HETEROTROPHIC GROWTH OF ULVA-LACTUCA (CHLOROPHYCEAE)

The effect of external glucose (51 mM) and acetate (13 mM) on growth and photosynthetic capacity of Ulva lactuca L. was tested in laboratory cultures over 41 days in the dark and in dim light (0.9-mu-mol photons.m-2.s-1) at 7-8-degrees-C. Glucose and acetate had a significant positive effect on growth rate, chlorophyll content, and quantum yield for discs grown in the dark and in dim light. The carbon gain from heterotrophic uptake was low and only allowed U. lactuca to maintain a specific growth rate of 0.005 day-1 compared to 0.06-0.1 day-1 at higher light intensities. However, plants with added organic substrate maintained a normal chlorophyll content and were able to photosynthesize whereas control plants lost pigmentation and photosynthetic capability after 41 days in both dim light and darkness, probably because disorganization of the photosynthetic apparatus. This suggests that the ecological significance of heterotrophic uptake is to allow U. lactuca to survive during prolonged low light conditions with an intact photosynthetic apparatus.