Title：
Different mechanisms of bicarbonate use affect carbon isotope composition in Ottelia guayangensis and Vallisneria denseserrulata in a karst stream
Macrophytes live in extremely different carbon environments compared to terrestrial plants. In response to bicarbonate dominated inorganic carbon in natural aquatic systems, the ability for HCO3- use has been developed in many macrophytes to alleviate a limitation by CO2 in aquatic systems. In the present study, all present macrophytes in a karst stream were investigated, and we found that seven of nine species were able to use HCO3- and two species at the source of the karst stream were restricted to CO2 use. Two different inhibitors were used to determine the HCO3- use mechanisms in Ottelia guanyangensis and Vallisneria denseserrulata, the only species coexisting from up- to down-stream. External carbonic anhydrase (CAext) mediated HCO3- use was present in O. guanyangensis up-stream, and then O. guanyangensis shifted to a combined mechanism of CAext and anion exchange (AE) mediated HCO3- use down-stream. In these two species, high HCO3- concentration induced the ability to use HCO3-, but high CO2 concentration inhibited this ability. From up-stream to down-stream, the decreasing CO2 induced increasing proportion of HCO3- use that led to changes in the delta C-13 in O. guanyangensis. However, a constant CAext and AE mediated HCO3- use caused a relatively stable delta C-13 in V. denseserrulata from river source to 330 m and then increased the proportion of HCO3- use at the stream outlet associated with the change in delta C-13.
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