Effects of nutrients and fish on periphyton and plant biomass across a European latitudinal gradient

Replicated, factorial mesocosm experiments were conducted across Europe to study the effects of nutrient enrichment and fish density on macrophytes and on periphyton chlorophyll a (chl-a) with regard to latitude. Periphyton chl-a densities and plant decline were significantly related to nutrient loading in all countries. Fish effects were significant in a few sites only, mostly because of their contribution to the nutrient pool. A saturation-response type curve in periphyton chl-a with nutrients was found, and northern lakes achieved higher densities than southern lakes. Nutrient concentration and phytoplankton chl-a necessary for a 50% plant reduction followed a latitudinal gradient. Total phosphorus values for 50% plant disappearance were similar from Sweden (0.27 mg L⁻¹) to northern Spain (0.35 mg L⁻¹), but with a sharp increase in southern Spain (0.9 mg L⁻¹). Planktonic chl-a values for 50% plant reduction increased monotonically from Sweden (30 μg L⁻¹) to València (150 μg L⁻¹). Longer plant growing-season, higher light intensities and temperature, and strong water-level fluctuations characteristic of southern latitudes can lead to greater persistence of macrophyte biomass at higher turbidities and nutrient concentration than in northern lakes. Results support the evidence that latitudinal differences in the functioning of shallow lakes should be considered in lake management and conservation policies.     

Melittin induces HII phase formation in cardiolipin model membranes

The interaction of melittin with bovine heart cardiolipin model membranes was investigated via binding assays, 31P-NMR, freeze-fracture electron microscopy, small angle X-ray diffraction and fluorescence based fusion assays. A strong binding (Kd less than 10(-7) M) appeared to be accompanied by the formation of large structures, resulting from a fusion process of extremely fast initial rate. As the melittin content is increased, bilayer structure is gradually lost and from a cardiolipin to melittin ratio of about 6 the lipid starts to organize itself in an hexagonal HII phase. At lower temperatures (T less than 40 degrees C) the coexistence of another structure is observed, characterized by a broad isotropic 31P-NMR signal and giving rise to sharp X-ray reflections, most probably a cubic phase, as suggested also be freeze-fracture images, showing orderly stacked particles. The results are discussed in relation to contrasting observations on the structural changes induced by melittin in the zwitterionic phospholipid system of dipalmitoylphosphatidylcholine (Dufourcq. J. et al. (1986) Biochim. Biophys. Acta 859, 33-48). The biological relevance of the observations with respect to the process of protein insertion into membranes is indicated.     

Autoinducer-2 detection among commensal oral streptococci is dependent on pH and boric acid

Autoinducer-2, considered a universal signaling molecule, is produced by many species of bacteria; including oral strains. Structurally, autoinducer-2 can exist bound to boron (borated autoinducer-2). Functionally, autoinducer-2 has been linked to important bacterial processes such as virulence and biofilm formation. In order to test production of autoinducer-2 by a given bacterial strain, a bioassay using marine bioluminescent bacteria Vibrio harveyi as a reporter for autoinducer-2 has been designed. We hypothesize that pH adjustment and addition of boron are required for optimal bioluminescence and accurate autoinducer-2 detection. Using this reporter strain we tested autoinducer-2 activity from two oral commensal species, Streptococcus gordonii DL1 and Streptococcus oralis 34. Spent broth was collected and adjusted to pH 7.5 and supplemented with boric acid prior to measuring autoinducer- 2 activity. Results show that low pH inhibits bioluminescence of the reporter strain, but pH 7.5 allows for bioluminescence induction and proper readings of autoinducer-2 activity. Addition of boric acid also has a positive effect on bioluminescence allowing for a more sensitive detection of autoinducer-2 activity. Our data suggests that although autoinducer-2 is present in spent broth, low pH and/or low levels of boric acid become an obstacle for proper autoinducer-2 detection. For proper autoinducer-2 detection, we propose a protocol using this bioassay to include pH adjustment and boric acid addition to spent broth. Studies on autoinducer-2 activity in several bacteria species represent an important area of study as this universal signaling molecule is involved in critical bacterial phenotypes such as virulence and biofilm formation.     

Rapid alteration of the phosphoproteome in the moss Physcomitrella patens after cytokinin treatment

Cytokinin hormones are crucial regulators of a large number of processes in plant development. Recently, significant progress has been made toward the elucidation of the molecular details of cytokinin that has led to a model for signal transduction involving a phosphorylation cascade. However, the current knowledge of cytokinin action remains largely unknown and does not explain the different roles of this hormone. To gain further insights into this aspect of cytokinin action and the inducible phosphorelay, we have produced the first large-scale map of a phosphoproteome in the moss Physcomitrella patens. Using a protocol that we recently published (Heintz, D.; et al. Electrophoresis 2004, 25, 1149-1159) that combines IMAC, MALDI-TOF-MS, and LC-MS/MS, a total of 172 phosphopeptide sequences were obtained by a peptide de novo sequencing strategy. Specific P. patens EST and raw genomic databases were interrogated, and protein homology searches resulted in the identification of 112 proteins that were then classified into functional categories. In addition, the temporal dynamics of the phosphoproteome in response to cytokinin stimulation was studied at 2, 4, 6, and 15 min after hormone addition. We identified 13 proteins that were not previously known targets of cytokinin action. Among the responsive proteins, some were involved in metabolism, and several proteins of unknown function were also identified. We have mapped the time course of their activation in response to cytokinin and discussed their hypothetical biological significance. Deciphering these early induced phosphorylation events has shown that the cytokinin effect can be rapid (few minutes), and the duration of this effect can be variable. Also phosphorylation events can be differentially regulated. Taken together our proteomic study provides an enriched look of the multistep phosphorelay system mediating cytokinin response and suggests the existence of a multidirectional interaction between cytokinin and numerous other pathways.