Epilithic bacterial and algal colonization in a stream run, riffle, and pool: a test of biomass covariation

Epilithic bacterial and algal biomass were compared among a run, riffle, and pool along an open-canopy section of a third-order, temperate stream. Epilithic biofilms were sampled after 3, 7, 14, 21, 28, and 35 days colonization on unglazed ceramic tiles that were attached to plastic trays (n = 3) placed across each of the three habitats (i.e., run, riffle, pool). The diverse habitats and sampling regime were selected to provide a range in algal biomass so that potential covariation between epilithic bacterial and algal biomass could be assessed. There were significant differences among habitats and among trays within each habitat for both chlorophyll a and AFDM. Chlorophyll a and AFDM increased in the run and pool throughout the colonization period. In the riffle, chlorophyll a and AFDM increased rapidly early in colonization, then decreased. Epilithic bacterial biomass increased rapidly with no significant differences among the three habitats throughout colonization. Further, bacterial biomass did not correlate with either chlorophyll a or AFDM in any of the three habitats or on any of the sampling days. These results suggest that epilithic algal and bacterial biomass may be regulated by independent controls in some stream environments.     

Isolation and analysis of a fur mutant of Neisseria gonorrhoeae.

The pathogenic Neisseria spp. produce a number of iron-regulated gene products that are thought to be important in virulence. Iron-responsive regulation of these gene products has been attributed to the presence in Neisseria spp. of the Fur (ferric uptake regulation) protein. Evidence for the role of Fur in neisserial iron regulation has been indirect because of the inability to make fur null mutations. To circumvent this problem, we used manganese selection to isolate missense mutations of Neisseria gonorrhoeae fur. We show that a mutation in gonococcal fur resulted in reduced modulation of expression of four well-studied iron-repressed genes and affected the iron regulation of a broad range of other genes as judged by two-dimensional polyacrylamide gel electrophoresis (PAGE). All 15 of the iron-repressed spots observed by two-dimensional PAGE were at least partially derepressed in the fur mutant, and 17 of the 45 iron-induced spots were affected by the fur mutation. Thus, Fur plays a central role in regulation of iron-repressed gonococcal genes and appears to be involved in regulation of many iron-induced genes. The size and complexity of the iron regulons in N. gonorrhoeae are much greater than previously recognized.     

Genetic interactions indicate a role for Mdg1p and the SH3 domain protein Bem1p in linking the G-protein mediated yeast pheromone signalling pathway to regulators of cell polarity

The pheromone signal in the yeastSaccharomyces cerevisiae is transmitted by the and subunits of the mating response G-protein. TheSTE20 gene, encoding a protein kinase required for pheromone signal transduction, has recently been identified in a genetic screen for high-gene-dosage suppressors of a partly defective G mutation. The same genetic screen identifiedBEM1, which encodes an SH3 domain protein required for polarized morphogenesis in response to pheromone, and a novel gene, designatedMDG1 (multicopy suppressor ofdefectiveG-protein). TheMDG1 gene was independently isolated in a search for multicopy suppressors of abem1 mutation. TheMDG1 gene encodes a predicted hydrophilic protein of 364 amino acids with a molecular weight of 41 kDa that has no homology with known proteins. A fusion of Mdg1p with the green fluorescent protein fromAequorea victoria localizes to the plasma membrane, suggesting that Mdg1p is an extrinsically bound membrane protein. Deletion ofMDG1 causes sterility in cells in which the wild-type G has been replaced by partly defective G derivatives but does not cause any other obvious phenotypes. The mating defect of cells deleted forSTE20 is partially suppressed by multiple copies ofBEM1 andCDC42, which encodes a small GTP-binding protein that binds to Ste20p and is necessary for the development of cell polarity. Elevated levels ofSTE20 andBEM1 are capable of suppressing a temperature-sensitive mutation inCDC42. This complex network of genetic interactions points to a role for Bem1p and Mdg1p in G-protein mediated signal transduction and indicates a functional linkage between components of the pheromone signalling pathway and regulators of cell polarity during yeast mating.     

Construction of hermes shuttle vectors: a versatile system useful for genetic complementation of transformable and non-transformableNeisseria mutants

A versatile shuttle system has been developed for genetic complementation with cloned genes of transformable and non-transformableNeisseria mutants. By random insertion of a selectable marker into the conjugativeNeisseria plasmidptetM25.2, a site within this plasmid was identified that is compatible with plasmid replication and with conjugative transfer of plasmid. Regions flanking the permissive insertion site of ptetM25.2 were cloned inEscherichia coli and served as a basis for the construction of the Hermes vectors. Hermes vectors are composed of anE. coli replicon that does not support autonomous replication inNeisseria, e.g. ColE1, p15A, orori _fd, fused with a shuttle consisting of a selectable marker and a multiple cloning site flanked by the integration region of ptetM25.2. Complementation of a non-transformableNeisseria strain involves a three-step process: (i) insertion of the desired gene into a Hermes vector; (ii) transformation of Hermes into aNeisseria strain containing ptetM25.2 to create a hybrid ptetM25.2 via gene replacement by the Hermes shuttle cassette; and (iii) conjugative transfer of the hybrid ptetM25.2 into the finalNeisseria recipient. Several applications for the genetic manipulation of pathogenicNeisseriae are described.     

Genetically-based plant resistance traits affect arthropods,fungi, and birds

We examine how the distribution of a leafgalling aphid (Pemphigus betae) affects other species associated with natural stands of hybrid cottonwoods (Populus angustifolia x P. fremontii). Aphid transfers on common-garden clones and RFLP analysis show that resistance to aphids in cottonwoods is affected by plant genotype. Because susceptible trees typically support thousands of galls, while adjacent resistant trees have few or none, plant resistance traits that affect the distribution of this abundant herbivore may directly and/or indirectly affect other species. We found that the arthropod community of aphid-susceptible trees had 31% greater species richness and 26% greater relative abundance than aphid-resistant trees. To examine direct and indirect effects of plant resistance traits on other organisms, we experimentally excluded aphids and found that abundances and/or foraging behavior of arthropods, fungi, and birds were altered. First, exclusion of gall aphids on susceptible trees resulted in a 24% decrease in species richness and a 28% decrease in relative abundance of the arthropod community. Second, exclusion of aphids also caused a 2- to 3-fold decrease in foraging and/or presence of three taxa of aphid enemies: birds, fungi, and insects. Lastly, aphidexclussion resulted in a 2-fold increase in inquilines (animals who live in abodes properly belonging to another). We also found that fungi and birds responded to variation in gall density at the branch level. We conclude plant resistance traits affect diverse species from three trophic levels supporting a bottom-up influence of plants on community structure.     

Misting and nitrogen fertilization of shoots of a saltmarsh grass: effects upon fungal decay of leaf blades

We conducted a 12-week field manipulation experiment in which we raised the nitrogen availability (ammonium sulfate fertilization to roots) and/or water potential (freshwater misting) of decaying leaf blades of a saltmarsh grass (smooth cordgrass, Spartina alterniflora) in triplicate 11-m² plots, and compared the manipulated plots to unmanipulated, control plots. The ascomycetous fungi that dominate cordgrass leaf decomposition processes under natural conditions exhibited a boosting (>2-fold) of living standing crop (ergosterol content) by misting at the 1 st week after tagging of senescent leaves, but afterwards, living-fungal standing crop on misted blades was equivalent to that on control blades, confirming prior evidence that Spartina fungi are well adapted to natural, irregular wetting. Misting also caused 2-fold sharper temporal declines than control in instantaneous rates of fungal production (ergosterol synthesis), 5-fold declines in density of sexual reproductive structures that were not shown by controls, and 2-fold higher rates of loss of plant organic mass. Extra nitrogen gave a long-term boost to living-fungal standing crop (about 2-fold at 12 weeks), which was also reflected in rates of fungal production at 4 weeks, suggesting that saltmarsh fungal production is nitrogen-limited. Although bacterial and green-microalgal crops were boosted by manipulations of nitrogen and/or water, their maximal crops remained 0.3 or 2% (bacteria or green microalgae, respectively) of contemporaneous living-fungal crop. The fungal carbon-productivity values obtained, in conjunction with rates of loss of plant carbon, hinted that fungal yield can be high (>50%), and that it is boosted by high availability of nitrogen. We speculate that one partial cause of high fungal yield could be subsidy of fungal growth by dissolved organic carbon from outside decomposing leaves.     

Aphid honeydew and its effect on the phyllosphere microflora of Picea abies (L.) Karst

Aphids of the genus Cinara, feeding on Norway spruce, excrete copious amounts of honeydew, a carbon-rich waste product, which accumulates locally on needles and twigs. We investigated the role of honeydew as a potential source of energy which might promote the growth of micro-organisms in the phyllosphere of conifer trees. To approach this question, we followed the population dynamics of Cinara spp. in a natural forest stand over two seasons. We also studied the amounts of honeydew produced by individual aphids and identified potential parameters which might influence honeydew production. Finally, we determined the growth of micro-organisms on infested and uninfested needles of Norway spruce during the growing season. Confined to Picea abies, the investigated Cinara species only became abundant in midsummer, when needles and shoots were expanding. The populations showed only a single peak in abundance, the timing and magnitude of which may vary from year to year due to weather conditions, changes in plant quality in a yearly cycle or the impact of natural enemies. The amount of honeydew produced by individual aphids was dependent on the developmental stage of the aphid, the nutritional supply of its host plant and on the developmental state of the Norway spruce (e.g. bud burst, end of shoot extension). The presence of honeydew significantly increased the growth of bacteria, yeast and filamentous fungi on the surface of needles and there was a pronounced seasonal trend, with the highest abundance in midsummer correlating with the period of peak aphid abundance. Taken together, these findings indicate that aphids have an influence on microbial ecology in the phyllosphere of trees. The implication of our study, from interactions at the population level to effects and potential consequences for C and N fluxes at the level of forest ecosystems, is discussed.     

Pliocene climates: the nature of the problem

The Pliocene may have been the last time period when global temperatures were greater than present. The Pliocene data base is sufficiently complete to provide a valuable test for climate model predictions for warm time periods. This paper reviews some key issues with respect to understanding and verifying theories for the origin of Pliocene warmth. There are two main factors cited to explain Pliocene warmth—higher CO₂ levels or higher levels of ocean heat transport. The two explanations may not be exclusive; for example CO₂ increases may drive ocean heat transport changes. However, initial proxy-CO₂ reconstructions suggest that the CO₂ perturbation is very small (~100 ppm) to effect such large changes in climate. Considerably more data are needed to evaluate the magnitude of Pliocene CO₂ changes. Although more modeling work is also required, it is necessary to continue evaluation of input data used to force model results (particularly sea surface temperatures). Continued simulations and evaluation of pCO₂ and SST results should enable us to better understand how the earth responds during a global warming and determine whether our models are properly simulating such changes.