Effects of the salinity on the biosynthesis of the polysaccharides of the marine microalgae Porphyridium cruentum

The effects of the cultivation medium salinity on the PolySaccharides (PS) production by the red microalgae Porphyridium cruentum was deeply investigated in this study. An original experimental strategy using a system of mini photobioreactors run in parallel was developed for that purpose. It allowed to study the impact of the medium salinity on the main polysaccharide fractions of Porphyridium cruentum, i.e. the floridean starch, the Bound and Released PolySaccharides (BPS and RPS) (i.e. ExoPolySaccharides or EPS) including also the intracellular Low Weight Carbohydrates (LWC). It was demonstrated that the microalgae can grow in a wide range of salinity levels, with an optimal value of c.a. 25 g·L−1 for a maximal productivity of 0.22 g DW·L−1·d−1. This was associated to a maximum of productivity for the EPS fraction (c.a. 45 mg BPS·L−1·d−1, and c.a. 10 mg RPS·L−1·d−1). The BPS followed the biomass modulation tendency, whereas the RPS remained almost at constant values. The other PS fractions, i.e. the starch and the LWC, with its major identified component the floridoside (a compatible solute) followed opposite modulation scheme with a point of convergence situated at the optimal EPS productivity value. The higher the salinity, the higher the low weight carbohydrates accumulation (18.63 %DW, compared to only 4 %DW for the lowest salinity level), mainly due to the accumulation of the floridoside. The lower the salinity, the higher the starch accumulation (15 %DW). Our results suggested interesting metabolic regulations occurring during the medium salinity modulation. It was hypothesized that the salinity was able to trigger specific biosynthetic pathways for the precursors (with a bascule between starch and floridoside depending of the osmolarity of the medium) of the EPS of Porphyridium cruentum.