Extensive Identification of Bacterial Riboflavin Transporters and Their Distribution across Bacterial Species

Riboflavin, the precursor for the cofactors flavin mononucleotide (FMN) and flavin adenine dinucleotide, is an essential metabolite in all organisms. While the functions for de novo riboflavin biosynthesis and riboflavin import may coexist in bacteria, the extent of this co-occurrence is undetermined. The RibM, RibN, RfuABCD and the energy-coupling factor-RibU bacterial riboflavin transporters have been experimentally characterized. In addition, ImpX, RfnT and RibXY are proposed as riboflavin transporters based on positional clustering with riboflavin biosynthetic pathway (RBP) genes or conservation of the FMN riboswitch regulatory element. Here, we searched for the FMN riboswitch in bacterial genomes to identify genes encoding riboflavin transporters and assessed their distribution among bacteria. Two new putative riboflavin transporters were identified: RibZ in Clostridium and RibV in Mesoplasma florum. Trans-complementation of an Escherichia coli riboflavin auxotroph strain confirmed the riboflavin transport activity of RibZ from Clostridium difficile, RibXY from Chloroflexus aurantiacus, ImpX from Fusobacterium nucleatum and RfnT from Ochrobactrum anthropi. The analysis of the genomic distribution of all known bacterial riboflavin transporters revealed that most occur in species possessing the RBP and that some bacteria may even encode functional riboflavin transporters from two different families. Our results indicate that some species possess ancestral riboflavin transporters, while others possess transporters that appear to have evolved recently. Moreover, our data suggest that unidentified riboflavin transporters also exist. The present study doubles the number of experimentally characterized riboflavin transporters and suggests a specific, non-accessory role for these proteins in riboflavin-prototrophic bacteria.     

Evidence of Intense Archaeal and Bacterial Methanotrophic Activity in the Black Sea Water Column

In the northwestern Black Sea, methane oxidation rates reveal that above shallow and deep gas seeps methane is removed from the water column as efficiently as it is at sites located off seeps. Hence, seeps should not have a significant impact on the estimated annual flux of ~4.1 × 10⁹ mol methane to the atmosphere [W. S. Reeburgh, B. B. Ward, S. C. Wahlen, K. A. Sandbeck, K. A. Kilatrick, and L. J. Kerkhof, Deep-Sea Res. 38(Suppl. 2):S1189-S1210, 1991]. Both the stable carbon isotopic composition of dissolved methane and the microbial community structure analyzed by fluorescent in situ hybridization provide strong evidence that microbially mediated methane oxidation occurs. At the shelf, strong isotope fractionation was observed above high-intensity seeps. This effect was attributed to bacterial type I and II methanotrophs, which on average accounted for 2.5% of the DAPI (4′,6′-diamidino-2-phenylindole)-stained cells in the whole oxic water column. At deep sites, in the oxic-anoxic transition zone, strong isotopic fractionation of methane overlapped with an increased abundance of Archaea and Bacteria, indicating that these organisms are involved in the oxidation of methane. In underlying anoxic water, we successfully identified the archaeal methanotrophs ANME-1 and ANME-2, eachof which accounted for 3 to 4% of the total cell counts. ANME-1 and ANME-2 appear as single cells in anoxicwater, compared to the sediment, where they may form cell aggregates with sulfate-reducing bacteria (A. Boetius, K. Ravenschlag, C. J. Schubert, D. Rickert, F. Widdel, A. Giesecke, R. Amann, B. B. Jørgensen, U. Witte, and O. Pfannkuche, Nature 407:623-626, 2000; V. J. Orphan, C. H. House, K.-U. Hinrichs, K. D. McKeegan, and E. F. DeLong, Proc. Natl. Acad. Sci. USA 99:7663-7668, 2002).     

Archaeal and Bacterial Communities Respond Differently to Environmental Gradients in Anoxic Sediments of a California Hypersaline Lake,the Salton Sea

Sulfidic, anoxic sediments of the moderately hypersaline Salton Sea contain gradients in salinity and carbon that potentially structure the sedimentary microbial community. We investigated the abundance, community structure, and diversity of Bacteria and Archaea along these gradients to further distinguish the ecologies of these domains outside their established physiological range. Quantitative PCR was used to enumerate 16S rRNA gene abundances of Bacteria, Archaea, and Crenarchaeota. Community structure and diversity were evaluated by terminal restriction fragment length polymorphism (T-RFLP), quantitative analysis of gene (16S rRNA) frequencies of dominant microorganisms, and cloning and sequencing of 16S rRNA. Archaea were numerically dominant at all depths and exhibited a lesser response to environmental gradients than that of Bacteria. The relative abundance of Crenarchaeota was low (0.4 to 22%) at all depths but increased with decreased carbon content and increased salinity. Salinity structured the bacterial community but exerted no significant control on archaeal community structure, which was weakly correlated with total carbon. Partial sequencing of archaeal 16S rRNA genes retrieved from three sediment depths revealed diverse communities of Euryarchaeota and Crenarchaeota, many of which were affiliated with groups previously described from marine sediments. The abundance of these groups across all depths suggests that many putative marine archaeal groups can tolerate elevated salinity (5.0 to 11.8% [wt/vol]) and persist under the anaerobic conditions present in Salton Sea sediments. The differential response of archaeal and bacterial communities to salinity and carbon patterns is consistent with the hypothesis that adaptations to energy stress and availability distinguish the ecologies of these domains.The vast majority of cultured Archaea isolates are characterized as extremophiles, which thrive under environmental extremes of temperature, pH, salinity, and oxygen availability. Unlike Bacteria, these organisms are well defined by select physiologies or catabolic activities. Cultivated halophilic archaea are obligate aerobes, and with a few exceptions (58), most 16S rRNA gene sequences affiliated with this physiological group have been recovered primarily from environments with oxygen present. Thermophilic archaea, many of which utilize hydrogen-based metabolisms, have temperature requirements that preclude their survival and growth in more moderate environments. Other archaeal physiological groups include acidophiles, which thrive in acidic and mostly high-temperature environments, the obligate anaerobic methanogens, which are capable of competing with Bacteria when more energetically favorable electron acceptors are not available (i.e., sulfate), and methane-oxidizing archaea, which require methane for energy production. Recent work on several Crenarchaeota isolates points to nitrification as their primary energy metabolism, but these organisms have been detected in cold, predominantly aerobic environments, such as open ocean waters and soil (47), and in hyperthermophilic environments (24).Several archaeal groups identified using only 16S rRNA genes, for which no current isolates exist, have been detected in anaerobic sediments of the marine subsurface (6), estuaries (42), freshwater (46), and salt lakes (29). While their physiology and catabolism remain a source of speculation, the environmental distribution patterns of these mesophilic, presumably anaerobic, groups seemingly exclude the physiological and catabolic types outlined above. That is, the persistence of diverse archaeal populations in anoxic sediments at moderate temperature and salinity and at circumneutral pH with only trace levels of methane strongly suggests that alternative metabolic or physiological activities must characterize these populations.Saline lakes are ubiquitous and can be found on all continents. Although many saline lakes are labeled “extreme” environments, microbial diversity within their sediments is often equivalent to that reported for studies of freshwater and marine systems (28). Most studies of the microbial ecology within saline lakes have focused on gradients within the water column, with very few studies on patterns within the sediments. Specifically, these studies have examined how changes in water column salinity lead to shifts in microbial productivity and diversity (8). However, particle-associated microbial communities are known to differ fundamentally from water column or free-living populations (1, 18). These observed differences could be explained by the type and strength of environmental gradients that microbial communities in sediments experience, as opposed to those encountered by pelagic communities.Sediments contain strong environmental gradients, such as time (e.g., sediment age at depth), nutrient and carbon availability, and the dominant terminal electron-accepting process (TEAP) resulting from the sequential use of available oxidants by the microbial community (41). These gradients can lead to changes in the dominant microbial groups (i.e., a shift from sulfate reducers to methanogens with depth and age). Many saline lakes are highly productive and shallow and experience large fluctuations in water level due to climatic changes or to changes in inflows due to urban and agricultural activities. Changes in lake level can lead to dramatic shifts in mixing regimens, nutrient cycling, and water chemistry. Historic fluctuations in water column salinity are often recorded within the sediments in the form of evaporite deposits, which may act as additional sources of ionic loading of the water column (62). These sedimentary salinity gradients may modulate the metabolic activity of some microbial groups. For example, Oren (44) proposed bioenergetic constraints as a possible explanation for the reduced activity or absence of some microbial groups within high-salinity environments. Thus, saline lake sediments are excellent natural laboratories in which to study changes and adaptations of microbial communities due to large-scale changes in environmental gradients.The Salton Sea is a large (980 km²), eutrophic, moderately hypersaline (48 to 50 g liter⁻¹), terminal lake located 69 m below sea level in the Salton Basin, CA. Several large lakes have formed in the Salton Basin over geologic history, the most recent of which was Lake Cahuilla ca. 300 years ago (7). The current lake was unintentionally created in 1905-1907, when the Colorado River flooded the Salton Basin for a period of 16 months. Profundal sediments are highly sulfidic, and sulfate reduction is suspected to be the dominant TEAP within these sediments (54). Based on elemental analysis (51) and ¹³⁷Cs activity (37) of sediment layers, a depth of ∼22 cm marks the point when flooding of the Salton Basin occurred. Sediment above this depth represents the ca. 102 years of historical change within the Salton Sea, including a shift from a water column salinity of 35 g liter⁻¹ to the hypersaline conditions that currently exist. Sediments below this depth consist of low-carbon, gypsum-rich evaporite deposits that were present on the older dry lake bed prior to the formation of the current lake. A previous study reported several strong geochemical gradients within pore water across this relatively small depth range (62).In this work, a suite of cultivation-independent techniques and geochemical analyses was utilized to correlate shifts in abundance, community structure, and diversity of Archaea and Bacteria in Salton Sea sediments with changes in environmental gradients. Large differences in abundance and community structure patterns of Archaea and Bacteria were found along the gradients. In addition, the majority of archaeal sequences retrieved were affiliated with previously described but as yet uncultivated groups identified from various marine sedimentary environments. This indicates that these groups are able to tolerate the higher salinity and anaerobic conditions characteristic of Salton Sea sediments. Fundamental differences between the metabolic capacities and ecologies of Archaea and Bacteria are discussed to explain these patterns.     

Biosynthesis of the scFv Antibody to Human Ferritin in Plant and Bacterial Producers

A recombinant scFv antibody against human spleen ferritin was expressed as a barstar-fused protein in Escherichia coli and in Nicotiana tabacum plants and suspension cell cultures. As demonstrated by immunoblotting with antibarstar antibodies, direction of the recombinant protein to the endomembrane system of plant cells ensured its stability and solubility. Production of the recombinant protein did not differ between parental transgenic plants and their first-generation progeny. Fusion with barstar allowed not only immunochemical detection of the recombinant scFv antibody, but also their purification from the plant material by affinity chromatography with barnase-His₆ immobilized on a metal-affinity carrier.     

Microbial Diversity and Potential Sulfide Producers in the Karazhanbas Oilfield (Kazakhstan)

Microbiology - Biogenic sulfide production in oilfields results in lower quality of oil and gas and in corrosion of oil-producing equipment. Bactericidal agents are not used at the Karazhanbas...     

True Chemotaxis in Oxygen Gradients of the Sulfur-Oxidizing Bacterium Thiovulum majus

Observations of free-swimming Thiovulum majus cells show that these bacteria exhibit a phobic response as well as true chemotaxis in oxygen gradients. Both phenomena of their chemotactic behavior are integrated into a single model of helical klinotaxis, which is demonstrated by computer simulations.     

Behaviors and Strategies of Bacterial Navigation in Chemical and Nonchemical Gradients

Navigation of cells to the optimal environmental condition is critical for their survival and growth. Escherichia coli cells, for example, can detect various chemicals and move up or down those chemical gradients (i.e., chemotaxis). Using the same signaling machinery, they can also sense other external factors such as pH and temperature and navigate from both sides toward some intermediate levels of those stimuli. This mode of precision sensing is more sophisticated than the (unidirectional) chemotaxis strategy and requires distinctive molecular mechanisms to encode and track the preferred external conditions. To systematically study these different bacterial taxis behaviors, we develop a continuum model that incorporates microscopic signaling events in single cells into macroscopic population dynamics. A simple theoretical result is obtained for the steady state cell distribution in general. In particular, we find the cell distribution is controlled by the intracellular sensory dynamics as well as the dependence of the cells'' speed on external factors. The model is verified by available experimental data in various taxis behaviors (including bacterial chemotaxis, pH taxis, and thermotaxis), and it also leads to predictions that can be tested by future experiments. Our analysis help reveal the key conditions/mechanisms for bacterial precision-sensing behaviors and directly connects the cellular taxis performances with the underlying molecular parameters. It provides a unified framework to study bacterial navigation in complex environments with chemical and non-chemical stimuli.     

Effects of Imposed Salinity Gradients on Dissimilatory Arsenate Reduction, Sulfate Reduction, and Other Microbial Processes in Sediments from Two California Soda Lakes

Salinity effects on microbial community structure and on potential rates of arsenate reduction, arsenite oxidation, sulfate reduction, denitrification, and methanogenesis were examined in sediment slurries from two California soda lakes. We conducted experiments with Mono Lake and Searles Lake sediments over a wide range of salt concentrations (25 to 346 g liter⁻¹). With the exception of sulfate reduction, rates of all processes demonstrated an inverse relationship to total salinity. However, each of these processes persisted at low but detectable rates at salt saturation. Denaturing gradient gel electrophoresis analysis of partial 16S rRNA genes amplified from As(V) reduction slurries revealed that distinct microbial populations grew at low (25 to 50 g liter⁻¹), intermediate (100 to 200 g liter⁻¹), and high (>300 g liter⁻¹) salinity. At intermediate and high salinities, a close relative of a cultivated As-respiring halophile was present. These results suggest that organisms adapted to more dilute conditions can remain viable at high salinity and rapidly repopulate the lake during periods of rising lake level. In contrast to As reduction, sulfate reduction in Mono Lake slurries was undetectable at salt saturation. Furthermore, sulfate reduction was excluded from Searles Lake sediments at any salinity despite the presence of abundant sulfate. Sulfate reduction occurred in Searles Lake sediment slurries only following inoculation with Mono Lake sediment, indicating the absence of sulfate-reducing flora. Experiments with borate-amended Mono Lake slurries suggest that the notably high (0.46 molal) concentration of borate in the Searles Lake brine was responsible for the exclusion of sulfate reducers from that ecosystem.     

Bacterial Community Composition in the Water Column of a Lake Formed by a Former Uranium Open Pit Mine

Mining of pyrite minerals is a major environmental issue involving both biological and geochemical processes. Here we present a study of an artificial lake of a former uranium open pit mine with the aim to connect the chemistry and bacterial community composition (454-pyrosequencing of 16S rRNA genes) in the stratified water column. A shift in the water chemistry from oxic conditions in the epilimnion to anoxic, alkaline, and metal and sulfide-rich conditions in the hypolimnion was corresponded by a strong shift in the bacterial community, with few shared operational taxonomic units (OTU) between the water layers. The epilimnetic bacterial community of the lake (～20?years old) showed similarities to other temperate freshwater lakes, while the hypolimnetic bacterial community showed similarity to extreme chemical environments. The epilimnetic bacterial community had dominance of Actinobacteria and Betaproteobacteria. The hypolimnion displayed a higher bacterial diversity and was dominated by the phototrophic green sulphur bacterium of the genus Chlorobium (ca. 40?% of the total community). Deltaproteobacteria were only represented in the hypolimnion and the most abundant OTUs were affiliated with ferric iron and sulfate reducers of the genus Geobacter and Desulfobulbus, respectively. The chemistry is clearly controlling, especially the hypolimnetic, bacterial community but the community composition also indicates that the bacteria are involved in metal cycling in the lake.     

Landscape Position Influences Microbial Composition and Function via Redistribution of Soil Water across a Watershed

Subalpine forest ecosystems influence global carbon cycling. However, little is known about the compositions of their soil microbial communities and how these may vary with soil environmental conditions. The goal of this study was to characterize the soil microbial communities in a subalpine forest watershed in central Montana (Stringer Creek Watershed within the Tenderfoot Creek Experimental Forest) and to investigate their relationships with environmental conditions and soil carbonaceous gases. As assessed by tagged Illumina sequencing of the 16S rRNA gene, community composition and structure differed significantly among three landscape positions: high upland zones (HUZ), low upland zones (LUZ), and riparian zones (RZ). Soil depth effects on phylogenetic diversity and β-diversity varied across landscape positions, being more evident in RZ than in HUZ. Mantel tests revealed significant correlations between microbial community assembly patterns and the soil environmental factors tested (water content, temperature, oxygen, and pH) and soil carbonaceous gases (carbon dioxide concentration and efflux and methane concentration). With one exception, methanogens were detected only in RZ soils. In contrast, methanotrophs were detected in all three landscape positions. Type I methanotrophs dominated RZ soils, while type II methanotrophs dominated LUZ and HUZ soils. The relative abundances of methanotroph populations correlated positively with soil water content (R = 0.72, P < 0.001) and negatively with soil oxygen (R = −0.53, P = 0.008). Our results suggest the coherence of soil microbial communities within and differences in communities between landscape positions in a subalpine forested watershed that reflect historical and contemporary environmental conditions.     

Synthetic Statistical Approach Reveals a High Degree of Richness of Microbial Eukaryotes in an Anoxic Water Column

Molecular surveys suggest that communities of microbial eukaryotes are remarkably rich, because even large clone libraries seem to capture only a minority of species. This provides a qualitative picture of protistan richness but does not measure its real extent either locally or globally. Statistical analysis can estimate a community's richness, but the specific methods used to date are not always well grounded in statistical theory. Here we study a large protistan molecular survey from an anoxic water column in the Cariaco Basin (Caribbean Sea). We group individual 18S rRNA gene sequences into operational taxonomic units (OTUs) using different cutoff values for sequence similarity (99 to 50%) and systematically apply parametric models and nonparametric estimators to the OTU frequency data to estimate the total protistan diversity. The parametric models provided statistically sound estimates of protistan richness, with biologically meaningful standard errors, maximal data usage, and extensive model diagnostics and were preferable to the available nonparametric tools. Our clone library exceeded 700 clones but still covered only a minority of species and less than half of the larger protistan clades. Our estimates of total protistan richness portray the target community as very rich at all OTU levels, with hundreds of different populations apparently co-occurring in the small (3-liter) volume of our sample, as well as dozens of clades of the highest taxonomic order. These estimates are among the first for microbial eukaryotes that are obtained using state-of-the-art statistical methods and can serve as benchmark numbers for the local diversity of protists.     

Fish Farming Effects on Chemical and Microbial Variables of the Water Column: A Spatio-temporal Study Along the Mediterranean Sea

The effects of fish farming on chemical and biological variables of the water column were investigated in three coastal locations along the Mediterranean (Spain, Italy and Greece). Analyses of nutrients, chlorophyll a, particulate organic carbon and nitrogen (POC and PON), heterotrophic bacteria and cyanobacteria (Synechococcus) were carried out on integrated samples (0–30 m depth) taken at various distances from fish farms and the respective reference sites. At one of these sites, water samples were collected by means of Niskin bottles at 09:00, 13:00 and 16:00 h and at three different depths (0 m, 10 m and bottom). Integrated sampling showed no significant changes with distance in any of the variables measured, whereas all variables except PON showed significant changes between the three locations studied. However, changes were found in samples taken by Niskin bottles, with increased concentrations of NH₄⁺, POC, PON and decreased cyanobacteria densities at the surface layer in the immediate vicinity of the cages. Samples taken at the cages in the afternoon showed increased PO₄³⁻ concentrations in comparison to those taken in the morning. Dilution and grazing are probably both responsible for the lack of detectable signs of eutrophication.     

Coral Reef Disturbance and Recovery Dynamics Differ across Gradients of Localized Stressors in the Mariana Islands

The individual contribution of natural disturbances, localized stressors, and environmental regimes upon longer-term reef dynamics remains poorly resolved for many locales despite its significance for management. This study examined coral reefs in the Commonwealth of the Northern Mariana Islands across a 12-year period that included elevated Crown-of-Thorns Starfish densities (COTS) and tropical storms that were drivers of spatially-inconsistent disturbance and recovery patterns. At the island scale, disturbance impacts were highest on Saipan with reduced fish sizes, grazing urchins, and water quality, despite having a more favorable geological foundation for coral growth compared with Rota. However, individual drivers of reef dynamics were better quantified through site-level investigations that built upon island generalizations. While COTS densities were the strongest predictors of coral decline as expected, interactive terms that included wave exposure and size of the overall fish assemblages improved models (R² and AIC values). Both wave exposure and fish size diminished disturbance impacts and had negative associations with COTS. However, contrasting findings emerged when examining net ecological change across the 12-year period. Wave exposure had a ubiquitous, positive influence upon the net change in favorable benthic substrates (i.e. corals and other heavily calcifying substrates, R² = 0.17 for all reeftypes grouped), yet including interactive terms for herbivore size and grazing urchin densities, as well as stratifying by major reeftypes, substantially improved models (R² = 0.21 to 0.89, lower AIC scores). Net changes in coral assemblages (i.e., coral ordination scores) were more sensitive to herbivore size or the water quality proxy acting independently (R² = 0.28 to 0.44). We conclude that COTS densities were the strongest drivers of coral decline, however, net ecological change was most influenced by localized stressors, especially herbivore sizes and grazing urchin densities. Interestingly, fish size, rather than biomass, was consistently a better predictor, supporting allometric, size-and-function relationships of fish assemblages. Management implications are discussed.     

Distribution of living coccolithophores in the California Current system,southern California borderland

The distribution of living coccolithophores in the California Current system of southern California at 10 m water depth was investigated on two dates in March and June, 1982. Six closely spaced stations were sampled in March, of which three were resampled in June. Thirty-six euphotic species were identified of which four,Emiliania huxleyi, Umbilicosphaera sibogae, Gephyrocapsa oceanica, andRhabdosphaera longistylis, respectively, were the most abundant. Both the “cold” and “warm” morphotypes ofE. huxleyi were present, in varying proportions. Large ranges in community structure, diversity (0.35–2.64 natural bels), and standing crop (1.0 × 10⁴–6.2 × 10⁵cells/l) were recorded. This range of end-member values is approximately that found in the open ocean from 0° to about 65° latitude.The distributions of four coccolithophore assemblages recognized in March samples from the Borderland area appear to reflect the following distinct water masses: (1) California Current; (2) Southern California Counter Current; (3) Transitional Zone; (4) Near Shore. The coccolithophore assemblages from the June stations were more uniform, indicating that the Borderland was experiencing more stable conditions than in March.     

Bacterial Community Structure Along Moisture Gradients in the Parafluvial Sediments of Two Ephemeral Desert Streams

Microorganisms inhabiting stream sediments mediate biogeochemical processes of importance to both aquatic and terrestrial ecosystems. In deserts, the lateral margins of ephemeral stream channels (parafluvial sediments) are dried and rewetted, creating periodically wet conditions that typically enhance microbial activity. However, the influence of water content on microbial community composition and diversity in desert stream sediments is unclear. We sampled stream margins along gradients of wet to dry sediments, measuring geochemistry and bacterial 16S rRNA gene composition, at streams in both a cold (McMurdo Dry Valleys, Antarctica) and hot (Chihuahuan Desert, New Mexico, USA) desert. Across the gradients, sediment water content spanned a wide range (1.6–37.9% w/w), and conductivity was highly variable (12.3–1,380 μS cm⁻²). Bacterial diversity (at 97% sequence similarity) was high and variable, but did not differ significantly between the hot and cold desert and was not correlated with sediment water content. Instead, conductivity was most strongly related to diversity. Water content was strongly related to bacterial 16S rRNA gene community composition, though samples were distributed in wet and dry clusters rather than as assemblages shifting along a gradient. Phylogenetic analyses showed that many taxa from wet sediments at the hot and cold desert site were related to, respectively, halotolerant Gammaproteobacteria, and one family within the Sphingobacteriales (Bacteroidetes), while dry sediments at both sites contained a high proportion of taxa related to the Acidobacteria. These results suggest that bacterial diversity and composition in desert stream sediments is more strongly affected by hydrology and conductivity than temperature.     

Eph Family Receptors and Their Ligands Distribute in Opposing Gradients in the Developing Mouse Retina

The Eph family of receptor tyrosine kinases and their ligands can be divided into two specificity subclasses: the Eck-related receptors and their GPI-anchored ligands, and the Elk-related receptors and their transmembrane ligands. Previous reports demonstrated that Eck- and Elk-related receptors in the retina distribute in high temporal–low nasal and high ventral–low dorsal gradients, respectively. While others have focused on complementary ligand gradients in the retinal axon target, the tectum, we report that ligands from each subclass also distribute in gradients opposing those of their corresponding receptors within the retina itself. Moreover, ligand gradients in the retina precede ganglion cell genesis. These results support an intraretinal role for Eph family members in addition to their previously proposed role in the development of retinotectal topography. The distinct distributions of Eph family members suggest that each subclass specifies positional information along independent retinal axes.     

News & Notes: The Effect of Ethanol and Oxygen on the Growth of Zymomonas mobilis and the Levels of Hopanoids and Other Membrane Lipids

Zymomonas mobilis (ATCC 29191) was grown either aerobically or anaerobically in the presence of 2% (wt/vol) glucose and 0, 3, or 6% (vol/vol) ethanol. The rates of growth and the composition of hopanoids, cellular fatty acids, and other lipids in the bacterial membranes were quantitatively analyzed. The bacterium grew in the presence of 3% and 6% ethanol and was more ethanol tolerant when grown anaerobically. In the absence of ethanol, hopanoids comprised about 30% (by mass) of the total cellular lipids. Addition of ethanol to the media caused complex changes in the levels of hopanoids and other lipids. However, there was not a significant increase in any of the hopanoid lipid classes as ethanol concentration was increased. As previously reported, vaccenic acid was the most abundant fatty acid in the lipids of Z. mobilis, and its high constitutive levels were unaffected by the variations in ethanol and oxygen concentrations. A cyclopropane fatty acid accounted for 2.6–6.4 wt % of the total fatty acids in all treatments. Received: 12 November 1996 / Accepted: 25 February 1997