Operational patterns affecting lactic acid production in ultrafiltration cell recycle bioreactor

Lactic acid production with cell recycling on an ultrafiltration tubular membrane reactor was studied; higher lactic acid concentrations as well as productivities were obtained under long-term fermentations compared with other high cell density systems. Different operational conditions, namely dilution rates and start-up modes, were assessed. Performances were very different at the three different dilution rates tested (D = 0.20 h(-1), D = 0.40 h(-1), or D = 0.58 h(-1)). The different behaviours are discussed and factors responsible for them are presented. The best way to operate for lactic acid production is chosen, the dilution rate of D = 0.40 h(-1) being the one providing the best overall performance. On the other hand, results show that of the two start-up modes tested, continuous start (membrane open) permits higher permeabilities throughout the operational runs than batch start (membrane closed). Operational stability was found to be directly associated with membranes that work at "steady state," the membrane permeability being kept around 15 L/m(2) h. Optimized cell bleed can improve time of operation if such membrane permeability can be maintained for a longer time. A comparison of results with those obtained in other lactic acid production systems is presented; such comparison shows that this tubular ultrafiltration membrane cell recycle reactor presents three important advantages: (1) concomitant lactic acid concentrations and productivities; (2) long periods of operation at reasonable permeabilities; and (3) good mechanical stability permitting the use of steam sterilization. (c) 1995 John Wiley & Sons, Inc.     

Anaerobic digestion of Ulva sp. 1. Relationship between Ulva composition and methanisation

Ulva often represents the main component of mass algal growths, and its composition and degradability make it a relatively good methanisation substrate. In ‘green tides’ Ulva sp. from Brittany, the low content oflignin-type components (polyphloroglucinols: 1.3% dry weight), and the large hemicellulosic fraction (9% dry weight) favour the substrate's accessibility to enzymes. Anaerobic degradation with a batch orcompletely stirred system is technically possible. However, the methane yield reached only 0.20 m³ kg⁻¹ volatile solids and the epuration rate 50% volatile solids in experiments in batch or completely stirred reactors. More generally, methanisation comes up against various practical obstacles: seasonal growth of Ulva, low density of alga in suspension for loading the digester, high S concentration leading to the production of a biogas with a high H2S content, and, finally, the existence of a refractory or slowly degradable part, which requires a compromise between productivity and biological yield. This revised version was published online in August 2006 with corrections to the Cover Date.     

Hyperbolic Modeling of Starburst Dendrimers

Starburst dendrimers are highly branched oligomers. A rigid dendritic hydrocarbon, C₁₁₃₄H₁₁₄₆, has recently been synthesized. It consists of 94 phenylacetylene units displayed in a self-similar two-dimensional skeleton isomorphous to the three-coordinated Bethe lattice. The three-dimensional representation of phenylacetylene dendrimer shows a globular architecture with large voids and niches in its interior, characteristic of hyperbolic surfaces. This work investigates the geometrical scaling behavior of this starburst dendrimer using the symmetry properties of a Bethe lattice embedded in the hyperbolic plane. The results for C₁₁₃₄H₁₁₄₆ provide its density profile and an upper bound for its macromolecular size. This revised version was published online in June 2006 with corrections to the Cover Date.     

The membrane-bound high-molecular-mass cytochromes c from Desulfovibrio gigas and Desulfovibrio vulgaris Hildenborough; EPR and Mössbauer studies

The high-molecular-mass cytochromes c (Hmcs) from the sulfate-reducing bacteria Desulfovibrio gigas and Desulfovibrio vulgaris (Hildenborough) were found to be strongly bound to the cytoplasmic membrane. After detergent solubilization they were shown to be water soluble and to be similar to those previously isolated from the soluble fractions in terms of N-terminal sequence, molecular mass, UV-visible and EPR spectroscopies. In D. gigas, higher amounts of Hmc can be obtained from the membranes than from the soluble fraction. This enabled further characterization of both cytochromes. The apparent heme reduction potentials of both Hmcs, determined at pH 7.5 through visible and EPR redox titrations, span a large range of redox potentials, approximately between 0 and –280?mV, and can be roughly divided into three groups: four to five hemes have E ⁰s of –30?mV to –100?mV, three to four hemes have E ⁰s around –170?mV, and seven to eight hemes have a lower E ⁰ of –250 to –280?mV. Several of these redox potentials are strongly pH dependent. Mössbauer studies of oxidized and reduced D. vulgaris Hmc show that this protein contains two high-spin hemes in both oxidation states. The rate of reduction of both Hmcs with the periplasmic hydrogenases from the corresponding organisms is extremely slow.     

Hydrological cycle and interannual variability of the aquatic community in a temporary saline lake (Fuente de Piedra,Southern Spain)

Fuente de Piedra is a shallow, temporary saline lake whoseseasonal behavior is strongly dependent on the annual hydrologicalbudget. In this study, we outline the characteristics of Fuente dePiedra Lake for two years that had different hydrological budgets.The high precipitations in 1989–90 caused the lake not to dry asusual, and decreased both salinity and the amplitude of changes.There were also differences in nutrient dynamics, with generallylower concentrations of soluble reactive phosphorus and ammonium,whereas in the more humid year nitrate showed a distinct maximum inwinter. Winter bloom chlorophyll a concentrations were alsomuch higher in 1989–90 (>600 g l-1) but there wasalso a winter productive phase that was presumably poorly coupledwith consumption processes that predominate in spring. Planktonicassemblages were different between years. Highly halotolerantphytoplankton species (Dunaliella salina and D. viridis) became scarcer, and especially two previouslyunrecorded diatoms (Cyclotella sp. and Chaetoceros sp.)became dominant in the bloom time in the wet year. The speciesrichness of the zooplankton increased in the wet year, with newspecies appearing that were not collected during 1987–88(Branchinectella media, Daphnia mediterranea, Macrothrix sp.,Arctodiaptomus salinus, Cyclops sp., Sigara sp...).There was also a much higher development of macrophytes (Ruppiadrepanensis, Althenia orientalis, Lamprothamnium papulosum)and bird populations, especially flamingoes (Phoenicopterusruber).Important interannual variations in this sort of system pointto the need for long term studies to observe the whole range ofstates that define the lake as an entity.     

A non-linear model of phosphorus flux in the phytoplankton of a temperate eutrophic reservoir

Phosphorus fluxes in phytoplankton, between intracellular andexternal compartments have been measured, in the epilimnion ofatemperate eutrophic reservoir, by means of phosphorusdepletionexperiments performed in situ. Flow rates betweencompartments were plotted against the phosphorus concentrationofthe compartments' origin of the flows and fitted to saturationfunctions.A value of phosphorus cell subsistence quota for the wholecommunity has been computed from the X-intercept values ofeveryfunction. The obtained figures are 10–100 times higher,dependingon the reference value, than those measured running theclassicalsingle species growth response experiments in chemostats.We propose a model using five compartments and a series offlowrates between them in order to simulate the general phosphoruscycling in freshwater phytoplankton. Short term simulation ofphosphorus content in every compartment shows a fluctuatingpatternaround rates of equilibrium, in agreement with the pattern ofvariation observed in situ for the selectedcompartments.     

BIOMASS AND DYNAMICS OF GROWTH OF ULVA SPECIES IN PALMONES RIVER ESTUARY1

During the last decade, the Palmones River estuary has undergone severe eutrophication followed by a green tide episode; two species of Ulva, rotundata Blid. and Ulva curvata (Kütz.) De Toni, were the main macroalgae responsible for this bloom. From November 1993 to December 1994, we followed the biomass, the growth dynamics, and tissue elemental composition (C:N:P)of Ulva species, as well as some physicochemical variables in the estuary. Maximum biomass (up to 375 g dry wt·m^?2 in some spots, corresponding to a thallus area index of nearly 17 m²Ulva·m^?2 sediment) were observed in June and December. However, the biomass varied among the sampling stations. Water nitrate, ammonia, and phosphate showed high concentrations throughout the year, with extremely high transient pulses, sustaining the high growth rates observed. Growth rates were estimated directly in the field. The rates were generally higher in Ulva discs maintained in net cages than those estimated by changes in biomass standing stock between two consecutive samplings. The difference between both estimates was used to quantify the importance of the processes causing loss of biomass, which were attributable to grazing, exported biomass, and thallus decomposition under anaerobic conditions resulting from extreme self-shading. Maximum chlorophyll content was found in winter, whereas the minimum was in spring. Atomic N:P ratios were generally higher in the algae than in the water. However, the absolute concentrations of tissue N and P were always higher than the critical levels for maximum growth, which suggests that growth was not limited by inorganic N or P availability. The results suggested that the increase in nutrient loading in the river may have triggered the massive development of green algae and that light limitation and temperature stress in summer seem to be the main factors controlling the abundance of Ulva in the estuary. In addition to light availability and thermal stress, the different loss processes may have a decisive role in the dynamics of Ulva biomass.     

Five new microsatellite polymorphisms at the q21 region of human chromosome 21

Five clones, containing polymorphic CA-repeat sequences, have been isolated from a specific human chromosome 21 phage library and have been localised to band q21 of chromosome 21 using a somatic cell hybrid panel. These highly repetitive sequences (D21S1263, D21S1264, D21S1415, D21S1417 and D21S1420) have been characterised in the CEPH reference parents and have heterozygosities ranging from 0.30 to 0.81 and an average polymorphism information content (PIC) of 0.62. The relative order of these markers, based on the somatic cell hybrid panel, is cen-D21S1417, D21S1420-D21S1263, D21S1415-D21S1264-tel. The most polymorphic marker (D21S1264) has been included in the chromosome 21 genetic map. They have also been localised in the CEPH/ Généthon YAC panel, providing a refined localisation of these polymorphic sequences. These five CA-repeat markers should provide a better characterisation of the q21 region of chromosome 21.     

Knowledge-based modeling of the D-lactate dehydrogenase three-dimensional structure

A three-dimensional structure of the NAD-dependent D -lactate dehydrogenase of Lactobacillus bulgaricus is modeled using the structure of the formate dehydrogenase of Pseudomonas sp. as template. Both sequences share only 22% of identical residues. Regions for knowledge-based modeling are defined from the structurally conserved regions predicted by multiple alignment of a set of related protein sequences with low homology. The model of the D -LDH subunit shows, as for the formate dehydrogenase, an α/β structure, with a catalytic domain and a coenzyme binding domain. It points out the catalytic histidine (His-296) and supports the hypothetical catalytic mechanism. It also suggests that the other residues involved in the active site are Arg-235, possibly involved in the binding of the carboxyl group of the pyruvate, and Phe-299, a candidate for stabilizing the methyl group of the substrate. © 1995 Wiley-Liss, Inc.