Bacterial community enhances flocculation efficiency of Ettlia sp. by altering extracellular polymeric substances profile.

Graphical abstract Highlights • Self-flocculating Ettlia sp. has high extracellular protein/carbohydrate ratio (EPS P /EPS C). • Growth-promoting and flocculation-inducing bacterial groups were detected. • Filamentous EPS matrices connecting cells appeared exclusively in xenic culture. • More compact EPS structure was introduced by the bacterial community. • Bacteria was not decisive but facilitating factor for Ettlia flocculation. Abstract This study examined the effects of a bacterial community and extracellular polymeric substances (EPS) on Ettlia sp. flocculation. The growth rate, flocculation efficiency (FE), bacterial community, and EPS profile of axenic and xenic Ettlia cultures were monitored during 46 days of cultivation. For the xenic culture, with a great abundance of growth-promoting and flocculation-inducing bacteria, the biomass density was 18.75% higher and its FE reached 100% in the mid-stationary phase. Moreover, microscopic observation and a quantitative analysis of the EPS revealed the exclusive presence of long filamentous EPS and more compact structure in the xenic Ettlia culture, possibly explaining its better FE. Notwithstanding, for the axenic culture, despite a lower biomass density and reduced abundance of EPS, its FE reached 92.54% in the mid-stationary phase. Thus, the role of the bacterial community was found to be supportive rather than vital for the high settleability of the self-flocculating Ettlia microalgal culture. [ABSTRACT FROM AUTHOR]