William Sunda, NOAA

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

Iron is a critical nutrient in photosynthesis and limits phytoplankton growth in large regions of the ocean. Most of the iron in phytoplankton occurs in iron-containing proteins in the photosynthetic apparatus, and thus interactions between cellular iron, light, growth rate are predicted. In agreement with this prediction, decreasing light intensity increased the cellular iron:carbon (Fe:C) ratio needed to support a given growth rate by 2- to 3-fold in both a coastal diatom Thalassiosira pseudonana, and a coastal cyanobacterium  ynechococcus bacillaris due to an increase in iron-containing photosynthetic units. However, although the light responses were similar, the cellular Fe:C ratios needed to support a given growth rate were 5 – 8-fold higher in the cyanobacterium than in the diatom, a pattern seen in other Syechococcus isolates and eukaryotic phytoplankton. Due to the high iron requirement for growth and low light acclimation, we might expect Synechococcus to be at a competitive disadvantage in many low-iron and low-light environments. Indeed, it decreases rapidly with depth within the ocean’s deep chlorophyll maximum, where iron and light levels are low and lower-iron requiring eukaryotic algae typically dominate the phytoplankton biomass.