Abstract
The rates of uptake of five amino acids - alanine, glutamate, glycine, leucine and serine - by axenic cultures of the cyanobacterium Planktothrix rubescens were measured over a range of irradiances using the 14C-labelled amino acids at the nanomolar concentrations observed in Lake Zürich. The rates in the light exceeded the dark rates by as much as two- to ninefold. The light-affinity constants for stimulation were similar, indicating a similar process for each of the five amino acids. The Ek (light saturation irradiance) for light stimulation was only 1 μmol m-2 s-1, less than the compensation point for photosynthesis and autotrophic growth, and much lower than the Ek for either process. The Ek for amino acid uptake was also less than the irradiance at which filaments obtain neutral buoyancy, which determines the depth at which they stratify and the irradiance they receive. This indicates that stimulation of amino acid uptake by light of low irradiances provides a mechanism for supplementing growth of filaments stratifying deep in the metalimnion, which, while able to grow at low irradiances, are often left with insufficient light to sustain them. Acetate uptake was also stimulated by light, but the kinetics differed
Walsby, Anthony E