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Biomining using microalgae has emerged as a sustainable option to extract rare earth elements (REEs). This study aims to (i) explore the capability of REEs recovery from bauxite by microalgae, (ii) assess the change of biochemical function affected by bauxite, and (iii) investigate the effects of operating conditions (i.e., aeration rate, pH, hydraulic retention time) to REEs recovery. The results showed that increasing bauxite in microalgae culture increases REEs recovery in biomass and production of biochemical compounds (e.g., pigments and Ca-Mg ATPase enzyme) up to 10 %. The optimum pulp ratio of bauxite in the microalgae culture ranges from 0.2 % to 0.6 %. Chlorella vulgaris was the most promising, with two times higher in REEs recovery in biomass than the other species. REEs accumulated in microalgae biomass decreased with increasing pH in the culture. This study establishes a platform to make the scaling up of REEs biomining by microalgae plausible., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 The Author(s). Published by Elsevier Ltd.. All rights reserved.)

The effects of microalgae harvesting methods on microalgal biomass quality were evaluated using three species namely the freshwater green alga Chlorella vulgaris, marine red alga Porphyridium purpureum and marine diatom Phaeodactylum tricornutum. Harvesting efficiencies of polyacrylamide addition, alkaline addition, and centrifugation ranged from 85 to 95, 59-92 and 100%, respectively, across these species. Morphology of the harvested cells (i.e. compromised cell walls) was significantly impacted by alkaline pH-induced flocculation for all three species. Over 50% of C. vulgaris cells were compromised with alkaline pH compared to < 10% with polyacrylamide and centrifugation. The metabolic profiles varied depending on harvesting methods. Species-specific decrease of certain metabolites was observed. These results suggest that the method of harvest can alter the metabolic profile of the biomass amongst the three harvesting methods, polyacrylamide addition showed higher harvesting efficiency with less compromised cells and higher retention of industry important biochemicals., (Copyright © 2022 Elsevier Ltd. All rights reserved.)

Flocculation is a low-cost harvesting technique for microalgae biomass production, but flocculation efficiency is species dependent. In this study, we investigated the efficacy of two synthetic (polyacrylamide) and one natural (chitosan) flocculants against three algal species: the cyanobacterium Synechocystis sp., the freshwater Chlorella vulgaris, and the marine Phaeodactylum tricornutum at laboratory and pilot scales to evaluate harvesting efficiency, biomass integrity and media recycling. Growth phase affected the harvesting efficiency of the eukaryotic microalgae. The flocculation was optimal at stationary phase with high flocculation efficiency achieved using polyacrylamides at 24-36 mg/g dry weight. The effect of the flocculants on the harvested biomass was investigated. The flocculated Synechocystis sp. showed a higher proportion of compromised cells compared to C. vulgaris and P. tricornutum likely due to differences in cell walls composition. Compromised cells could lead to the release of valuable products into the surrounding growth media during flocculation. The residual culture media was recycled once with no impact on cell growth for all treatments and algal species. The flocculation technique was demonstrated at pilot-scale using 350 L microalgal suspension, showing an efficiency of 82-90% at a polyacrylamide dosage of 6.5-10 mg/L. This efficiency and the biomass quality are comparable to the laboratory-scale results. Overall, results indicate that polyacrylamide flocculants work on a wide range of species without the need for pre-treatment. The information generated in this study can contribute to making the microalgae industry more competitive., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier B.V. All rights reserved.)

This study describes the impacts of inorganic carbon limitation on the photosynthetic efficiency and operation of photosynthetic electron transport pathways in the biofuel-candidate microalga Nannochloropsis oculata. Using a combination of highly-controlled cultivation setup (photobioreactor), variable chlorophyll a fluorescence and transient spectroscopy methods (electrochromic shift (ECS) and P 700 redox kinetics), we showed that net photosynthesis and effective quantum yield of Photosystem II (PSII) decreased in N. oculata under carbon limitation. This was accompanied by a transient increase in total proton motive force and energy-dependent non-photochemical quenching as well as slightly elevated respiration. On the other hand, under carbon limitation the rapid increase in proton motive force (PMF, estimated from the total ECS signal) was also accompanied by reduced conductivity of ATP synthase to protons (estimated from the rate of ECS decay in dark after actinic illumination). This indicates that the slow operation of ATP synthase results in the transient build-up of PMF, which leads to the activation of fast energy dissipation mechanisms such as energy-dependent non-photochemical quenching. N. oculata also increased content of lipids under carbon limitation, which compensated for reduced NAPDH consumption during decreased CO 2 fixation. The integrated knowledge of the underlying energetic regulation of photosynthetic processes attained with a combination of biophysical methods may be used to identify photo-physiological signatures of the onset of carbon limitation in microalgal cultivation systems, as well as to potentially identify microalgal strains that can better acclimate to carbon limitation., (Copyright © 2018 Elsevier B.V. All rights reserved.)

Multi-wavelength chlorophyll fluorescence analysis was utilised to examine the photosynthetic efficiency of the biofuel-producing alga Nannochloropsis oculata, grown under two light regimes; low (LL) and high (HL) irradiance levels. Wavelength dependency was evident in the functional absorption cross-section of Photosystem II (σII(λ)), absolute electron transfer rates (ETR(II)), and non-photochemical quenching (NPQ) of chlorophyll fluorescence in both HL and LL cells. While σII(λ) was not significantly different between the two growth conditions, HL cells upregulated ETR(II) 1.6-1.8-fold compared to LL cells, most significantly in the wavelength range of 440-540 nm. This indicates preferential utilisation of blue-green light, a highly relevant spectral region for visible light in algal pond conditions. Under these conditions, the HL cells accumulated saturated fatty acids, whereas polyunsaturated fatty acids were more abundant in LL cells. This knowledge is of importance for the use of N. oculata for fatty acid production in the biofuel industry., (Copyright © 2014 Elsevier Ltd. All rights reserved.)

Diatoms (Bacillariophyceae) are important to primary productivity of aquatic ecosystems. This algal group is also a valuable source of high value compounds that are utilised as aquaculture feed. The productivity of diatoms is strongly driven by light and CO2 availability, and macro- and micronutrient concentrations. The light dependency of biomass productivity and metabolite composition is well researched in diatoms, but information on the impact of light quality, particularly the productivity return on energy invested when using different monochromatic light sources, remains scarce. In this work, the productivity return on energy invested of improving growth rate, photosynthetic activity, and metabolite productivity of the diatom Chaetoceros muelleri under defined wavelengths (blue, red, and green) as well as while light is analysed. By adjusting the different light qualities to equal photosynthetically utilisable radiation, it was found that the growth rate and photosynthetic oxygen evolution was unchanged under white, blue, and green light, but it was lower under red light. Blue light improved the productivity return on energy invested for biomass, total protein, total lipid, total carbohydrate, and in fatty acids production, which would suggest that blue light should be used for aquaculture feed production. [ABSTRACT FROM AUTHOR]

This study evaluates the repurposing of expired isopropanol (IPA) COVID-19 disinfectant (64% w/w) to pretreat algal biomass for enhancing methane (CH 4) yield. The impact of harvesting methods (centrifugation and polymer flocculation) and microwave pretreatment on CH 4 production from Scenedesmus sp. microalgal biomass were also investigated. Results show minimal impact of harvesting methods on the CH 4 yield, with wet centrifuged and polymer-harvested biomass exhibiting comparable and low CH 4 production at 66 and 74 L/kg volatile solid , respectively. However, microalgae drying significantly increased CH 4 yield compared to wet biomass, attributed to cell shrinkage and enhanced digestibility. Consequently, microwave and IPA pretreatment significantly enhanced CH 4 production when applied to dried microalgae, yielding a 135% and 212% increase, respectively, compared to non-pretreated wet biomass. These findings underscore the advantage of using dried Scenedesmus sp. over wet biomass and highlight the synergistic effect of combining oven drying with IPA treatment to boost CH 4 production whilst reducing COVID-19 waste. [Display omitted] • CH 4 production from microalgal (Scenedesmus sp.) biomass was demonstrated. • Pretreatment enhanced CH 4 production from microalgal biomass. • Oven dried biomass results in higher CH 4 production than wet biomass. • Drying and alcohol pretreatment increased CH 4 yield by >210% over wet microalgae. [ABSTRACT FROM AUTHOR]

The commercialisation of valuable plant triterpenoids faces major challenges, including low abundance in natural hosts and costly downstream purification procedures. Endeavours to produce these compounds at industrial scale using microbial systems are gaining attention. Here, we report on a strategy to enrich the biomass of the biotechnologically-relevant Chlamydomonas reinhardtii strain UVM4 with valuable triterpenes, such as squalene and (S)-2,3-epoxysqualene. C. reinhardtii UVM4 was subjected to the elicitor compounds methyl jasmonate (MeJA) and methyl-β-cyclodextrine (MβCD) to increase triterpene yields. MeJA treatment triggered oxidative stress, arrested growth, and altered the photosynthetic activity of the cells, while increasing squalene, (S)-2,3-epoxysqualene, and cycloartenol contents. Applying MβCD to cultures of C. reinhardtii lead to the sequestration of the two main sterols (ergosterol and 7-dehydroporiferasterol) into the growth medium and the intracellular accumulation of the intermediate cycloartenol, without compromising cell growth. When MβCD was applied in combination with MeJA, it counteracted the negative effects of MeJA on cell growth and physiology, but no synergistic effect on triterpene yield was observed. Together, our findings provide strategies for the triterpene enrichment of microalgal biomass and medium. [ABSTRACT FROM AUTHOR]

Abstract Terpenoids are a large and diverse class of naturally occurring metabolites serving many industrial applications and natural roles. Economically important terpenoids are often produced in low abundance from their natural sources, making their industrial-scale production challenging or uneconomical, therefore engineered microorganisms are frequently used as heterologous production platforms. Photosynthetic microorganisms, such as the green alga Chlamydomonas reinhardtii , represent promising systems to produce terpenoids in a cost-effective and sustainable manner, but knowledge about the regulation of their terpenoid metabolism remains limited. Here we report on the investigation of the phytohormone methyl jasmonate (MeJA) as elicitor of algal terpenoid synthesis. We treated C. reinhardtii cells in mid-exponential growth phase with three different concentrations of MeJA (0.05, 0.5 and 1 mM). The highest concentration of MeJA affected the photosynthetic activity of the cells, arrested the growth and up-regulated key genes of the 2- C -methyl-D-erythritol 4-phosphate (MEP) pathway, leading to a significant increase in intermediates of this pathway, squalene and (S)-2,3-epoxysqualene, while the abundance of cycloartenol, and two main sterols (ergosterol and 7-dehydroporiferasterol) decreased. These data suggest the redirection of the carbon flux towards the synthesis of yet uncharacterised triterpenoid secondary metabolites upon MeJA treatment. Our results offer important new insights into the regulation of the triterpenoid metabolism in C. reinhardtii and raise important questions on hormonal signalling in microalgae. Phytohormone treatment is tested for the first time in algae, where it holds great potential for identifying key transcriptional regulators of the MEP pathway as targets for future metabolic engineering studies for improve production of high-value triterpenoids. Highlights • First report of triterpenoid elicitation by a phytohormone in C. reinhardtii. • MeJA treatment (1 mM) arrested the growth and up-regulated genes of the MEP pathway. • Highlighting a specialised pathway branching immediately upstream of cycloartenol. • Accumulation of an uncharacterised triterpenoid secondary metabolite (C 30 H 52 O 3). • Plant-like MeJA response machinery is putatively absent in C. reinhardtii. [ABSTRACT FROM AUTHOR]