A comprehensive review on marine algal polysaccharide-mediated siRNA delivery systems for biofuel production

Marine algae remain promising feedstock for renewable biofuel production, yet metabolic bottlenecks such as limited carbon allocation to lipid synthesis, competition from starch pathways, and variable nitrogen assimilation continue to constrain productivity. Small interfering RNA delivered gene silencing offers a targeted route to modulate these pathways, although its application in algae is limited by molecular instability, inconsistent uptake, and poor intracellular retention. This review evaluates marine polysaccharides including alginate, carrageenan, fucoidan, and ulvan as siRNA delivery carriers designed for algal systems, highlighting the structural features that underpin their performance. Alginate contains guluronic rich blocks that support ionic crosslinking with divalent cations to form stable hydrogels that protect and gradually release siRNA. Carrageenan and fucoidan contain dense sulfate groups that promote strong electrostatic binding and stabilisation of siRNA in aquatic culture conditions. Ulvan provides rhamnose and glucuronic acid residues that assist nanoparticle formation and support efficient cellular internalisation. Mechanistic studies in Nannochloropsis and Chlamydomonas show that siRNA mediated knockdown of lipid pathway enzymes such as acetyl CoA carboxylase and diacylglycerol acyltransferase can increase lipid accumulation by around fifteen to thirty five percent. Silencing starch biosynthesis genes further redirects carbon flux towards fatty acid pathways, supported by metabolic flux modelling that predicts enhanced malonyl CoA availability. Critical discussion is included on species dependent uptake variability, ecological considerations, and techno economic constraints linked to polysaccharide extraction and nanoparticle formulation. Emerging advances such as CRISPR RNAi hybrid strategies, AI assisted nanocarrier optimization, and programmable algal gene circuits further strengthen the potential of this platform. Future progress will increasingly rely on integrating polysaccharide based nanocarriers with advances in synthetic biology, dynamic gene circuit design, and AI assisted process modelling. Together, these approaches can enable scalable and precision controlled metabolic engineering in algae, supporting industrial biofuel production and strengthening the technological pathway toward next generation renewable energy systems.