Iron (Fe) is an essential nutrient element that limits primary productivity in vast regions of the oceans. Ongoing increases in ocean CO₂ concentrations can affect both Fe availability and its requirement by phytoplankton, potentially impacting carbon fixation and growth of Fe-limited phytoplankton. The cyanobacterium Synechococcus is one of the most ubiquitous phytoplankton groups in the ocean and the strategies for oceanic and coastal Synechococcus to cope with low Fe stress may be different owing to large differences in iron concentrations between oceanic and coastal waters. We cultured the oceanic Synechococcus sp. WH8102 and coastal Synechococcus sp. WH5701 under different Fe concentrations and pCO₂ to investigate how Fe limitation affects their response to increased CO₂ levels. The growth of the coastal strain was more limited by low iron concentrations than that of the oceanic strain, similar to the behavior of eukaryotic phytoplankton. High pCO₂ promoted the growth rate of both strains only under Fe-limitation. This effect was larger in the oceanic species, and was largely due to greater allocation of iron for iron containing photosynthetic proteins. High pCO₂ also downregulated CO₂ concentrating mechanisms (CCMs) and reduced intracellular oxidative stress in Fe-limited oceanic strain WH8102 further benefiting cellular growth rates. Therefore, it is likely that oceanic and coastal Synechococcus may perform differently in the future changing oceans, and oceanic Synechococcus may benefit more from increased CO₂ concentrations in Fe-limited oligotrophic oceans.

 

iron limitation, pCO2, photosynthesis, Synechococcus