Impact of growth medium salinity on galactoxylan exopolysaccharides of Porphyridium purpureum

Porphyridium purpureum is a red saline microalga rich in protein and floridean starch with the ability to excrete high amounts of sulfated polysaccharides (sEPS) into the growth medium. This microalga has been used for feeding fish in aquaculture . P. purpureum is easily cultivated and could change its growth rate and composition in response to environmental variations. Thus, the effect of growth medium salinity (18, 32, and 50 g/L NaCl) on P. purpureum cell growth, biomass composition, and extracellular polysaccharides production yield and chemical structure were evaluated. A maximum growth of an estimated 5.7 × 10 6 cells/mL achieved at a salinity of 32 g/L NaCl, after 19 days of growth. Different from the almost unchanged biomass composition of algal cells, the yield and structure of sEPS were affected by the salinity of the growth media. The salinity of the growth media led to a slight change of the sulfation pattern of the glucuronoglucogalactoxylan, as the sEPS produced by P. purpureum growing at lower salinity tended to be more sulfated in the O -3 position of xylose and the O -6 position of glucose, whereas the sEPS at higher salinity tended to be more sulfated in the O -4 position of xylose and glucose. The sEPS produced at higher salinity also revealed higher linear 2-Gal, 3-Gal, and 4-Gal residues. In all samples, the most representative sugars residues were constituted by t-Xyl, t-Xyl-4S, 3-Xyl, 4-Xyl, t-Glc, 3-Glc-6S, t-Gal, and 2,3,4-Gal. The sEPS showed immunostimulatory effect on B lymphocytes in vitro and could be produced at large scale at an outdoor 800 L-flat panel photobioreactor with higher yield. These results pave the way for the potential use of P. purpureum sEPS to enhance humoral immune activity of fish in aquaculture. • Microalga Porphyridium purpureum was able to grow at 18, 32 and 50 g/L NaCl. • Culture at 32 g/L NaCl resulted in higher growth rates and sulfated extracellular polysaccharides (sEPS) production yield. • Different salinity of the growth medium slightly changed the pattern of sEPS sulfation. • sEPS produced at all culture salinities showed an immunostimulatory response, mainly due to the presence of sulfates. • P. purpureum showed feasibility to produce sEPS at large scale (800 L) with higher yield.