Strains, growth conditions and growth physiology
Cultures of the psychrophilic Chlamydomonas sp. UWO 241 (CCMP1619) and a mesophilic strain, Chlamydomonas raudensis SAG 49.72, were grown in Bold’s basal medium (BBM) (Nichols & Bold, 1965) under ambient CO2 conditions in 250 mL Pyrex tubes submerged in temperature-regulated aquaria as described in Morgan-Kiss et al. (2008). The mesophilic SAG 49.72 was chosen for comparison as it has been used in several comparative studies with UWO 241 (Pocock, Vetterli, & Falk, 2011; Szyszka-Mroz et al., 2019; Szyszka-Mroz et al., 2015; Szyszka, Ivanov, & Hüner, 2007). Cultures were grown under either control (C) conditions or exposed to one of three long-term stress treatments: high light (HL), low temperature (LT) or high salt (HS). For control conditions, cultures were grown under temperature/light regimes of 8oC/50 μmol -2s-1 and 20oC/50 μmol-2 s-1 for UWO 241 and SAG 49.72, respectively, and NaCl levels of 0.43 mM for both strains (Table 1). Long-term stress conditions were chosen based on previous studies (Morgan-Kiss et al., 2006; Pocock et al., 2011; Szyszka et al., 2007) to reflect the maximum level of a particular stress to which the organism could fully acclimate, that is achieve exponential growth and high photochemical activity (maximum photosynthetic efficiency values, FV/FM, above 0.5). Long-term stress conditions for UWO 241 and SAG 49.72, respectively, were: i) high light, 250 μmol m-2 s-1 and 500 μmol m-2 s-1; ii) low temperature, 2° C and 11 ° C; iii) high salinity, 700 mM NaCl and 100 mM NaCl.
Growth kinetics were monitored as change in optical density at 750 nm. All other measurements were performed on mid-log phase cultures. Chlorophyll a and b concentrations were determined from whole cell extractions in 90% acetone according to Jeffry & Humphrey (1975).