Biotechnology Journal International

Microalgae remain scientifically compelling biofuel feedstocks because they can convert light and carbon into chemically diverse biomass without an intrinsic requirement for fertile agricultural land, while some production systems can also use saline water, wastewater nutrients and concentrated carbon dioxide streams. Yet the field has not translated laboratory demonstrations into routine production of competitively priced, low-carbon commodity fuels. This critical narrative review evaluates why that gap persists and how the research agenda has changed. Literature published from 1 January 2015 to 22 June 2026 was examined, with earlier seminal studies retained where necessary to establish mechanisms, process benchmarks and the evolution of sustainability arguments. Evidence was synthesised across strain physiology and engineering, cultivation, harvesting and dewatering, lipid-based biodiesel, hydrothermal liquefaction, pyrolysis, anaerobic digestion, carbohydrate-derived fuels, wastewater and carbon integration, techno-economics and life-cycle assessment. The central finding is that microalgal fuel performance is governed by cross-stage interactions rather than by a single favourable trait such as lipid content. Stress-induced lipid accumulation can reduce biomass productivity; controlled photobioreactors can improve culture performance while increasing capital and energy burdens; and downstream choices determine whether dilute wet biomass becomes an energetic liability or a useful feedstock. Hydrothermal liquefaction is particularly compatible with wet whole biomass, whereas conventional biodiesel benefits from mature chemistry but is constrained by selective lipid recovery and drying or extraction requirements. Wastewater, nutrient recycling, carbon utilisation and coproduct valorisation can improve system performance, but benefits are strongly context dependent and may be overstated when environmental-service credits or high-value coproduct revenues are assumed without scale-consistent markets. Pilot-informed life-cycle studies continue to show substantial sensitivity to electricity supply, infrastructure, productivity, nutrient source, dewatering and allocation choices. The most defensible near-term direction is therefore not a universal algal-fuel process, but location-specific, integrated biorefineries in which fuel production is co-designed with nutrient recovery, carbon management and realistic coproduct pathways. Progress towards commercial relevance requires continuous outdoor validation, harmonised assessment, wet-processing strategies and evidence that integrated gains persist at scale.