Enzymatic regulation of photosynthetic and light- independent carbon fixation in Laminaria setchellii (Phaeophyta), Ulva lactuca (Chlorophyta) and Iridaea cordata (Rhodophyta)

Carbon is acquired through photosynthetic and non-photosynthetic processes in marine algae, However, little is known about the biochemical regulation of these metabolic pathways along the thallus of seaweeds. Consequently, th e objective of this study was to assess the distribution of in vivo carboxy lation pathways and to relate them to the in vitro activity of ribulose 1,5 -bisphosphate carboxylase/oxygenase (RUBISCO), phosphoenolpyruvate carboxyk inase (PEPCK), and phosphoenolpyruvate carboxylase (PEPC) in the Phaeophyte Laminaria setchellii, the Chlorophyte Ulva lactuca, and the Rhodophyte Iri daea cordata. Chlorophyll-a levels did not vary in U. lactuca and I. cordat a. However, pigment levels were significantly lower in the meristematic reg ion of L. setchellii probably as a result of a lack of differentiation of t he chloroplasts in this region. Similarly, net photosynthesis did not vary in the thallus of U. lactuca and L cordata. while it increased from the sti pe and meristem towards the lamina of L. setchellii. In contrast to photosy nthesis, light-independent carbon fixation rates were significantly greater in the meristematic region of L. setchellii suggesting a compensating mech anism for carbon incorporation in photosynthetically limited tissue. The ac tivity of RUBISCO and PEPCK followed a pattern similar to that of in vivo c arboxylation processes indicating that in vivo carbon assimilation is regul ated by the activity of the carboxylating enzymes throughout the thallus of L. setchellii.