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Iron limits carbon fixation in much of the modern ocean due to the very low solubility of ferric iron in oxygenated ocean waters. We examined iron-limitation of growth rate under varying light intensities in the coastal cyanobacterium  Synechococcus bacillaris , a descendent of the oxygenic phototrophs that evolved ca. 3 billion years ago when the ocean was reducing and iron was present at much higher concentrations as soluble Fe(II). Decreasing light intensity increased the cellular iron:carbon (Fe:C) ratio needed to support a given growth rate, indicating that iron and light may co-limit the growth of  Synechococcus  in the ocean, as shown previously for eukaryotic phytoplankton. The cellular Fe:C ratios needed to support a given growth rate were 5- to 8-fold higher than ratios for coastal eukaryotic algae growing under the same light conditions. The higher iron requirements for growth in the coastal cyanobacterium may be largely caused by the high demand for iron in photosynthesis, and to higher ratios of iron-rich photosystem I to iron-poor photosystem II in  Synechococcus  than in eukaryotic algae. This high iron requirement may also be vestigial and represent an adaptation to the much higher iron levels in the ancient reducing ocean. Due to the high cellular iron requirement for photosynthesis and growth, and for low light acclimation,  Synechococcus  may be excluded from many low-iron and low-light environments. Indeed, it decreases rapidly with depth within the ocean’s deep chlorophyll maximum (DCM) where iron and light levels are low, and lower-iron requiring picoeukaryotes typically dominate the biomass of phytoplankton community within the mid to lower DCM.

High iron requirement for growth, photosynthesis, and low-light acclimation in the coastal cyanobacterium Synechococcus bacillaris