Wind‐driven thermocline movements affect the colonisation and growth of Achnanthidium minutissimum,a ubiquitous benthic diatom in lakes

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Bacteria may induce the secretion of mucin‐like proteins by the diatom Phaeodactylum tricornutum

Benthic diatoms live in photoautotrophic/heterotrophic biofilm communities embedded in a matrix of secreted extracellular polymeric substances. Closely associated bacteria influence their growth, aggregation, and secretion of exopolymers. We have studied a diatom/bacteria model community, in which a marine Roseobacter strain is able to grow with secreted diatom exopolymers as a sole source of carbon. The strain influences the aggregation of Phaeodactylum tricornutum by inducing a morphotypic transition from planktonic, fusiform cells to benthic, oval cells. Analysis of the extracellular soluble proteome of P. tricornutum in the presence and absence of bacteria revealed constitutively expressed newly identified proteins with mucin‐like domains that appear to be typical for extracellular diatom proteins. In contrast to mucins, the proline‐, serine‐, threonine‐rich (PST) domains in these proteins were also found in combination with protease‐, glucosidase‐ and leucine‐rich repeat‐domains. Bioinformatic functional predictions indicate that several of these newly identified diatom‐specific proteins may be involved in algal defense, intercellular signaling, and aggregation.     

Coexistence of two Cyclotella diatom species in the plankton of Lake Baikal

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Comparison of population growth and photosynthetic apparatus changes in response to different nutrient status in a diatom and a coccolithophore

In many marine ecosystems, diatoms dominate in nutrient‐rich coastal waters while coccolithiophores are found offshore in areas where nutrients may be limiting. In lab‐controlled batch cultures, mixed‐species competition between the diatom Phaeodactylum tricornutum and the coccolithophore Emiliana huxleyi and the response of each species were examined under nitrate (N) and phosphate (P) starvation. Based on the logistic growth model and the Lotka–Volterra competition model, E. huxleyi showed higher competitive abilities than P. tricornutum under N and P starvation. For both species, cell growth was more inhibited by P starvation, while photosynthetic functions (chl a fluorescence parameters) and cellular constituents (pigments) were impaired by N starvation. The decline of photosynthetic functions occurred later in E. huxleyi (day 12) than in P. tricornutum (day 9); this time difference was associated with greater damage of the photosynthetic apparatus in P. tricornutum compared with E. huxleyi. Xanthophyll cycle pigment accumulation and the transformation from diadinoxanthin to diatoxanthin was more active in E. huxleyi than P. tricornutum, under similar N and P starvation. We concluded that E. huxleyi and P. tricornutum have different mechanisms to allocate resources and energy under nutrient starvation. It appears that E. huxleyi has a more economic strategy to adapt to nutrient depleted environments than P. tricornutum. These findings provided additional evidence explaining how N versus P limitation differentially support diatom and coccolithophore blooms in natural environments.     

Physical and chemical characterization of the polysaccharide capsule of the marine bacterium,Hyphomonas strain MHS-3

Hyphomonas MHS-3 is a biphasic, marine bacterium that synthesizes an exopolysaccharide (EPS) capsule, which has a role in attaching the adherent, prosthecate developmental stages to solid substrata. To correlate structure with function, we characterized this integral EPS. It has a relatively homogeneous molecular weight of approximately 60000 daltons, is acidic, and putatively contains large concentrations ofN-acetylgalactosamine (GalNAc). The theoretical identity of the anionic component of the polymer, and the similarities betweenHyphomonas MHS-3 EPS and other adhesive marine/aquatic bacterial EPS are discussed.     

Biofilm formation and toxin production provide a fitness advantage in mixed colonies of environmental yeast isolates

Microbes can engage in social interactions ranging from cooperation to warfare. Biofilms are structured, cooperative microbial communities. Like all cooperative communities, they are susceptible to invasion by selfish individuals who benefit without contributing. However, biofilms are pervasive and ancient, representing the first fossilized life. One hypothesis for the stability of biofilms is spatial structure: Segregated patches of related cooperative cells are able to outcompete unrelated cells. These dynamics have been explored computationally and in bacteria; however, their relevance to eukaryotic microbes remains an open question. The complexity of eukaryotic cell signaling and communication suggests the possibility of different social dynamics. Using the tractable model yeast, Saccharomyces cerevisiae, which can form biofilms, we investigate the interactions of environmental isolates with different social phenotypes. We find that biofilm strains spatially exclude nonbiofilm strains and that biofilm spatial structure confers a consistent and robust fitness advantage in direct competition. Furthermore, biofilms may protect against killer toxin, a warfare phenotype. During biofilm formation, cells are susceptible to toxin from nearby competitors; however, increased spatial use may provide an escape from toxin producers. Our results suggest that yeast biofilms represent a competitive strategy and that principles elucidated for the evolution and stability of bacterial biofilms may apply to more complex eukaryotes.     

Spermatogenesis and auxospore structure in the multipolar centric diatom Hydrosera

Spermatogenesis and auxospore development were studied in the freshwater centric diatom Hydrosera triquetra. Spermatogenesis was unusual, lacking depauperating cell divisions within the spermatogonangium. Instead, a series of mitoses occurred within an undivided cell to produce a multinucleate plasmodium with peripheral nuclei, which then underwent meiosis. 32 or 64 sperm budded off from the plasmodium leaving a large residual cell containing all the chloroplasts. Similar development apparently occurs in Pleurosira, Aulacodiscus, and Guinardia, these being so distantly related that independent evolution of plasmodial spermatogenesis seems likely. After presumed fertilization, the Hydrosera egg cell expanded distally to form a triangular end part. However, unlike in other triangular diatoms (Lithodesmium, Triceratium), the development of triradiate symmetry was not controlled by the “canonical” method of a perizonium that constrains expansion to small terminal areas of the auxospore wall. Instead, the auxospore wall lacked a perizonium and possessed only scales and a dense mat of thin, apparently entangled strips of imperforate silica. No such structures have been reported from any other centric diatoms, the closest analogs being instead the incunabular strips of some raphid diatoms (Nitzschia and Pinnularia). Whether these silica structures are formed by the normal method (intracellular deposition within a silica deposition vesicle) is unknown. As well as being more rounded than vegetative cells, the initial cell is aberrant in its structure, since it has a less polarized distribution of the “triptych” pores characteristic of the species.     

Characterization of the extracellular matrix of Phaeodactylum tricornutum (Bacillariophyceae): structure,composition, and adhesive characteristics

The extracellular matrix of the ovoid and fusiform morphotypes of Phaeodactylum tricornutum (Bohlin) was characterized in detail. The structural and nanophysical properties were analyzed by microscopy. Of the two morphotypes, only the ovoid form secretes adhesive mucilage; light microscopy and scanning electron microscopy images showed that the mucilage was secreted from the girdle band region of the cell as cell‐substratum tethers, accumulating on the surface forming a biofilm. After 7 d, the secreted mucilage became entangled, forming adhesive strands that crisscrossed the substratum surface. In the initial secreted mucilage atomic force microscopy identified a high proportion of adhesive molecules without regular retraction curves and some modular‐like adhesive molecules, in the 7 d old biofilm, the adhesive molecules were longer with fewer adhesive events but greater adhesive strength. Chemical characterization was carried out on extracted proteins and polysaccharides. Differences in protein composition, monosaccharide composition, and linkage analysis are discussed in relation to the composition of the frustule and secreted adhesive mucilage. Polysaccharide analysis showed a broad range of monosaccharides and linkages across all fractions with idiosyncratic enrichment of particular monosaccharides and linkages in each fraction. 3‐linked Mannan was highly enriched in the cell frustule fractions indicating a major structural role, while Rhamnose and Fucose derivatives were enriched in the secreted fractions of the ovoid morphotype suggesting involvement in cell adhesion. Comparison of SDS‐PAGE of extracellular proteins showed two major bands for the ovoid morphotype and four for the fusiform morphotype of which only one appeared to be common to both morphotypes.     

Stimulated rejuvenation of dormant Aulacoseira granulata (Bacillariophyta) by Gloeocystis planctonica (Chlorophyta) in a eutrophic river

The abundance of Aulacoseira granulata (Ehrenburg) Simonsen and Gloeocystis planctonica (West & G.S.West) Lemmermann was assessed during the summers of 2005 and 2010 in the eutrophic Fox River, Wisconsin, USA. In both years, a mid‐summer bloom of G. planctonica was followed by the rapid growth of A. granulata. Laboratory experiments in which A. granulata was grown in cell‐free filtrate of a G. planctonica culture revealed that the growth of A. granulata was stimulated in the G. planctonica‐treated medium relative to controls. This effect was detected when dormant A. granulata cells were used as the source culture for the experiment but not when actively growing cells were used. Dormant A. granulata also grew more rapidly in river water collected after the 2010 G. planctonica bloom relative to river water collected before the bloom. These results suggest that the summer bloom of A. granulata in the river was stimulated by G. planctonica. This relationship can be described as stimulated rejuvenation, an interaction where the transition of an algal resting stage into active growth is triggered by exposure to another species.     

Biofilm formation by a fimbriae-deficient mutant of Actinobacillus actinomycetemcomitans

Actinobacillus actinomycetemcomitans strain 310-TR produces fimbriae and forms a tight biofilm in broth cultures, without turbid growth. The fimbriae-deficient mutant 310-DF, constructed in this study, was grown as a relatively fragile biofilm at the bottom of a culture vessel. Scanning electron microscopy revealed that on glass coverslips, 310-TR formed tight and spherical microcolonies, while 310-DF produced looser ones. These findings suggest that fimbriae are not essential for the surface-adherent growth but are required for enhancing cell-to-surface and cell-to-cell interactions to stabilize the biofilm. Treatment of the 310-DF biofilm with either sodium metaperiodate or DNase resulted in significant desorption of cells from glass surfaces, indicating that both carbohydrate residues and DNA molecules present on the cell surface are also involved in the biofilm formation.     

Molecular phylogeny of the diatom genera Amphora and Halamphora (Bacillariophyta) with a focus on morphological and ecological evolution

The taxonomic history of the diatom genus Amphora is one of a broad early morphological concept resulting in the inclusion of a diversity of taxa, followed by an extended period of revision and refinement. The introduction of molecular systematics has increased the pace of revision and has largely resolved the relationships between the major lineages, indicating homoplasy in the evolution of amphoroid symmetry. Within the two largest monophyletic lineages, the genus Halamphora and the now taxonomically refined genus Amphora, the intrageneric morphological and ecological relationships have yet to be explored within a phylogenetic framework. Critical among this is whether the range of morphological features exhibited within these diverse genera are reflective of evolutionary groupings or, as with many previously studied amphoroid features, are nonhomologous when examined phylogenetically. Presented here is a four‐marker molecular phylogeny that includes 31 taxa from the genus Amphora and 77 taxa from the genus Halamphora collected from fresh, brackish, and salt waters from coastal and inland habitats of the United States and Japan. These phylogenies illustrate complex patterns in the evolution of frustule morphology and ecology within the genera and the implications of this on the taxonomy, classification, and organization of the genera are discussed.     

Decreased photosynthesis and growth with reduced respiration in the model diatom Phaeodactylum tricornutum grown under elevated CO2 over 1800 generations

Studies on the long‐term responses of marine phytoplankton to ongoing ocean acidification (OA) are appearing rapidly in the literature. However, only a few of these have investigated diatoms, which is disproportionate to their contribution to global primary production. Here we show that a population of the model diatom Phaeodactylum tricornutum, after growing under elevated CO₂ (1000 μatm, HCL, pH_T: 7.70) for 1860 generations, showed significant differences in photosynthesis and growth from a population maintained in ambient CO₂ and then transferred to elevated CO₂ for 20 generations (HC). The HCL population had lower mitochondrial respiration, than did the control population maintained in ambient CO₂ (400 μatm, LCL, pH_T: 8.02) for 1860 generations. Although the cells had higher respiratory carbon loss within 20 generations under the elevated CO₂, being consistent to previous findings, they downregulated their respiration to sustain their growth in longer duration under the OA condition. Responses of phytoplankton to OA may depend on the timescale for which they are exposed due to fluctuations in physiological traits over time. This study provides the first evidence that populations of the model species, P. tricornutum, differ phenotypically from each other after having been grown for differing spans of time under OA conditions, suggesting that long‐term changes should be measured to understand responses of primary producers to OA, especially in waters with diatom‐dominated phytoplankton assemblages.     

Colonization and extinction dynamics of a declining migratory bird are influenced by climate and habitat degradation

Uncovering the mechanisms involved in the decline of long‐distance migrants remains one of the most pressing issues in European conservation. Since the 1980s, the British breeding population of Garden Warbler Sylvia borin has declined by more than 25%. Here we use data from repeated bird surveys of woodland sites in the 1980s and in 2003–2004 to show that, although the overall population declined between the two periods, the probability of occupancy for this species increased at high latitudes and decreased at low latitudes. Range shifts such as this arise from a change in the ratio of colonizations to extinctions at the range margins, and we therefore related colonization and local extinction at the patch level to concurrent changes in temperature and habitat. The probability of patch colonization by this species was significantly lower where the percentage cover of vegetation in the understorey had declined, reducing habitat quality for this species. The probability of local extinction was significantly correlated with increasing mean May temperature, which may reflect a change in phenology, making breeding conditions less suitable. Changed regimes of grazing and woodland management could be used to increase habitat suitability and thereby increase colonization probability at the local scale, which may in turn increase the probability of patch occupancy despite future climatic unsuitability.     

Optimization of growth conditions and fatty acid analysis for three freshwater diatom isolates

Diatoms are a group of highly abundant and diverse aquatic algae species. They contain high lipid content along with many bioactive compounds that can be exploited for biotechnological applications. Despite these attractive attributes, diatoms are underrepresented in production projects due to difficulties in their cultivation. To optimize the growth of three freshwater diatom isolates, Cyclotella sp., Synedra sp. and Navicula sp., an orthogonal assay on N, P, Si and Fe, as well as temperature and pH, was designed using traditional single‐factor tests. We also studied the effect of using nanosilica as an alternate Si source on growth and found that the diatom isolates studied achieved their highest growth rates under different combinations of nutrient and environmental conditions. Silica had the greatest influence on growth, followed by phosphate and iron. The optimized growth conditions for Synedra sp. were N: 30 mg L^?1, P: 3 mg L^?1, Si: 14.8 mg L^?1, Fe: 0.448 mg L^?1, temperature 25°C and pH 8. For Navicula sp.: N: 20 mg L^?1, P: 2.5 mg L^?1, Si: 19.7 mg L^?1, Fe: 0.112 mg L^?1, temperature 30°C and pH 7.5–8. For Cyclotella sp.: N: 20 mg L^?1, P: 2.5 mg L^?1, Si: 19.7 mg L^?1, Fe: 0.448 mg L^?1, temperature 30°C and pH 7.5–8. Nano silica negatively affected growth in Navicula sp. and Cyclotella sp., but no such effect was observed in Synedra sp. Fatty acid profiling showed C16:0, C16:1(n ? 7), C18:0 and C20:5(n ? 3) as major fatty acids, with no significant differences in fatty acid methyl ester profiles between traditional and modified media. This work gives us a new insight into the growth requirements of freshwater diatom species, which are less studied than marine species.