Arthrospira platensis, or spirulina, is a cyanobacterial species mainly exploited for its nutritional qualities and able to produce exopolysaccharides (EPS) under specific conditions. In this work we identify that the bicarbonate concentration is a key parameter that induces EPS production by the cells. Using flocculation/flotation harvesting experiments, in addition to atomic force microscopy to characterize the biophysical properties of EPS, we show that EPS produced by A. platensis cells form a soft, adhesive gel in the medium that forms aggregates in which cells are entrapped and thus bioflocculated. This results in the creation of microalgae flocs in which ascending microbubbles produced in the flotation process get trapped, thus enhancing the efficiency of the microalgae separation from water. This interdisciplinary study provides new insights into culture conditions that positively influence the EPS production, while allowing both a high final biomass concentration and a harvesting efficiency superior to 90% to be reached.