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Marine microalgae produce extracellular metabolites such as exopolysaccharides (EPS) with potentially beneficial biological applications to human health, especially antioxidant and antitumor properties, which can be increased with changes in crop trophic conditions. This study aimed to develop the autotrophic and heterotrophic culture of Tetraselmis suecica (Kylin) Butcher in order to increase EPS production and to characterize its antioxidant activity and cytotoxic effects on tumor cells. The adaptation of autotrophic to heterotrophic culture was carried out by progressively reducing the photoperiod and adding glucose. EPS extraction and purification were performed. EPS were characterized by Fourier-transform infrared spectroscopy and gas chromatography-mass spectrometry. The antioxidant capacity of EPS was analyzed by the 2,2’-azino-bis (3-ethylbenzothiazoline-6-sulphonic acid) (ABTS) method, and the antitumor capacity was measured by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay, showing high activity on human leukemia, breast and lung cancer cell lines. Although total EPS showed no cytotoxicity, acidic EPS showed cytotoxicity over the gingival fibroblasts cell line. Heterotrophic culture has advantages over autotrophic, such as increasing EPS yield, higher antioxidant capacity of the EPS and, to the best of our knowledge, this is the first probe that T. suecica EPS have cytotoxic effects on tumor cells; therefore, they could offer greater advantages as possible natural nutraceuticals for the pharmaceutical industry.

Microalgae are successfully adapted to different environments where they metabolize inorganic and, in some cases, organic compounds. The modification of culture conditions can induce the synthesis of exopolysaccharides with potential health benefits. This study aims to adapt the autotrophic culture of the red alga Porphyridium purpureum to a heterotrophic culture in order to compare the biochemical composition of biomass and the production of exopolysaccharides (EPS). The adaptation of autotrophic to heterotrophic culture was carried out by progressively reducing the photoperiod and adding glucose. EPS extraction and purification were carried out. EPS were characterized by Fourier transform infrared spectroscopy and gas chromatography-mass spectrometry. The antioxidant capacity of both biomasses was analyzed by the 2,2′-azino-bis (3- ethylbenzothiazoline-6-sulphonic acid) (ABTS) method and 2,2-diphenyl-1-picrylhydrazyl (DPPH) assay. Results for both biomass and EPS showed significant differences in biochemical composition and antioxidant activity under the two growing conditions. Furthermore, the heterotrophic EPS (HT-EPS) seems to favor the accumulation of glucuronic acid. Our results are the first adaptation from autotrophic to heterotrophic culture of P. purpureum. These trophic variations show that P. purpureum can adapt to heterotrophic culture and present interesting differences in its biochemical composition, being an alternative in the increase of metabolites, mainly carbon compounds, of interest to the industry.

The use of functional feed additives is an important approach to both, prevent and fight, viral diseases in aquaculture. In this regard, microalgae-derived products, and, more specifically, microalgal exopolysaccharides (EPSs), have attracted attention, since multiple biotechnological applications are being described for these molecules. Furthermore, depending on culture conditions, the composition and, therefore, properties of EPSs can vary. In the present study, the antiviral activity of EPSs from Tetraselmis suecica and Porphyridium cruentum cultured under autotrophic and heterotrophic conditions has been evaluated in vitro against Viral Haemorrhagic Septicaemia Virus (VHSV), an important pathogen in fish farming. Results showed that EPSs from both species have anti-VHSV activity. T. suecica EPSs from autotrophic cultures showed the strongest effect, since both, adsorption and post-adsorption phases of the VHSV multiplication cycle were affected. In contrast, both, autotrophic and heterotrophic P. cruentum EPSs showed anti-VHSV activity only after the adsorption phase. These results pave the way to use these EPSs to fight VHSV infections, and animate to evaluate the EPS antiviral activity against other viral pathogens relevant to the aquaculture industry.