Production and composition of extracellular polymeric substances by a unicellular strain and natural colonies of Microcystis: Impact of salinity and nutrient stress

Abstract

The transfer of toxic cyanobacterial Microcystis blooms from freshwater to estuaries constitutes a serious environmental problem worldwide that is expected to expand in scale and intensity with anthropogenic and climate change. The formation and maintenance of Microcystis in colonial form is conditioned to the presence of extracellular polymeric substances (EPS). In this study, we attempted to better understand how the mucilaginous colonial form of Microcystis evolves under environmental stress conditions. In particular, we studied and compared the production and the composition of EPS fractions (attached and free) from natural colonies of a Microcystis bloom and from a unicellular M. aeruginosa strain under salinity and nutrient stress (representing a land-sea continuum). Our results highlighted a greater production of EPS from the natural colonies of Microcystis than the unicellular one under nutrient and combined stress conditions dominated by the attached form. In comparison to the unicellular Microcystis, EPS produced by the colonial form were characterized by high molecular weight polysaccharides which were enriched in uronic acids and hexosamines, notably for the free fraction in response to increased salinities. This complex extracellular matrix gives the cells the ability to aggregate and allows the colonial cyanobacterial population to cope with osmotic shock.

FIGURE 1

Phytoplankton community sampled in Pen Mur freshwater reservoir at bloom and post‐bloom and at the end of the two batch experiments (‘Nutrient+’ and ‘Nutrient−’) at salinity S = 0, S = 5, S = 10, S = 15 and S = 20.

FIGURE 2

Free and attached EPS (A) yield, (B) proteins, lipids and polysaccharides composition and (C) monosaccharide composition of the polysaccharide fraction from the unicellular M. aeruginosa PCC 7806 strain and natural colonies of Microcystis during bloom and post‐bloom at salinity zero.

FIGURE 3

Microscopic photographs of a representative colony of M. aeruginosa and M. wesenbergii at salinity S = 0 and S = 20 showing the difference of thickness and boundary limit of the mucilage.

FIGURE 4

Free and attached EPS (A) yield, (B) proteins, lipids and polysaccharides composition and (C) monosaccharide composition of the polysaccharide fraction from the unicellular M. aeruginosa PCC 7806 strain and natural colonies of Microcystis at the end of the two batch experiments (Nutrient+ and Nutrient−) at salinity S = 0, S = 5, S = 10, S = 15 and S = 20. ND, not detected.