Energy partitioning between the CO2 concentrating mechanism and N assimilation in the cyanobacterium Synechoccus UTEX 2380: repercussions on cell composition and stoichiometry

Nitrogen is available in nature in various chemical forms. In the sea, NO3 - and NH4 + are the main N sources for primary producers. When growth is energy limited, the utilization of either N forms has important energetic implications and may alter the energy budget of cells and thus on their physiology and ecological fitness. In this thesis, the cyanobacterium Synechococcus was used as the model organism because: 1) Synechococcus is among the main contributors to oceanic primary production; 2) cyanobacteria may be especially sensitive to the changes in energy availability due to the intrinsic limitation of their Rubisco and their inefficient photosynthetic light use. Synechococcus sp. UTEX LB 2380 was cultured semicontinuously under either limiting N (22 μM either NO3 - or NH4 +, 100 μmol photons m-2 s-1) or limiting light (550 μM either NO3 - or NH4 +, 100 μmol photons m-2 s-1). I studied cell composition and elemental stoichiometry by FTIR and TXRF spectrometry, GC-based elemental analysis; photosynthesis and C acquisition by PAM fluorometry, oxygen electrode, silicon oil centrifugation; N assimilation enzyme kinetics by spectrophotometric methods. This thesis was designed to test four main hypotheses: 1) growth in the presence of NH4 + at limiting energy affords higher growth rate that growth in equimoalr concentrations of NO3 -; 2) in the above condition, NH4 +-grown cells have a higher energy availability than their NO3 - grown counterparts and use this additional energy to produce richer biomass (in terms of energetic content); 3) under limiting energy, CO2 acquisition and fixation compete for energy; overall, C acquisition and assimilation is the priory sink for energy; growth on NH4 + allows a somewhat greater metabolic plasticity and an overall greater primary productivity; 4) I also tested the hypothesis that, under energy limitation, the machinery of N assimilation would be altered to minimize cost. My results confirmed all these hypotheses.