PHOSPHOENOLPYRUVATE CARBOXYKINASE ACTIVITY IN ASCOPHYLLUM NODOSUM (PHAEOPHYCEAE)1

PEP-dependent⁴ CO₂-fixation by extracts of Ascophyllum nodosum (L.) Le Jol. is reported. The carboxylation of PEP is Mn²⁺ dependent and ATP is shown to be a product. IDP was found to be less efficient as a phosphate acceptor than ADP and 3-mercaptopicolinic acid inhibited the carboxylation reaction. Extracts decarboxylated OAA only in the presence of ATP and had high activities of MDH and GOT. This evidence, together with the probable absence of PEPC, PEPCTrP, and PC in A. nodosum extracts, favors the view that PEPCK is responsible for the light-independent CO₂-fixation observed in this alga.     

PHLOROTANNINS AS CHEMICAL DEFENSE AGAINST MACROALGAL EPIPHYTES ON ASCOPHYLLUM NODOSUM

Brock, E., Åberg, P., & Pavia, H. Department of Marine Botany, Göteborg University, Box 461, SE 40530 Göteborg, Sweden. Phlorotannins are a group of polyphenolic acids present in brown algae. They have been shown to act as defense against herbivores and UV-B radiation. They have also been suggested to function as a defense against epiphytes. In this study the effect of the presence of macroalgal epiphytes on the phlorotannin content in Ascophyllum nodosum was analysed. Six adult plants of A. nodosum were collected from each of four sites in the archipelago on the west coast of Sweden. Four apical parts from each of the 24 plants were analysed for their phlorotannin content. For each plant, two samples were without any visible macroalgal epiphyte growth and two samples were 50-100% covered by epiphytes, mainly of the orders Ceramiales and Ectocarpales. We were not able to detect any general effect of epiphytes on the phlorotannin content. There was, however, a significant interaction between the two factors algal individual and epiphytes, where apical parts with epiphytes present had either higher or lower levels of phlorotannin than samples without epiphytes. Furthermore, there was a general, significant difference in the phlorotannin content among sites. The results suggest that the relationship between macroalgal epiphytes and phlorotannin production in A. nodosum is complex and influenced by several interacting factors.     

PROPERTIES OF PHOSPHOENOLPYRUVATE CARBOXYKINASE FROM ASCOPHYLLUM NODOSUM (PHAEOPHYCEAE)1

Phosphoenolpyruvate carboxykinase was purified to electrophoretic homogeneity from the brown alga. Ascophyllum nodosum (L.) le Jol. Its molecular mass was 60 kDa as determined by gel filtration. The pH optimum of the carboxylating reaction was 7.9 and the apparent K_m for PEP⁴, ADP and HCO₃^- were 0.036, 0.0116 and 50 mol · m^-3, respectively. Rates of light and dark carbon fixation are also reported for A. nodosum apices and it is shown that rates of in vitro PEPCK activity would account for the observed rate of dark fixation. The physiological role of PEPCK is discussed in relation to light-independent carbon fixation.     

SEPARATING POPULATION STRUCTURE FROM POPULATION HISTORY IN ASCOPHYLLUM NODOSUM (FUCALES)

Ascophyllum nodosum is dominant seaweed along many rocky intertidal shores throughout the North Atlantic. Next to the kelps, fucalean taxa such as Ascophyllum are the largest macrophytes and provide important habitat for invertebrates. Understanding the underlying genetic structure of natural populations over a range of spatial scales can reveal how the causes of structure may change with scales. Separating population history from population structure may also be elucidated. The analysis is based on six polymorphic microsatellite loci and> 1000 individuals. Strong genetic structure at small spatial scale was found and is consistent with demographic models based on long-lived individuals, low recruitment and many sib matings. At large spatial scales only weak population differentiation was found. This is consistent with recent recolonization of the North Atlantic following the last glacial maximum.