Production of exopolymers (EPS) by cyanobacteria: impact on the carbon-to-nutrient ratio of the particulate organic matter

International audience ; Freshwater cyanobacteria can produce large amount of mucilage, particularly during large 11 blooms. The production of these carbon-rich exopolymers (EPS) should influence the carbon-to-12 nutrient ratios of the organic matter (OM), which are regularly used as a proxy for the herbivorous 13 food quality. However, little is known about the consequences of EPS production on the carbon-to-14 nutrient ratio of the OM. Two EPS forms can be distinguished: the free fraction composed of Soluble 15 Extracellular Polymeric Substances (S-EPS) and the particulate fraction corresponding to the 16 Transparent Exopolymer Particles (TEP). The aim of the study was to determine whether the TEP and 17 S-EPS productions by cyanobacteria influence the carbon-to-nutrient ratios of the particulate OM 18 (POM). Five cyanobacteria species were grown in batch culture and characterized in terms of 19 photosynthetic activity, EPS production, and C, N, P contents. The variability in EPS production was 20 compared with the variability in stoichiometry of the POM. Most of cyanobacteria live in association 21 with heterotrophic bacteria (HB) within the mucilage. The effect of the presence/absence of HB on 22 EPS production and the carbon-to-nutrient ratios of the POM was also characterized for the 23 cyanobacteria Microcystis aeruginosa. We showed that TEP production increased the carbon-to-24 nutrient ratios of the POM in the absence of HB, while the stoichiometry did not significantly change 25 when HB were present. The C:N ratio of the POM decreased with production of S-EPS by the five 26 species. Lastly, the three colonial species (Chroococcales) tend to produce more TEP than the two 27 filamentous species (Oscillatoriales), with the two picocyanobacteria being the most productive of 28 both TEP and S-EPS. 29 30 2