Abstract: As an important limiting factor for primary production in oligotrophic oceans, iron (Fe) is essential for phytoplankton, involved in essential physiological processes such as photosynthesis, respiration and anti-oxidative stress. Ongoing increases in seawater CO₂ concentrations could lead to the intensification of ocean acidification (OA), affecting the bioavailability of Fe in seawater. Increasing light intensity in shallower mixed layer caused by seawater warming could influence phytoplankton physiology and also change Fe bioavailability. The coupling of these three factors could impact the growth of phytoplankton. As a major contributor to primary production, Prochlorococcus is widely distributed in oligotrophic oceans. In this study, Prochlorococcus NATL1A was used as the model species to explore the coupling effects of Fe, light and OA. Low bioavailable inorganic Fe (Fe’) concentration, low light or over-saturating high light irradiation all reduced growth and carbon fixation, and the reduction in growth and carbon fixation was exacerbated in the case of Fe and light co-limitation and low Fe coupled with high light. Increase of the CO₂ partial pressure (pCO₂) from 400 ppm to 800 ppm had positive effects on the growth and carbon fixation of Fe limited Prochlorococcus, and the effects became more obvious with the increase of light intensity. The results suggest that OA may increase primary production contributed by Fe-limited Prochlorococcus in oligotrophic oceans. Biochemical and molecular analysis revealed that the beneficial effects of OA were mainly due to the reduction of oxidative stress which might be attributed to increase of CO₂ as an electron acceptor and thus reducing the generation of ROS under Fe-limited and high light conditions. At the meantime, cells optimized the expression of various electron transfer pathways to achieve maximum energy metabolism intracellularly.

Key words: Prochlorococcus; Fe limitation; light; ocean acidification