Haloferax mediterranei R4 for bioremediation of desalination and textile wastes: a step towards their valorisation

Haloferax mediterranei has shown strong potential for bioremediating brines contaminated with nitrogenous compounds, oxychlorides, and metals, making it a promising candidate for treating saline wastewater. Therefore, this study aims to evaluate its capacity to bioremediate industrial wastewaters (IWWs) from desalination and textile industries. After characterising IWWs and formulating suitable media, growth was compared with two control media: an optimal medium (NH₄⁺, 20% salts) and a control medium providing basic nutrients and ionic strength (NO₃^-, 15% salts). Raw IWWs were heterogeneous and unsuitable per se for optimal growth, requiring supplementation with minimal essential nutrients (C, N, P, and Fe). H. mediterranei grown in IWWs media exhibited reduced growth and increased cell pigmentation, indicating stress. Particularly, textile residues induced tenfold higher carotenoid production than the optimal medium and only fourfold less than the carotenoid production medium. Carotenoid extract composition showed variations within different IWWs, but bacterioruberin revealed as the main natural pigment in all cases. Elemental biomass analysis showed Fe and Zn accumulation, as metalloprotein cofactors for stress tolerance, and Ca, related to exopolysaccharide production. Despite the stress, H. mediterranei effectively removed 60-90% of the NO₃^- in the wastes-among the higher removal rate values reported for matrices under comparable salinities and initial NO₃^- loads. While further optimisation of waste mixtures is needed to improve nutrient balance and eliminate the need for supplementation, our findings suggest that IWWs from both industries can support H. mediterranei growth, enabling a cost-effective, industrially viable bioremediating strategy with the added value from pigment production. KEY POINTS: Haloferax mediterranei removed up to 90% of nitrates from industrial wastes (IWWs) IWWs bioremediation enhanced carotenoid production, a valuable biomolecule Extremophile-based strategy enables circular reuse of high-salinity IWWs.