The carbon-concentrating mechanisms (CCMs) of cyanobacteria counteract the low CO2 affinity and CO2:O2 selectivities of the Rubisco of these photolithotrophs and the relatively low oceanic CO2 availability. CCMs have a significant energy cost; if light is limiting, the use of N sources whose assimilation demands less energy could permit a greater investment of energy into CCMs and inorganic C (Ci) assimilation. To test this, we cultured Synechococcus sp. UTEX LB 2380 under either N or energy limitation, in the presence of NO3- or NH4+. When growth was energy-limited, NH4+-grown cells had a 1.2-fold higher growth rate, 1.3-fold higher dissolved inorganic carbon (DIC)-saturated photosynthetic rate, 19% higher linear electron transfer, 80% higher photosynthetic 1/K1/2(DIC), 2.0-fold greater slope of the linear part of the photosynthesis versus DIC curve, 3.5-fold larger intracellular Ci pool, and 2.3-fold higher Zn quota than NO3--grown cells. When energy was not limiting growth, there were not differences between NH4+- and NO3--grown cells, except for higher linear electron transfer and larger intracellular Ci pool.We conclude that, when energy limits growth, cells that use the cheaper N source divert energy from N assimilation to C acquisition and assimilation; this does not happen when energy is not limiting.

Dipartimento di Scienze della Vita e dell'Ambiente Università Politecnica delle Marche Ancona 60131 Italy Institute of Microbiology ASCR Algatech Trebon Czech Republic National Research Council Institute of Marine Science Venezia Italy

Division of Plant Sciences University of Dundee at the James Hutton Institute Invergowrie Dundee DD2 5DA UK Plant Functional Biology and Climate Change Cluster University of Technology Sydney Ultimo NSW 2007 Australia

Marine Biology Institute Science Center Shantou University Shantou Guangdong 515063 China Dipartimento di Scienze della Vita e dell'Ambiente Università Politecnica delle Marche Ancona 60131 Italy

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