Bioethanol from macroalgae: Prospects and challenges

Burgeoning dependence on fossil fuels for transport and industrial sectors has been posing challenges such as depletion of fossil fuel reserves, enhanced greenhouse gas (GHG) footprint, with the imminent changes in the climate, etc. This has necessitated an exploration of sustainable, eco-friendly and carbon neutral energy alternatives. Recent studies on biofuels indicate that algal biomass, particularly from marine macroalgae (seaweeds) have the potential to supplement oil fuel. Marine macroalgae are fast growing and carbohydrate rich biomass having advantage over other biofuel feedstock in terms of land dependence, freshwater requirements, not competing with food crops, which were the inherent drawback of the first- and second-generation feedstock. The present communication reviews the macroalgal feedstock availability, screening and selection of viable feedstock based on the biochemical composition, process involved, scope and opportunities in bioethanol production as well as technology interventions. The prospect of bioethanol production from algal feedstock of Central West Coast of India has been evaluated taking into account challenges (feedstock sustenance, technical feasibility, economic viability) in order to achieve energy sustainability. The green algae exhibited growth during all seasons and highest total carbohydrate was recorded from green seaweed Ulva lactuca (62.15 +/- 12.8%). Elemental (CHN) analyses of seaweed samples indicate 25.31-37.95% of carbon, 4.52-6.48% hydrogen and 1.88-4.36% Nitrogen. Highest carbon, hydrogen and nitrogen content were recorded respectively from G.pusillum (C: 37.95%), G. pusillum (H: 6.48%) and E.intestinalis (N: 4.36%). Green seaweeds are rich in cellulose content (>10%) compared to other seaweeds (2-10%). Higher cellulose content was estimated in U.lactuca (14.03 +/- 0.14%), followed by E. intestinalis (12.10 +/- 0.53%) and C.media (10.53 +/- 0.17%). Cellulose is a glucan present in green seaweeds, which can easily be hydrolysed through enzyme and subsequently fermented to produce bioethanol. Lower sugar removal in acid hydrolysate neutralization process (Na2CO3) was recorded in U.lactuca (39.8%) and E.intestinalis (14.7%). Highest ethanol yield of 1.63 g and 0.49 g achieving 25.8% and 77.4% efficiency in SHF (Separate Hydrolysis and Fermentation) and SSF (Simultaneous Saccharification and Fermentation) process respectively was recorded for green alga E. intestinalis.