In the pursuit of advancing sustainable biofuel production, the co-cultivation of microalgae and oleaginous yeast has emerged as a promising strategy to enhance lipid yield. This study aimed to optimize the flocculation process using different flocculants to improve the harvesting efficiency of three distinct co-culture combinations: Chlorella sorokiniana with Rhodotorula glutinis, Scenedesmus obliquus with Yarrowia lipolytica, and Chlorella protothecoides with Trichosporon pullulans. The co-cultures were flocculated using a selection of flocculants, including Chitosan, Ferric chloride, Ferrous sulphate, and Aluminium sulphate. A systematic investigation was conducted using varying parameters such as pH, flocculant concentration, and flocculation time utilizing the Central Composite Design Methodology (RSM) to achieve optimal flocculation efficiency, Results demonstrated that Chitosan achieved the maximum efficiency of 96 % at 0.1 gL-1, while Al2(SO4)3 reached up to 95 % at 0.5 gL-1 under pH 6.0. In contrast, FeCl3 and FeSO4 exhibited a lower flocculation efficiency of 79 % and 92 %, respectively. The experimental results and the model output indicate the significant interactions between variables that impact flocculation efficiency. The regression models demonstrated predictive solid capabilities, with R2 values exceeding 0.94 with chitosan for all the cocultures, confirming that the quadratic model best fits the experimental results. The zeta potential measurements highlighted the negative zeta potential at lower pH levels, indicating reduced electrostatic repulsion and increased flocculation efficiency. These results provide valuable insight into selecting appropriate flocculants and determining optimal conditions to enhance biomass recovery processes in biorefinery applications.

Phosphorus is one of the crucial elements required for the proper functioning of metabolic processes in microalgae. Despite the crucial role of phosphate (P), the dynamics of polyphosphate accumulation with respect to nutrient availability remain unknown in freshwater microalgae. We have investigated three freshwater microalgal strains - Chlorella pyrenoidosa, Scenedesmus obliquus, and Chlamydomonas reinhardtii under varied phosphate treatments to understand the phosphate metabolic responses and polyphosphate dynamics. Our results show that the accumulation of polyP in microalgae is very dynamic and mainly depends on the availability of extracellular phosphate. Reduced P availability showed algal species-specific reduction in the polyphosphate storage with a decline in growth and total chlorophyll content. Further, an increase in lipid and carbohydrate content with a substantial decrease in protein was observed under P stress, suggesting preferential utilization of stored polyP to support cell survival. Species-specific differences in the fatty acid profiles were also observed in the GC analysis among all three algal strains, indicating varied mechanisms happening among the species to adapt and protect themselves against cellular damage under P stress. Our results suggest the existence of natural variability among the selected algal strains in their ability to accumulate polyP and metabolites with respect to P-varied conditions. Among the three microalgal species, Scenedesmus obliquus showed notably enhanced accumulation of polyphosphate, highlighting its potential application as P-rich biofertilizer.

Microalgae being the primary feedstock source, its cells contain lipids with a high amount of triacylglyceride (TAG) potential for biodiesel production. Two microalgal strains of Chlorella sorokiniana and Scenedesmus obliquus were selected for the mono and co-culture growth in the BG 11 medium for 15 days. To enhance lipid productivity, three different disruption techniques, i.e., autoclave, microwave, and sonication, were selected. Later in the study, the transesterified FAME (fatty acid methyl ester) composition of mono- and co-culture was analyzed via gas chromatography with an FID (flame ionization detector) detector to reveal the fatty acids used for biodiesel production. The results show that biomass and lipid productivity from co-cultures were higher than monocultures, with 2.173 g L-1 biomass and 18% of lipid productivity enhanced from sonication disruption technique. The FAME composition determines C14 to C18 fatty acids, mainly leading to the sonication disruption method with co-culture obtaining the value of 78% of saturated fatty acid. Thus, this study reveals the impact of co-cultivation compared to mono-cultivation including the process of disruption of cells for extracting more lipid recovery for biodiesel production

Co-cultivation and effective downstream processing of microalgae and oleaginous yeast are potential strategies to enhance lipid yield. The findings of this work suggest that each cultivable partner should be screened and optimized for the best mutualistic co-cultures. In this case, T. pullulans was found to be the most effective yeast culture with microalgae S. obliquus, C. sorokiniana, and C. protothecoides along with additional cell disruption techniques. Co-cultures of C.protothecoides and T.pullulans had the highest relative lipid yield (RLY) of 68.71%, 59.32%, 55.33%, 53.19%, and 47.3% with sonication, microwave, osmotic shock, freeze-drying, and autoclave, respectively. Sonication was concluded as the best disruption technique with five out of nine co-cultures showing significant lipid enhancement. Even though T. pullulans was the best yeast partner with disruption techniques, co-culture of Y. lipolytica and S.obliquus (RLY 37.11%) produced lipids which was significantly higher than that of their respective monocultures (S. obliquus RLY 23.96%, Y. lipolytica RLY 4.96%) without any disruption process. R. glutinis and Y. lipolytica produced significantly enhanced lipid yield with C.sorokiniana and S. obliquus with sonication (RLY 51.44%) and osmotic shock (RLY 57.70%), respectively. FAME analysis reported that the co-cultures produced higher percentage of total FAME nearly 100% in the case of C. sorokiniana and R. glutinis with high content of saturated fatty acids (SFAs). The presence of biofuel precursors like palmitic acid, linoleic acid, oleic acid, and heptadecanoic acid, confirms their suitability for biofuel production.

Recent development in strategies to overcome the environmental adversities are focused mainly on lowering the carbon footprints and to decrease the global temperature rise. Bioprocesses based on microalgae are promising due to a large spectrum of possible products, like platform chemicals, proteins and higher value products that can directly impart a fall in the carbon emission. Another promising approach towards sustainability is the integrated bio refinery cultivation of microalgae for waste water treatment with simultaneous production of fuels (alcohol and biodiesel), high value chemicals (Astaxanthin, Lutein, polyunsaturated fatty acids (PUFA), monounsaturated fatty acids (MUFA)etc.), proteins etc, but the technology readiness level are rather low and therefore are not established. However, for sustainable, economical and feasible bioprocess strategies using microalgae intra and extracellular bio-components extraction and cell wall disruption techniques should be optimized whereas challenges like high energy consumption involving these stages in the process makes it futile. The efficiency of cell disruption techniques and product extraction varies within different microalgae and depends mainly on the growth conditions and cell wall composition. The current review describes the cell wall structure and its composition of green freshwater microalgae such as Botryococccus braunii, Scenedesmus obliquus, Desmodesmus sp., Chlamydomonas reinhardtii, Chlorella vulgaris, Haematococcus pluvialis and marine forms like Dunaliella salina and Nannocholoropsis gaditana. Further, it highlights the green cell lysis technologies to disrupt cell walls and the future perspectives and challenges of algae in integrated wastewater treatment, CO2 capture, and energy-efficient algal cell wall disruption for utilizing microalgae for fuel production is discussed.

Major environmental issues of current decades focus on lowering the carbon footprints and global temperature rise. Conversion of microalgae biomass to fuel is a promising platform for carbon-neutral biofuels which would decrease the dependency on petrochemicals. The current review describes a general characteristic of freshwater microalgae cell walls. The conventional techniques used to disrupt cell walls and extraction of intra as well as extracellular bio components consume energy. The conventional cell disruption techniques do not result in uniform disruption efficiency in all algal species due to differences in cell wall thickness. Further, the conventional cell disruption methodology applies energy in water rather than the algal cell. For example, microwaves, sonication, and other physio-chemical methods spend significant energy to heat water. The current chapter focuses on solvent-free green technologies such as enzymatic, cell lysis by bacteria, ultraviolet light, high-pressure gases, hot water pretreatment, osmotic shock, milking, and in situ extraction to disrupt cell walls. Further, it also highlights the extraction and fractionation bio components for fuel conversion.

Microalgae have been explored for sustainable production of biofuel and chemicals. Microalgae is promising feed stock for the production of several oleochemicals. It has the ability to utilize a variety of low cost feed stocks, accumulated large quantities of lipids and variety of value added products in their biomass. One of the major obstacles associated with the conversion of algae into value-added products is harvesting. The harvesting of algae is the most problematic area due to its low sedimentation rate, low biomass concentration, and high capital costs. Harvesting of algae is carried out by different physical, chemical, mechanical, biological, and electrolytic methods such as sedimentation, centrifugation, microstraining, dissolved air flotation, electrolytic flotation, chemical flocculation, bioflocculation, autoflocculation, Filtration. This review highlights the various methods of microalgae harvesting with advantages and future perspective of sustainable and cost-effective harvesting of microalgae.