Marine microalgal biofuels: toward a systems-based circular biofactory

Marine microalgae are frequently promoted as sustainable biofuel feedstocks because of their halotolerance, high photosynthetic efficiency, and limited land requirements, yet commercial deployment remains elusive. This gap is primarily systemic rather than biological, reflecting the fragmented development of strain engineering, harvesting, conversion, and sustainability assessment. This review reframes marine microalgae as circular biofactories and advances a system-centric paradigm for integrated biorefineries. We synthesise recent advances in metabolic and genetic engineering, low-energy harvesting, and thermochemical and biochemical conversion, highlighting how cross-stage interdependencies dominate overall performance. We further discuss how artificial intelligence, digital twins, nutrient recycling, carbon utilisation, and high-value coproducts enable predictive optimisation and techno-economic viability. This perspective provides a road map for translating marine microalgal biofuels from laboratory promise to industrial relevance.