Photosynthetic responses of Ulva lactuca at very low light

Changes in photosynthetic variables and dark respiration were measured in Ulva lactuca acchmated to continuous photon f l u densities between 0 and 55prnol n ~-~s -' at ?OC for up to 80d. Photosynthetic light harvesting efficiency ( a ) was lughest in U. lactuca acclimated to intermedate light levels of 8.8 and 25 prnol m-2s-1 and was associated with maximum chlorophyll density and light absorptance. Below 2.5 pm01 m-'s-', energy capture was insufficient to maintan the maximum light harvesting apparatus and at 55 pm01 m-2s-i, light was no longer fully lirnihng to photosynthesis. The light harvesting efficiency (m) was linearly related to light absorptance (A). The quantum efficiency (0, = a ~-' ) was, therefore, relatively constant and independent of the light acclimation level (0.045 to 0.059 m01 C [m01 photon absorbed]-'). The light compensation point for photosynthesis (I,) is given by the ratio between dark respiration (r) and light harvesting efficiency (a). Dark respiration rates increased with incident light level. As a result of combined changes in r and a, I, had minimum values (1.5 to 2.3 pm01 m-2s-') in plants acclimated to 8.8 vmol m-2s-' and it increased in the dark and particularly in the high light acclimated plants (5.3 to 7.6 pm01 m-2s-'). Photosynthetic capacity (P,) was lmearly related to light harvesting efficiency (cc), suggesting that photoadaptation mainly occurred by changes in number of photosynthetic units. U. lactuca growing in continuous low light possessed a photosynthetic capacity in excess of photosynthetic performance and would benefit from short-term exposure to high light. This may be of selective advantage, considering that natural light levels are widely fluctuating.