Glutamine synthetase and nitrogen cycling in colonies of the marine diazotrophic cyanobacteria Trichodesmium spp

Abstract

We examined freshly collected samples of the colonial planktonic cyanobacterium Trichodesmium thiebautii to determine the pathways of recently fixed N within and among trichomes. High concentrations of glutamate and glutamine were found in colonies. Glutamate and glutamine uptake rates and concentrations in cells were low in the early morning and increased in the late morning to reach maxima near midday; then uptake and concentration again fell to low values. This pattern followed that previously observed for T. thiebautii nitrogenase activity. Our results suggest that recently fixed nitrogen is incorporated into glutamine in the N2-fixing trichomes and may be passed as glutamate to non-N2-fixing trichomes. The high transport rates and concentrations of glutamate may explain the previously observed absence of appreciable uptake of NH4+, N03-, or urea by Trichodesmium spp. Immunolocalization, Western blots (immunoblots), and enzymatic assays indicated that glutamine synthetase (GS) was present in all cells during both day and night. GS appeared to be primarily contained in cells of T. thiebautii rather than in associated bacteria or cyanobacteria. Double immunolabeling showed that cells with nitrogenase (Fe protein) contained levels of the GS protein that were twofold higher than those in cells with little or no nitrogenase. GS activity and the uptake of glutamine and glutamate dramatically decreased in the presence of the GS inhibitor methionine sulfoximine. Since no glutamate dehydrogenase activity was detected in this species, GS appears to be the primary enzyme responsible for NH3 incorporation.

NUTRIENT CONTROLS ON NITROGEN UPTAKE AND METABOLISM BY NATURAL POPULATIONS AND CULTURES OF TRICHODESMIUM (CYANOBACTERIA)

“…Nitrogenase is only present in about 10% of the cells along a filament or within colony of Trichodesmium Carpenter 1991, Lin et al 1998). Although GS enzyme appears to be more abundant in cells containing nitrogenase (Carpenter et al 1992), N 2 fixing cells of Trichodesmium cannot be physically separated from those that are not fixing N 2 , and therefore it is difficult to determine from whole filament homogenates the extent to which GS activity varies among N 2 fixing and nonfixing cells. A large constitutive GS pool may make it difficult to detect changes in a smaller GS pool that is regulated in response to N.…”

Coevolution of metal availability and nitrogen assimilation in cyanobacteria and algae

“…The eight electrons required for each catalytic cycle are supplied by ferredoxin, a small [2Fe-2S]-cluster protein that accepts electrons from the photosynthetic electron transport chain (Schrautemeier & Bohme, 1985). The product NH 3 gas is rapidly converted into the soluble and directed into glutamine and glutamate biosynthesis (Wolk et al, 1976;Meeks et al, 1978;Carpenter et al, 1992) (see Ammonium Assimilation section).…”

Regulation of nitrogen metabolism in the marine diazotroph Trichodesmium IMS101 under varying temperatures and atmospheric CO2 concentrations

“…is assimilated via the glutamine synthetase/glutamate synthase (GS/GOGAT) pathway (Carpenter et al, 1992;Kramer et al, 1996;Mulholland and Capone, 2000). In field samples of T. thiebautii, glutamine synthetase (GS) transcript levels increased to a maximum in the afternoon during the period of maximum N2 fixation (Kramer et al, 1996), while GS activity remained steady throughout the diel cycle (Carpenter et al, 1992). A positive correlation between the abundance of GS protein and the nitrogenase enzyme was observed in Trichodesmium spp.…”

“…A positive correlation between the abundance of GS protein and the nitrogenase enzyme was observed in Trichodesmium spp. (Carpenter et al, 1992). Moreover, GS was suggested to play an important regulatory role in N2 fixation either directly or indirectly, by preventing feedback inhibition from accumulated metabolites (Flores and Herrero, 1994;Mulholland and Capone, 2000).…”