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General Session 3: Biological oceanography & global change |
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Decrease of Trichodesmium N2 fixation rate with ocean acidification projected by an optimal resource allocating cellular model
Monday 9th @ 1425-1445
Multi-function Hall
Ya-Wei Luo* , State Key Laboratory of Marine Environmental Science, Xiamen University
Dalin Shi, State Key Laboratory of Marine Environmental Science, Xiamen University
Haizheng Hong, State Key Laboratory of Marine Environmental Science, Xiamen University
Presenter Email: ywluo@xmu.edu.cn
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| Anthropogenic CO2 release induces the pCO2 increase and the pH decrease of seawater, an effect collectively termed ocean acidification (OA). Previous studies suggest that the predominant effect of OA on Trichodesmium, a major marine N2-fixing genus, is the saved energy from CO2 concentrating mechanism (CMM) and thus the stimulation of its growth. However, it is unclear how the accompanying decrease of pH can impact Trichodesmium growth. Here we use a cellular model to show that the net effect of OA is to inhibit the growth and N2 fixation of Trichodesmium. We construct an optimality-based Trichodesmium cellular model to quantitatively explain and simulate cellular energy and iron (Fe) allocations to light reaction (energy harvest), CCM, carbon fixation and N2 fixation, assuming that Trichodesmium always reallocates available resources to maximize its growth rate. In the model, OA reduces CCM energy expenditure, meanwhile increases energy expenditure on anti-stress and reduces the efficiency of nitrogenase, the enzyme catalyzing N2 fixation. The values of model parameters are mainly from theoretical estimates and from a series of culture experiments under combination of different levels of Fe, pCO2 and pH. The model is able to accurately reproduce the decreasing growth and N2 fixation rates of the culture experiments with OA, revealing that the positive effect from CCM energy saving is offset by costs incurred by reduced pH. This inhibition is even more profound when Fe is limiting, when some Fe has to be allocated to N2 fixation to compensate for the reduced efficiency of nitrogenase. Our results predict a ubiquitous reduction of oceanic N2 fixation, particularly in the Fe-limiting regions such as the South Pacific. |
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