Molecular characteristics of microalgal extracellular polymeric substances were different among phyla and correlated with the extracellular persistent free radicals

Extracellular polymeric substance (EPS) plays essential roles in microalgal adaptation to the external environment and aggregate formation. The molecular characteristics of EPS and extracellular persistent free radicals (PFRs) of 15 microalgal species belonging to three phyla were analyzed using Fourier-transform ion cyclotron resonance mass spec-trometry (FT-ICR-MS), three-dimensional fluorescence excitation-emission matrices combined with parallel factor analysis (EEM-PARAFAC), and Electron Paramagnetic Resonance Spectrometer (EPR). Lignin accounted for the highest proportion of EPS for Cyanophyta and the proportion of lipids was higher for Bacillariophyta. The presence of PFRs was detected on the cell surfaces of all microalgae species (excluding Cyclotella sp.). The intensity of carbon -centered PFRs was positively correlated with the proportions of humic-like component and lignin, but was negatively correlated with the proportion of lipids in microalgal EPS. Following EPS extraction, carbon-and oxygen-centered free radicals were still detectable on the surface of microalgae. Given the high intensity of PFRs produced by Cyanophyta, the level of PFRs in eutrophic lakes and reservoirs predominated by Cyanophyta may be considerably high. Other or-ganisms in the water column, such as bacteria and zooplankton are bound to be stressed by elevated level of PFRs. The ecological functions and environmental risks of PFRs carried by microalgae still need to be explored in follow-up research.

Automated summary

Extracellular polymeric substance (EPS) and extracellular persistent free radicals (PFRs) play important roles in microalgal adaptation to the external environment and aggregate formation. The molecular characteristics of EPS and PFRs in various microalgal species were analyzed, revealing that lignin accounted for the highest proportion of EPS for Cyanophyta and the proportion of lipids was higher for Bacillariophyta. PFRs were detected on the cell surfaces of all microalgae species (excluding Cyclotella sp.), and the intensity of carbon-centered PFRs was correlated with the proportions of humic-like component and lignin.