Iron quantification was carried out in microtiter plates with a modified spectrophotometric ferrozine assay(Stookey,1970),as described previously (Muehe et al.,2009).Samples from photoferrotrophic cultures were acidified with 1 M HCI to stabilize Fe(II). Ferrozine measurements were preformed in triplicate and measured with a spectrometric plate reader(FlashScan 550;Analytik Jena,Germany or Multiskan Go,Thermo Fisher Scientific).