Effect of increased carbon dioxide and iron limitation on coastal and oceanic strains of Synechococcus

Abstract

Iron (Fe) is an essential nutrient that limits primary productivity in vast regions of the oceans. Ongoing increases in ocean carbon dioxide (CO2) concentrations can affect both Fe availability and its requirement by phytoplankton, potentially impacting carbon fixation and the 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 Fe concentrations between oceanic and coastal waters. We cultured the oceanic Synechococcus strain WH8102 and coastal Synechococcus strain WH5701 under different pCO2 levels and concentrations of bioavailable dissolved inorganic iron species (Fe′) to investigate how Fe limitation affects their response to increased CO2 levels. The growth of the coastal strain was more limited by low Fe' concentrations than that of the oceanic strain. High pCO2 significantly promoted the growth rate of both strains only under low Fe' concentrations. At similar degrees of Fe limitation, this effect was larger in the oceanic species and was largely due to a reduced growth demand for Fe‐containing photosynthetic proteins. High pCO2 also down‐regulated CO2‐concentrating mechanisms and reduced intracellular oxidative stress in the Fe‐limited oceanic strain, further benefiting cellular growth rates. Therefore, ongoing and future increases in CO2 concentrations may affect the growth rate and species composition of Synechococcus in Fe‐limited regions of the ocean.