Transition from Anoxygenic to Oxygenic Photosynthesis in a Microcoleus chthonoplastes Cyanobacterial Mat

Benthic cyanobacterial mats with the filamentous Microcoleus chthonoplastes as the dominant phototroph grow in oxic hypersaline environments such as Solar Lake, Sinai. The cyanobacteria are in situ exposed to chemical variations between 200 μmol of sulfide liter⁻¹ at night and 1 atm pO₂ during the day. During experimental H₂S to O₂ transitions the microbial community was shown to shift from anoxygenic photosynthesis, with H₂S as the electron donor, to oxygenic photosynthesis. Microcoleus filaments could carry out both types of photosynthesis concurrently. Anoxygenic photosynthesis dominated at high sulfide levels, 500 μmol liter⁻¹, while the oxygenic reaction became dominant when the sulfide level was reduced below 100 to 300 μmol liter⁻¹ (25 to 75 μmol of H₂S liter⁻¹). An increasing inhibition of the oxygenic photosynthesis was observed upon transition to oxic conditions from increasing sulfide concentrations. Oxygen built up within the Microcoleus layer of the mat even under 5 mmol of sulfide liter⁻¹ (500 μmol of H₂S liter⁻¹) in the overlying water. The implications of such a localized O₂ production in a highly reducing environment are discussed in relation to the evolution of oxygenic photosynthesis during the Proterozoic era.     

Primary and Bacterial Secondary Production in a Southwestern Reservoir

Rates of primary and bacterial secondary production in Lake Arlington, Texas, were determined. The lake is a warm (annual temperature range, 7 to 32°C), shallow, monomictic reservoir with limited macrophyte development in the littoral zone. Samples were collected from six depths within the photic zone from a site located over the deepest portion of the lake. Primary production and bacterial production were calculated from NaH¹⁴CO₃ and [methyl-³H]thymidine incorporation, respectively. Peak instantaneous production ranged between 14.8 and 220.5 μg of C liter⁻¹ h⁻¹. There were two distinct periods of high rates of production. From May through July, production near the metalimnion exceeded 100 μg of C liter⁻¹ h⁻¹. During holomixis, production throughout the water column was in excess of 100 μg of C liter⁻¹ h⁻¹ and above 150 μg of C liter⁻¹ h⁻¹ near the surface. Annual areal primary production was 588 g of C m⁻². Bacterial production was markedly seasonal. Growth rates during late fall through spring were typically around 0.002 h⁻¹, and production rates were typically 5 μg of C liter⁻¹ h⁻¹. Growth rates were higher during warmer parts of the year and reached 0.03 h⁻¹ by August. The maximum instantaneous rate of bacterial production was approximately 45 μg of C liter⁻¹ h⁻¹. Annual areal bacterial production was 125 g of C m⁻². Temporal and spatial distributions of bacterial numbers and activities coincided with temporal and spatial distributions of primary production. Areal primary and bacterial secondary production were highly correlated (r = 0.77, n = 15, P < 0.002).     

Estimating Bacterioplankton Production by Measuring [3H]thymidine Incorporation in a Eutrophic Swedish Lake

Bacterioplankton abundance, [³H]thymidine incorporation, ¹⁴CO₂ uptake in the dark, and fractionated primary production were measured on several occasions between June and August 1982 in eutrophic Lake Norrviken, Sweden. Bacterioplankton abundance and carbon biomass ranged from 0.5 × 10⁹ to 2.4 × 10⁹ cells liter⁻¹ and 7 to 47 μg of C liter⁻¹, respectively. The average bacterial cell volume was 0.185 μm³. [³H]thymidine incorporation into cold-trichloroacetic acid-insoluble material ranged from 12 × 10⁻¹² to 200 × 10⁻¹² mol liter⁻¹ h⁻¹. Bacterial carbon production rates were estimated to be 0.2 to 7.1 μg of C liter⁻¹ h⁻¹. Bacterial production estimates from [³H]thymidine incorporation and ¹⁴CO₂ uptake in the dark agreed when activity was high but diverged when activity was low and when blue-green algae (cyanobacteria) dominated the phytoplankton. Size fractionation indicated negligible uptake of [³H]thymidine in the >3-μm fraction during a chrysophycean bloom in early June. We found that >50% of the ³H activity was in the >3-μm fraction in late August; this phenomenon was most likely due to Microcystis spp., their associated bacteria, or both. Over 60% of the ¹⁴CO₂ uptake in the dark was attributed to algae on each sampling occasion. Algal exudate was an important carbon source for planktonic bacteria. Bacterial production was roughly 50% of primary production.     

Impact of Different In Vitro Electron Donor/Acceptor Conditions on Potential Chemolithoautotrophic Communities from Marine Pelagic Redoxclines

Anaerobic or microaerophilic chemolithoautotrophic bacteria have been considered to be responsible for CO₂ dark fixation in different pelagic redoxclines worldwide, but their involvement in redox processes is still not fully resolved. We investigated the impact of 17 different electron donor/acceptor combinations in water of pelagic redoxclines from the central Baltic Sea on the stimulation of bacterial CO₂ dark fixation as well as on the development of chemolithoautotrophic populations. In situ, the highest CO₂ dark fixation rates, ranging from 0.7 to 1.4 μmol liter⁻¹ day⁻¹, were measured directly below the redoxcline. In enrichment experiments, chemolithoautotrophic CO₂ dark fixation was maximally stimulated by the addition of thiosulfate, reaching values of up to 9.7 μmol liter⁻¹ CO₂ day⁻¹. Chemolithoautotrophic nitrate reduction proved to be an important process, with rates of up to 33.5 μmol liter⁻¹ NO₃⁻ day⁻¹. Reduction of Fe(III) or Mn(IV) was not detected; nevertheless, the presence of these potential electron acceptors influenced the development of stimulated microbial assemblages. Potential chemolithoautotrophic bacteria in the enrichment experiments were displayed on 16S ribosomal complementary DNA single-strand-conformation polymorphism fingerprints and identified by sequencing of excised bands. Sequences were closely related to chemolithoautotrophic Thiomicrospira psychrophila and Maorithyas hadalis gill symbiont (both Gammaproteobacteria) and to an uncultured nitrate-reducing Helicobacteraceae bacterium (Epsilonproteobacteria). Our data indicate that this Helicobacteraceae bacterium could be of general importance or even a key organism for autotrophic nitrate reduction in pelagic redoxclines.     

Physiology and Phylogeny of Green Sulfur Bacteria Forming a Monospecific Phototrophic Assemblage at a Depth of 100 Meters in the Black Sea

The biomass, phylogenetic composition, and photoautotrophic metabolism of green sulfur bacteria in the Black Sea was assessed in situ and in laboratory enrichments. In the center of the western basin, bacteriochlorophyll e (BChl e) was detected between depths of 90 and 120 m and reached maxima of 54 and 68 ng liter⁻¹. High-pressure liquid chromatography analysis revealed a dominance of farnesyl esters and the presence of four unusual geranyl ester homologs of BChl e. Only traces of BChl e (8 ng liter⁻¹) were found at the northwestern slope of the Black Sea basin, where the chemocline was positioned at a significantly greater depth of 140 m. Stable carbon isotope fractionation values of farnesol indicated an autotrophic growth mode of the green sulfur bacteria. For the first time, light intensities in the Black Sea chemocline were determined employing an integrating quantum meter, which yielded maximum values between 0.0022 and 0.00075 μmol quanta m⁻² s⁻¹ at the top of the green sulfur bacterial layer around solar noon in December. These values represent by far the lowest values reported for any habitat of photosynthetic organisms. Only one 16S rRNA gene sequence type was detected in the chemocline using PCR primers specific for green sulfur bacteria. This previously unknown phylotype groups with the marine cluster of the Chlorobiaceae and was successfully enriched in a mineral medium containing sulfide, dithionite, and freshly prepared yeast extract. Under precisely controlled laboratory conditions, the enriched green sulfur bacterium proved to be capable of exploiting light intensities as low as 0.015 μmol quanta m⁻² s⁻¹ for photosynthetic ¹⁴CO₂ fixation. Calculated in situ doubling times of the green sulfur bacterium range between 3.1 and 26 years depending on the season, and anoxygenic photosynthesis contributes only 0.002 to 0.01% to total sulfide oxidation in the chemocline. The stable population of green sulfur bacteria in the Black Sea chemocline thus represents the most extremely low-light-adapted and slowest-growing type of phototroph known to date.     

Kinetic Parameters of Denitrification in a River Continuum

Kinetic parameters for nitrate reduction in intact sediment cores were investigated by using the acetylene blockage method at five sites along the Swale-Ouse river system in northeastern England, including a highly polluted tributary, R. Wiske. The denitrification rate in sediment containing added nitrate exhibited a Michaelis-Menten-type curve. The concentration of nitrate for half-maximal activity (K_map) by denitrifying bacteria increased on passing downstream from 13.1 to 90.4 μM in the main river, but it was highest (640 μM) in the Wiske. The apparent maximal rate (V_maxap) ranged between 35.8 and 324 μmol of N m⁻² h⁻¹ in the Swale-Ouse (increasing upstream to downstream), but it was highest in the Wiske (1,194 μmol N m⁻² h⁻¹). A study of nitrous oxide (N₂O) production at the same time showed that rates ranged from below the detection limit (0.05 μmol of N₂O-N m⁻² h⁻¹) at the headwater site to 27 μmol of N₂O-N m⁻² h⁻¹ at the downstream site. In the Wiske the rate was up to 570 μmol of N₂O-N m⁻² h⁻¹, accounting for up to 80% of total N gas production.     

Streptomyces thermoautotrophicus sp. nov., a Thermophilic CO- and H2-Oxidizing Obligate Chemolithoautotroph

The novel thermophilic CO- and H₂-oxidizing bacterium UBT1 has been isolated from the covering soil of a burning charcoal pile. The isolate is gram positive and obligately chemolithoautotrophic and has been named Streptomyces thermoautotrophicus on the basis of G+C content (70.6 ± 0.19 mol%), a phospholipid pattern of type II, MK-9(H₄) as the major quinone, and other chemotaxonomic and morphological properties. S. thermoautotrophicus could grow with CO (t_d = 8 h), H₂ plus CO₂ (t_d = 6 h), car exhaust, or gas produced by the incomplete combustion of wood. Complex media or heterotrophic substrates such as sugars, organic acids, amino acids, and alcohols did not support growth. Molybdenum was required for CO-autotrophic growth. For growth with H₂, nickel was not necessary. The optimum growth temperature was 65°C; no growth was observed below 40°C. However, CO-grown cells were able to oxidize CO at temperatures of 10 to 70°C. Temperature profiles of burning charcoal piles revealed that, up to a depth of about 10 to 25 cm, the entire covering soil provides a suitable habitat for S. thermoautotrophicus. The K_m was 88 μl of CO liter⁻¹ and V_max was 20.2 μl of CO h⁻¹ mg of protein⁻¹. The threshold value of S. thermoautotrophicus of 0.2 μl of CO liter⁻¹ was similar to those of various soils. The specific CO-oxidizing activity in extracts with phenazinemethosulfate plus 2,6-dichlorophenolindophenol as electron acceptors was 246 μmol min⁻¹ mg of protein⁻¹. In exception to other carboxydotrophic bacteria, S. thermoautotrophicus CO dehydrogenase was able to reduce low potential electron acceptors such as methyl and benzyl viologens.     

Potential Importance of Fish Predation and Zooplankton Grazing on Natural Populations of Freshwater Bacteria

The rates of ingestion of natural bacterial assemblages by natural populations of zooplankton (>50 μm in size) were measured during a 19-day period in eutrophic Frederiksborg Slotssø, Denmark, as well as in experimental enclosures (containing 5.3 m³ of lake water). The fish and nutrients of the enclosures were manipulated. In enclosures without fish, large increases in ingestion by zooplankton >140 μm in size were found (up to 3 μg of C liter⁻¹ h⁻¹), compared with values less than 0.3 μg of C liter⁻¹ h⁻¹ in the enclosures with fish and in the open lake. Daphnia cucullata and D. galeata dominated the community of zooplankton of >140 μm. Ingestion rates for zooplankton between 50 and 140 μm decreased after a period of about 8 days, in all enclosures and in the lake, to values below 0.1 μg of C liter⁻¹ h⁻¹. On the last 2 sampling days, somewhat higher values were observed in the enclosures with fish present. The >50-μm zooplankton ingested 48 to 51% of the bacterial net secondary production in enclosures without fish, compared to 4% in the enclosures with added fish. Considering the sum of bacterial secondary production plus biomass change, 35 to 41% of the available bacteria were ingested by zooplankton of >50 μm in the enclosures without fish, compared with 4 to 6% in the enclosures with added fish and 21% in the open lake. Fish predation reduced the occurrence of zookplankton sized >50 μm and thus left a large proportion of the available bacteria to zooplankton sized <50 μm. In fact, there were 4.6 × 10³ to 5.0 × 10³ flagellates (4 to 8 μm in size) ml⁻¹ in the enclosures with fish added as well as in the lake, compared with 0.5 × 10² to 2.3 × 10² ml⁻¹ in the enclosures without fish. This link in the food chain was reduced when fish predation on zooplankton was eliminated and a direct route of dissolved organic matter, via the bacteria to the zooplankton, was established.     

A New Sensitive Bioassay for Determination of Microbially Available Phosphorus in Water

The content of assimilable organic carbon has been proposed to control the growth of microbes in drinking water. However, recent results have shown that there are regions where it is predominantly phosphorus which determines the extent of microbial growth in drinking waters. Even a very low concentration of phosphorus (below 1 μg of P liter⁻¹) can promote extensive microbial growth. We present here a new sensitive method to determine microbially available phosphorus concentrations in water down to 0.08 μg of P liter⁻¹. The method is a bioassay in which the analysis of phosphorus in a water sample is based on maximum growth of Pseudomonas fluorescens P17 when the energy supply and inorganic nutrients, with the exception of phosphorus, do not limit bacterial growth. Maximum growth (CFU) in the water sample is related to the concentration of phosphorus with the factor 373,200 ± 9,400 CFU/μg of PO₄-P. A linear relationship was found between cell growth and phosphorus concentration between 0.05 to 10 μg of PO₄-P liter⁻¹. The content of microbially available phosphorus in Finnish drinking waters varied from 0.1 to 10.2 μg of P liter⁻¹ (median, 0.60 μg of P liter⁻¹).     

Assessing Phytoplankton and Bacterioplankton Production During Early Spring in Lake Erken, Sweden

The spring development of both phytoplankton and bacterioplankton was investigated between 18 April and 7 May 1983 in mesotrophic Lake Erken, Sweden. By using the lake as a batch culture, our aim was to estimate, via different methods, the production of phytoplankton and bacterioplankton in the lake and to compare these production estimates with the actual increase in phytoplankton and bacterioplankton biomass. The average water temperature was 3.5°C. Of the phytoplankton biomass, >90% was the diatom Stephanodiscus hantzchii var. pusillus, by the peak of the bloom. The ¹⁴C and O₂ methods of estimating primary production gave equivalent results (r = 0.999) with a photosynthetic quotient of 1.63. The theoretical photosynthetic quotient predicted from the C/NO₃ N assimilation ratio was 1.57. The total integrated incorporation of [¹⁴C]bicarbonate into particulate material (>1 μm) was similar to the increase in phytoplankton carbon determined from cell counts. Bacterioplankton increased from 0.5 × 10⁹ to 1.52 × 10⁹ cells liter⁻¹ (~0.5 μg of C liter⁻¹ day⁻¹). Estimates of bacterioplankton production from rates of [³H]thymidine incorporation were ca. 1.2 to 1.7 μg of C liter⁻¹ day⁻¹. Bacterial respiration, measured by a high-precision Winkler technique, was estimated as 4.8 μg of C liter⁻¹ day⁻¹, indicating a bacterial growth yield of 25%. The bulk of the bacterioplankton production was accounted for by algal extracellular products. Gross bacterioplankton production (production plus respiration) was 20% of gross primary production, per square meter of surface area. We found no indication that bacterioplankton production was underestimated by the [³H]thymidine incorporation method.     

Differential responses of the sunn4 and rdn1-1 super-nodulation mutants of Medicago truncatula to elevated atmospheric CO2

Background and AimsNitrogen fixation in legumes requires tight control of carbon and nitrogen balance. Thus, legumes control nodule numbers via an autoregulation mechanism. ‘Autoregulation of nodulation’ mutants super-nodulate are thought to be carbon-limited due to the high carbon-sink strength of excessive nodules. This study aimed to examine the effect of increasing carbon supply on the performance of super-nodulation mutants.MethodsWe compared the responses of Medicago truncatula super-nodulation mutants (sunn-4 and rdn1-1) and wild type to five CO₂ levels (300–850 μmol mol⁻¹). Nodule formation and nitrogen fixation were assessed in soil-grown plants at 18 and 42 d after sowing.Key ResultsShoot and root biomass, nodule number and biomass, nitrogenase activity and fixed nitrogen per plant of all genotypes increased with increasing CO₂ concentration and reached a maximum at 700 μmol mol⁻¹. While the sunn-4 mutant showed strong growth retardation compared with wild-type plants, elevated CO₂ increased shoot biomass and total nitrogen content of the rdn1-1 mutant up to 2-fold. This was accompanied by a 4-fold increase in nitrogen fixation capacity in the rdn1-1 mutant.ConclusionsThese results suggest that the super-nodulation phenotype per se did not limit growth. The additional nitrogen fixation capacity of the rdn1-1 mutant may enhance the benefit of elevated CO₂ for plant growth and N₂ fixation.     

Methane Oxidation by Nitrosococcus oceanus and Nitrosomonas europaea

Chemolithotrophic ammonium-oxidizing and nitrite-oxidizing bacteria including Nitrosomonas europaea, Nitrosococcus oceanus, Nitrobacter sp., Nitiospina gracilis, and Nitrococcus mobilis were examined as to their ability to oxidize methane in the absence of ammonium or nitrite. All ammonium oxidizers tested had the ability to oxidize significant amounts of methane to CO₂ and incorporate various amounts into cellular components. None of the nitrite-oxidizing bacteria were capable of methane oxidation. The methane-oxidizing capabilities of Nitrosococcus oceanus and Nitrosomonas europaea were examined with respect to ammonium and methane concentrations, nitrogen source, and pH. The addition of ammonium stimulated both CO₂ production and cellular incorporation of methane-carbon by both organisms. Less than 0.1 mM CH₄ in solution inhibited the oxidation of ammonium by Nitrosococcus oceanus by 87%. Methane concentrations up to 1.0 mM had no inhibitory effects on ammonium oxidation by Nitrosomonas europaea. In the absence of NH₄-N, Nitrosococcus oceanus achieved a maximum methane oxidation rate of 2.20 × 10⁻² μmol of CH₄ h⁻¹ mg (dry weight) of cells⁻¹, which remained constant as the methane concentration was increased. In the presence of NH₄-N (10 ppm [10 μg/ml]), its maximum rate was 26.4 × 10⁻² μmol of CH₄ h⁻¹ mg (dry weight) of cells⁻¹ at a methane concentration of 1.19 × 10⁻² mM. Increasing the methane concentration above this level decreased CO₂ production, whereas cellular incorporation of methane-carbon continued to increase. Nitrosomonas europaea showed a linear response throughout the test range, with an activity of 196.0 × 10⁻² μmol of CH₄ h⁻¹ mg (dry weight) of cells ⁻¹ at a methane concentration of 1.38 × 10⁻¹ mM. Both nitrite and nitrate stimulated the oxidation of methane. The pH range was similar to that for ammonium oxidation, but the points of maximum activity were at lower values for the oxidation of methane.     

Continuous Production of Long-Side-Chain Poly-β-Hydroxyalkanoates by Pseudomonas oleovorans

Shake flask experiments showed that Pseudomonas oleovorans began to be growth inhibited at 4.65 g of sodium octanoate liter^-1, with total inhibition at 6 g liter^-1. In chemostat studies with 2 g of ammonium sulfate and 8 g of octanoate liter^-1 in the feed, the maximum specific growth rate was 0.51 h^-1, and the maximum specific rate of poly-β-hydroxyalkanoate (PHA) production was 0.074 g of PHA g of cellular protein^-1 h^-1 at a dilution rate (D) of 0.25 h^-1. When the specific growth rate (μ) was <0.3 h^-1, the PHA composition was relatively constant with a C₄/C₆/C₈/C₁₀ ratio of 0.1:1.7:20.7:1.0. At μ > 0.3 h^-1, a decrease in the percentage of C₈ with a concomitant increase in C₁₀ monomers as μ increased was probably due to the effects of higher concentrations of unmetabolized octanoate in the fermentor. At D = 0.24 h^-1 and an increasing carbon/nitrogen ratio, the percentage of PHA in the biomass was constant at 13% (wt/wt), indicating that nitrogen limitation did not affect PHA accumulation. Under carbon-limited conditions, the yield of biomass from substrate was 0.76 g of biomass g of octanoate^-1 consumed, the yield of PHA was 0.085 g of PHA g of octanoate^-1 used, and 7.9 g of octanoate was consumed for each gram of NH₄⁺ supplied. The maintenance coefficient was 0.046 g of octanoate g of biomass^-1 h^-1. Replacement of sodium octanoate with octanoic acid appeared to result in transport-limited growth due to the water insolubility of the acid.     

Modulation of Affinity of a Marine Pseudomonad for Toluene and Benzene by Hydrocarbon Exposure

Trace (microgram liter⁻¹) quantities of either toluene or benzene injected into an amino-acid-limited continuous culture of Pseudomonas sp. strain T2 were utilized immediately with affinities of 2.6 and 6.8 liters g of cells⁻¹ h⁻¹, respectively, and yielded large amounts of organic products, carbon dioxide, and cells. The immediate utilization of hydrocarbons by hydrocarbon-deprived organisms helps to establish the nutritional value of nonpolar substrates in the environment. The observation of small Michaelis constants for toluene transport led to tests of metabolic competition between hydrocarbons; however, competitive inhibition of toluene metabolism was not found for benzene, naphthalene, xylene, dodecane, or amino acids. Benzene and terpenes were inhibitory at milligram liter⁻¹ concentrations. Toluene was metabolized by a strongly inducible system when compared with benzene. The capacity of toluene to effect larger affinity values increased with exposure time and concentration. The kinetics of induction suggested saturation phenomena, resulting in an induction constant, K_ind, of 96 μg of toluene liter⁻¹. Maximal induction of amino-acid-grown cells required about 80 h, with the affinity reaching 317 liters g of cells⁻¹ h⁻¹.     

Aerobic Fermentation of D-Xylose to Ethanol by Clavispora sp

Eleven strains of an undescribed species of Clavispora fermented D-xylose directly to ethanol under aerobic conditions. Strain UWO(PS)83-877-1 was grown in a medium containing 2% D-xylose and 0.5% yeast extract, and the following results were obtained: ethanol yield coefficient (ethanol/D-xylose), 0.29 g g⁻¹ (57.4% of theoretical); cell yield coefficient (dry biomass/D-xylose), 0.25 g g⁻¹; maximum ethanol concentration, 5.9 g liter⁻¹; maximum volumetric ethanol productivity, 0.11 g liter⁻¹ h⁻¹. With initial D-xylose concentrations of 40, 60, and 80 g liter⁻¹, maximum ethanol concentrations of 8.8, 10.9, and 9.8 g liter⁻¹ were obtained, respectively (57.2, 57.1, and 48.3% of theoretical). Ethanol was found to inhibit the fermentation of D-xylose (K_p = 0.58 g liter⁻¹) more than the fermentation of glucose (K_p = 6.5 g liter⁻¹). The performance of this yeast compared favorably with that reported for some other D-xylose-fermenting yeasts.     

Biosynthesis of Poly-β-Hydroxyalkanoates from Pentoses by Pseudomonas pseudoflava

The potential of Pseudomonas pseudoflava to produce poly-β-hydroxyalkanoates (PHAs) from pentoses was studied. This organism was able to use a hydrolysate from the hemicellulosic fraction of poplar wood as a carbon and energy source for its growth. However, in batch cultures, growth was inhibited completely at hydrolysate concentrations higher than 30% (vol/vol). When P. pseudoflava was grown on the major sugars present in hemicelluloses in batch cultures, poly-β-hydroxybutyric acid (PHB) accumulated when glucose, xylose, or arabinose was the sole carbon source, with the final PHB content varying from 17% (wt/wt) of the biomass dry weight on arabinose to 22% (wt/wt) of the biomass dry weight on glucose and xylose. Specific growth rates were 0.58 h⁻¹ on glucose, 0.13 h⁻¹ on xylose, and 0.10 h⁻¹ on arabinose, while the specific PHB production rates based on total biomass ranged from 0.02 g g⁻¹ h⁻¹ on arabinose to 0.11 g g⁻¹ h⁻¹ on glucose. PHB weight-average molecular weights were 640,000 on arabinose and 1,100,000 on glucose and xylose. The absolute amount of PHB in the cells decreased markedly when nitrogen limitation was relaxed by feeding ammonium sulfate at the end of the PHB accumulation stage of the arabinose and xylose fermentations. Copolymers of β-hydroxybutyric and β-hydroxyvaleric acids were produced when propionic acid was added to shake flasks containing 10 g of glucose liter⁻¹. The β-hydroxyvaleric acid monomer content attained a maximum of 45 mol% when the initial propionic acid concentration was 2 g liter⁻¹.     

Hydrogen Metabolism by Decomposing Cyanobacterial Aggregates in Big Soda Lake, Nevada

Hydrogen production by incubated cyanobacterial epiphytes occurred only in the dark, was stimulated by C₂H₂, and was inhibited by O₂. Addition of NO₃⁻ inhibited dark, anaerobic H₂ production, whereas the addition of NH₄⁺ inhibited N₂ fixation (C₂H₂ reduction) but not dark H₂ production. Aerobically incubated cyanobacterial aggregates consumed H₂, but light-incubated rates (3.6 μmol of H₂ g⁻¹ h⁻¹) were statistically equivalent to dark uptake rates (4.8 μmol of H₂ g⁻¹ h⁻¹), which were statistically equivalent to dark, anaerobic production rates (2.5 to 10 μmol of H₂ g⁻¹ h⁻¹). Production rates of H₂ were fourfold higher for aggregates in a more advanced stage of decomposition. Enrichment cultures of H₂-producing fermentative bacteria were recovered from freshly harvested, H₂-producing cyanobacterial aggregates. Hydrogen production in these cyanobacterial communities appears to be caused by the resident bacterial flora and not by the cyanobacteria. In situ areal estimates of dark H₂ production by submerged epiphytes (6.8 μmol of H₂ m⁻² h⁻¹) were much lower than rates of light-driven N₂ fixation by the epiphytic cyanobacteria (310 μmol of C₂H₄ m⁻² h⁻¹).     

Rapid Methane Oxidation in a Landfill Cover Soil

Methane oxidation rates observed in a topsoil covering a retired landfill are the highest reported (45 g m⁻² day⁻¹) for any environment. This microbial community had the capacity to rapidly oxidize CH₄ at concentrations ranging from <1 ppm (microliters per liter) (first-order rate constant [k] = −0.54 h⁻¹) to >10⁴ ppm (k = −2.37 h⁻¹). The physiological characteristics of a methanotroph isolated from the soil (characteristics determined in aqueous medium) and the natural population, however, were similar to those of other natural populations and cultures: the Q₁₀ and optimum temperature were 1.9 and 31°C, respectively, the apparent half-saturation constant was 2.5 to 9.3 μM, and 19 to 69% of oxidized CH₄ was assimilated into biomass. The CH₄ oxidation rate of this soil under waterlogged (41% [wt/vol] H₂O) conditions, 6.1 mg liter⁻¹ day⁻¹, was near rates reported for lake sediment and much lower than the rate of 116 mg liter⁻¹ day⁻¹ in the same soil under moist (11% H₂O) conditions. Since there are no large physiological differences between this microbial community and other CH₄ oxidizers, we attribute the high CH₄ oxidation rate in moist soil to enhanced CH₄ transport to the microorganisms; gas-phase molecular diffusion is 10⁴-fold faster than aqueous diffusion. These high CH₄ oxidation rates in moist soil have implications that are important in global climate change. Soil CH₄ oxidation could become a negative feedback to atmospheric CH₄ increases (and warming) in areas that are presently waterlogged but are projected to undergo a reduction in summer soil moisture.     

Carbon gain and photosynthetic response of chrysanthemum to photosynthetic photon flux density cycles

Most models of carbon gain as a function of photosynthetic irradiance assume an instantaneous response to increases and decreases in irradiance. High- and low-light-grown plants differ, however, in the time required to adjust to increases and decreases in irradiance. In this study the response to a series of increases and decreases in irradiance was observed in Chrysanthemum × morifolium Ramat. “Fiesta” and compared with calculated values assuming an instantaneous response. There were significant differences between high- and low-light-grown plants in their photosynthetic response to four sequential photosynthetic photon flux density (PPFD) cycles consisting of 5-minute exposures to 200 and 400 micromoles per square meter per second (μmol m⁻²s⁻¹). The CO₂ assimilation rate of high-light-grown plants at the cycle peak increased throughout the PPFD sequence, but the rate of increase was similar to the increase in CO₂ assimilation rate observed under continuous high-light conditions. Low-light leaves showed more variability in their response to light cycles with no significant increase in CO₂ assimilation rate at the cycle peak during sequential cycles. Carbon gain and deviations from actual values (percentage carbon gain over- or underestimation) based on assumptions of instantaneous response were compared under continuous and cyclic light conditions. The percentage carbon gain overestimation depended on the PPFD step size and growth light level of the leaf. When leaves were exposed to a large PPFD increase, the carbon gain was overestimated by 16 to 26%. The photosynthetic response to 100 μmol m⁻² s⁻¹ PPFD increases and decreases was rapid, and the small overestimation of the predicted carbon gain, observed during photosynthetic induction, was almost entirely negated by the carbon gain underestimation observed after a decrease. If the PPFD cycle was 200 or 400 μmol m⁻² s⁻¹, high- and low-light leaves showed a carbon gain overestimation of 25% that was not negated by the underestimation observed after a light decrease. When leaves were exposed to sequential PPFD cycles (200-400 μmol m⁻² s⁻¹), carbon gain did not differ from leaves exposed to a single PPFD cycle of identical irradiance integral that had the same step size (200-400-200 μmol m⁻² s⁻¹) or mean irradiance (200-300-200 μmol m⁻² s⁻¹).     

Transport, Compartmentation, and Metabolism of Homoserine in Higher Plant Cells : Carbon-13- and Phosphorus-31-Nuclear Magnetic Resonance Studies

The transport, compartmentation, and metabolism of homoserine was characterized in two strains of meristematic higher plant cells, the dicotyledonous sycamore (Acer pseudoplatanus) and the monocotyledonous weed Echinochloa colonum. Homoserine is an intermediate in the synthesis of the aspartate-derived amino acids methionine, threonine (Thr), and isoleucine. Using ¹³C-nuclear magnetic resonance, we showed that homoserine actively entered the cells via a high-affinity proton-symport carrier (K_m approximately 50–60 μm) at the maximum rate of 8 ± 0.5 μmol h⁻¹ g⁻¹ cell wet weight, and in competition with serine or Thr. We could visualize the compartmentation of homoserine, and observed that it accumulated at a concentration 4 to 5 times higher in the cytoplasm than in the large vacuolar compartment. ³¹P-nuclear magnetic resonance permitted us to analyze the phosphorylation of homoserine. When sycamore cells were incubated with 100 μm homoserine, phosphohomoserine steadily accumulated in the cytoplasmic compartment over 24 h at the constant rate of 0.7 μmol h⁻¹ g⁻¹ cell wet weight, indicating that homoserine kinase was not inhibited in vivo by its product, phosphohomoserine. The rate of metabolism of phosphohomoserine was much lower (0.06 μmol h⁻¹ g⁻¹ cell wet weight) and essentially sustained Thr accumulation. Similarly, homoserine was actively incorporated by E. colonum cells. However, in contrast to what was seen in sycamore cells, large accumulations of Thr were observed, whereas the intracellular concentration of homoserine remained low, and phosphohomoserine did not accumulate. These differences with sycamore cells were attributed to the presence of a higher Thr synthase activity in this strain of monocot cells.