Human faecal microbiota display variable patterns of glycerol metabolism

Significant amounts of glycerol reach the colon microbiota daily through the diet and/or by in situ microbial production or release from desquamated epithelial cells. Some gut microorganisms may anaerobically reduce glycerol to 1,3-propanediol (1,3-PDO), with 3-hydroxypropanal as an intermediate. Accumulation of the latter intermediate may result in the formation of reuterin, which is known for its biological activity (e.g. antimicrobial properties). To date, glycerol metabolism in mixed cultures from the human colon has received little attention. Using in vitro batch incubations of faeces from 10 human individuals, we demonstrated that glycerol addition (140 mM) significantly affects the metabolism and composition of the microbial community. About a third of the samples exhibited rapid glycerol conversion, yielding proportionally higher levels of acetate and 1,3-PDO. In contrast, a slower glycerol metabolism resulted in higher levels of propionate. Furthermore, rapid glycerol metabolism correlated with significant shifts in the Lactobacillus-Enterococcus community, which were not observed in slower glycerol-metabolizing samples. As the conversion of glycerol to 1,3-PDO is a highly reducing process, we infer that the glycerol metabolism may act as an effective hydrogen sink. Given the importance of hydrogen-consuming processes in the gut, this work suggests that glycerol may have potential as a tool for modulating fermentation kinetics and profiles in the gastrointestinal tract.     

Stable Isotope Ratios as a Tool for Assessing Changes in Carbon and Nutrient Sources in Antarctic Terrestrial Ecosystems

Samples of an angiosperm species, nine lichen species and a terrestrial alga, were collected from a variety of Antarctic terrestrial habitats, and were analysed for C and N stable isotope composition. Collections were made along natural gradients, the marine gradient, running from the sea coast inland and the moisture gradient, determined by melt water and precipitation runoff, and running towards the sea coast. Considerable variation in stable isotope ratios was found; δ¹³C values ranged between −16 and −32‰ and δ¹⁵N values between −23 and +23‰ The variation in stable carbon isotope ratios could be attributed in part to species specific differences, but differences in water availability also played a role, as was shown for the terrestrial alga Prasiola crispa and the lichen species Usnea antarctica. The differences in the isotope ratios of nitrogen could be retraced to the origin of nitrogen: marine or terrestrial. The nitrogen stable isotope ratios were influenced by both the marine gradient from the sea inland and the melt water and precipitation flow running in the opposite direction, towards the sea. This was shown for the lichen species Turgidosculum complicatulum and the angiosperm species Deschampsia antarctica. The variation in the C and N stable isotope ratios can be used to determine sources and pathways of N and changes in the water availability in Antarctic terrestrial ecosystems. Contrary to earlier reports the use of stable N isotope ratios is possible in this case because of the relative simplicity of the structure of the Antarctic terrestrial ecosystems.     

Chemoautotrophic Carbon Fixation Rates and Active Bacterial Communities in Intertidal Marine Sediments

Chemoautotrophy has been little studied in typical coastal marine sediments, but may be an important component of carbon recycling as intense anaerobic mineralization processes in these sediments lead to accumulation of high amounts of reduced compounds, such as sulfides and ammonium. We studied chemoautotrophy by measuring dark-fixation of ¹³C-bicarbonate into phospholipid derived fatty acid (PLFA) biomarkers at two coastal sediment sites with contrasting sulfur chemistry in the Eastern Scheldt estuary, the Netherlands. At one site where free sulfide accumulated in the pore water right to the top of the sediment, PLFA labeling was restricted to compounds typically found in sulfur and ammonium oxidizing bacteria. At the other site, with no detectable free sulfide in the pore water, a very different PLFA labeling pattern was found with high amounts of label in branched i- and a-PLFA besides the typical compounds for sulfur and ammonium oxidizing bacteria. This suggests that other types of chemoautotrophic bacteria were also active, most likely Deltaproteobacteria related to sulfate reducers. Maximum rates of chemoautotrophy were detected in first 1 to 2 centimeters of both sediments and chemosynthetic biomass production was high ranging from 3 to 36 mmol C m⁻² d⁻¹. Average dark carbon fixation to sediment oxygen uptake ratios were 0.22±0.07 mol C (mol O₂)⁻¹, which is in the range of the maximum growth yields reported for sulfur oxidizing bacteria indicating highly efficient growth. Chemoautotrophic biomass production was similar to carbon mineralization rates in the top of the free sulfide site, suggesting that chemoautotrophic bacteria could play a crucial role in the microbial food web and labeling in eukaryotic poly-unsaturated PLFA was indeed detectable. Our study shows that dark carbon fixation by chemoautotrophic bacteria is a major process in the carbon cycle of coastal sediments, and should therefore receive more attention in future studies on sediment biogeochemistry and microbial ecology.     

Field research on the effects of UV-B filters on terrestrial Antarctic vegetation

Patches of vegetation of six common species growing on Léonie Island (67°35 S, 68°20 W), Antarctic Peninsula region were covered with either UV-B transparent perspex screens or UV-B absorbing screens. Uncovered plots served as a control. Temperature and relative humidity were monitored during the austral summer under and outside the screens. The mean effective PSII quantum efficiency showed significant differences among the species, but not between the UV-B treatments. It was concluded that the temperature and the moisture status of the vegetation obscured any possible influence of UV-B treatment on the tteffective PSII quantum efficiency. he usefulness of various UV-B exclusion and supplementation methods used to study the influence of UV-B in the field is discussed.     

Isolation,characterization and localization of extracellular polymeric substances from the cyanobacterium Arthrospira platensis strain MMG-9

Arthrospira platensis is a cyanobacterium known for its nutritional value and secondary metabolites. Extracellular polymeric substances (EPS) are an important trait of most cyanobacteria, including A. platensis. Here, we extracted and analysed different fractions of EPS from a locally isolated strain of A. platensis. Three different fractions of EPS were distinguished. These were EPS released into the medium (REPS), EPS loosely bound to the organism (LEPS) and EPS tightly bound to the organism (TEPS), which were extracted by different procedures. The LEPS fraction was smaller than the other two fractions. The EPS of A. platensis exhibited high diversity. Total protein and carbohydrate content was determined in each of these fractions. The largest amount of total carbohydrates and total proteins was in the TEPS fraction. Eight sugar moieties were detected and analysed in all EPS fractions using HPAE-PAD. Fructose, mannose and ribose were rare sugar residues in all fractions of EPS. With the exception of fructose, all sugars tested for were detected in TEPS. The amount of sugars detected was significantly higher in TEPS compared with the two other fractions, especially for galactose, xylose and glucose. The EPS were localized by confocal laser scanning microscopy (CLSM) after staining with different fluorescent dyes and it was found that A. platensis possessed a thick and smooth layer of EPS around the spiral trichomes.