Adaptation of the toxic freshwater cyanobacterium Microcystis aeruginosa to salinity is achieved by the selection of spontaneous mutants

Abstract

SUMMARY The cyanobacterium Microcystis aeruginosa causes most of the harmful toxic blooms in freshwater ecosystems. Some strains of M. aeruginosa tolerate low‐medium levels of salinity, and because salinization of freshwater aquatic systems is increasing worldwide it is relevant to know what adaptive mechanisms allow tolerance to salinity. The mechanisms involved in the adaptation of M. aeruginosa to salinity (acclimation vs. genetic adaptation) were tested by a fluctuation analysis design, and then the maximum capacity of adaptation to salinity was studied by a ratchet protocol experiment. Whereas a dose of 10 g NaCl L−1 completely inhibited the growth of M. aeruginosa, salinity‐resistant genetic variants, capable of tolerating up to 14 g NaCl L−1, were isolated in the fluctuation analysis experiment. The salinity‐resistant cells arose by spontaneous mutations at a rate of 7.3 × 10−7 mutants per cell division. We observed with the ratchet protocol that three independent culture populations of M. aeruginosa were able to adapt to up to 15.1 g L−1 of NaCl, suggesting that successive mutation‐selection processes can enhance the highest salinity level to which M. aeruginosa cells can initially adapt. We propose that increasing salinity in water reservoirs could lead to the selection of salinity‐resistant mutants of M. aeruginosa.

The limit of resistance to salinity in the freshwater cyanobacterium Microcystis aeruginosa is modulated by the rate of salinity increase

“…Strains Ma1Vc and Ma5Vc showed a lethal dose value of 7 gL ‐1 NaCl, whereas the value for the MaAVc strain was 10 gL ‐1 NaCl, as observed previously for this strain (Melero‐Jiménez et al, 2019).…”

“…Therefore, it was assumed that the strains had no previous evolutionary history of salinity exposure. Culture conditions were similar to those described in our previous studies (Bañares‐España et al, 2016, Carrillo et al, 2003; Costas et al, 2007; Fernández‐Arjona et al, 2013; López‐Rodas et al, 2007, 2009; Huertas, Rouco, López‐Rodas, & Costas, 2010; Huertas et al, 2011; Rouco et al, 2014; García‐Villada et al, 2004; Martín‐Clemente, Melero‐Jiménez, Bañares‐España, Flores‐Moya, & García‐Sánchez, 2019; Melero‐Jiménez, et al, 2019). Cultures were grown axenically in 250 mL ventilated cell culture flasks sealed with filter screw‐caps (Greiner; Bio‐One), containing 60 mL of half‐diluted BG11 medium (Sigma‐Aldrich Chemie).…”

“…In order to analyze the effects of environmental deterioration rate on the limit of resistance to salinity of each of the three strains of M. aeruginosa , a ratchet experiment was performed as described by Huertas et al (2010, 2011), Rouco et al (2014), Melero‐Jiménez et al (2019), and Martín‐Clemente et al (2019). Two ratchet experiments were carried out to simulate fast and slow environmental deterioration rates (Figure 1).…”

“…In a previous study, using an eco‐evolutionary approach, we demonstrated that the selection of spontaneous mutants allowed the survival of the freshwater cyanobacterium Microcystis aeruginosa (Kützing) Kützing when exposed to lethal salinity concentrations (Melero‐Jiménez, Martín‐Clemente, García‐Sánchez, Flores‐Moya, & Bañares‐España, 2019). Moreover, we determined the limit of resistance to this selective agent at a single rate of salinity increase (Melero‐Jiménez et al, 2019). However, the rate of environmental change (salinity increment) could determine the limit of resistance to salinity in this species.…”

“…On the contrary, only a few studies have been performed submitting cell populations to a selective agent for a high number of generations. In these studies, a fluctuation analysis design based on Luria and Delbrük (1943) was applied indicating that selection of new genotypes of M. aeruginosa (i.e., adaptation) occurred under exposition to different selective agents (Fernández‐Arjona et al, 2013; García‐Villada et al, 2004; Huertas, Rouco, López‐Rodas, & Costas, 2011; López‐Rodas et al, 2007; Rouco et al, 2014), including high salinity (Melero‐Jiménez et al, 2019). However, we want to explore two more questions; could the limit of resistance to a selective agent be different depending on the rate of environmental deterioration?…”