Combined effects of CO₂ and light on the N₂-fixing cyanobacterium Trichodesmium IMS101: A mechanistic view

The marine diazotrophic cyanobacterium Trichodesmium responds to elevated atmospheric CO₂ partial pressure (pCO₂) with higher N₂ fixation and growth rates. To unveil the underlying mechanisms, we examined the combined influence of pCO₂ (150 and 900 μatm) and light (50 and 200 μmol photons m^-2 s^-1) on Trichodesmium IMS101. We expand on a complementary study that demonstrated that while elevated pCO₂ enhanced N₂ fixation and growth, oxygen evolution and carbon fixation increased mainly as a response to high light. Here, we investigated changes in the photosynthetic fluorescence parameters of photosystem II, in ratios of the photosynthetic units (photosystem I:photosystem II), and in the pool sizes of key proteins involved in the fixation of carbon and nitrogen as well as their subsequent assimilation. We show that the combined elevation in pCO₂ and light controlled the operation of the CO₂-concentrating mechanism and enhanced protein activity without increasing their pool size. Moreover, elevated pCO₂ and high light decreased the amounts of several key proteins (NifH, PsbA, and PsaC), while amounts of AtpB and RbcL did not significantly change. Reduced investment in protein biosynthesis, without notably changing photosynthetic fluxes, could free up energy that can be reallocated to increase N₂ fixation and growth at elevated pCO₂ and light. We suggest that changes in the redox state of the photosynthetic electron transport chain and posttranslational regulation of key proteins mediate the high flexibility in resources and energy allocation in Trichodesmium. This strategy should enable Trichodesmium to flourish in future surface oceans characterized by elevated pCO₂, higher temperatures, and high light.