Outdoor microalgae cultivation in airlift photobioreactor at high irradiance and temperature conditions: effect of batch and fed-batch strategies, photoinhibition, and temperature stress

Recent advancements in wastewater treatment via anaerobic fermentation process: A systematic review

This manuscript delves into the realm of wastewater treatment, with a particular emphasis on anaerobic fermentation processes, especially dark, photo, and dark-photo fermentation processes, which have not been covered and overviewed previously in the literature regarding the treatment of wastewater. Moreover, the study conducts a bibliometric analysis for the first time to elucidate the research landscape of anaerobic fermentation utilization in wastewater purification. Furthermore, microorganisms, ranging from microalgae to bacteria and fungi, emphasizing the integration of these agents for enhanced efficiency, are all discussed and compared. Various bioreactors, such as dark and photo fermentation bioreactors, including tubular photo bioreactors, are scrutinized for their design and operational intricacies. The results illustrated that using clostridium pasteurianum CH4 and Rhodopseudomonas palustris WP3-5 in a combined dark-photo fermentation process can treat wastewater to a pH of nearly 7 with over 90% COD removal. Also, integrating Chlorella sp and Activated sludge can potentially treat synthetic wastewater to COD, P, and N percentage removal rates of 99%,86%, and 79%, respectively. Finally, the paper extends to discuss the limitations and future prospects of dark-photo fermentation processes, offering insights into the road ahead for researchers and scientists.

Overexpression of putative glutathione peroxidase from Neopyropia-associated microorganisms in Chlamydomonas to respond to abiotic stress

Lipid accumulation in microalgae can be substantially enhanced by exposing the microalgae to abiotic stress, thus increasing biofuel production. However, this also generates reactive oxygen species (ROS), which disrupts cell metabolism and reduces their productivity. Previous mRNA sequencing analyses in Neopyropia yezoensis and its associated microorganisms elucidated a putative glutathione peroxidase (PuGPx) gene. Here, this putative glutathione peroxidase was overexpressed in the microalga Chlamydomonas reinhardtii, which increased cell growth and survival rates compared to the control group under abiotic stress. Additionally, increased lipid accumulation was observed under salinity stress, high-temperature stress, and hydrogen peroxide (H₂O₂)-induced oxidative stress. These results suggest that PuGPx plays a protective role against abiotic stress in C. reinhardtii and stimulates lipid accumulation, which could be considered advantageous in terms of biofuel production.

Microalgal remediation and valorisation of polluted wastewaters for zero-carbon circular bioeconomy

Overexploitation of natural resources to meet human needs has considerably impacted CO₂ emissions, contributing to global warming and severe climatic change. This review furnishes an understanding of the sources, brutality, and effects of CO₂ emissions and compelling requirements for metamorphosis from a linear to a circular bioeconomy. A detailed emphasis on microalgae, its types, properties, and cultivation are explained with significance in attaining a zero-carbon circular bioeconomy. Microalgal treatment of a variety of wastewaters with the conversion of generated biomass into value-added products such as bio-energy and pharmaceuticals, along with agricultural products is elaborated. Challenges encountered in large-scale implementation of microalgal technologies for low-carbon circular bioeconomy are discussed along with solutions and future perceptions. Emphasis on the suitability of microalgae in wastewater treatment and its conversion into alternate low-carbon footprint bio-energies and value-added products enforcing a zero-carbon circular bioeconomy is the major focus of this review.

Process engineering strategy for large scale outdoor cultivation of Tetradesmus obliquus CT02 coupled with pH guided CO2 feeding

A novel CO₂ tolerant microalga Tetradesmus obliquus CT02, was previously evaluated to be a suitable bio refinery platform for synthesis of bioactive molecules, biodiesel, and biofertilizer. In the present study, a process engineering strategy was developed targeting improved growth performance of the strain at large scale under fluctuating outdoor environmental conditions. The strategy relies on maintaining pH of the culture at its optimal value via cascade control with CO₂ feeding. The strategy was developed at laboratory scale bubble column photobioreactor under diurnal variation of simulated sunlight intensity and was further validated through growth performance of the strain under outdoor conditions in a 100 L airlift bioreactor. Under laboratory condition, 53.3% and 85.16% improvement in biomass concentration (1.87 g L^-1) and productivity (114.8 mg L^-1 day^-1) was achieved as compared to the uncontrolled pH, respectively. The strategy demonstrated a significant improvement in biomass concentration and productivity by 225.7% and 121.6% respectively, compared to the pH uncontrolled batch, even under outdoor fluctuating environmental condition.

Production, Processing, and Protection of Microalgal n-3 PUFA-Rich Oil

Microalgae have been increasingly considered as a sustainable “biofactory” with huge potentials to fill up the current and future shortages of food and nutrition. They have become an economically and technologically viable solution to produce a great diversity of high-value bioactive compounds, including n-3 polyunsaturated fatty acids (PUFA). The n-3 PUFA, especially eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), possess an array of biological activities and positively affect a number of diseases, including cardiovascular and neurodegenerative disorders. As such, the global market of n-3 PUFA has been increasing at a fast pace in the past two decades. Nowadays, the supply of n-3 PUFA is facing serious challenges as a result of global warming and maximal/over marine fisheries catches. Although increasing rapidly in recent years, aquaculture as an alternative source of n-3 PUFA appears insufficient to meet the fast increase in consumption and market demand. Therefore, the cultivation of microalgae stands out as a potential solution to meet the shortages of the n-3 PUFA market and provides unique fatty acids for the special groups of the population. This review focuses on the biosynthesis pathways and recombinant engineering approaches that can be used to enhance the production of n-3 PUFA, the impact of environmental conditions in heterotrophic cultivation on n-3 PUFA production, and the technologies that have been applied in the food industry to extract and purify oil in microalgae and protect n-3 PUFA from oxidation.

Functional Diversity Facilitates Stability Under Environmental Changes in an Outdoor Microalgal Cultivation System

Functionally uniform monocultures have remained the paradigm in microalgal cultivation despite the apparent challenges to avoid invasions by other microorganisms. A mixed microbial consortium approach has the potential to optimize and maintain biomass production despite of seasonal changes and to be more resilient toward contaminations. Here we present a 3-year outdoor production of mixed consortia of locally adapted microalgae and bacteria in cold temperate latitude. Microalgal consortia were cultivated in flat panel photobioreactors using brackish Baltic Sea water and CO₂ from a cement factory (Degerhamn, Cementa AB, Heidelberg Cement Group) as a sustainable CO₂ source. To evaluate the ability of the microbial consortia to maintain stable biomass production while exposed to seasonal changes in both light and temperature, we tracked changes in the microbial community using molecular methods (16S and 18S rDNA amplicon sequencing) and monitored the biomass production and quality (lipid, protein, and carbohydrate content) over 3 years. Despite changes in environmental conditions, the mixed consortia maintained stable biomass production by alternating between two different predominant green microalgae (Monoraphidium and Mychonastes) with complementary tolerance to temperature. The bacterial population was few taxa co-occured over time and the composition did not have any connection to the shifts in microalgal taxa. We propose that a locally adapted and mixed microalgal consortia, with complementary traits, can be useful for optimizing yield of commercial scale microalgal cultivation.

Irradiance-driven 20-hydroxyecdysone production and morphophysiological changes in Pfaffia glomerata plants grown in vitro

Pfaffia glomerata possesses potential pharmacological and medicinal properties, mainly owing to the secondary metabolite 20-hydroxyecdysone (20E). Increasing production of biomass and 20E is important for industrial purposes. This study aimed to evaluate the influence of irradiance on plant morphology and production of 20E in P. glomerata grown in vitro. Nodal segments of accessions 22 and 43 (Ac22 and Ac43) were inoculated in culture medium containing MS salts and vitamins. Cultures were maintained at 25 ± 2 °C under a 16-h photoperiod and subjected to irradiance treatments of 65, 130, and 200 μmol m^-2 s^-1 by fluorescent lamps. After 30 days, growth parameters, pigment content, stomatal density, in vitro photosynthesis, metabolites content, and morphoanatomy were assessed. Notably, Ac22 plants exhibited 10-fold higher 20E production when cultivated at 200 μmol m^-2 s^-1 than at 65 μmol m^-2 s^-1, evidencing the importance of light quantity for the accumulation of this metabolite. 20E production was twice as high in Ac22 as in Ac43 plants although both accessions responded positively to higher irradiance. Growth under 200 μmol m^-2 s^-1 stimulated photosynthesis and consequent biomass accumulation, but lowered carotenoids and anthocyanins. Furthermore, increasing irradiance enhanced the number of palisade and spongy parenchyma cells, enhancing the overall growth of P. glomerata. Graphical abstract.

Bioprocess strategies for enhancing the outdoor production of Nannochloropsis gaditana: an evaluation of the effects of pH on culture performance in tubular photobioreactors

A priority of the industrial applications of microalgae is the reduction of production costs while maximizing algae biomass productivity. The purpose of this study was to carry out a comprehensive evaluation of the effects of pH control on the production of Nannochloropsis gaditana in tubular photobioreactors under external conditions while considering the environmental, biological, and operational parameters of the process. Experiments were carried out in 3.0 m³ tubular photobioreactors under outdoor conditions. The pH values evaluated were 6.0, 7.0, 8.0, 9.0, and 10.0, which were controlled by injecting pure CO₂ on-demand. The results have shown that the ideal pH for microalgal growth was 8.0, with higher values of biomass productivity (P_b) (0.16 g L^-1 d^-1), and CO₂ use efficiency ([Formula: see text]) (74.6% w w^-1); [Formula: see text]_/biomass value obtained at this pH (2.42 [Formula: see text] g_biomass^-1) was close to the theoretical value, indicating an adequate CO₂ supply. At this pH, the system was more stable and required a lower number of CO₂ injections than the other treatments. At pH 6.0, there was a decrease in the P_b and [Formula: see text]; cultures at pH 10.0 exhibited a lower P_b and photosynthetic efficiency as well. These results imply that controlling the pH at an optimum value allows higher CO₂ conversions in biomass to be achieved and contributes to the reduction in costs of the microalgae production process.

Alternative fertilizer-based growth media support high lipid contents without growth impairment in Scenedesmus obliquus BR003

Nitrogen (N) sources have been target in microalgae cultivation studies, considering their nutritional impact on growth and high costs. Here, we have evaluated the growth of Scenedesmus obliquus BR003, applying alternative low-cost culture media containing ammonium and urea, or combinations of both N sources. The culture media were applied for indoor and outdoor cultivation, followed by growth analyses and metabolic characterization. The alternative culture media B4 and L4 supported higher biomass production (1.4 g L^-1) compared to BG11 (nitrate-based medium). In addition, the lipid percentage was higher for B4 (ammonium-based culture medium), reaching up to 25% DW. High contents of carbohydrates (60%) and proteins (40%) were also obtained in media with ammonium and urea, respectively. Considering the lower costs of alternative fertilizer-based media, using ammonium and/or urea as N sources, and the high lipid content observed, we suggest these media as viable for large-scale production of S. obliquus.

Current practices and challenges in using microalgae for treatment of nutrient rich wastewater from agro-based industries

Considerable research activities are underway involving microalgae species in order to treat industrial wastewater to address the waste-to-bioenergy economy. Several studies of wastewater treatment using microalgae have been primarily focused on removal of key nutrients such as nitrogen and phosphorus. Although the use of wastewater would provide nutrients and water for microalgae growth, the whole process is even more complex than the conventional microalgae cultivation on freshwater media. The former one adds several gridlocks to the system. These gridlocks are surplus organic and inorganic nutrients concentration, pH of wastewater, wastewater color, total dissolved solids (TDS), microbial contaminants, the scale of photobioreactor, batch versus continuous system, harvesting of microalgae biomass etc. The present review discusses, analyses, and summarizes key aspects involved in the treatment of wastewaters from distillery, food/snacks product processing, and dairy processing industry using microalgae along with sustainable production of its biomass. This review further evaluates the bottlenecks for individual steps involved in the process such as pretreatment of wastewater for contaminants removal, concentration tolerance/dilutions, harvesting of microalgae biomass, and outdoor scale-up. The review also describes various strategies to optimize algal biomass and lipid productivities for various wastewater and photobioreactor type. Moreover, the review emphasizes the potential of co-cultivation of microorganism such as yeast and bacteria along with microalgae in the treatment of industrial wastewater.