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.The marine filamentous N₂-fixing (diazotrophic) cyanobacteria Trichodesmium spp. bloom extensively in the oligotrophic subtropical and tropical oceans (Carpenter and Capone, 2008). Trichodesmium contributes 25% to 50% of the estimated rates of N₂ fixation in these areas, where the new nitrogen inputs stimulate carbon and nitrogen cycling (Capone and Subramaniam, 2005; Mahaffey et al., 2005). The increases in atmospheric CO₂ partial pressure (pCO₂) and the subsequent impacts on ocean acidification are predicted to influence diazotrophs and specifically Trichodesmium.The reported sensitivity of Trichodesmium to changes in pCO₂ prompted further investigation into the cellular responses and underlying mechanisms, specifically when combined with other environmental parameters such as temperature, nutrient availability, and light. Elevated pCO₂ significantly increased growth and N₂ fixation rates of Trichodesmium cultures (Barcelos é Ramos et al., 2007; Hutchins et al., 2007; Levitan et al., 2007, 2010). The physiological response was also characterized by changes in inorganic carbon acquisition, limited flexibility of carbon-nitrogen ratios, and conservation of photosynthetic activities with increased pCO₂. These manifestations suggested that ATP and reductants [ferredoxin, NAD(P)H] are reallocated in the cells (Levitan et al., 2007, 2010; Kranz et al., 2009, 2010).In Trichodesmium, as in all cyanobacteria, the metabolic pathways of respiration and photosynthesis share several cellular complexes/proteins such as the plastoquinone (PQ) pool, succinate dehydrogenase, and ferredoxin (Fig. 1; Kana, 1993; Bergman et al., 1997; Lin et al., 1998). Energetic currencies [reduced ferredoxin, ATP, NAD(P)H] are also shared and can be allocated and utilized according to cellular requirements. N₂ fixation by nitrogenase and the subsequent assimilation of NH₄⁺ by Gln synthetase requires carbon skeletons from the tricarboxylic acid reactions. Moreover, linear and pseudocyclic photosynthesis can also generate additional ATP and reductants essential for N₂ fixation (Fig. 1; Berman-Frank et al., 2001).Open in a separate windowFigure 1.Schematic representation of major cellular complexes involved in energy flow [electron, ATP, NAD(P)H, carbon skeletons] in Trichodesmium IMS101. Dashed arrows represent movement direction of electrons, and solid arrows represent directions of protons, ATP, and NAD(P)H. Measured protein subunits are represented by gray diamonds. See Kranz et al. (2010) for measurements of O₂ evolution, inorganic carbon fixation, and fluxes of N₂ fixation.To understand the regulation of these metabolic pathways in Trichodesmium under varying pCO₂ levels and light intensities, we designed an experiment to characterize changes in the fluxes of carbon, nitrogen, and oxygen (O₂), related protein pool sizes, and variable fluorescence parameters of PSII. Elevated atmospheric pCO₂ combined with enhanced sea surface temperatures are forecast to stabilize thermal stratification, resulting in a shallower, more acidified, upper mixed layer characterized by higher mean light intensities (Doney, 2006). Thus, Trichodesmium IMS101 cultures were acclimated to past and future pCO₂ levels (150 and 900 μatm) at low and high light (50 and 200 μmol photons m⁻² s⁻¹).In the first part of this combined report (Kranz et al., 2010), we examined the physiological responses to the different acclimation conditions. The combination of elevated pCO₂ and light enhanced the production of particulate organic carbon and nitrogen (270% and 390% increase, respectively) as well as growth rates (180% increase; percentages are calculated from Kranz et al., 2010). Generally, the pCO₂-dependent stimulation was higher in cultures acclimated to low light. The pCO₂ effect was also reflected in other measured physiological parameters, particularly the diel patterns of N₂ fixation and the integrated N₂ fixation rates during the day, which increased approximately 30-fold between the low-pCO₂/low-light and the high-pCO₂/high-light acclimations (Kranz et al., 2010). While at high light, elevated pCO₂ extended the period of high N₂ fixation, which lasted from 5 h after the onset of light throughout the end of the photoperiod, the high-pCO₂ contribution to the integrated N₂ fixation was more significant at low light (Kranz et al., 2010). Light, but not pCO₂, influenced gross photosynthesis as measured by PSII O₂ evolution, which increased by approximately 250% in high-light-acclimated cultures. To supply the Calvin cycle with sufficient CO₂, Trichodesmium possesses a CO₂-concentrating mechanism mainly based on HCO₃⁻ uptake (Kranz et al., 2009, 2010). When Trichodesmium was acclimated to elevated pCO₂ (900 μatm), a decline in the cellular affinity to dissolved inorganic carbon was observed (Kranz et al., 2009), while the specific uptake of CO₂ showed a 9-fold increase between the low-pCO₂/low-light and the high-pCO₂/high-light acclimations (Kranz et al., 2010).Proteins are fundamental cellular components that influence the underlying mechanisms subsequently reflected in the cells’ physiology. In this study, we extend the experimental results presented by Kranz et al. (2010) by examining the influence of pCO₂ at different light regimes on the photosynthetic fluorescence parameters of PSII and on the pool sizes of key proteins involved in carbon and nitrogen fixation and their subsequent assimilation processes.