Elemental stoichiometry and photophysiology regulation of Synechococcus sp. PCC7002 under increasing severity of chronic iron limitation.

Iron (Fe) is an essential co-factor for many metabolic enzymes of photoautotrophs. Although Fe limits phytoplankton productivity in broad areas of the ocean, phytoplankton have adapted their metabolism and growth to survive in these conditions. Using the euryhaline cyanobacterium Synechococcus sp. PCC7002 we investigated the physiological responses to long-term acclimation to four levels of Fe availability representative of the contemporary ocean (36.7, 3.83, 0.47 and 0.047 pM Fe'). With increasing severity of Fe limitation, Synechococcus sp. cells gradually decreased their volume and growth while increasing their energy allocation into organic carbon and nitrogen cellular pools. Furthermore, total cellular content of pigments decreased. Additionally, with increasing severity of Fe limitation, intertwined responses of photosystem II functional cross-section (σPSII), re-oxidation time of the plastoquinone primary acceptor QA (τ), and non-photochemical quenching revealed a shift in the photophysiological response between mild to strong Fe limitation compared to severe limitation. Under mild and strong Fe limitation there was a decrease in linear electron transport accompanied by progressive loss of state transitions. Under severe Fe limitation, state transitions seemed to be largely supplanted by alternative electron pathways. In addition, mechanisms to dissipate energy excess and minimize oxidative stress associated with high irradiances increased with increasing severity of Fe limitation. Overall, our results establish the sequence of physiological strategies adopted by the cells under increasing severity of chronic Fe limitation, within a range of Fe concentrations relevant to modern ocean biogeochemistry.