Effect of salinity on growth and microbial diversity in cultures of Scenedesmus almeriensis produced at a pilot scale

<b>Introduction:</b> Freshwater scarcity represents a major constraint for the sustainable industrial-scale cultivation of microalgae. This study investigates the feasibility of producing <i>Scenedesmus almeriensis</i> using seawater in 3.1 m³ tubular photobioreactors under winter-spring conditions. The appearance of algal predators represents a significant challenge in industrial facilities, and this research also explores whether seawater can serve as a strategic water source for more resilient and efficient production systems. <b>Methods:</b> Biomass productivity and microbial diversity were compared between freshwater and seawater-based cultures under batch and semi-continuous regimes at dilution rates of 0.1, 0.2, and 0.3 day^-1. The production was carried out in duplicate using identical tubular photobioreactors. Analytical determinations included measuring biomass concentration, chlorophyll fluorescence, and oxygen production via photorespirometry. Microbial diversity was assessed through microscopy and metagenomic analysis (18S and 16S rDNA) to identify taxonomic classifications and potential biotic contaminants. <b>Results and Discussion:</b> Maximum biomass concentrations reached 0.60 and 2.15 g·L^-1 in freshwater and seawater, respectively. Production using seawater led to a higher biomass productivity (0.18 g·L^-1·day^-1) compared to freshwater (0.06 g·L^-1·day^-1) at a fixed dilution rate of 0.1 day^-1. Seawater cultures exhibited greater stability and higher photosynthetic efficiency, with <i>Scenedesmus</i> dominating up to 70% of the microalgal community due to reduced contamination by zooplankton, fungi, and ciliates. In contrast, freshwater cultures were rapidly degraded by rotifers and anaerobic fungi, leading to a culture crash when dilution rates were increased. These findings highlight the potential of seawater to act as a biological barrier against contaminants while significantly reducing freshwater requirements in industrial microalgae production.