Production and composition of Extracellular Polymeric Substances (EPS) 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 an attempt to better understand how mucilaginous colonial form may evolved under environmental stress condition, 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 without or under stress condition. 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.

Morphological and physiological impacts of salinity on colonial strains of the cyanobacteria Microcystis aeruginosa

“…aeruginosa have been described in several publications including an enhanced production of mucilage, an increase in colony size, the reduction of intercellular space, the increase of cellular density within the colony (Kruk et al, 2017;Reignier et al, 2023;Sampognaro et al, 2020) and the decrease of growth rate (Kruk et al, 2017;Orr et al, 2004;Robson & Hamilton, 2003;Sellner et al, 1988).…”

“…In their natural habitat, Microcystis colonies produce mucilage composed of exopolysaccharides, proteins, lipids, and nucleic acids to form scums (Liu et al, 2018;Reignier et al, 2023). Although the ecological impact of cyanobacteria colonies is still debated, some environmental conditions have been identified as proliferation factors: eutrophication of waters, a high concentration of calcium, magnesium, or lead, an increase in temperature, or the presence of other cyanobacteria and heterotrophic bacteria (Drugă et al, 2019;Sampognaro et al, 2020;Xiao et al, 2017).…”