Variable HCO ₃^- affinity of Elodea canadensis Michaux in response to different HCO ₃^- and CO₂ concentrations during...

Elodea canadensis grows over a wide range of inorganic carbon, nutrient, and light conditions in lakes and streams. Affinity for HCO ₃^- use during photosynthesis ranged from strong to weak in Elodea collected from seven localities with different HCO ₃^- and CO₂ concentrations. The response to HCO ₃^- was also very plastic in plants grown in the laboratory at high HCO ₃^- concentrations and CO₂ concentrations varying from 14.8 to 2,200 μM. Bicarbonate affinity was markedly reduced with increasing CO₂ concentrations in the growth medium so that ultimately HCO ₃^- use was not detectable. High CO₂ concentrations also decreased CO₂ affinity and induced high CO₂ compensation points (360μM CO₂) and tenfold higher half-saturation values (∼800 μM CO₂).The variable HCO ₃^- affinity is probably environmentally based. Elodea is a recently introduced species in Denmark, where it reproduces only vegetatively, leaving little opportunity for genetic variation. More important, local populations in the same water system had different HCO ₃^- affinities, and a similar variation was created by exposing one plant collection to different laboratory conditions.Bicarbonate use enabled Elodea to photosynthesize rapidly in waters of high alkalinity and enhanced the carbon-extracting capacity by maintaining photosynthesis above pH 10. On the other hand, use of HCO ₃^- represents an investment in transport apparatus and energy which is probably not profitable when CO₂ is high and HCO ₃^- is low. This explanation is supported by the findings that HCO ₃^- affinity was low in field populations where HCO ₃^- was low (0.5 and 0.9 m M) or CO₂ was locally high, and that HCO ₃^- affinity was suppressed in the laboratory by high CO₂ concentrations.