Abstract
Acclimation to variable CO2 was studied in floating leaves of the freshwater monocot Ottelia cordata grown in either low or high CO2. The most striking anatomical variations responding to high CO2 included the enlarged upper epidermal cells and the decreased area of epidermal chloroplasts. Stomata that distributed on the upper surface, and the stomatic chamber area, showed no significant response to high CO2. pH-drift experiments indicated that floating leaves of O. cordata were able to use bicarbonate regardless of CO2 concentrations. Photosynthetic enzyme activities and patterns of organic acids fluctuation confirmed that floating leaves of O. cordata can operate CAM only at low CO2, and perform C4-like metabolism at both high and low CO2. Overall, the present results imply that the floating leaves of O. cordata does not just rely on the atmospheric CO2 for its inorganic carbon, but is also dependent on CO2 and bicarbonate in the water. By showing these effects of CO2 variation, we highlight the need for further experimental studies on the regulatory mechanisms in O. cordata floating leaves, that prevent futile cycling among the three CO2 concentrating mechanisms (bicarbonate use, C4, and CAM metabolism) and the strategy for exploiting atmospheric CO2, as well as studies on the detailed biochemical pathway for C4 and CAM metabolism in this species.
Highlights
Since preindustrial times, the atmospheric CO2 concentration has increased from ~280 ppm to currently over 400 ppm and is predicted to reach 800 ppm by the year 2100 (IPCC, 2014)
Increasing the atmospheric CO2 will increase the availability of dissolved inorganic carbon (DIC) and decrease the pH in some aquatic ecosystems; this will increase the proportion of CO2 and HCO3- within the pool of DIC, within a given pH range (Pedersen et al, 2013)
The light intensity condition was chosen as a trade-off between having sufficient light for photosynthesis to avoid the effects induced by low light levels on anatomy and photosynthetic physiology, and not so much light to avoid causing photodamage when at low CO2 concentrations
Summary
The atmospheric CO2 concentration has increased from ~280 ppm to currently over 400 ppm and is predicted to reach 800 ppm by the year 2100 (IPCC, 2014). In some cases such as the productive lakes, the aquatic ecosystems are pulled out of the air-equilibrium either by rapid photosynthesis that decrease CO2 concentrations or rapid decomposition of litter in the sediment that increase CO2 concentrations in the water. This can generate diel changes in CO2 concentrations of over 100-fold as well as the seasonal changes (Maberly, 1996; Schippers et al, 2004). Photosynthesis in submerged aquatic plants is more sensitive and responsive to increasing CO2 than HCO3- (Maberly and Madsen, 1998)
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