Enhanced lipid production in thermo-tolerant mutants of Chlorella pyrenoidosa NCIM 2738

Microalgae-based flue gas CO2 sequestration for cleaner environment and biofuel feedstock production: a review

Anthropogenic CO2 emissions are the prime cause of global warming and climate change, promoting researchers to develop suitable technologies to reduce carbon footprints. Among various CO2 sequestration technologies, microalgal-based methods are found to be promising due to their easier operation, environmental benefits, and simpler equipment requirements. Microalgae-based carbon capture and storage (CCS) technology is essential for addressing challenges related to the use of industrial-emitted flue gases. This review focuses on the literature concerning the microalgal application for CO2 sequestration. It highlights the primary physiochemical parameters that affect microalgal-based CO2 biofixation, including light exposure, microalgal strain, temperature, inoculum size, pH levels, mass transfer, CO2 concentration, flow rate, cultivation system, and mixing mechanisms. Moreover, the inhibition effect of different flue gas components including NOx, SOx, and Hg on growth kinetics is discussed to enhance the capacity of microalgal-based CO2 biofixation, along with deliberated challenges and prospects for future development. Overall, the review indicated microalgal-based flue gas CO2 fixation rates range from 80 mg L-1 day-1 to over 578 mg L-1 day-1, primarily influenced by physiochemical parameters and flue gas composition. This article summarizes the mechanisms and stages of microalgal-based CO2 sequestration and provides a comprehensive review based on international interest in this green technology.

Chemical mutagenesis and thermal selection of coral photosymbionts induce adaptation to heat stress with trait trade-offs

Despite the relevance of heat-evolved microalgal endosymbionts to coral reef restoration, to date, few Symbiodiniaceae strains have been thermally enhanced via experimental evolution. Here, we investigated whether the thermal tolerance of Symbiodiniaceae can be increased through chemical mutagenesis followed by thermal selection. Strains of Durusdinium trenchii, Fugacium kawagutii and Symbiodinium pilosum were exposed to ethyl methanesulfonate to induce random mutagenesis, and then underwent thermal selection at high temperature (31/33°C). After 4.6-5 years of experimental evolution, the in vitro thermal tolerance of these strains was assessed via reciprocal transplant experiments to ambient (27°C) and elevated (31/35°C) temperatures. Growth, photosynthetic efficiency, oxidative stress and nutrient use were measured to compare thermal tolerance between strains. Heat-evolved D. trenchii, F. kawagutii and S. pilosum strains all exhibited increased photosynthetic efficiency under thermal stress. However, trade-offs in growth rates were observed for the heat-evolved D. trenchii lineage at both ambient and elevated temperatures. Reduced phosphate and nitrate uptake rates in F. kawagutii and S. pilosum heat-evolved lineages, respectively, suggest alterations in nutrition resource usage and allocation processes may have occurred. Increased phosphate uptake rates of the heat-evolved D. trenchii strain indicate that experimental evolution resulted in further trade-offs in this species. These findings deepen our understanding of the physiological responses of Symbiodiniaceae cultures to thermal selection and their capacity to adapt to elevated temperatures. The new heat-evolved Symbiodiniaceae developed here may be beneficial for coral reef restoration efforts if their enhanced thermal tolerance can be conferred in hospite.

Thermal-tolerant potential of ordinary Chlorella pyrenoidosa and the promotion of cell harvesting by heterotrophic cultivation at high temperature

During the heterotrophic cultivation of microalgae, a cooled process against temperature rise caused by the metabolism of exogenous organic carbon sources greatly increases cultivation cost. Furthermore, microalgae harvesting is also a cost-consuming process. Cell harvesting efficiency is closely related to the characteristics of the algal cells. It may be possible to change cell characteristics through controlling culture conditions to make harvesting easier. In this study, the mesophilic Chlorella pyrenoidosa was found to be a thermal-tolerant species in the heterotrophic mode. The cells could maintain their maximal specific growth rate at 40°C and reached 1.45 day−1, which is equivalent to that of cultures at 35°C but significantly higher than those cultured at lower temperatures. Interestingly, the cells cultured at 40°C were much easier to be harvested than those at lower temperatures. The harvesting efficiency of the cells cultured at 40°C reached 96.83% after sedimentation for 240 min, while the cells cultured at lower temperatures were reluctant to settle. Likely, the same circumstance occurred when cells were harvested by centrifugation or flocculation. The promotion of cell harvesting for cells cultured at high temperatures was mainly attributed to increased cell size and decreased cell surface charge. To the best of our knowledge, this is the first report that cells cultured at high temperatures can promote microalgae harvesting. This study explores a new approach to simplify the cultivation and harvesting of microalgae, which effectively reduces the microalgae production cost.

Physiological insights into enhanced lipid accumulation and temperature tolerance by Tetraselmis suecica ultraviolet mutants

High outdoor temperatures significantly inhibit the growth and lipid production of the industrially promising marine microalga Tetraselmis suecica, which is viewed as a potential feedstock for high-value bioproducts and biofuels. To overcome this limitation, T. suecica was subjected to ultraviolet irradiation to generate mutants capable of being productive at higher temperatures. The top two high-lipid mutants UV-25 and UV-31 isolated at 25 °C and 31 °C, respectively, were compared to the wild type (WT) to delineate physiological alterations and shed light on the mutants' increased biomass and lipid productivity. At 25 °C, UV-25 and UV-31 exhibited lipid productivity of 36.12 and 31.33 mg/L day, which were 1.4- and 1.2-fold higher than WT, respectively. This increase in lipid biosynthesis correlated well with increased carotenoid content in UV-25 (2.2-fold) and UV-31 (3.6-fold), indicating an improved capacity to quench reactive oxygen species. At 31 °C, the growth and lipid accumulation of UV-31 remained high, signifying adaptation to higher temperatures. This is attributed to a well-coordinated modulation of the mutant's cellular metabolism through an increase in galactose and phosphatidylglycerol levels, as well as in protein, all of which contributed to its performance at elevated temperatures. The study successfully established a UV mutagenesis strategy for producing superior- performing microalgae strains with industrially desired traits, paving the way for future outdoor cultivation deployment.

Random Mutagenesis as a Promising Tool for Microalgal Strain Improvement towards Industrial Production

Microalgae have become a promising novel and sustainable feedstock for meeting the rising demand for food and feed. However, microalgae-based products are currently hindered by high production costs. One major reason for this is that commonly cultivated wildtype strains do not possess the robustness and productivity required for successful industrial production. Several strain improvement technologies have been developed towards creating more stress tolerant and productive strains. While classical methods of forward genetics have been extensively used to determine gene function of randomly generated mutants, reverse genetics has been explored to generate specific mutations and target phenotypes. Site-directed mutagenesis can be accomplished by employing different gene editing tools, which enable the generation of tailor-made genotypes. Nevertheless, strategies promoting the selection of randomly generated mutants avoid the introduction of foreign genetic material. In this paper, we review different microalgal strain improvement approaches and their applications, with a primary focus on random mutagenesis. Current challenges hampering strain improvement, selection, and commercialization will be discussed. The combination of these approaches with high-throughput technologies, such as fluorescence-activated cell sorting, as tools to select the most promising mutants, will also be discussed.

Use of reverse osmosis reject from drinking water plant for microalgal biomass production

The present study evaluates the use of reverse osmosis (RO) reject, termed as ROR, for microalgal biomass production. The supplementation of ROR from two different sources, namely domestic RO unit (ROR1) and commercial-scale RO plant (ROR2), showed a synergistic effect on the growth and biochemical composition of Chlorella pyrenoidosa. Among the tested ROR1 doses, the highest biomass production (1.27±0.06 g L^-1) was observed with 25% ROR1 supplemented growth media. In contrast, the lipid content (28.85±3.13% of TS) in C. pyrenoidosa at 50% ROR1 dose was almost twice that in BG11 (positive control). Interestingly, the microalgae showed relatively higher biomass production (1.37±0.07 g L^-1) and higher lipid content (33.23±3.92% of TS) when 50% ROR2 was used in growth media. At the same time, the estimated carbohydrate and protein contents were 28.41±0.73 and 29.75±0.31% of TS, respectively. Furthermore, the lipid productivity (28.98±2.79 mg L^-1 d^-1) was relatively higher than the nutrient media (12.35±1.34 mg L^-1 d^-1). The present findings revealed that the RO reject from drinking water purifiers can efficiently be utilized for lipid-rich microalgal biomass production. Hence, the dependency on freshwater resources for mass scale microalgae cultivation through recycling of RO reject can be reduced.

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.

Sustainable development of microalgal biotechnology in coastal zone for aquaculture and food

Region-specific Research and Development (R&D) of microalga-derived product systems are crucial if "biotech's green gold" is to be explored in a rational and economically viable way. Coastal zones, particularly the locations around the equator, are typically considered to be optimum cultivation sites due to stable annual temperature, light, and ready availability of seawater. However, a 'cradle-to-grave' assessment of the development of microalgal biotechnology in these areas, not only under the laboratory conditions, but also in the fields has not yet been demonstrated. In this study, to evaluate the viability of microalga-derived multi-product technology, we showed the development of microalgal biotechnology in coastal zones for aquaculture and food. By creating and screening a (sub)tropical microalgal collection, a Chlorella strain MEM25 with a robust growth in a wide range of salinities, temperatures, and light intensities was identified. Evaluation of the economic viability and performance of different scale cultivation system designs (500 L and 5000 L closed photobioreactors and 60,000 L open race ponds, ORPs) at coastal zones under geographically specific conditions showed the stable and robust characteristics of MEM25 across different production system designs and various spatial and temporal scales. It produces high amounts of proteins and polyunsaturated fatty acids (PUFAs) in various conditions. Feeding experiments reveal the nutritional merits of MEM25 as food additives where PUFAs and essential amino acids are enriched and the algal diet improves consumers' growth. Economic evaluation highlights an appreciable profitability of MEM25 production as human or animal food using ORP systems. Therefore, despite the pros and cons, sound opportunities exist for the development of market-ready multiple-product systems by employing region-specific R&D strategies for microalgal biotechnology.

Harnessing the Power of Mutagenesis and Adaptive Laboratory Evolution for High Lipid Production by Oleaginous Microalgae and Yeasts

Oleaginous microalgae and yeasts represent promising candidates for large-scale production of lipids, which can be utilized for production of drop-in biofuels, nutraceuticals, pigments, and cosmetics. However, low lipid productivity and costly downstream processing continue to hamper the commercial deployment of oleaginous microorganisms. Strain improvement can play an essential role in the development of such industrial microorganisms by increasing lipid production and hence reducing production costs. The main means of strain improvement are random mutagenesis, adaptive laboratory evolution (ALE), and rational genetic engineering. Among these, random mutagenesis and ALE are straight forward, low-cost, and do not require thorough knowledge of the microorganism’s genetic composition. This paper reviews available mutagenesis and ALE techniques and screening methods to effectively select for oleaginous microalgae and yeasts with enhanced lipid yield and understand the alterations caused to metabolic pathways, which could subsequently serve as the basis for further targeted genetic engineering.

Embedding photosynthetic biorefineries with circular economies: Exploring the waste recycling potential of Arthrospira sp. to produce high quality by-products

This study was conducted with the aim of embedding circular economies (waste recycling) with photosynthetic biorefineries, for production of commercially viable by-products. Since nitrogen source constitute the major input costs for commercial Arthrospira sp. production, the use of nitrogen rich wastewater for Arthrospira sp. cultivation could significantly reduce their production costs. This study evaluated the effects of high concentrations (8.5-120 mM) of alternative nitrogen sources (urea, ammonium and nitrite) on the biochemical, pigment and proteomic profile of Arthrospira sp., under batch and continuous conditions. Arthrospira sp. cells fed with urea were quantified with modified biochemical and proteomic profile compared to the nitrate fed cells. No inhibitory effect of urea was observed on the biomass even at 120 mM. Nitrite fed cells exhibited comparable biochemical and proteomic profiles as nitrate fed cells. These results clearly indicated at the possibility of using urea rich wastewater streams for profitable cultivation of Arthrospira sp.

Assessment of transient effects of alternative nitrogen sources in continuous cultures of Arthrospira sp. using proteomic, modeling and biochemical tools

The ability of cyanobacterium Arthrospira sp. to assimilate waste nitrogen sources (ammonium and urea) makes it an important candidate for wastewater management. The aim of this work was to evaluate a cultivation approach based on continuous-transitional-feeding regime (nitrate-ammonium-nitrate) in a photobioreactor to assess the effects of ammonium salts on Arthrospira sp. PCC 8005 metabolism. Using a comprehensive biochemical, proteomic and stoichiometric profiling of biomass, this study demonstrated that the proposed cultivation approach could increase the proteins and pigments yields by 20-30%, compared to the respective yields obtained from wild-type Arthrospira sp. strain A light-energy-transfer model was used to predict the biomass and oxygen productivities of Arthrospira sp. cultivated under transitional-feeding regime. 95 ± 2% match was observed between the experimental and simulated productivities. This study thus opened new avenues for use of ammonium rich wastewater for commercial production of high value pigments, biofuel and bioplastics using Arthrospira sp.

Augmented lipid accumulation in ethyl methyl sulphonate mutants of oleaginous microalga for biodiesel production

The aim of this work was to generate high lipid accumulating mutants of Chlorella minutissima (CM) using ethyl methyl sulphonate (EMS) as a random chemical mutagen. Amid the 5% surviving cells after exposure to EMS (2M), three fast growing mutants (CM2, CM5, CM7) were selected and compared with wild type for lipid productivity and biochemical composition. Among these mutants, CM7 showed the maximum biomass (2.4g/L) and lipid content (42%) as compared to wild type (1.5g/L; 27%). Further, the mutant showed high photosynthetic pigments with low starch content signifying the re-allocation of carbon flux to lipid. The obtained mutant showed no visible morphological changes in comparison to its WT. The fatty acid profile showed increase in monounsaturated fatty acids while decreased saturated and polyunsaturated fatty acids signifying good quality biodiesel. The mutant strain thus obtained can be optimized further and applied for enhanced biodiesel production.