Taxonomic and Environmental Variability in the Elemental Composition and Stoichiometry of Individual Dinoflagellate and Diatom Cells from the NW Mediterranean Sea

Here we present, for the first time, the elemental concentration, including C, N and O, of single phytoplankton cells collected from the sea. Plankton elemental concentration and stoichiometry are key variables in phytoplankton ecophysiology and ocean biogeochemistry, and are used to link cells and ecosystems. However, most field studies rely on bulk techniques that overestimate carbon and nitrogen because the samples include organic matter other than plankton organisms. Here we used X-ray microanalysis (XRMA), a technique that, unlike bulk analyses, gives simultaneous quotas of C, N, O, Mg, Si, P, and S, in single-cell organisms that can be collected directly from the sea. We analysed the elemental composition of dinoflagellates and diatoms (largely Chaetoceros spp.) collected from different sites of the Catalan coast (NW Mediterranean Sea). As expected, a lower C content is found in our cells compared to historical values of cultured cells. Our results indicate that, except for Si and O in diatoms, the mass of all elements is not a constant fraction of cell volume but rather decreases with increasing cell volume. Also, diatoms are significantly less dense in all the measured elements, except Si, compared to dinoflagellates. The N:P ratio of both groups is higher than the Redfield ratio, as it is the N:P nutrient ratio in deep NW Mediterranean Sea waters (N:P = 20–23). The results suggest that the P requirement is highest for bacterioplankton, followed by dinoflagellates, and lowest for diatoms, giving them a clear ecological advantage in P-limited environments like the Mediterranean Sea. Finally, the P concentration of cells of the same genera but growing under different nutrient conditions was the same, suggesting that the P quota of these cells is at a critical level. Our results indicate that XRMA is an accurate technique to determine single cell elemental quotas and derived conversion factors used to understand and model ocean biogeochemical cycles

Distinct Gene Number-Genome Size Relationships for Eukaryotes and Non-Eukaryotes: Gene Content Estimation for Dinoflagellate Genomes

The ability to predict gene content is highly desirable for characterization of not-yet sequenced genomes like those of dinoflagellates. Using data from completely sequenced and annotated genomes from phylogenetically diverse lineages, we investigated the relationship between gene content and genome size using regression analyses. Distinct relationships between log10-transformed protein-coding gene number (Y′) versus log10-transformed genome size (X′, genome size in kbp) were found for eukaryotes and non-eukaryotes. Eukaryotes best fit a logarithmic model, Y′ = ln(-46.200+22.678X′, whereas non-eukaryotes a linear model, Y′ = 0.045+0.977X′, both with high significance (p<0.001, R2>0.91). Total gene number shows similar trends in both groups to their respective protein coding regressions. The distinct correlations reflect lower and decreasing gene-coding percentages as genome size increases in eukaryotes (82%–1%) compared to higher and relatively stable percentages in prokaryotes and viruses (97%–47%). The eukaryotic regression models project that the smallest dinoflagellate genome (3×106 kbp) contains 38,188 protein-coding (40,086 total) genes and the largest (245×106 kbp) 87,688 protein-coding (92,013 total) genes, corresponding to 1.8% and 0.05% gene-coding percentages. These estimates do not likely represent extraordinarily high functional diversity of the encoded proteome but rather highly redundant genomes as evidenced by high gene copy numbers documented for various dinoflagellate species

Are Algae Relevant to the Detritus-Based Food Web in Tank-Bromeliads?

We assessed the occurrence of algae in five species of tank-bromeliads found in contrasting environmental sites in a Neotropical, primary rainforest around the Nouragues Research Station, French Guiana. The distributions of both algal abundance and biomass were examined based on physical parameters, the morphological characteristics of bromeliad species and with regard to the structure of other aquatic microbial communities held in the tanks. Algae were retrieved in all of the bromeliad species with mean densities ranging from ∼102 to 104 cells/mL. Their biomass was positively correlated to light exposure and bacterial biomass. Algae represented a tiny component of the detrital food web in shaded bromeliads but accounted for up to 30 percent of the living microbial carbon in the tanks of Catopsis berteroniana, located in a highly exposed area. Thus, while nutrient supplies are believed to originate from wind-borne particles and trapped insects (i.e., allochtonous organic matter), our results indicate that primary producers (i.e., autochtonous organic matter) are present in this insectivorous bromeliad. Using a 24-h incubation of size-fractionated and manipulated samples from this plant, we evaluated the impact of mosquito foraging on algae, other microorganisms and rotifers. The prey assemblages were greatly altered by the predation of mosquito larvae. Grazing losses indicated that the dominant algal taxon, Bumilleriopsis sp., like protozoa and rotifers, is a significant part of the diet of mosquito larvae. We conclude that algae are a relevant functional community of the aquatic food web in C. berteroniana and might form the basis of a complementary non-detrital food web

Active Suppression of Early Immune Response in Tobacco by the Human Pathogen Salmonella Typhimurium

The persistence of enteric pathogens on plants has been studied extensively, mainly due to the potential hazard of human pathogens such as Salmonella enterica being able to invade and survive in/on plants. Factors involved in the interactions between enteric bacteria and plants have been identified and consequently it was hypothesized that plants may be vectors or alternative hosts for enteric pathogens. To survive, endophytic bacteria have to escape the plant immune systems, which function at different levels through the plant-bacteria interactions. To understand how S. enterica survives endophyticaly we conducted a detailed analysis on its ability to elicit or evade the plant immune response. The models of this study were Nicotiana tabacum plants and cells suspension exposed to S. enterica serovar Typhimurium. The plant immune response was analyzed by looking at tissue damage and by testing oxidative burst and pH changes. It was found that S. Typhimurium did not promote disease symptoms in the contaminated plants. Live S. Typhimurium did not trigger the production of an oxidative burst and pH changes by the plant cells, while heat killed or chloramphenicol treated S. Typhimurium and purified LPS of Salmonella were significant elicitors, indicating that S. Typhimurium actively suppress the plant response. By looking at the plant response to mutants defective in virulence factors we showed that the suppression depends on secreted factors. Deletion of invA reduced the ability of S. Typhimurium to suppress oxidative burst and pH changes, indicating that a functional SPI1 TTSS is required for the suppression. This study demonstrates that plant colonization by S. Typhimurium is indeed an active process. S. Typhimurium utilizes adaptive strategies of altering innate plant perception systems to improve its fitness in the plant habitat. All together these results suggest a complex mechanism for perception of S. Typhimurium by plants

Functional Effects of Parasites on Food Web Properties during the Spring Diatom Bloom in Lake Pavin: A Linear Inverse Modeling Analysis

This study is the first assessment of the quantitative impact of parasitic chytrids on a planktonic food web. We used a carbon-based food web model of Lake Pavin (Massif Central, France) to investigate the effects of chytrids during the spring diatom bloom by developing models with and without chytrids. Linear inverse modelling procedures were employed to estimate undetermined flows in the lake. The Monte Carlo Markov chain linear inverse modelling procedure provided estimates of the ranges of model-derived fluxes. Model results support recent theories on the probable impact of parasites on food web function. In the lake, during spring, when ‘inedible’ algae (unexploited by planktonic herbivores) were the dominant primary producers, the epidemic growth of chytrids significantly reduced the sedimentation loss of algal carbon to the detritus pool through the production of grazer-exploitable zoospores. We also review some theories about the potential influence of parasites on ecological network properties and argue that parasitism contributes to longer carbon path lengths, higher levels of activity and specialization, and lower recycling. Considering the “structural asymmetry” hypothesis as a stabilizing pattern, chytrids should contribute to the stability of aquatic food webs

Molecular heterogeneity in Aphanizomenon flos-aquae and Anabaena flos-aquae (Cyanophyta): a synchrotron-based Fourier-transform infrared study of lake micropopulations

Synchrotron-based Fourier-transform infrared (FTIR) microspectroscopy was used to characterize micropopulations of the cyanobacteria Anabaena flos-aquae and Aphanizomenon flos-aquae in mixed phytoplankton samples taken from the epilimnion and hypolimnion of a stratified eutrophic lake (Rostherne Mere, UK). All FTIR spectra (normalized to amide I) showed a closely similar sequence of 10 bands over the wavenumber range 4,000-900 cm -1 and were assigned to a range of vibrationally active chemical groups. Interspecific differences between Anabaena and Aphanizomenon were revealed using correlation and multivariate analysis (principal component analysis, PCA) of the set of individual band intensities. Differences between species (separately for epilimnion and hypolimnion) were also analysed by comparing band intensity means, and by performing PCA over a broad spectral region (1,750-900 cm -1 ). Comparison of intensity means showed that spectra obtained from epilimnetic algae had significant differences in bands 5-7 and in bands 4, 9 and 10. However, in the spectra obtained from the hypolimnion algae, a different pattern was observed, with significant differences between the species restricted to bands 7-10. PCA of the spectral region 1,750-900 cm -1 also demonstrated interspecific differences in relation to carbohydrate and protein concentrations, with Anabaena displaying higher carbohydrate levels in both the epilimnion and hypolimnion. Intraspecific comparisons of band intensity means for epilimnion and hypolimnion subpopulations demonstrated clear differences in Aphanizomenon , where significant differences were observed between epilimnion (higher carbohydrate concentration compared with protein) and hypolimnion (lower carbohydrate concentration compared with protein) subpopulations. Anabaena however showed only minor differences. PCA comparison of the spectral region 1,750-900 cm -1 , followed by data visualization using scores plots and loading plots, also demonstrated quantitative differences. Scores plots resolved the epilimnion and hypolimnion subpopulations in Aphanizomenon (but not Anabaena ), with the loading plot showing large differences in carbohydrates, supporting the analysis using discrete peak intensities. © 2006 British Phycological Society