Integrated (bio)hydrogen production from Tetraselmis indica: A sustainable environmental biorefinery approach

The high global energy demand and also the environmental toll of fossil-fuel dependency have led to an accelerated search for sustainable, carbon-neutral energy carriers. (Bio)Hydrogen (H 2 ) has emerged as a novel candidate due to its high energy density and the absence of carbon byproducts during combustion. Among marine biological systems, the recently described halophilic microalga Tetraselmis indica (T. indica ) stands out for its robust metabolic plasticity and tolerance to environmental fluctuations. However, despite its potential, species-specific genomic and proteomic characterization of its [FeFe]-hydrogenase remains under-researched compared to its terrestrial counterparts. This review provides a comprehensive understanding of recent advancements in the use of T. indica for H 2 synthesis, evaluating its fundamental metabolic pathways with a primary focus on direct and indirect (bio)photolysis, as well as the influence of critical parameters such as light intensity and salinity stress. In addition, the euryhaline nature of T. indica has been explored in the present review for its potential to reduce freshwater footprints while enabling integrated phycoremediation as a dual-benefit strategy for domestic wastewater. This review also summarizes and critically discusses the techno-economic assessment (TEA) and life cycle assessment (LCA) reported for scaling up microalgal systems, with emphasis on their relevance to T. indica . Despite the proven efficiency of T. indica in removing nutrients from wastewater, a critical gap in biorefinery modeling remains. To address these knowledge gaps, the present review critically synthesizes the existing literature investigated phycoremediation and H 2 evolution, which are largely treated as sequential or isolated processes, by systematically evaluating available published data on their simultaneous occurrence within single-stage reactor systems. Based on the current literatures, this review outlines the metabolic and operational configurations required for concurrent nutrient removal and biohydrogen production. Furthermore, bridging the remaining genomic and operational gaps through integrated bioreactor design and multi-omics analysis is essential to assess the economic feasibility and scale-up potential of T. indica as a large-scale bioenergy feedstock.