Anaerobic Metabolism in the N-Limited Green Alga Selenastrum minutum

Abstract

The green alga Selenastrum minutum (Naeg.) Collins is able to assimilate NH4+ in the dark under anaerobic conditions (GC Vanlerberghe, AK Horsey, HG Weger, DH Turpin [1989] Plant Physiol 91: 1551-1557. In the present study, analysis of metabolites following addition of NH4+ to cells acclimated to anaerobic conditions has shown the following. There was a transient decline in adenylate energy charge from 0.6 to 0.4 followed by a recovery back to -0.6. This was accompanied by a rapid increase in pyruvate/phosphoenolpyruvate and fructose-1,6-bisphosphate/ fructose-6-phosphate ratios indicating activation of pyruvate kinase and 6-phosphofructokinase, respectively. There was also an increase in fructose-2,6-bisphosphate, which, since this alga lacks pyrophosphate dependent 6-phosphofructokinase can be inferred to inhibit gluconeogenic fructose-1,6-bisphosphatase.These changes resulted in an increase in the rate of anaerobic starch breakdown. Anaerobic NH4+ assimilation also resulted in a two-fold increase in the rate of production of the major fermentative end-products in this alga, D-lactate and ethanol. There was no change in the rate of accumulation of the fermentative endproduct succinate but malate accumulated under anoxia during NH4+ assimilation. A rapid increase in Gin and decline in Glu indicates that primary NH4+ assimilation under anoxia was via glutamine synthetase-glutamate synthase. Almost all N assimilated under these conditions was sequestered in alanine. These results allow us to propose a model for the regulation of carbon metabolism during anaerobic NH4+ assimilation.

Amino acid and ureide transport in the xylem of symbiotic soybean plants during short-term flooding of the root system in the presence of different sources of nitrogen

“…Elevated levels of amino acids in the xylem sap (reported in more detail in this study) indicate extensive absorption and assimilation of NH 4 + in the roots. Furthermore, the production of alanine, the principal product of anaerobic N metabolism, is greatly enhanced in the presence of NH 4 + , a phenomenon reported previously by Vanleberghe and colleagues (Vanlerberghe and Turpin, 1990;Vanlerberghe et al, 1991). The question arises as to why this apparently efficient assimilation of NH 4 + does not lead to dry matter accumulation under hypoxia above that seen with the N-free treatment.…”

Anaerobic Metabolism in the N-Limited Green Alga Selenastrum minutum

“…When anaerobic cells were supplied with NH4', NH4' assimilation occurred, although at a rate much lower than under aerobic conditions. During anaerobic NH4' assimilation, Ala became ' a significant fermentation product (accounting for 23% of anaerobic starch breakdown) and the rate of Ala synthesis was similar to observed rates of N uptake from the medium (17).…”

“…A sample calculation is provided in Table II. The amino acids reported are the major constituents in these cells (14,17). We were unable to assay '5N abundance of the amide group of Gln because all of the Gln underwent deamination and cyclization to pyroglutamate during the N-DMTBS derivatization.…”

“…They were then incubated 30 min in the dark under either aerobic or anaerobic conditions, prior to addition of '5NH4' (16,17). '5NH4CI (99 atom % '5N, Merck, Sharp and Dohme, Montreal, Canada) was added at a concentration of 5 mm.…”

“…We recently detailed the anaerobic metabolism in N-limited cells of the green alga Selenastrum minutum (16,17). In the absence of an external N source, the major fermentation products were lactate, ethanol, and succinate (16).…”