Algal Strain and culture condition
The algal strain used in this study was ArthrospiraPlatensis maintained in Zarrouk’s medium with the following macro and micro ingredients (mM): NaHCO3 (200), K2HPO4 (3.7), NaNO3(30), K2SO4 (5.7), NaCl (18), CaCl2 2H2O (0.27), FeSO47H2O (0.036), Na2EDTA 2H2O (0.215), NaOH (0.1), H3BO3 (0.045), MnCl24H2O (0.009), ZnSO4 7H2O (0.001), NaMoO4 2H2O (0.000083), and CuSO4 5H2O (0.00032) (Zarrouk, 1966). Unless stated otherwise, Zarrouk´s medium with 200 mM NaHCO3 was used for all culture cultivations. The seed culture was sourced from a private collection and cultivated at 30 °C for one week to reach a biomass concentration of 0.75 gDW L-1 (optical density at 750nm, OD750, equal to 1.5). Under most conditions the DW to OD750ratio was about 0.5 gDW L-1 per OD750which is regarded as normal for this organism. Although some variation within the relationship between OD and DW can occur with differing temperature conditions (Jahnke et al., 2011; Torzillo et al., 1991a), such variations were generally minor in the experiments reported here and only occurred at the latter stages of growth at 35 °C where the cultures displayed clear instabilities (declines) in pigmentation. Lower pigmentation, especially in the red spectral region, can lead to lower refractive indices at 750 nm, lower light scattering, and thus high DW/OD ratios, as observed. The DW measurements involve collecting algal cells on pre-rinsed glass fiber filters (1.5 µm pore size, 47 mm ProWeigh Filters, Cole-Parmer) by filtration, washing three times in deionized water, and then drying to a constant weight at 60 °C for 48 h. The specification for the drying method calls for a water content of no more than 3% after 24 h. The remaining salt content after this procedure was generally less than 2%, and to that extent our DW is equivalent to ash-free DW (AFDW).
Arthrospira platensis , the most common commercial strain, is interchangeably referred to as “Spirulina ” in the literature. We have used Arthrospira throughout this article to refer to the organism under study here and literature results cited forArthrospira platensis .