Microalgae have gained significant attention as sustainable sources of high value compounds, such as bioactive polysaccharides that are usually rich in sulfated groups and exhibit antioxidant properties. Here, 14 new microalgae strains of the genera Tetraselmis, Dunaliella, and Nannochloropsis, isolated from Greek coastal lagoons were analyzed to quantify and characterize their polysaccharide content. Heterogeneity was observed regarding the content of their total sugars (5.5–40.9 g/100 g dry biomass). The strains with a total sugar content above 20% were analyzed concerning the content of total, α- and β-glucans. Tetraselmis verrucosa f. rubens PLA1-2 and T. suecica T3-1 were rich in β-glucans (11%, and 8.1%, respectively). The polysaccharides of the two Tetraselmis strains were isolated and they were mainly composed of glucose and galactose. The isolated polysaccharides were fractionated using ion-exchange-chromatography. The anionic fraction from T. verrucosa f. rubens PLA1-2 was rich in sulfated polysaccharides, had antioxidant capacity, and exhibited healing properties. The anionic polysaccharides from the two Tetraselmis strains did not negatively influence the viability of human cells, while exhibiting antiviral properties against the replication of Hepatitis C Virus (HCV), with median efficient concentrations (EC50) at a range of 210–258 μg/mL.

In the pursuit of sustainable sources for food, energy, and health products, microalgae have gained attention. In the present study, the lagoonal system of the Nestos River delta was selected as a sampling point in order to search for opportunistic and robust species. Two new strains of Tetraselmis are described with regards to their taxonomic features (as observed using light and transmission electron microscopy and molecular phylogenetics) and their biochemical properties (total lipid, total protein, and total carbohydrate content, photosynthetic pigments, and antioxidant capacity). The studied strains were identified as representatives of Tetraselmis verrucosa f. rubens. Furthermore, both strains exhibited an interesting biochemical profile coupled with high growth rates and promising antioxidant activity, without the use of enhancement and induction culture methods, warranting further investigation and showing potential for biotechnological use.

Hypersaline solar saltworks represent unique ecological niches that harbor extremophilic microalgae with considerable biotechnological potential. Within these environments, members of the genus Dunaliella are particularly noteworthy due to their remarkable metabolic plasticity and ability to accumulate high-value biomolecules. In the present study, we investigated the biodiversity of Dunaliella in hypersaline saltworks by isolating and identifying autochthonous strains and assessing their growth kinetics and biomass biochemical composition in the context of potential biotechnological applications. Specifically, sixteen strains of Dunaliella were isolated from evaporation and crystallizer ponds of the Kalloni saltworks in Lesvos, Greece, and subjected to an integrative characterization combining morphological observations, molecular phylogenetics, growth kinetics, and biochemical profiling. Phylogenetic analyses based on four genetic markers (18S, ITS, rbcL, tufA) consistently resolved the isolates into three distinct clades: one corresponding to Dunaliella salina/D. minutissima, one to D. parva, and a third representing a clearly divergent lineage. Growth assays revealed marked variability in cell density, biomass productivity and specific growth rate, with certain strains exhibiting enhanced proliferation under controlled conditions. Biochemical analyses demonstrated distinct allocation patterns, with evaporation pond isolates comparatively enriched in proteins (up to 60.8% DW), whereas crystallizer pond isolates accumulated higher levels of carbohydrates (up to 19.0% DW), carotenoids (up to 7.34% mg g⁻¹ DW) and phenolic compounds (up to 8.68% mg GAE g⁻¹ DW). Antioxidant assays (FRAP, TEAC) further indicated significantly elevated reducing and radical scavenging activities among crystallizer isolates. These findings expand current knowledge on the biodiversity of autochthonous Dunaliella strains and support their potential as sustainable sources of bioactive compounds for applications in the agri-food, nutraceutical, pharmaceutical, and cosmeutical sectors.

Hydroponic systems are gaining prominence in sustainable agriculture, yet their nutrient-rich effluents remain an underexplored source of microbial biodiversity with potential biotechnological interest. In this study, shotgun metagenomic sequencing was employed to profile, with a high taxonomic resolution, the photosynthetic microbial community in hydroponic effluent before and after a natural algal bloom, revealing pronounced shifts in microbial composition. Notably, relative abundance increased sixfold for Chlamydomonas reinhardtii and tenfold for Bigelowiella natans. Four dominant microalgal strains (PR1–PR4) were subsequently isolated and characterized through integrative morphological and molecular taxonomy, with phylogenetic analyses based on four genetic markers (18S rRNA, ITS, rbcL and tufA) confirming that each isolate represents a distinct lineage within Chlorophyceae families, including Chlorella sp., Chlamydomonas sp., and Scenedesmus sp. Growth kinetics under three temperature regimes, typical of Greek environmental conditions from spring to autumn (15 °C, 23 °C, 32 °C), demonstrated broad ecological plasticity and rapid biomass production, highlighting strains with strong adaptive resilience. Biochemical profiling of the isolates revealed significant inter-strain differences in primary and secondary metabolite content, including proteins (up to 43% DW), lipids (up to 31% DW), carbohydrates (up to 44% DW), photosynthetic pigments, phenolics, flavonoids, and antioxidant activity. The observed metabolic diversity of autochthonous microalgal strains from hydroponic environments, combined with their high growth rates, underscores their potential for applications in bioremediation, bioenergy, and the development of value-added products within a circular bioeconomy framework.