**1. Introduction**

Microalgae can develop in autotrophic, mixotrophic and heterotrophic conditions due to their physiological characteristics and plasticity of adaptation on our planet [1], differentiating them by the type of energy and the source of carbon to be used [2]. The cultivation of microalgae is mostly performed in autotrophy using light as an energy source, which is transformed by photosynthesis into chemical energy for the storage of polysaccharides and lipids [3,4].

In a heterotrophic culture, microalgae have the ability to grow and metabolize organic carbon sources with limited irradiance [5], changing their metabolism to generate energy by breathing or using an organic substrate under heterotrophic conditions [6]. It has several advantages over an autotrophic crop: (i) it does not require lighting, (ii) high biomass yield, (iii) high growth rates and (iv) increased synthesis of metabolites of scientific and biotechnological interest [5–7]. These metabolites depend on changes in culture media; therefore, optimization of organic carbon, macronutrient and micronutrient concentrations is sought in order to obtain the best yields in terms of productivity and biomass [1,8].

Crop types and systems provide advances both for the production of biomass with high nutritional value and economic viability and for the search, extraction and characterization of new products [9,10]. Heterotrophic culture has a potential market for the production of high-value metabolites with respect to autotrophic crops: (i) lipids with four times higher content of polyunsaturated fatty acids (PUFAs) than those cultured in autotrophic conditions [11–13], (ii) accumulation of up to 45% by dry weight of carbohydrates [11,14,15], (iii) a higher percentage of proteins for the biomass [16] and (iv) pigments (lutein and phycocyanin) with balanced C/N ratios and astaxanthin with limiting nitrogen [17–19].

*Tetraselmis suecica* is a marine green microalga widely used in aquaculture as live food for rotifers and copepods or Artemia in hatcheries [20]. This microalga can be cultivated in autotrophic and heterotrophic conditions [21]. It has antibacterial activity [22,23], probiotic properties [24] and has been proposed as a source of vitamin E for humans [25]. *T. suecica* produces exopolysaccharides [26]; however, their structural characteristics and biotechnological applications in human health remain unknown.

Polysaccharides are high-molecular weight molecules that contain repetitive structural units—monosaccharides- joined by glucosidic bonds, forming linear or branched structures. This structural variability has biotechnological interest [27]. Its applications are promising due to its immunomodulatory, antimicrobial, antiviral, antioxidant and antitumoral capacities [28]. Algal polysaccharides are free radical scavengers and, therefore, have antioxidant effects and prevent oxidative damage in living organisms [29]. The antioxidant activity of polysaccharides has been related to the presence of sulfates and uronic acids in them [28,30]. In this sense, in vitro antiproliferative activity in human cancer cells lines [31] and in vivo inhibition of Graffi myeloid tumor growth in hamsters [32] have been demonstrated with marine algal polysaccharides. Therefore, the prospecting of natural-origin compounds such as polysaccharides is a source for the prevention of diseases that counteract the toxic effects of synthetic compounds.

The aim of this work was to develop the crop of *T. suecica* under autotrophic and heterotrophic conditions, comparing the differences between the biochemical composition of the algal biomass and their yield exopolysaccharides (EPS). Furthermore, the functional groups, monosaccharides characterization and the antioxidant activity and cytotoxic effects on tumor cells and healthy cells of the exopolysaccharides were assessed.
