Photosynthesis of Ulva sp. II. utilization of CO₂ and HCO^-₃ when submerged

The photosynthetic capacity of submerged Ulva sp. when utilizing CO₂ and HCO^-₃ as exogenous carbon forms has been investigated and compared with ambient carbon concentrations in sea water. Saturating concentrations of HCO^{- ₃} and CO₂ were 1200 and 100 μM, respectively at saturating light, and photosynthetic rates under such conditions averaged 700 μmol O₂·g DW^-1 ·h^-1. The HCO^-₃ concentration of sea water (≈2500μM), was thus found to be saturating for photosynthesis of Ulva. At the CO₂ concentration of sea water (≈ 10 μM), the contribution of this carbon form to photosynthesis could be 27% at the most. Under conditions of slow water movement, the relative importance of CO₂ utilization would probably be minimized in favour of HCO^-₃ utilization. It is concluded that HCO^-₃ uptake is not limiting photosynthesis for Ulva under natural conditions.