Methanogenesis and microbial lipid synthesis in anoxic salt marsh sediments

In anoxic salt marsh sediments of Sapelo Island, GA, USA, the vertical distribution of CH₄ production was measured in the upper 20 cm of surface sediments in ten locations. In one section of high marsh sediments, the concentration and oxidation of acetate in sediment porewaters and the rate and amount of¹⁴C acetate and¹⁴CO₂ incorporation into cellular lipids of the microbial population were investigated. CH₄ production rates ranged from <1 to 493 nM CH₄ gram sediment⁻¹ day⁻¹ from intact subcores incubated under nitrogen. Replacement with H₂ stimulated the rate of methane release up to nine fold relative to N₂ incubations. Rates of lipid synthesis from CO₂ averaged 39.2 ×10⁻²nanomoles lipid carbon cm³ sediment⁻¹ hr⁻¹, suggesting that CO₂ may be an important carbon precursor for microbial membrane synthesis in marsh sediments under anoxic conditions. Qualitative measurements of lipid synthesis rates from acetate were found to average 8.7 × 10⁻² nanomoles. Phospholipids were the dominant lipids synthesized by both substrates in sediment cores, accounting for an average of 76.6% of all lipid radioactivity. Small amounts of ether lipids indicative of methanogenic bacteria were observed in cores incubated for 7 days, with similar rates of synthesis for both CO₂ and acetate. The low rate of ether lipid synthesis suggests that either methanogen lipid biosynthesis is very slow or that methanogens represent a small component of total microbial lipid synthesis in anoxic sediments. present address: The University of Maryland,, Chesapeake Biological Laboratory, Box 38, Solomons, MD 20688, USA     

Nitrate addition stimulates microbial decomposition of organic matter in salt marsh sediments

Salt marshes sequester carbon at rates more than an order of magnitude greater than their terrestrial counterparts, helping to mitigate climate change. As nitrogen loading to coastal waters continues, primarily in the form of nitrate, it is unclear what effect it will have on carbon storage capacity of these highly productive systems. This uncertainty is largely driven by the dual role nitrate can play in biological processes, where it can serve as a nutrient‐stimulating primary production or a thermodynamically favorable electron acceptor fueling heterotrophic metabolism. Here, we used a controlled flow‐through reactor experiment to test the role of nitrate as an electron acceptor, and its effect on organic matter decomposition and the associated microbial community in salt marsh sediments. Organic matter decomposition significantly increased in response to nitrate, even at sediment depths typically considered resistant to decomposition. The use of isotope tracers suggests that this pattern was largely driven by stimulated denitrification. Nitrate addition also significantly altered the microbial community and decreased alpha diversity, selecting for taxa belonging to groups known to reduce nitrate and oxidize more complex forms of organic matter. Fourier Transform‐Infrared Spectroscopy further supported these results, suggesting that nitrate facilitated decomposition of complex organic matter compounds into more bioavailable forms. Taken together, these results suggest the existence of organic matter pools that only become accessible with nitrate and would otherwise remain stabilized in the sediment. The existence of such pools could have important implications for carbon storage, since greater decomposition rates as N loading increases may result in less overall burial of organic‐rich sediment. Given the extent of nitrogen loading along our coastlines, it is imperative that we better understand the resilience of salt marsh systems to nutrient enrichment, especially if we hope to rely on salt marshes, and other blue carbon systems, for long‐term carbon storage.     

Production and fate of methylated sulfur compounds from methionine and dimethylsulfoniopropionate in anoxic salt marsh sediments

Anoxic salt marsh sediments were amended with dl-methionine and dimethylsulfoniopropionate (DMSP). Microbial metabolism of methionine yielded methane thiol (MSH) as the major volatile organosulfur product, with the formation of lesser amounts of dimethylsulfide (DMS). Biological transformation of DMSP resulted in the rapid release of DMS and only small amounts of MSH. Experiments with microbial inhibitors indicated that production of MSH from methionine was carried out by procaryotic organisms, probably sulfate-reducing bacteria. Methane-producing bacteria did not metabolize methionine. The involvement of specific groups of organisms in DMSP hydrolysis could not be determined with the inhibitors used, because DMSP was hydrolyzed in all samples except those which were autoclaved. Unamended sediment slurries, prepared from Spartina alterniflora sediments, contained significant (1 to 10 muM) concentrations of DMS. Endogenous methylated sulfur compounds and those produced from added methionine and DMSP were consumed by sediment microbes. Both sulfate-reducing and methane-producing bacteria were involved in DMS and MSH consumption. Methanogenesis was stimulated by the volatile organosulfur compounds released from methionine and DMSP. However, apparent competition for these compounds exists between methanogens and sulfate reducers. At low (1 muM) concentrations of methionine, the terminal S-methyl group was metabolized almost exclusively to CO(2) and only small amounts of CH(4). At higher (>100 muM) concentrations of methionine, the proportion of the methyl-sulfur group converted to CH(4) increased. The results of this study demonstrate that methionine and DMSP are potential precursors of methylated sulfur compounds in anoxic sediments and that the microbial community is capable of metabolizing volatile methylated sulfur compounds.     

Effects of sulphur dioxide fumigation on lipid biosynthesis in pine needles

Pine needle tissues were shown to incorporate acetate [1-¹⁴C] into phospho-, galacto- and neutral lipids. The major incorporation of the label among these lipids was always in the phosphatidyl choline (PC) fraction. The amount of label among the other lipid fractions varied depending on the age and source of the needle tissues (lodgepole or jack pine). In general, the biosynthesis of these lipids was more efficient in the developing than in the fully developed tissues. Treatment of the needle tissues with either gaseous or aqueous SO₂ markedly inhibited their lipid biosynthesis. These effects were more pronounced in the developing than in the fully developed needles. Fumigation with gaseous SO₂ showed that SO₂ concentration and length of exposure determine the extent to which the lipid biosynthetic capacity of the tissues is affected. Lipid biosynthetic capacity was partially or completely recovered when plants were removed from the SO₂ environment. Plants exposed to moderate SO₂ concentrations (0.18–0.20 ppm) for a period of 24 hr recovered faster than those exposed to near lethal SO₂ concentrations (0.34–0.37 ppm) for only 1 hr.     

Diversity, composition, and geographical distribution of microbial communities in California salt marsh sediments

The Pacific Estuarine Ecosystem Indicators Research Consortium seeks to develop bioindicators of toxicant-induced stress and bioavailability for wetland biota. Within this framework, the effects of environmental and pollutant variables on microbial communities were studied at different spatial scales over a 2-year period. Six salt marshes along the California coastline were characterized using phospholipid fatty acid (PLFA) analysis and terminal restriction fragment length polymorphism (TRFLP) analysis. Additionally, 27 metals, six currently used pesticides, total polychlorinated biphenyls and polycyclic aromatic hydrocarbons, chlordanes, nonachlors, dichlorodiphenyldichloroethane, and dichlorodiphenyldichloroethylene were analyzed. Sampling was performed over large (between salt marshes), medium (stations within a marsh), and small (different channel depths) spatial scales. Regression and ordination analysis suggested that the spatial variation in microbial communities exceeded the variation attributable to pollutants. PLFA analysis and TRFLP canonical correspondence analysis (CCA) explained 74 and 43% of the variation, respectively, and both methods attributed 34% of the variation to tidal cycles, marsh, year, and latitude. After accounting for spatial variation using partial CCA, we found that metals had a greater effect on microbial community composition than organic pollutants had. Organic carbon and nitrogen contents were positively correlated with PLFA biomass, whereas total metal concentrations were positively correlated with biomass and diversity. Higher concentrations of heavy metals were negatively correlated with branched PLFAs and positively correlated with methyl- and cyclo-substituted PLFAs. The strong relationships observed between pollutant concentrations and some of the microbial indicators indicated the potential for using microbial community analyses in assessments of the ecosystem health of salt marshes.     

Biogenesis and functions of lipid droplets in plants: Thematic Review Series: Lipid Droplet Synthesis and Metabolism: from Yeast to Man

The compartmentation of neutral lipids in plants is mostly associated with seed tissues, where triacylglycerols (TAGs) stored within lipid droplets (LDs) serve as an essential physiological energy and carbon reserve during postgerminative growth. However, some nonseed tissues, such as leaves, flowers and fruits, also synthesize and store TAGs, yet relatively little is known about the formation or function of LDs in these tissues. Characterization of LD-associated proteins, such as oleosins, caleosins, and sterol dehydrogenases (steroleosins), has revealed surprising features of LD function in plants, including stress responses, hormone signaling pathways, and various aspects of plant growth and development. Although oleosin and caleosin proteins are specific to plants, LD-associated sterol dehydrogenases also are present in mammals, and in both plants and mammals these enzymes have been shown to be important in (steroid) hormone metabolism and signaling. In addition, several other proteins known to be important in LD biogenesis in yeasts and mammals are conserved in plants, suggesting that at least some aspects of LD biogenesis and/or function are evolutionarily conserved.     

Drought impacts on biogeochemistry and microbial processes in salt marsh sediments: a flow-through reactor approach

The effects of drought on salt marsh sediments from Sapelo Island, Georgia, were examined in flow-through reactor experiments. Three hydrological treatments were employed: a continuously flooded anoxic control, a periodic drought treatment that experienced alternate periods of flooding and drying, and a severe drought treatment, where sediment was exposed to drought (drying) for several weeks and then flooded; the effect of both buffered and non-buffered flooding solutions were examined. In permanently anoxic sediments as well as in sediments exposed to drought, organic carbon oxidation was dominated by SO₄ ^2? reduction (SR) and SR rates increased over time. The shift from anoxic to oxic conditions in drought treatments significantly altered sediment geochemistry and pathways of microbial metabolism. Drought conditions favored suboxic mineralization processes, such as Fe(III) reduction and denitrification, which was fueled by NH₄ ⁺ oxidation promoted by O₂ delivered during drought conditions. Other major drought-induced changes included pH decrease, and altered concentrations of solid phase adsorbed metals.     

DGAT enzymes are required for triacylglycerol synthesis and lipid droplets in adipocytes

The total contribution of the acyl CoA:diacylglycerol acyltransferase (DGAT) enzymes, DGAT1 and DGAT2, to mammalian triacylglycerol (TG) synthesis has not been determined. Similarly, whether DGAT enzymes are required for lipid droplet (LD) formation is unknown. In this study, we examined the requirement for DGAT enzymes in TG synthesis and LDs in differentiated adipocytes with genetic deletions of DGAT1 and DGAT2. Adipocytes with a single deletion of either enzyme were capable of TG synthesis and LD formation. In contrast, adipocytes with deletions of both DGATs were severely lacking in TG and did not have LDs, indicating that DGAT1 and DGAT2 account for nearly all TG synthesis in adipocytes and appear to be required for LD formation during adipogenesis. DGAT enzymes were not absolutely required for LD formation in mammalian cells, however; macrophages deficient in both DGAT enzymes were able to form LDs when incubated with cholesterol-rich lipoproteins. Although adipocytes lacking both DGATs had no TG or LDs, they were fully differentiated by multiple criteria. Our findings show that DGAT1 and DGAT2 account for the vast majority of TG synthesis in mice, and DGAT function is required for LDs in adipocytes, but not in all cell types.     

Lipid synthesis inhibitors: Effect on epidermal lipid conformational changes and percutaneous permeation of levodopa

A combination of lipid synthesis inhibitors was used to enhance the in vitro and in vivo permeation of levodopa (LD) across rat epidermis, and their influence on epidermal lipids was investigated using attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy. Rat epidermis was treated with ethanol and a combination of atorvastatin (750 μg/7 cm²), cerulenin (20 μg/7 cm²), and β-chloroalanine (600 μg/7 cm²) for sustaining the reduced content of epidermal cholesterol, fatty acids (as triglycerides), and ceramide (as sphingosine), respectively, in viable rat skin. This treatment resulted in significant (P<.05) synthesis inhibition of skin lipids up to 48 hours and 6-fold enhancement in the in vitro permeation of LD. The effective plasma concentration of LD was achieved within 1 hour and maintained over 48 hours after topical application to rat epidermis treated with a combination of these lipid synthesis inhibitors. ATR-FTIR studies of inhibitor(s)-treated rat epidermis revealed a significant decrease (P<.05) in peak height and area for both asymmetric and symmetric C−H stretching absorbances, suggesting extraction of lipids. However, an insignificant (P<.05) shift in the frequency of these peaks suggested no fluidization of epidermal lipids by lipid synthesis inhibitors. A direct correlation was observed between epidermal lipid synthesis inhibition, decrease in peak height or area, and percutaneous permeation of LD. Skin lipid synthesis inhibition by a combination of lipid synthesis inhibitors seems to offer a feasible approach for enhancing the transcutaneous delivery of LD. Published: October 24, 2005     

Prenyl lipid and fatty-acid synthesis in isolatedAcetabularia chloroplasts

A gentle procedure allowed the isolation of intact and highly active chloroplasts from the unicellular green algaAcetabularia mediterranea. These chloroplasts incorporated carbon from NaH¹⁴CO₃ into fatty acids and prenyl lipids at a rate of about 20–50 nmol carbon· (mg chlorophyll)⁻¹·h⁻¹. Most of the fatty acids formed in vitro were esterified in galactolipids. The main prenyl lipids synthesized were the chlorophyll side chain, intermediates of the carotenogenic path, α-and β-carotene, as well as lutein. Large amounts of [1-¹⁴C]acetate were incorporated, but exclusively into fatty acids.Isopentenyl diphosphate was a good substrate for prenyl-lipid formation in hypotonically treated chloroplasts. The envelope of intact chloroplasts, however, was impermeable to this compound. Intermediates of the mevalonate pathway were not accepted as precursors under conditions whereisopentenyl diphosphate was well incorporated. The results show that the lipid biosynthetic pathways in the plastids ofAcetabularia, a member of the ancient family of Dasycladaceae, are very similar to those in higher-plant plastids. Dedicated to Professor Hans Mohr on the occasion of his 60th birthday     

Succinate metabolism related to lipid synthesis in the housefly Musca domestica L

The metabolism of succinate was examined in the housefly Musca domestica L. The labeled carbons from [2,3-¹⁴C]succinate were readily incorporated into cuticular hydrocarbon and internal lipid, whereas radioactivity from [1,4-¹⁴C]succinate was not incorporated into either fraction. Examination of the incorporation of [2,3-¹⁴C]succinate, [1-¹⁴C]acetate, and [U-¹⁴C]proline into hydrocarbon by radio-gas-liquid chromatography showed that each substrate gave a similar labeling pattern, which suggested that succinate and proline were converted to acetyl-CoA prior to incorporation into hydrocarbons. Carbon-13 nuclear magnetic resonance showed that the labeled carbons from [2,3-¹³C]succinate enriched carbons 1, 2, and 3 of hydrocarbons with carbon-carbon coupling showing that carbons 2 and 3 of succinate were incorporated as an intact unit. Radio-high-performance liquid chromatographic analysis of [2,3-¹⁴C]succinate metabolism by mitochondrial preparations showed that in addition to labeling fumarate, malate, and citrate, considerable radioactivity was also present in the acetate fraction. The data show that succinate was not converted to methylmalonate and did not label hydrocarbon via a methylmalonyl derivative. Malic enzyme was assayed in sonicated mitochondria prepared from the abdomens and thoraces of 1- and 4-day-old insects; higher activity was obtained with NAD⁺ in mitochondria prepared from thoraces, whereas NADP⁺ gave higher activity with abdomen preparations. These data document the metabolism of succinate to acetyl-CoA and not to a methylmalonyl unit prior to incorporation into lipid in the housefly and establish the role of the malic enzyme in this process.     

Biogeochemistry and microbial ecology of methane oxidation in anoxic environments: a review

Evidence supporting a key role for anaerobic methane oxidation in the global methane cycle is reviewed. Emphasis is on recent microbiological advances. The driving force for research on this process continues to be the fact that microbial communities intercept and consume methane from anoxic environments, methane that would otherwise enter the atmosphere. Anaerobic methane oxidation is biogeochemically important because methane is a potent greenhouse gas in the atmosphere and is abundant in anoxic environments. Geochemical evidence for this process has been observed in numerous marine sediments along the continental margins, in methane seeps and vents, around methane hydrate deposits, and in anoxic waters. The anaerobic oxidation of methane is performed by at least two phylogenetically distinct groups of archaea, the ANME-1 and ANME-2. These archaea are frequently observed as consortia with sulfate-reducing bacteria, and the metabolism of these consortia presumably involves a syntrophic association based on interspecies electron transfer. The archaeal member of a consortium apparently oxidizes methane and shuttles reduced compounds to the sulfate-reducing bacteria. Despite recent advances in understanding anaerobic methane oxidation, uncertainties still remain regarding the nature and necessity of the syntrophic association, the biochemical pathway of methane oxidation, and the interaction of the process with the local chemical and physical environment. This review will consider the microbial ecology and biogeochemistry of anaerobic methane oxidation with a special emphasis on the interactions between the responsible organisms and their environment. This revised version was published online in August 2006 with corrections to the Cover Date.     

Acyl lipid changes in photosynthetic mutants of Rhodopseudomonas sphaeroides correlate with blocks in bacteriochlorophyll synthesis

Abstract In comparison with the wild-type, mutants of Rhodopseudomonas sphaeroides defective in bacteriochlorophyll synthesis fail to alter their lipid composition on shifting from non-photosynthetic to photosynthetic growth conditions. The earlier the lesion in the bacteriochlorophyll synthetic pathway, the more severe the effect on membrane lipid composition, indicating that acyl lipid and pigment syntheses are co-ordinated and linked to pigment-protein complex assembly.     

Effect of growth temperature on ether lipid biochemistry in Archaeoglobus fulgidus

The archaea are distinguished by their unique isoprenoid ether lipids, which typically consist of the sn-2,3-diphytanylglycerol diether or sn-2,3-dibiphytanyldiglycerol tetraether core modified with a variety of polar headgroups. However, many hyperthermophilic archaea also synthesize tetraether lipids with up to four pentacyclic rings per 40-carbon chain, presumably to improve membrane thermal stability at temperatures up to∼110 °C. This study aimed to correlate the ratio of tetraether to diether core lipid, as well as the presence of pentacyclic groups in tetraether lipids, with growth temperature for the hyperthermophilic archaeon, Archaeoglobus fulgidus. Analysis of the membrane core lipids of A. fulgidus using APCI–MS analysis revealed that the tetraether-to-diether lipid ratio increases from 0.3 ± 0.1 for cultures grown at 70°C to 0.9 ± 0.1 for cultures grown at 89°C. Thin-layer chromatography (TLC) followed by APCI–MS analysis provided evidence for no more than one pentacycle in the hydrocarbon chains of tetraether lipid from cultures grown at 70°C and up to 2 pentacycles in the tetraether lipid from cultures grown at higher temperatures. Analysis of the polar lipid extract using TLC and negative-ion ESI–MS suggested the presence of diether and tetraether phospholipids with inositol, glycosyl, and ethanolamine headgroup chemistry.     

High confidence proteomic analysis of yeast LDs identifies additional droplet proteins and reveals connections to dolichol synthesis and sterol acetylation

Accurate protein inventories are essential for understanding an organelle’s functions. The lipid droplet (LD) is a ubiquitous intracellular organelle with major functions in lipid storage and metabolism. LDs differ from other organelles because they are bounded by a surface monolayer, presenting unique features for protein targeting to LDs. Many proteins of varied functions have been found in purified LD fractions by proteomics. While these studies have become increasingly sensitive, it is often unclear which of the identified proteins are specific to LDs. Here we used protein correlation profiling to identify 35 proteins that specifically enrich with LD fractions of Saccharomyces cerevisiae. Of these candidates, 30 fluorophore-tagged proteins localize to LDs by microscopy, including six proteins, several with human orthologs linked to diseases, which we newly identify as LD proteins (Cab5, Rer2, Say1, Tsc10, YKL047W, and YPR147C). Two of these proteins, Say1, a sterol deacetylase, and Rer2, a cis-isoprenyl transferase, are enzymes involved in sterol and polyprenol metabolism, respectively, and we show their activities are present in LD fractions. Our results provide a highly specific list of yeast LD proteins and reveal that the vast majority of these proteins are involved in lipid metabolism.     

Changes in lipid content and composition during the development of N-nitrosodiethylamine induced hepatocarcinoma

Alterations in lipid content and composition in the N-nitrosodiethylamine-induced hepatocarcinoma were investigated. Rats were administrated with N-nitrosodiethylamine in the drinking water for 12 weeks followed by normal tap water for another 6 weeks. The cholesterol content in the liver was increased shortly after the administration of N-nitrosodiethylamine and remained elevated after the removal of the nitrosoamine from the water. The phosphatidylethanolamine level was elevated during N-nitrosodiethylamine administration with a concomitant reduction in phosphatidylcholine level. Lysophosphatidylcholine and sphingomyelin levels were increased during the last four weeks of the study. The level of phosphatidylinositol was substantially reduced after eight weeks of N-nitrosodiethylamine treatment, and remained low during the post-treatment period. We postulate that changes in lysophosphatidylcholine and sphingomyelin may be a compensatory mechanism for maintaining the asymmetrical distribution of choline-containing lipids in the outer leaflet of the membrane. The elevated level of cholesterol may be a useful indicator for the early detection of N-nitrosodiethylamine-induced hepatocarcinoma.     

Peroxisomes, lipid metabolism, and human disease

In the past few years, much has been learned about the metabolic functions of peroxisomes. These studies have shown that peroxisomes play a major role in lipid metabolism, including fatty acid β-oxidation, etherphospholipid biosynthesis, and phytanic acid α-oxidation. This article describes the current state of knowledge concerning the role of peroxisomes in these processes, especially in relation to various peroxisomal disorders in which there is an impairment in peroxisomal lipid metabolism.     

海岸盐沼湿地土壤硫循环中的微生物及其作用

硫及硫化合物的动态循环是海岸盐沼湿地的重要组成部分,硫酸盐还原菌(SRB)和硫氧化菌(SOB)是推动硫循环的重要微生物。硫酸盐还原菌把硫酸盐还原为硫化物,同时消耗土壤中的有机物质;硫氧化菌把还原性硫化合物氧化为硫酸盐,缓解土壤中硫化物的积累,它们共同维持硫循环的动态平衡。本文综述了海岸盐沼湿地土壤中硫的存在形式、硫的地球化学循环以及在硫循环过程中扮演重要角色的硫酸盐还原菌和硫氧化菌的生物多样性、活性测定方法及其生态学意义等的最新研究进展,并提出了存在的问题及研究展望。     

Seasonal changes in polar lipids in fronds of the ferns Dryopteris filix-mas and Matteuccia struthiopteris

The distribution pattern of total lipids, glyco- and phospholipids, and one betaine lipid (DGTS) in the fronds of the ferns Dryopteris filix-mas and Matteuccia struthiopteris was studied. The lipid composition of the embryo leaflets forming a bud, or treble clef, and that of fully opened leaves changed throughout the growth season. The maximum amount of DGTS in clefs and mature leaves was detected at the beginning of the season. By midsummer, the DGTS content decreased, dropping to zero in the fully opened leaves, and then increased again. The amount of DGTS in the clefs collected in October versus those collected in May was somewhat higher in the case of Dryopteris filix-mas and almost twofold lower in the case of Matteuccia struthiopteris. The ratio between polar lipids contained in the clefs and mature leaves throughout the growth season was determined.     

Up-regulation of lipid metabolism and glycine betaine synthesis are associated with choline-induced salt tolerance in halophytic seashore paspalum

Choline may affect salt tolerance by regulating lipid and glycine betaine (GB) metabolism. This study was conducted to determine whether alteration of lipid profiles and GB metabolism may contribute to choline regulation and genotypic variations in salt tolerance in a halophytic grass, seashore paspalum (Paspalum vaginatum). Plants of Adalayd and Sea Isle 2000 were subjected to salt stress (200-mM NaCl) with or without foliar application of choline chloride (1 mM). Genotypic variations in salt tolerance and promotive effects of choline application on salt tolerance were associated with both the up-regulation of lipid metabolism and GB synthesis. The genotypic variations in salt tolerance associated with lipid metabolism were reflected by the differential accumulation of phosphatidylcholine and phosphatidylethanolamine between Adalayd and Sea Isle 2000. Choline-induced salt tolerance was associated with of the increase in digalactosyl diacylglycerol (DGDG) content including DGDG (36:4 and 36:6) in both cultivars of seashore paspalum and enhanced synthesis of phosphatidylinositol (34:2, 36:5, and 36:2) and phosphatidic acid (34:2, 34:1, and 36:5), as well as increases in the ratio of digalactosyl diacylglycerol: monogalactosyl diacylglycerol (DGDG:MGDG) in salt-tolerant Sea Isle 2000. Choline regulation of salt tolerance may be due to the alteration in lipid metabolism in this halophytic grass species.