyme activities increased when exposed to low- to medium-NP concentrations but were inhibited at high concentration (4 mg l-1) after 96 hours of exposure. These results suggested that CV acclimated better, exhibited more efficient and rapid responses to NP-induced oxidative stress and its photosynthetic activity recovered faster from NP-induced damage than SC. All four of the Chlorella species, including CV, 1uoai, 2f5aia and WW1, displayed a rapid and high NP removal ability, and CV showed the highest removal and degradation ability, with nearly all NP having been removed from the medium, while 2f5aia had the lowest NP degradation ability. The growth and NP biodegradation were significantly enhanced by temperature and light intensity but decreased by initial biomass of CV, suggesting that the biodegradation ability was positively correlated with photosynthetic and metabolic activities. These results suggested that CV was the most suitable species for NP removal. The NP removal by CV was not affected by immobilization, although the immobilized cells had a slight decrease in NP biodegradation ability, probably due to a decrease in growth and a reduction of NP bioavailability, as some NP was adsorbed onto alginate matrix. The mechanism involved in the NP removal by immobilized cells was similar to that by free cells, which included adsorption onto algal cells and alginate matrix, absorption within cells and cellular biodegradation. The NP biodegradation could be enhanced by increasing the cell number per bead but the NP removal was not significantly affected. On the other hand, increases of bead density improved the NP removal and biodegradation in a short incubation period but such improvement disappeared with the prolonged incubation time. The present study demonstrated that the toxicity of NP to microalgae was species-specific and microalgae could adapt to NP-induced stress through regulation of their physiological and biochemical responses, as well as through NP biodegradation. Both free and alginate immobilized CV, the most tolerant microalgal species, could effectively remove and degrade NP and such efficiency could be further enhanced by proper adjustment of biomass, temperature and light intensity, but not nutrients.
- Microalgae, Nonylphenol, Toxicology, Immobilized microorganisms, Environmental aspects