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

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Abstract
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.