Characterization of light-dependent hydrogen production by new green microalga Parachlorella kessleri in various conditions

Temperature modulated sustainable on/off photosynthesis switching of microalgae towards hydrogen evolution

Despite great progress in the active interfacing between various abiotic materials and living organisms, the development of a smart polymer matrix with modulated functionality of algae towards the application of green bioenergy is still rare. Herein, we design a thermally sensitive poly(N-isopropylacrylamide)-co-poly(butyl acrylate) with an LCST (ca. 25 °C) as a chassis, which could co-assemble with algal cells based on hydrophobic interaction to generate a new type of robust hybrid hydrogel living material. By modulating the temperature to 30 °C, the volume of the polymer matrix is shrunk by 9 times, which allows the formation of physical shading and metabolism changing of the algae, and then triggers the functionality switching of the algae from photosynthetic oxygen production to hydrogen production. By contrast, by decreasing the temperature to 20 °C, the hybrid living materials go into a sol state where the algae behave normally with photosynthetic oxygen production. In particular, due to the proliferation of the algae in living materials, a long-term and exponential enhancement in the amount of hydrogen produced is achieved. Overall, it is anticipated that our investigations could provide a new paradigm for the development of polymer/living organism-based hybrid living materials with synergistic functionality boosting green biomanufacturing.

Algae in wastewater treatment, mechanism, and application of biomass for production of value-added product

The pollutants can enter water bodies at various point and non-point sources, and wastewater discharge remains a major pathway. Wastewater treatment effectively reduces contaminants, it is expensive and requires an eco-friendly and sustainable alternative approach to reduce treatment costs. Algae have recently emerged as a potentially cost-effective method to remediate toxic pollutants through the mechanism of biosorption, bioaccumulation, and intracellular degradation. Hence, before discharging the wastewater into the natural environment better solutions for environmental resource recovery and sustainable developments can be applied. More importantly, algae are a potential feedstock material for various industrial applications such as biofuel production. Currently, researchers are developing algae as a source for pharmaceuticals, biofuels, food additives, and bio-fertilizers. This review mainly focused on the potential of algae and their specific mechanisms involved in wastewater treatment and energy recovery systems leading to important industrial precursors. The review is highly beneficial for scientists, wastewater treatment plant operators, freshwater managers, and industrial communities to support the sustainable development of natural resources.

Cultivation and Biorefinery of Microalgae (Chlorella sp.) for Producing Biofuels and Other Byproducts: A Review

Microalgae-based carbon dioxide (CO2) biofixation and biorefinery are the most efficient methods of biological CO2 reduction and reutilization. The diversification and high-value byproducts of microalgal biomass, known as microalgae-based biorefinery, are considered the most promising platforms for the sustainable development of energy and the environment, in addition to the improvement and integration of microalgal cultivation, scale-up, harvest, and extraction technologies. In this review, the factors influencing CO2 biofixation by microalgae, including microalgal strains, flue gas, wastewater, light, pH, temperature, and microalgae cultivation systems are summarized. Moreover, the biorefinery of Chlorella biomass for producing biofuels and its byproducts, such as fine chemicals, feed additives, and high-value products, are also discussed. The technical and economic assessments (TEAs) and life cycle assessments (LCAs) are introduced to evaluate the sustainability of microalgae CO2 fixation technology. This review provides detailed insights on the adjusted factors of microalgal cultivation to establish sustainable biological CO2 fixation technology, and the diversified applications of microalgal biomass in biorefinery. The economic and environmental sustainability, and the limitations and needs of microalgal CO2 fixation, are discussed. Finally, future research directions are provided for CO2 reduction by microalgae.

Regulation of biohydrogen production by protonophores in novel green microalgae Parachlorella kessleri

The green microalgae Parachlorella kessleri RA-002 isolated in Armenia can produce biohydrogen (H₂) during oxygenic photosynthesis. Addition of protonophores, carbonyl cyanide m-chlorophenylhydrazone (CCCP) and 2,4-dinitrophenol (DNF) enhances H₂ yield in P. kessleri. The maximal H₂ yield of ~2.20 and 2.08 mmol L^-1 was obtained in the presence of 15 μM CCCP and 50 μM DNF, respectively. During dark conditions H₂ production by P. kessleri was not observed even in the presence of protonophores, indicating that H₂ formation in these algae was mediated by light conditions. The enhancing effect of protonophores can be coupled with dissipation of proton motive force across thylakoid membrane in P. kessleri, facilitating the availability of protons and electrons to [Fe-Fe]-hydrogenase, which led to formation of H₂. At the same time H₂ production was not observed in the presence of diuron (3-(3,4-dichlorophenyl)-1,1-dimethylurea), a specific inhibitor of PS II. Moreover, diuron inhibits H₂ yield in P. kessleri in the presence of protonophores. The inhibitory effect of diuron coupled with suppression of electron transfer from PS II. The results showed that in these algae operates PS II-dependent pathway of H₂ generation. This study is important for understanding of the mechanisms of H₂ production by green microalgae P. kessleri and developing of its biotechnology.

An efficient Photobioreactors/Raceway circulating system combined with alkaline-CO2 capturing medium for microalgal cultivation

High efficiency of microalgal growth and CO₂ fixation in a Photobioreactors (PBRs)/Raceway circulating (PsRC) system combined with alkaline-CO₂ capturing medium and operation was established and investigated. Compared with a pH 6 medium, the average biomass productivity of Chlorella sp. AT1 cultured in a pH 11 medium at 2 L min^-1 circulation rate for 7 days increased by about 2-fold to 0.346 g L^-1 d^-1. The maximum amount of CO₂ fixation and CO₂ utilization efficiency of Chlorella sp. AT1 could be obtained at a PBRs to Raceway ratio of 1:10 in an indoor-simulated PsRC system. A similar result was also shown in an outdoor PsRC system with a 10-ton scale for microalgal cultivation. Under the appropriate circulation rate, the stable growth performance of Chlorella sp. AT1 cultured by long-term semi-continuous operation in the 10-ton outdoor PsRC system was observed, and the total amount of CO₂ fixation was approximately 1.2 kg d^-1 with 50% CO₂ utilization efficiency.