Oilfield-produced water as a medium for the growth of Chlorella pyrenoidosa outdoor in an arid region

The Strategies Microalgae Adopt to Counteract the Toxic Effect of Heavy Metals

Besides biomass production, some microalgae have been used to treat wastewater contamination. However, in general, high concentrations of heavy metals significantly inhibit algal growth. We thus need to find ways to promote the resistance of microalgae to heavy metals, increase their growth rate under stress, and achieve coupling of heavy metal removal and biomass production simultaneously. In this review, mechanisms for removal of heavy metals by microalgae are proposed. Effects of exogenous chemical additives (dissolved organic matters, formaldehyde, sulphate, phosphate, nitric oxide donors, etc.) on algal biosorption to heavy metals are summarized. Genetic manipulation and microalgal strain selection strategies are also introduced, especially for the acid-tolerant strains with high biosorption efficiencies to Cr(VI) and Cd2+ at low pH conditions. Recent advances in (semi)continuous heavy-metal-bioremediation and biomass-production coupled system with immobilized microalgae, as well as challenges and solutions to the commercialization and industrialization of the coupled system were discussed.

Bioremediation of dissolved organic compounds in produced water from oil and gas operations using Chlorella sorokiniana: a sustainable approach

The sustainable treatment of petroleum-derived produced water (PW), a significant byproduct of oil and gas extraction, presents a persistent problem due to the presence of organic pollutants. This study examines the potential of the microalga Chlorella sorokiniana (C. sorokiniana) for the bioremediation of dissolved organic pollutants in PW. The primary objectives were to evaluate the efficacy of C. sorokiniana in decreasing the levels of dissolved organic contaminants while examining its growth and survival in such a complex environment. The cultivation of C. sorokiniana in photobioreactors containing synthetic produced water (SPW), supplemented with synthetic municipal wastewater (SMW) to provide essential nutrients, was carried out under controlled laboratory conditions. Parameters such as biomass growth, lipid content, and the microalgae's capacity to metabolize organic compounds are monitored over time. The results indicate that, except for 100% PW, maximum biomass output after 16 days ranged from 733 to 1077 mg/L. Total organic carbon (TOC) removal efficiency increased with rising PW concentrations, peaking at 85% for 50% PW. The cultivation period resulted in substantial nitrogen and phosphorus removal from the enriched PW media, achieving a maximum nitrogen removal of 87% at 10% PW and a phosphorus removal of 98.5% at 40% PW. Lipid content ranged from 12 to 16% during this period. In conclusion, C. sorokiniana offers a promising and sustainable approach for the bioremediation of dissolved organic compounds in PW. This method provides an eco-friendly option to reduce the ecological impact associated with petroleum-derived PW.

Produced water treatment by semi-continuous sequential bioreactor and microalgae photobioreactor

Produced water (PW) from oil and gas exploration adversely affects aquatic life and living organisms, necessitating treatment before discharge to meet effluent permissible limits. This study first used activated sludge to pretreat PW in a sequential batch reactor (SBR). The pretreated PW then entered a 13 L photobioreactor (PBR) containing Scenedesmus obliquus microalgae culture. Initially, 10% of the PW mixed with 90% microalgae culture in the PBR. After the exponential growth of the microalgae, an additional 25% of PW was added to the PBR without extra nutrients. This study reported the growth performance of microalgae in the PBR as well as the reduction in effluent's total organic carbon (TOC), total dissolved solids (TDS), electrical conductivity (EC), and heavy metals content. The results demonstrated removal efficiencies of 64% for TOC, 49.8% for TDS, and 49.1% for EC. The results also showed reductions in barium, iron, and manganese in the effluent by 95, 76, and 52%, respectively.

Lipid production characteristics of a newly isolated microalga Asterarcys quadricellulare R-56 as biodiesel feedstock

In this study, a new microalgal strain, Asterarcys quadricellulare R-56, was isolated for biomass and lipid production. The effects of carbon and nitrogen sources and initial pH on the cell growth and lipid accumulation of strain R-56 were investigated. At 10 g L^-1 glucose, 0.6 g L^-1 sodium nitrate, and pH 7, the highest biomass of 4.18 g L^-1 and lipid content of 43.66% were obtained. Microalgae had a broad pH tolerance in the range of 5-11, and the pH of the culture medium was close to neutral at the end of cultivation. The maximum contents of chlorophyll, carbohydrate, and protein under the recommended culture conditions were 19.47 mg mL^-1, 21.80%, and 29.94%, respectively. Palmitic and palmitoleic acid contents in strain R-56 accounted for as high as 83.73% of total fatty acids. This study suggested that strain R-56 was a promising lipid producer for high-quality biodiesel production.

Genetic engineering to enhance microalgal-based produced water treatment with emphasis on CRISPR/Cas9: A review

In recent years, the increased demand for and regional variability of available water resources, along with sustainable water supply planning, have driven interest in the reuse of produced water. Reusing produced water can provide important economic, social, and environmental benefits, particularly in water-scarce regions. Therefore, efficient wastewater treatment is a crucial step prior to reuse to meet the requirements for use within the oil and gas industry or by external users. Bioremediation using microalgae has received increased interest as a method for produced water treatment for removing not only major contaminants such as nitrogen and phosphorus, but also heavy metals and hydrocarbons. Some research publications reported nearly 100% removal of total hydrocarbons, total nitrogen, ammonium nitrogen, and iron when using microalgae to treat produced water. Enhancing microalgal removal efficiency as well as growth rate, in the presence of such relevant contaminants is of great interest to many industries to further optimize the process. One novel approach to further enhancing algal capabilities and phytoremediation of wastewater is genetic modification. A comprehensive description of using genetically engineered microalgae for wastewater bioremediation is discussed in this review. This article also reviews random and targeted mutations as a method to alter microalgal traits to produce strains capable of tolerating various stressors related to wastewater. Other methods of genetic engineering are discussed, with sympathy for CRISPR/Cas9 technology. This is accompanied by the opportunities, as well as the challenges of using genetically engineered microalgae for this purpose.