• Energy Research

The pre-treatment of microalgae cell walls is known to be a key factor to enhance methane (CH 4) yields during anaerobic digestion. This study investigated the combined effects of two different biomass storage methods and physical pre-treatments on the anaerobic digestion for three different microalgae species. Acutodesmus obliquus, Chlorella vulgaris and Chlorella emersonii were cultivated in 80 L sleevebag photobioreactors (batch mode), and then subjected to different storage (cooling and freezing) and pre-treatment methods prior to anaerobic digestion using the biochemical methane potential (BMP) test. A. obliquus was selected to evaluate pre-treatment methods for further experimentation. Significantly higher CH 4 yields of cooled (4 °C) A. obliquus biomass were achieved through ultrasonication (+53% CH 4) and wet-milling (+51% CH 4). These methods were then applied in follow-up experiments to cooled (4 °C) biomass of C. emersonii and A. obliquus. Ultrasonication again led to significantly higher CH 4 yields for A. obliquus biomass (323 dm 3 kg −1 CH 4 yield calculated at standard gas conditions of 273 K, and 101.5 kPa per unit volatile solids, +41% CH 4), and C. emersonii biomass (308 dm 3 kg −1; +35% CH 4). In a third experiment series, frozen A. obliquus and C. vulgaris biomass were thawed prior to pre-treatment and BMP-testing. Among all BMP tests, the highest CH 4 yields were achieved with untreated, freeze-thawed C. vulgaris biomass (406 dm 3 kg −1); pre-treatment did not enhance CH 4 yields for C. vulgaris, but for A. obliquus (ultrasonication +20%). Pre-treatment was more effective for cooled than freeze-thawed microalgal biomass and combined effects acted strain dependently.

Contaminations are challenging for monocultures, as they impact the culture conditions and thus influence the growth of the target organism and the overall biomass composition. In phycology, axenic cultures comprising a single living species are commonly strived for both basic research and industrial applications, because contaminants reduce significance for analytic purposes and interfere with the safety and quality of commercial products. We aimed to establish axenic cultures of Limnospira fusiformis, known as the food additive “Spirulina”. Axenicity is strived because it ensures that pathogens or harmful microorganisms are absent and that the harvested biomass is consistent in terms of quality and composition. For the axenic treatment, we applied sterile filtration, ultrasonication, pH treatment, repeated centrifugation, and administration of antibiotics. For testing axenicity, we considered the most common verification method plate tests with Lysogeny Broth (LB) medium, which indicated axenicity after treatments were performed. In addition, we included plate tests with Reasoner’s 2A (R2A) agar and modified Zarrouk+ medium, the latter comparable to the biochemical properties of L. fusiformis’ cultivation medium. In contrast to LB plates, the other media, particularly Zarrouk+, indicated bacterial contamination. We conclude that LB-agar plates are inappropriate for contamination screening of extremophiles. Contamination was also verified by cultivation-independent methods like flow cytometry and 16S rRNA genome amplicon sequencing. We detected taxa of the phyla Proteobacteria, Bacteriodota, Firmicutes and to a lesser extent Verrucomicrobiota. Contaminants are robust taxa, as they survived aggressive treatments. Sequencing data suggest that some of them are promising candidates for in-depth studies to commercially exploit them.

Abstract Microalgal biomass as a feedstock for biogas production is linked to the parameters biomass productivity and biogas yield. Besides an easy-to-use strain for anaerobic digestion, the photobioreactor (PBR) design is important. A microalgae strain selection revealed Eustigmatos magnus (SAG 36.89) as the most promising strain yielding an average of 100 mg total suspended solids (TSS) L−1 day−1. The strain was tested in cost-effective sleevebag-PBR-systems of 10 cm, 20 cm and 30 cm diameter facing the light from the front or laterally. Highest mean productivity on a volumetric basis was measured in PBRs with the lowest diameter (104 and 117 mg L−1 day−1. The highest productivity per m−2 was achieved in 10 cm PBRs with front light configuration (9.35 g TSS m−2 day−1). The lateral light configuration of 10 cm PBRs had positive aspects such as the lowest mean water demand to produce 1 kg TSS (481 L−1 kg−1) and the lowest mean energy demand for medium separation of 1 kg TSS (106 Wh). The concentrated microalgal biomass was then subjected to ultrasonication and thermal pre-treatment (90 °C and 120 °C) and tested in BMP tests. Mesophilic anaerobic mono-digestion of untreated microalgae biomass led to a methane (CH4) yield of 343 L−1 kg−1 volatile solids (VS). Thermal pre-treatment at 120 °C resulted in significantly increased CH4 yields of 430 L−1 kg−1 VS. As thermal pre-treatment can be easily installed nearby a biogas plant it could be an interesting option for AD of microalgal biomass with only little investment.

L'épuisement des réserves de combustibles fossiles résulte de leur application dans les secteurs industriel et énergétique. En conséquence, des efforts considérables ont été consacrés à favoriser le passage des combustibles fossiles aux sources d'énergie renouvelables par le biais de progrès technologiques dans les processus industriels. Les microalgues peuvent être utilisées pour produire des biocarburants tels que le biodiesel, l'hydrogène et le bioéthanol. Les microalgues sont particulièrement adaptées à la production d'hydrogène en raison de leur taux de croissance rapide, de leur capacité à prospérer dans divers habitats, de leur capacité à résoudre les conflits entre la production de carburant et la production alimentaire, et de leur capacité à capturer et à utiliser le dioxyde de carbone atmosphérique. Par conséquent, la production de biohydrogène à base de microalgues a attiré une attention considérable en tant que carburant propre et durable pour atteindre la neutralité carbone et la durabilité dans la nature. À cette fin, le document de synthèse met l'accent sur les informations récentes relatives à la production de biohydrogène à base de microalgues, aux mécanismes de production durable d'hydrogène, aux facteurs affectant la production de biohydrogène par les microalgues, à la conception du bioréacteur et à la production d'hydrogène, aux stratégies avancées pour améliorer l'efficacité de la production de biohydrogène par les microalgues, ainsi qu'aux goulots d'étranglement et aux perspectives pour surmonter les défis. Cette revue vise à rassembler les avancées et les nouvelles connaissances apparues ces dernières années pour la production de biohydrogène à base de microalgues et à promouvoir l'adoption du biohydrogène comme alternative aux biocarburants hydrocarbonés conventionnels, accélérant ainsi l'objectif de neutralité carbone le plus avantageux pour l'environnement. El agotamiento de las reservas de combustibles fósiles ha sido el resultado de su aplicación en los sectores industrial y energético. Como resultado, se han dedicado esfuerzos sustanciales a fomentar el cambio de los combustibles fósiles a las fuentes de energía renovables a través de los avances tecnológicos en los procesos industriales. Las microalgas se pueden utilizar para producir biocombustibles como biodiesel, hidrógeno y bioetanol. Las microalgas son particularmente adecuadas para la producción de hidrógeno debido a su rápida tasa de crecimiento, su capacidad para prosperar en diversos hábitats, su capacidad para resolver conflictos entre la producción de combustibles y alimentos, y su capacidad para capturar y utilizar el dióxido de carbono atmosférico. Por lo tanto, la producción de biohidrógeno a base de microalgas ha atraído una atención significativa como combustible limpio y sostenible para lograr la neutralidad de carbono y la sostenibilidad en la naturaleza. Con este fin, el documento de revisión enfatiza la información reciente relacionada con la producción de biohidrógeno a base de microalgas, los mecanismos de producción sostenible de hidrógeno, los factores que afectan la producción de biohidrógeno por parte de las microalgas, el diseño del biorreactor y la producción de hidrógeno, las estrategias avanzadas para mejorar la eficiencia de la producción de biohidrógeno por parte de las microalgas, junto con los cuellos de botella y las perspectivas para superar los desafíos. Esta revisión tiene como objetivo recopilar los avances y nuevos conocimientos surgidos en los últimos años para la producción de biohidrógeno a base de microalgas y promover la adopción de biohidrógeno como alternativa a los biocombustibles de hidrocarburos convencionales, acelerando así el objetivo de neutralidad de carbono que es más ventajoso para el medio ambiente. The depletion of fossil fuel reserves has resulted from their application in the industrial and energy sectors. As a result, substantial efforts have been dedicated to fostering the shift from fossil fuels to renewable energy sources via technological advancements in industrial processes. Microalgae can be used to produce biofuels such as biodiesel, hydrogen, and bioethanol. Microalgae are particularly suitable for hydrogen production due to their rapid growth rate, ability to thrive in diverse habitats, ability to resolve conflicts between fuel and food production, and capacity to capture and utilize atmospheric carbon dioxide. Therefore, microalgae-based biohydrogen production has attracted significant attention as a clean and sustainable fuel to achieve carbon neutrality and sustainability in nature. To this end, the review paper emphasizes recent information related to microalgae-based biohydrogen production, mechanisms of sustainable hydrogen production, factors affecting biohydrogen production by microalgae, bioreactor design and hydrogen production, advanced strategies to improve efficiency of biohydrogen production by microalgae, along with bottlenecks and perspectives to overcome the challenges. This review aims to collate advances and new knowledge emerged in recent years for microalgae-based biohydrogen production and promote the adoption of biohydrogen as an alternative to conventional hydrocarbon biofuels, thereby expediting the carbon neutrality target that is most advantageous to the environment. وقد نتج استنفاد احتياطيات الوقود الأحفوري عن استخدامها في قطاعي الصناعة والطاقة. ونتيجة لذلك، تم تكريس جهود كبيرة لتعزيز التحول من الوقود الأحفوري إلى مصادر الطاقة المتجددة من خلال التقدم التكنولوجي في العمليات الصناعية. يمكن استخدام الطحالب الدقيقة لإنتاج الوقود الحيوي مثل الديزل الحيوي والهيدروجين والإيثانول الحيوي. الطحالب الدقيقة مناسبة بشكل خاص لإنتاج الهيدروجين بسبب معدل نموها السريع، وقدرتها على الازدهار في موائل متنوعة، والقدرة على حل النزاعات بين إنتاج الوقود والغذاء، والقدرة على التقاط واستخدام ثاني أكسيد الكربون في الغلاف الجوي. لذلك، اجتذب إنتاج الهيدروجين الحيوي القائم على الطحالب الدقيقة اهتمامًا كبيرًا كوقود نظيف ومستدام لتحقيق الحياد الكربوني والاستدامة في الطبيعة. ولتحقيق هذه الغاية، تؤكد ورقة المراجعة على المعلومات الحديثة المتعلقة بإنتاج الهيدروجين الحيوي القائم على الطحالب الدقيقة، وآليات إنتاج الهيدروجين المستدام، والعوامل التي تؤثر على إنتاج الهيدروجين الحيوي بواسطة الطحالب الدقيقة، وتصميم المفاعل الحيوي وإنتاج الهيدروجين، والاستراتيجيات المتقدمة لتحسين كفاءة إنتاج الهيدروجين الحيوي بواسطة الطحالب الدقيقة، إلى جانب الاختناقات ووجهات النظر للتغلب على التحديات. تهدف هذه المراجعة إلى جمع التقدم والمعرفة الجديدة التي ظهرت في السنوات الأخيرة لإنتاج الهيدروجين الحيوي القائم على الطحالب الدقيقة وتعزيز اعتماد الهيدروجين الحيوي كبديل للوقود الحيوي الهيدروكربوني التقليدي، وبالتالي تسريع هدف حياد الكربون الأكثر فائدة للبيئة.

Atelomixis influences phytoplankton composition in regions where day–night temperature variations are high. Although this physical process is shown as the most important driver in a number of tropical–subtropical lake systems, information on tropical high-mountain lakes is largely lacking. We therefore studied the phytoplankton community composition and the underlying limnological variables of the atelomictic tropical high-mountain Lake Wonchi (Ethiopia) for 13 months. Nutrient levels indicated oligotrophic conditions with ammonium being the primary nitrogen form. The phytoplankton community comprised 53 taxa distributed in five taxonomic divisions, which could be assigned to 15 codas based on phytoplankton functional group classification. Partial atelomixis and low nutrient concentrations seemed to be key variables in structuring phytoplankton community composition, which was quite stable and characterized by high diversity of heavy, immobile and fast-sinking planktonic desmids of the N A codon during mixing followed by diatoms (MP codon). A near-monospecific bloom of Peridinium cinctum (Lo codon) prevailed before the onset of lake turnover in December 2011 with conditions of nutrient depletion, which was later followed by the functional groups F–J during the episode of complete mixing and then replaced by the N A codon. Non-metric multidimensional scaling resulted in a 2-dimensional solution, which revealed clear segregation of phytoplankton community to five groups. Mixing regime of the water column, conductivity, total phosphorus, ammonium and zooplankton had significant influence on the observed seasonal pattern.

Rapidly expanding industrialization and the depletion of non-renewable fossil fuels have necessitated the discovery of feasible renewable alternatives to meet the rising energy demand while reducing carbon dioxide (CO2) emissions. The present global energy strategy is built on cost-effective and environmentally friendly alternatives; and production of microalgae has the ability to meet these requirements. Microalgae have been found as a promising and sustainable alternative for treating wastewater (WW) concurrently with biofuel production. One potential strategy, which uses microalgae for lowering the level of contamination in WW is called bioremediation. There are substantial gains to be made for both the economy and the environment through the integration of microalgae-based biofuel production with wastewater treatment (WWT). The use of microalgae that have a short life span, a high growth rate, and a high CO2 usage efficiency is one of the promising approaches for producing biomass from WW nutrients that involves the utilization of renewable resources. Microalgae are one of the most promising biomass resources for use in thermochemical conversion processes for the production of liquid and gaseous biofuels due to their advantages over other biomass feedstocks, such as sustainability, renewability, and productivity. Currently, technology and cost are the primary obstacles limiting industrial applicability, which necessitates an optimum downstream process to minimize production costs. Consequently, the concurrent utilization of microalgae for WWT and biofuel production has made these challenges practical and economically viable. This review provides an overview of microalgae and their bioremediation and bioenergy production applications. It also provides insight for future research to investigate additional possible applications of microalgal biomass. These applications could include not only the bioremediation process, but also the generation of revenues from microalgae through the incorporation of clean and green technology, which would provide long-term sustainability and environmental benefits.

Abstract Microalgae can be manipulated to accumulate certain cellular compounds of interest without the need for genetic modification - simply by controlling growth parameters such as nitrogen (N). Therefore, A. obliquus was batch-cultivated in a sleevebag photobioreactor system in N-deprived and N-rich medium to test the effect of N-status on CH 4 yield under anaerobic digestion. Two different dewatering methods, i.e., centrifugation and sedimentation were applied to each resulting biomass. For the N-deprived biomass, cellular protein content dropped by 42–49%, and lipids increased up to 20% relative to the N-rich biomass. The highest CH 4 yields were achieved with N-deprived, sedimented biomass (391 Nm 3 t − 1 VS = normalized gas volume in m 3 corrected to norm temperature and pressure per unit volatile solids), followed by N-rich sedimented biomass (361 Nm 3 t − 1 VS). Centrifugation led to lower CH 4 yields, where N-deprived biomass achieved 280 compared to 200 Nm 3 t − 1 VS for N-rich microalgae. Our data indicate that valuable organic material was lost to the supernatant during the centrifugation step. We conclude that not only the N-status of cultivation, but also the biomass dewatering method has an instrumental effect on CH 4 yield of microalgal biomass in anaerobic digestion.

Abstract Trophic cascade effects occur when a food web is disrupted by loss or significant reduction of one or more of its members. In East African Rift Valley lakes, the Lesser Flamingo is on top of a short food chain. At irregular intervals, the dominance of their most important food source, the cyanobacterium Arthrospira fusiformis, is interrupted. Bacteriophages are known as potentially controlling photoautotrophic bacterioplankton. In Lake Nakuru (Kenya), we found the highest abundance of suspended viruses ever recorded in a natural aquatic system. We document that cyanophage infection and the related breakdown of A. fusiformis biomass led to a dramatic reduction in flamingo abundance. This documents that virus infection at the very base of a food chain can affect, in a bottom-up cascade, the distribution of end consumers. We anticipate this as an important example for virus-mediated cascading effects, potentially occurring also in various other aquatic food webs.