Since the industrial revolution a large amount of anthropogenic carbon dioxide has emitted and dissolved in the ocean, resulting in ocean acidification. Iron (Fe) is an essential element that limits primary productivity in the vast area of the oligotrophic oceans. Previous studies have shown that ocean acidification (OA) can affect both Fe availability and Fe requirement by phytoplankton, which will likely affect the growth and carbon fixation of Fe-limited phytoplankton. Synechococcus are one of the most ubiquitous phytoplankton in the ocean. It has been reported that and the strategies for coastal and oceanic Synechococcus to cope with Fe stress are different. To investigate how Fe limitation modules the response of coastal and oceanic Synechococcus to ocean acidification, we cultured the oceanic strain Synechococcus sp. WH8102 and coastal strain Synechococcus sp. WH5701 under different pCO₂ and Fe concentrations. The results showed that high pCO₂ promoted the growth rate and carbon fixation rate of the two Synechococcus strains only under Fe-limited condition, but not Fe-replete condition. Particularly, the positive effect was more obvious in oceanic WH8102 than in coastal WH5701. Further chemical and biochemical analysis showed that OA greatly increased Fe use efficiency in Fe-limited WH8201 by allocating more Fe on Fe-containing photosynthetic proteins such as PsbA, PsaC and PetC and thus promoting carbon fixation. However, this phenomenon did not occur in coastal WH5701. In addition, high pCO₂ down regulated the efficiency of CO₂ concentrating mechanisms (CCMs), decreased respiration rate and reduced intracellular oxidative stress in Fe-limited WH8102. The energy and resources saved from these metabolic pathways may further contribute to the positive response of Fe-limited WH8102 to elevated pCO₂. On the contrary, only ROS level decreased in coastal WH5701 at Fe-limited and high pCO₂ conditions, which was consistent with the trivial effect of high pCO₂ on the growth and carbon fixation of WH5701. In the future changing oceans, oceanic and coastal Synechococcus may perform differently and cosmopolitan oceanic Synechococcus may benefit from ocean acidification in Fe-limited oligotrophic oceans.

Key words: Synechococcus, iron limitation, ocean acidification, photosynthesis