Expression of carboxylating enzymes in Laminaria setchellii (Phaeophyceae)

Ribulose 1,5-bisphosphate carboxylase-oxygenase (RUBISCO) and phosphoenolpy ruvate carboxykinase (PEPCK) catalyse the bulk of the carboxylation reactio n in marine algae; however, little is known about the expression of these e nzymes in the brown algae. The objective of this study was to assess the ph otosynthetic and light-independent carbon fixation (LICF) capacities along the thallus of Laminaria setchellii (Laminariales. Phycophyceae) and to rel ate the carboxylation patterns, to the activity and abundance of RUBISCO an d PEPCK. The translocation of photosynthetic products and the synthesis of specific amino acids through photosynthetic and LICF processes were also ev aluated. Photosynthetic rates and in vitro RUBISCO activity were correlated with the abundance of the RUBISCO enzyme, suggesting that differential pho tosynthetic capacity in different regions of the thallus is regulated by th e abundance of RUBISCO. Maximum LICF and PEPCK activity values were observe d in the meristematic region of L. setchellii and were positively correlate d with the abundance of the enzyme in the thallus. This also indicates that LICF capacity is regulated by PEPCK abundance. Pigment levels increased fr om the stipe and meristem towards the laminar region of the kelp, which is consistent with the observed increase in maximum photosynthesis. Collective ly, our results indicate that photosynthesis is limited in the meristematic region by low RUBISCO activity. enzyme abundance and reduced pigment level s. Translocation rates of photosynthetic products from the tip of the lamin a to the meristem were 10-fold greater in plants incubated in the light tha n in those kept in darkness, and the bulk of the labelling was determined a s aspartate. glutamate and alanine. The differences of in vivo and in vitro carboxylation along the thallus suggest the need for whole plant carboxyla tion measurements to achieve accurate estimates of primary productivity in L. setchellii and possibly other Laminariales.