Carbon dioxide as the exogenous inorganic carbon source for batrachospermum and lem an e a

Rates of photosynthesis at light saturation as a function of CO₂ concentration and rates of dark respiration, are reported for the freshwater red algae Lemanea mamillosa and Batracho- spermum sp., and are related to their ecological strategies. The CO₂ compensation concentration in these two algae is pH-independent in the range pH 6.5-8.9, and is of a magnitude which suggests that these algae are unable to use HCO₃“in photosynthesis, and lack a “CO₂ concentrating mechanism”. This is related to the relatively high CO₂/HCO₃ ⁻ ratio in the natural habitats of the algae, in contrast with the seawater in which most rhodophytes live and in which some red algae exhibit characteristics of HCO₃ “use. The absence of HCO₂ use is also discussed in relation to the “bipolar band growth” of florideophytes, and to the relative availability of C, N and P and light in these haptophytic algae.