Carbon dioxide as the exogenous inorganic carbon source forBatrachospermumandLemanea

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 Batrachospermum 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 HCO3 - 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.