Trade-offs between the costs and benefits of HCO3- use in an aquatic plant Ottelia alismoides.

HCO3- use is the most widespread CO2-concentrating mechanism (CCM) in aquatic plants, but the trade-offs between the costs and benefits of this process are unclear. This issue was addressed in the submerged freshwater plant Ottelia alismoides by measuring photosynthesis rates at different inorganic carbon concentrations with and without the addition of HCO3- inhibitors under pH 6.5, 7.5 and 8.5 (high CO2, medium CO2 and low CO2). We showed that depending on CO2 concentrations, O. alismoides uses HCO3- through extracellular carbonic anhydrase, H+-ATPase and anion exchange protein. HCO3- contributed more to photosynthesis as CO2 decreased, and significantly decreased carbon loss by reducing photorespiration. Based on a model building using photosynthesis, photorespiration and dark respiration, in 2 mM total inorganic carbon, under CO2 < 28 μM, the carbon conservation from HCO3- use reducing photorespiration, far exceeded the carbon investment by HCO3- use. When CO2 > 28 μM, although HCO3- use seemed not to be a cost-effective process, it still supported a part of photosynthesis. However, even under higher CO2 concentrations, HCO3- use reduced photorespiration, which improved nitrogen utilization efficiency in leaves, showing lower N content, higher C:N and lower activities of nitrogen assimilation-related enzymes in leaves, including nitrate reductase, glutamine synthetase, glutamate synthase and glutamate dehydrogenase. Our data suggest that CO2 concentration and nitrogen utilization efficiency determine the trade-offs between carbon costs and benefits during HCO3- use.