Variations in carboxylase activity in marine phytoplankton cultures, ß-carbolxylation in carbon flux studies

Abstract

Inorganic carbon may be assimilated through the Calvin-Benson cycle via the enzyme ribulose-1.5-bisphosphate carboxylase (Rubisco) andlor by 0-carboxylation [via the enzymes phosphoenolpyruvate carboxylase (PEPC), phosphoenolpyruvate carboxykinase (PEPCK) or pyruvate carboxylase]. Here, carboxylase activity measurements for marine phytoplankton are described. Two indices measuring carboxylase activity in marine phytoplankton were used. The first measures Rubisco activity per unit chlorophyll [R/Chl; nmol CO2 (pg chl a+b+c)-' h-'] while the second is the ratio of P-carboxylase activity to Rubisco activity, expressed as % (PC/R), which reflects the proportion of inorganic carbon fixed by these 2 groups of carboxylases. These ratios were studied in (1) different algal species in culture, (2) dunng the different growth phases of a culture, and (3) after a light-dark transition to measure the time response of carboxylase activities. These indices were different from one species to another at the same stage of growth. In autotrophic cells, P-carboxylation remained low (PC/R<40). The PCIR ratio increased significantly when R/Chl began to decrease at the end of the growth phase of a culture of Skeletonerna costaturn. The heterotrophic dinoflagellate Crypthecodinjurn cohnii, grown on an organic medium, incorporated inorganic carbon in the dark through PEPCK activity. The wlde range in PC/R ratio observed among the species confirm that in phytoplankton there may exist a continuum between autotrophy and heterotrophy. From a carbon budget point of view the 2 mechanisms are not equivalent. Rubisco fixation uses light as an energy source and results in gross production; 0-carboxylation also fixes inorganic carbon but as energy source uses metabolites synthesized by other pathways in the cell or from the external medium.

The effect of pCO2 on carbon acquisition and intracellular assimilation in four marine diatoms

“…Reinfelder et al (2000) demonstrated that PEPC activity was up-regulated at low CO 2 concentrations in T. weissflogii and that the measured PEPC activities contributed up to 50% to carbon fixation under zinc limitation. Even though the assay applied in their study does not exclude the anaplerotic role of PEPC, which is considered to be involved in the synthesis of amino-acid precursors (Descolas-Gros and Oriol, 1992), the observation that the 14 C labelled C 4 compound malate was so rapidly formed in T. weissflogii (Reinfelder et al, 2000;Morel et al, 2002;McGinn and Morel, 2008) indicates photosynthetic C 4 fixation rather than anaplerotic processes. However, recent studies (e.g.…”

Inorganic carbon uptake and intracellular assimilation by subarctic Pacific phytoplankton assemblages

“…The activities of Rubisco and PEPC were determined using 14 C-based assays modified from previous studies (Descolas-Gros and Oriol 1992;MacIntyre et al 1997). The assays measure the rate of 14 C incorporation into organic (acid stable) carbon products following the addition of H 14 CO 2 3 / 14 CO 2 and ribulose bisphosphate (RuBP) or phosphoenolpyruvate (PEP), respectively.…”

Effects of growth rate, CO2 concentration, and cell size on the stable carbon isotope fractionation in marine phytoplankton

“…Compared to diatoms, dinoflagellates have higher rates of dark respiration relative to growth rate (Geider and Osborne, 1989) -Gros and Oriol, 1992). Whether differences in ␤-carboxylation activity are reflected in p , however, depends on the ultimate fate of CO 2 involved in these reactions (no effect on p if CO 2 is released again shortly after fixation; Raven, 1997) and on fractionation by the carboxylating enzyme.…”