Although microalgae exploitation is very promising, process sustainability is undermined by biomass production and harvesting. Among the various bottlenecks of the production process, particular attention should be paid to the water footprint. Indeed, a huge volume of water is required in microalgae production. Water reuse can support both the water footprint and medium cost reduction, saving water and unconverted substrates. The present study reports preliminary results regarding the utilization of a water reuse strategy for two Chlorophyta microalgae under batch conditions. Growth parameters and chlorophyll content are monitored and the optimal amount of reused medium is assessed. The results show that 70% of the medium can be reused with no loss of specific growth rate and chlorophyll fraction for Pseudococcomyxa simplex in three consecutive batch cultivations. By contrast, a significant decline in Chlorella vulgaris growth was observed after the first cultivation in reused medium, across all tested conditions.

Microalgae are gaining interest as sustainable biofactories for the industrial-scale production of Phycocyanin (PC), a phycobiliprotein (PBP) with significant antioxidant, anti-inflammatory, and nutraceutical properties. Among them, red extremophilic species such as Galdieria spp. have emerged as particularly promising sources due to their ability to thrive in harsh conditions, enabling the production of high-purity C-Phycocyanin (C-PC) with reduced contamination risks. The growing demand for natural pigments, supported by a global market forecasted to expand at a compound average growth rate (CAGR) of 7% through 2030, calls for scalable, cost-efficient, and environmentally sustainable strategies.This review critically examines upstream and downstream approaches for C-PC production from Rhodophyta, including cultivation regimes (autotrophic, heterotrophic, and mixotrophic), extraction and purification techniques, with a focus on operational performance, purity index, and process sustainability. Finally, we highlight persistent methodological gaps, such as protocol heterogeneity and the lack of standardization, and propose integrated strategies to support future biotechnological applications and large-scale deployment. The development of unified quality benchmarks, robust production protocols, and biorefinery models is essential to unlock the full commercial potential of red algal-derived C-PC. Graphical abstract

Microalgae cultivation for biotechnological purposes is an expanding field. Their cultivation is usually performed on a liquid culture medium and require the presence of an energy source. Culture media are water-salt solutions containing nitrogen, sulfur, phosphate sources, with trace of transition metals. Light source supplies energy through photosynthesis process, resulting in metabolic reactions and cell growth. Thanks to the huge number of existing species and their diversity, it is possible to exploit alternative water sources with chemical and physical properties far from the commonly used freshwater media. Among these, oilfield or formation waters (AFW) are saline wastewater produced during oil drilling. They represent over 95 % of the total fluids collected from gas and oil wells during fuel extraction. This study exploits the adaptation capability of a Galdieria sulphuraria strain in different salinity conditions to build up a more feasible microalgae-based platform. Batch cultivation of G. sulphuraria was evaluated in a range from 0 to 20 g/L NaCl in terms of growth rate, metabolite content, photophysiology, and intracellular redox state. Moreover, the same strain has been tested for artificial formation water growth in semicontinuous cultivation mode. In these condition, NaCl preadaptation effect has been assessed too. Results clearly demonstrate that biomass composition is influenced from medium salinity. Strains preadapted in 0 to 20 g/L range of NaCl salinity show almost same cell growth profile and a slightly different ratio between lipids, carbohydrates, and proteins. Pigments content and photophysiology point out that variations of osmotic pressure and ionic strength in medium affect Photosystem II (PSII). Regulation of light harvesting is necessary to trigger metabolism shift toward synthesis of metabolites required in the adaptation process.