Fig. 2. Illustration of the effects of the addition of different concentrations of Fe(II) on the growth of Pseudanabaena sp. PCC7367 and Synechococcus sp. 7336.

Comparison of growth (a, b, e) and Fe(II) oxidation (c, d) of Pseudanabaena sp. PCC7367 (a, c) and Synechococcus sp. PCC7336 (b, d) between the open- and a closed-culture systems. The growth response of the two strains of saltwater Cyanobacteria in 22 mM NaHCO₃-buffered ASN-III media to different concentrations of Fe(II) was measured by Chl a determination every 2–3 days (a, b) for 21 days in closed anoxic culture bottles (solid lines) or open cell suspension flasks in an anoxic workstation (dashed lines). The Fe(II) starting concentrations were 20 µM (PCC7367 light green/PCC7336 yellow), 120 µM (PCC7367 green/PCC7336 orange) and 600 µM (PCC7367 dark green/PCC7336 red). The controls (grey) were grown in ASN-III media with an Fe(III) concentration of 15 µM under anoxic (solid/dashed line) and normal atmospheric conditions (dotted line). The oxidation of Fe(II) to Fe(III) was assessed in parallel using a spectrophotometric assay (c, d). Fe(II) measurements of abiotic controls are depicted in Supplementary Fig. 6. Cell doubling times (d) and their standard deviations during the exponential growth phase (e) were determined from day 1 to 13 for Pseudanabaena sp. PCC7367 and from day 1 to 17 for Synechococcus sp. PCC7336. The doubling time for the cultures with 600 µM Fe(II) in the open-culture system was split into a doubling time before green rust formation at day 4 and after. Data are presented as mean values ± SD with n = 4 biologically independent samples. Asterisks (*) represent a significant difference to the media control grown in the equivalent growth system (p < 0.05, Student’s t-test, two-tailed).