Neha Arora, a Post-Doctoral fellow at the Indian Institute of Technology, writes in that, despite microalgae having great potential for biofuels and mitigation of heavy metals, its utilization on a large scale is still a long road ahead.

The key factors that could expedite the economical production of biofuel and metal bioremediation include increasing the algal biomass productivity, utilization of copious/abundant feedstocks, and cost-effective extraction of lipids/carbohydrates, along with the utilization of residue biomass for high-value products. Utilization of seawater or brackish water that is not suitable for irrigation resolves the algal farming dilemma as there is no competition with the fresh water reserves, land, and nutrients to cultivate these microorganisms.

Neha Arora illustration

Figure courtesy Neha Arora. Click to enlarge.

To this end, Dr. Arora’s team’s research efforts were focused toward bioprospecting of high lipid accumulating microalgal strains from a local lake capable of growing in seawater. Among the isolated strains, Scenedesmus sp. IITRIND2 was able to tolerate natural seawater (35 g/L sea salts), showing rapid growth rate along with an accumulation of high lipid and carbohydrate content. A series of physiological and biochemical studies at both lab and pilot scale (in photobioreactors operated under summer outdoor conditions to mimic the real temperature and light fluctuations) provided detailed insights into the seawater acclimatization characteristics of the microalga. Such an adaptation was attributed to the increase in neutral sugars associated with structural and storage functions including glucose, mannose, galactose, fucose, and ribose.

The carbohydrate rearrangements may aid in rewiring the cellular components and membrane permeability in circumventing the detrimental effects of high salinity. Further, the applicability of biodiesel in conventional diesel engines was verified by analyzing its fuel properties using the American and European Union fuel standard protocols.

The researchers then set out to understand something that is a key interest to the algal biologists and industries: how precisely a fresh water microalga adapted to such high saline conditions. They designed a systematic workflow to decipher the changes in the microalga occurring at the gene, protein, and metabolite levels when cultivated in sea water as opposed to fresh water. The results showed that the microalga remodeled its membrane permeability by restricting ion channels, decreasing the surface potential and excreting extra polysaccharides as compared to fresh water medium.

Interestingly, an increase in cell size along with disorganization of cellular structure with large lipid accumulation and few starch granules in 100% ASW culture were visualized in electron micrographs of the microalga as compared to the control cells. Based on an “integrated omics approach” comprised of proteomics, metabolomics, and lipidomics, a salinity-driven metabolic pathway was hypothesized which aided in development of a metabolic model and in identifying genetic targets for engineering non-halotolerant algal species.

The next challenge was to utilize this particular microalga for remediation of toxic heavy metals present in the environment. This led us to the development of a synergistic modus operandi for mitigating heavy metals such as arsenic and cadmium along with biodiesel production.

Scenedesmus sp. IITRIND2 showed an exceptional capacity to tolerate 500 mg/L of both the arsenic forms (III, V) and 100 mg/L of cadmium. A high removal efficiency of 87% for arsenic (III, V) and 95% for cadmium was recorded at an initial metal concentration of 50 mg/L in a cultivation medium mimicking well water. The microalga also showed ~ 40-45% lipid content of dry cell weight when cultivated under the heavy metal stress. The biodiesel produced from the lipids also complied with the international biodiesel standards, thereby indicating the feasibility of such a hybrid approach.

The researchers hope eventually to go one step further by exploring the potential of Scenedesmus sp. IITRIND2 in open raceway ponds on Indian soil using real wastewaters and leachates. A small step toward this is being initiated by the group by cultivating this microalga on local diary wastewater obtained from Indian Institute of Technology Roorkee campus. The results obtained in terms of bioremediation of the wastewater are very promising, with a removal of ~ 75% total organic carbon, ~ 94% chemical oxygen demand, ~ 90% total nitrogen and ~ 93% total phosphorous along with mitigation of chlorine (~ 91%), fluorine (~ 89%) and bromide (~ 66%), respectively, in just 10 days of cultivation. The microalga also accumulated high intracellular lipids and carbohydrates which were converted into biodiesel and bioethanol.

These concerted efforts are focused towards the initialization of large-scale microalgae cultivation for biofuel and bioremediation which can promote its sustainable application in the near future. The researchers are of the opinion that robust and highly productive microalgal strains such as Scenedesmus sp. IITRIND2 could be an answer to save the planet from the energy crisis, climatic deterioration, and potable water contamination. They believe this study could be a foundation step towards the realization of an algal-based bio-economy and subsequently help algae become the “green mines” fueling the world’s needs.

These findings are described in the article entitled Elucidating the unique physiological responses of halotolerant Scenedesmus sp. cultivated in sea water for biofuel production, recently published in the journal Algal Research.

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