13C NMR evidence for bacteriochlorophyll c formation by the C5 pathway in green sulfur bacterium, Prosthecochloris

The 13C NMR spectra of the pheophorbide of bacteriochlorophyll c, formed in the presence of L-[1-13C]glutamate and [2-13C]glycine and [13C]bicarbonate in Prosthecochloris aestaurii, were analysed. The isotope in the glutamate was specifically incorporated into the eight carbon atoms in the tetrapyrrole macrocycle derived from the C-5 of 5-aminolevulinic acid, while no specific enrichment of these eight carbon atoms was observed in the spectrum of the pigment formed in the presence of [2-13C]glycine. These labelling patterns provide evidence for the operation of the C5 pathway of 5-aminolevulinic acid synthesis for bacteriochlorophyll c formation in the bacterium. The labelling of bacteriochlorophyll c by [13C]bicarbonate is consistent with its formation from 5-[1,4,5-13C]aminolevulinic acid formed by the C5 pathway from [1,2,5-13C]glutamic acid. It is proposed that this glutamate is the transamination product of 2-[1,2,5-13C]oxoglutaric acid, arising by carboxylation of [1,4-13C]succinyl-CoA with 13CO2 catalysed by 2-oxoglutaric acid synthase activity, and that the labelled succinyl-CoA is, in turn, derived by the fixation of 13CO2 by the reductive tricarboxylic acid cycle. The 13C chemical shifts of two sp2 quaternary carbons of bacteriopheophorbide c methyl ester (C-2 and C-4) were reassigned.     

Complete mineralization of benzene by aquifer microorganisms under strictly anaerobic conditions.

Benzene was mineralized to CO2 by aquifer-derived microorganisms under strictly anaerobic conditions. The degradation occurred in microcosms containing gasoline-contaminated subsurface sediment from Seal Beach, California, and anaerobic, sulfide-reduced defined mineral medium supplemented with 20 mM sulfate. Benzene, at initial concentrations ranging from 40 to 200 microM, was depleted in all microcosms and more than 90% of 14C-labeled benzene was mineralized to 14CO2.     

Complete mineralization of benzene by aquifer microorganisms under strictly anaerobic conditions.

Benzene was mineralized to CO2 by aquifer-derived microorganisms under strictly anaerobic conditions. The degradation occurred in microcosms containing gasoline-contaminated subsurface sediment from Seal Beach, California, and anaerobic, sulfide-reduced defined mineral medium supplemented with 20 mM sulfate. Benzene, at initial concentrations ranging from 40 to 200 microM, was depleted in all microcosms and more than 90% of 14C-labeled benzene was mineralized to 14CO2.     

Microbial mineralization of VC and DCE under different terminal electron accepting conditions

Production of 14CO2 from [1,2-14C] dichloroethene (DCE) or [1,2-14C] vinyl chloride (VC) was quantified in aquifer and stream-bed sediment microcosms to evaluate the potential for microbial mineralization as a pathway for DCE and VC biodegradation under aerobic, Fe(III)-reducing, SO4-reducing, and methanogenic conditions. Mineralization of [1,2-14C] DCE and [1,2-14C] VC to 14CO2 decreased under increasingly reducing conditions, but significant mineralization was observed for both sediments even under anaerobic conditions. VC mineralization decreased in the order of aerobic > Fe(III)-reducing > SO4-reducing > methanogenic conditions. For both sediments, VC mineralization was greater than DCE mineralization under all electron-accepting conditions examined. For both sediments, DCE mineralization was at least two times greater under aerobic conditions than under anaerobic conditions. Although significant microbial mineralization of DCE was observed under anaerobic conditions, recovery of 14CO2 did not differ substantially between anaerobic treatments.     

Tracer priming the bicarbonate pool

We have described a technique whereby the time necessary to reach an equilibrium enrichment of expired CO2 during a primed-constant infusion of [U-13C]glucose was shortened from 7 to 8 h to 1 hour or less. We applied the theory of the primed-constant infusion technique to the bicarbonate pool, with the "constant infusion" of labeled carbon dioxide originating from oxidation of the infused [13C]glucose rather than from a labeled infusion of bicarbonate.     

Anaerobic mineralization of toluene by enriched sediments with quinones and humus as terminal electron acceptors

The anaerobic microbial oxidation of toluene to CO(2) coupled to humus respiration was demonstrated by use of enriched anaerobic sediments from the Amsterdam petroleum harbor (APH) and the Rhine River. Both highly purified soil humic acids (HPSHA) and the humic quinone moiety model compound anthraquinone-2,6-disulfonate (AQDS) were utilized as terminal electron acceptors. After 2 weeks of incubation, 50 and 85% of added uniformly labeled [(13)C]toluene were recovered as (13)CO(2) in HPSHA- and AQDS-supplemented APH sediment enrichment cultures, respectively; negligible recovery occurred in unsupplemented cultures. The conversion of [(13)C]toluene agreed with the high level of recovery of electrons as reduced humus or as anthrahydroquinone-2,6-disulfonate. APH sediment was also able to use nitrate and amorphous manganese dioxide as terminal electron acceptors to support the anaerobic biodegradation of toluene. The addition of substoichiometric amounts of humic acids to bioassay reaction mixtures containing amorphous ferric oxyhydroxide as a terminal electron acceptor led to more than 65% conversion of toluene (1 mM) after 11 weeks of incubation, a result which paralleled the partial recovery of electron equivalents as acid-extractable Fe(II). Negligible conversion of toluene and reduction of Fe(III) occurred in these bioassay reaction mixtures when humic acids were omitted. The present study provides clear quantitative evidence for the mineralization of an aromatic hydrocarbon by humus-respiring microorganisms. The results indicate that humic substances may significantly contribute to the intrinsic bioremediation of anaerobic sites contaminated with priority pollutants by serving as terminal electron acceptors.     

Interaction of acetogens and methanogens in anaerobic freshwater sediments

Anaerobic decomposition processes in the profundal sediments of Blelham Tarn (English Lake District) are often limited during late summer by the input of organic carbon. The concentration of acetate in the interstitial water fell from about 100 microM (immediately after sedimentation of the spring diatom bloom) to a relatively constant value of about 20 microM in late summer, during which acetate utilization appeared to be balanced by production. Addition of chloroform and molybdate caused an accumulation of cold acetate in large sediment cores and of [14C]acetate in small cores to which [14C]bicarbonate had been added. In both cases chloroform caused the greater accumulation, implying that acetoclastic methanogens were the more active consumers. The conversion of 14CO2 to [14C]acetate was inversely related, with depth, to its conversion to 14CH4. Methanogenesis from CO2 decreased during late summer, whereas acetogenesis and acetoclastic methanogenesis increased over the same time period. The production of acetate from CO2 was generally equivalent to less than 10% of the acetate carbon utilized but could be as high as 25% of that value. Hydrogen consumption by acetogens could be as high as 50% of that utilized in methanogenesis. The role of acetogenic bacteria in anaerobic processes may therefore be of greater significance in lakes such as Blelham Tarn than in more eutrophic systems.     

Interaction of acetogens and methanogens in anaerobic freshwater sediments.

Anaerobic decomposition processes in the profundal sediments of Blelham Tarn (English Lake District) are often limited during late summer by the input of organic carbon. The concentration of acetate in the interstitial water fell from about 100 microM (immediately after sedimentation of the spring diatom bloom) to a relatively constant value of about 20 microM in late summer, during which acetate utilization appeared to be balanced by production. Addition of chloroform and molybdate caused an accumulation of cold acetate in large sediment cores and of [14C]acetate in small cores to which [14C]bicarbonate had been added. In both cases chloroform caused the greater accumulation, implying that acetoclastic methanogens were the more active consumers. The conversion of 14CO2 to [14C]acetate was inversely related, with depth, to its conversion to 14CH4. Methanogenesis from CO2 decreased during late summer, whereas acetogenesis and acetoclastic methanogenesis increased over the same time period. The production of acetate from CO2 was generally equivalent to less than 10% of the acetate carbon utilized but could be as high as 25% of that value. Hydrogen consumption by acetogens could be as high as 50% of that utilized in methanogenesis. The role of acetogenic bacteria in anaerobic processes may therefore be of greater significance in lakes such as Blelham Tarn than in more eutrophic systems.     

Reassessment of 14CO2 compartmentation and of [14C]formate oxidation in rat liver

Our previous report (Marsolais, C., Huot, S., David, F., Garneau, M., and Brunengraber, H. (1987) J. Biol. Chem. 262, 2604-2607) had concluded that a fraction of [14C]formate oxidation in liver occurs in the mitochondrion. This conclusion was based on the labeling patterns of urea and acetoacetate labeled via 14CO2 generated from [14C]formate and other [14C]substrates. We reassessed our interpretation in experiments conducted in (i) perifused mitochondria and (ii) isolated livers perfused with buffer containing [14C]formate, [14C]gluconolactone, 14CO2, or NaH13CO3, in the absence and presence of acetazolamide, an inhibitor of carbonic anhydrase. Our data show that the cytosolic pools of bicarbonate and CO2 are not in isotopic equilibrium when 14CO2 is generated in the cytosol or is supplied as NaH14CO3. We retract our earlier suggestion of a mitochondrial site of [14C]formate oxidation.     

Absence of microbial mineralization of lignin in anaerobic enrichment cultures.

The existence of anaerobic biodegradation of lignin was examined in mixed microflora. Egyptian soil samples, in which rapid mineralization of organic matter takes place in the presence of an important anaerobic microflora, were used to obtain the anaerobic enrichment cultures for this study. Specifically, 14CO2 or [14C]lignin wood was used to investigate the release of labeled gaseous or soluble degradation products of lignin in microbial cultures. No conversion of 14C-labeled lignin to 14CO2 or 14CH4 was observed after 6 months of incubation at 30 degrees C in anaerobic conditions with or without NO3-. A small increase in soluble radioactivity was observed in certain cultures, but it could not be related to the release of catabolic products during the anaerobic biodegradation of lignin.