Molecular oxygen is essential to freshwater microalgae but its metabolic by-product known as reactive oxygen species (ROS) is toxic at higher levels leading to oxidative stress. Freshwater microalgae are able to prevent the effect of ROS using antioxidant response mechanisms. This mini-review is aimed at providing the progress and prospects of the antioxidant response mechanisms exhibited by freshwater microalgae to counteract excess ROS. The mini-review shows that the production of ROS and antioxidant response mechanism varies across species of freshwater microalgae and is dependent on factors such as freshwater microalgae cell size, cell shape, cell density, growth stage and abiotic factors such as light, nutrient, and temperature. This study also shows that freshwater microalgae are able to prevent oxidative damage resulting from a higher level of ROS production by the production of antioxidants. This antioxidant can be harness as phytonutrients that can be used as a health-promoting compound for the prevention of the risk of heart disease, cancer, and other diseases. More studies need to be done to ascertain the bioactive compounds that make up the antioxidant system of freshwater microalgae against excess ROS production and how these bioactive compounds can be utilised.

Antibiotic residues have been detected in aquatic environments worldwide. Biofilms are one of the most successful life forms, and as a result are ubiquitous in natural waters. However, the response mechanism of freshwater biofilms to the stress of various antibiotic residues is still unclear. Here, the stress of veterinary antibiotic florfenicol (FF) and fluoroquinolone antibiotic ofloxacin (OFL) on freshwater biofilms were investigated by determining the changes in the key physicochemical and biological properties of the biofilms. The results showed that the chlorophyll a content in biofilms firstly decreased to 46–71% and then recovered to original content under the stress of FF and OFL with high, mid, and low concentrations. Meanwhile, the activities of antioxidant enzymes, including superoxide dismutase and catalase, increased between 1.3–6.7 times their initial values. FF was more toxic to the biofilms than OFL. The distribution coefficients of FF and OFL binding in extracellular polymeric substances (EPS)-free biofilms were 3.2 and 6.5 times higher than those in intact biofilms, respectively. It indicated that EPS could inhibit the FF and OFL accumulation in biofilm cells. The present study shows that the EPS matrix, as the house of freshwater biofilms, is the primary barrier that resists the stress from antibiotic residues.

This study aimed at studying the phycoremediation of petroleum-contaminated water using microalgae biofilm cultured in nutrient-rich water. Microalgae biofilm was grown in a photobioreactor containing water rich in calcium nitrate, manganese chloride, sodium potassium tartrate, calcium phosphate, and ammonium sulfate. Petroleum contaminated water was poured into a photobioreactor, and the substrate containing microalgae biofilm was inserted into the photobioreactor and allowed for eight weeks for biofilm formation. Physicochemical parameters (pH, turbidity, conductivity, sulfate, alkalinity, chloride, TDS, TSS, nitrate, salinity, iron, potassium, phosphate, chlorine, chromium, magnesium, zinc, COD, BOD, and total petroleum hydrocarbon (TPH) of the petroleum contaminated water before and after treatment were determined. The microalgae biofilm used for the treatment was characterized before and after treatment using a Scanning Electron Microscope, X-Ray Fluorescence, and Fourier-transform infrared spectroscopy. The phytochemical constituent of the microalgae biofilm was also determined before and after treatment of the petroleum-contaminated water. The result obtained shows highest removal efficiency of physicochemical parameters (turbidity (81%), conductivity (51.2), sulfate (17.5%), alkalinity 28.4%), chloride (14.6%), TDS (7.9), TSS (26%), nitrate (33%), salinity (23.4), iron (16%), potassium (22%), phosphate (28.2%), chlorine (14%), chromium (13.6%), magnesium (30.3%), zinc (40.5%), COD (8%), BOD (16.7%) and total petroleum hydrocarbon (15%)). The microalgae's characterization shows microalgae biofilm's ability to adsorb pollutants in petroleum-contaminated water due to the presence of microspores and larger surface area of the cells of the microalgae forming the biofilm or due to the absorption efficiency of the extracellular polymeric substances (EPS). The analysis of the microalgae biofilm's phytochemical parameters shows the involvement of the chemicals components in pollutants degradation and antioxidant response of the microalgae to counteract the oxidative effect resulting from the exposure of the microalgae to the contaminated water. NOVELTY STATEMENT This is the first study that attempts the phycoremediation of petroleum contaminated water using microalgae biofilm. The reduction efficiency of the parameters treated in this study is very low compared to that reported in the literature but increases with the retention day. This low reduction efficiency is attributed to the slow assimilation of organic and inorganic pollutants due to the initial growth condition. This study is the first to re-affirm that microalgae biofilm can phycoremediate petroleum-contaminated water by adsorption and assimilation due to the presence of microspores and a larger surface area the cells of the microalgae forming the biofilm or the extracellular polymetric surface covering the biofilm. Several studies have reported that phytochemicals present in microalgae play an antioxidant response role to prevent the microalgae from oxidative damage resulting from water pollution. However, this study is the first to strongly link phytochemicals to the enhancement of pollutants degradation and adsorption by microalgae biofilm.

Ammonia nitrogen (NH₄-N) contaminated wastewater has posed a great threat to the safety of water resources. In this study, air stripping was employed to remove and recover NH₄-N from acetylene purification wastewater (APW) in a polyvinylchloride manufacturing plant. Investigated parameters were initial APW pH, air flow rate, APW temperature and stripping time. The NH₄-N removal by air stripping has been modeled and the overall volumetric mass transfer coefficient (K_La) of the stripping process has been calculated from the model equation obtained. In addition, the ability of H₂SO₄ solution to absorb the NH₃ stripped was also investigated. The results indicated that under the experimental conditions, the APW temperature and its initial pH had significant effects on the NH₄-N removal efficiency and the K_La, while the effects of other factors were relatively minor. The removal efficiency and residual concentration of NH₄-N were about 91% and 12 mg/L, respectively, at the optimal operating conditions of initial APW pH of 12.0, air flow rate of 0.500 m³/(h·L), APW temperature of 60 °C and stripping time of 120 min. One volume of H₂SO₄ solution (0.2 mol/L) could absorb about 93% of the NH₃ stripped from 54 volumes of the APW.