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Microalgae cultivation in liquid foams is a promising concept which requires the use of a surfactant as a foam stabilizing agent. The biodegradable character of a surfactant is a key aspect regarding its applicability in a liquid foam-bed photobioreactor (LF-PBR), since it might influence microalgal growth and the stability of the foam-based cultivation. In this work, the effects of the biodegradable surfactants bovine serum albumin (BSA), Saponin and Tween 20 on the whole microbial community of microalgal cultures (i.e., microalgal and bacterial populations) were studied. The three surfactants enhanced bacterial and microalgal growth in non-axenic microalgal cultures, but they differed in their efficiency to sustain bacterial growth. In this sense, Saponin was proven to enhance the growth of S. obliquus-associated bacteria in microalgae-free cultures, and to sustain it even when other nutrients were lacking, suggesting that Saponin can be used as an energy and nutrients source by these bacteria. The degradation and consumption of Saponin by S. obliquus-associated bacteria was also confirmed by the foaming capacity decrease in Saponin-added bacterial cultures. The biodegradable character of BSA, Saponin and Tween 20 reduces their suitability to be used in a LF-PBR since they would not be able to maintain stable foaming.

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