EVOLUTIONARY ANALYSES OF THE NUCLEAR‐ENCODED PHOTOSYNTHETIC GENE psbO FROM TERTIARY PLASTID‐CONTAINING ALGAE IN DINOPHYTA1

Although the dinophytes generally possess red‐algal‐derived secondary plastids, tertiary plastids originating from haptophyte and diatom ancestors are recognized in some lineages within the Dinophyta. However, little is known about the nuclear‐encoded genes of plastid‐targeted proteins from the dinophytes with diatom‐derived tertiary plastids. We analyzed the sequences of the nuclear psbO gene encoding oxygen‐evolving enhancer protein from various algae with red‐algal‐derived secondary and tertiary plastids. Based on our sequencing of 10 new genes and phylogenetic analysis of PsbO amino acid sequences from a wide taxon sampling of red algae and organisms with red‐algal‐derived plastids, dinophytes form three separate lineages: one composed of peridinin‐containing species with secondary plastids, and the other two having haptophyte‐ or diatom‐derived tertiary plastids and forming a robust monophyletic group with haptophytes and diatoms, respectively. Comparison of the N‐terminal sequences of PsbO proteins suggests that psbO genes from a dinophyte with diatom‐derived tertiary plastids (Kryptoperidinium) encode proteins that are targeted to the diatom plastid from the endosymbiotic diatom nucleus as in the secondary phototrophs, whereas the fucoxanthin‐containing dinophytes (Karenia and Karlodinium) have evolved an additional system of psbO genes for targeting the PsbO proteins to their haptophyte‐derived tertiary plastids from the host dinophyte nuclei.     

Evolution and functional diversification of fructose bisphosphate aldolase genes in photosynthetic marine diatoms

Diatoms and other chlorophyll-c containing, or chromalveolate, algae are among the most productive and diverse phytoplankton in the ocean. Evolutionarily, chlorophyll-c algae are linked through common, although not necessarily monophyletic, acquisition of plastid endosymbionts of red as well as most likely green algal origin. There is also strong evidence for a relatively high level of lineage-specific bacterial gene acquisition within chromalveolates. Therefore, analyses of gene content and derivation in chromalveolate taxa have indicated particularly diverse origins of their overall gene repertoire. As a single group of functionally related enzymes spanning two distinct gene families, fructose 1,6-bisphosphate aldolases (FBAs) illustrate the influence on core biochemical pathways of specific evolutionary associations among diatoms and other chromalveolates with various plastid-bearing and bacterial endosymbionts. Protein localization and activity, gene expression, and phylogenetic analyses indicate that the pennate diatom Phaeodactylum tricornutum contains five FBA genes with very little overall functional overlap. Three P. tricornutum FBAs, one class I and two class II, are plastid localized, and each appears to have a distinct evolutionary origin as well as function. Class I plastid FBA appears to have been acquired by chromalveolates from a red algal endosymbiont, whereas one copy of class II plastid FBA is likely to have originated from an ancient green algal endosymbiont. The other copy appears to be the result of a chromalveolate-specific gene duplication. Plastid FBA I and chromalveolate-specific class II plastid FBA are localized in the pyrenoid region of the chloroplast where they are associated with β-carbonic anhydrase, which is known to play a significant role in regulation of the diatom carbon concentrating mechanism. The two pyrenoid-associated FBAs are distinguished by contrasting gene expression profiles under nutrient limiting compared with optimal CO2 fixation conditions, suggestive of a distinct specialized function for each. Cytosolically localized FBAs in P. tricornutum likely play a role in glycolysis and cytoskeleton function and seem to have originated from the stramenopile host cell and from diatom-specific bacterial gene transfer, respectively.     

Ribosomal RNA Analysis Indicates a Benthic Pennate Diatom Ancestry for the Endosymbionts of the Dinoflagellates Peridinium foliaceum and Peridinium balticum (Pyrrhophyta)

ABSTRACT. The establishment of chloroplasts as cellular organelles in the dinoflagellate, heterokont (stramenopile), haptophyte, and cryptophyte algae is widely accepted to have been the result of secondary endosymbiotic events, that is, the uptake of a photosynthetic eukaryote by a phagotrophic eukaryote. However, the circumstances that promote such associations between two phylogenetically distinct organisms and result in the integration of their genomes to form a single functional photosynthetic cell is unclear. The dinoflagellates Peridinium foliaceum and Peridinium balticum are unusual in that each contains a membrance-bound eukaryotic heterokont endosymbiont. These symbioses have been interpreted, through data derived from ultrastructural and biochemical investigations, to represent an intermediate stage of secondary endosymbiotic chloroplast acquistion. In this study we have examined the phylogenetic origin of the P. foliaceum and P. Balticum heterokont endosymbionts through analaysis of their nuclear small subunit ribosomal RNA genes. Our analyses clearly demonstrate both endosymbionts are pennate diatoms belonging to the family Bacillariaceae. Since members of the Bacillariaceae are usually benthic, living on shallow marine sediments, the manner in which establishment of a symbiosis between a planktonic flagellated dinoflagellate and a botton-dwelling diatom is discussed. In particular, specific environmentally associated life strategy stages of the host and symbiont, coupled with diatom food preferences by the dinoflagellate, may have been vital to the formation of this association.     

Stemming epigenetics in marine stramenopiles

Epigenetics include DNA methylation, the modification of histone tails that affect chromatin states, and small RNAs that are involved in the setting and maintenance of chromatin modifications. Marine stramenopiles (MAS), which are a diverse assemblage of algae that acquired photosynthesis from secondary endosymbiosis, include single-celled organisms such as diatoms as well as multicellular forms such as brown algae. The recent publication of two diatom genomes that diverged ~90 million years ago (mya), as well as the one of a brown algae that diverged from diatoms ~250 Mya, provide a great system of related, yet diverged set of organisms to compare epigenetic marks and their relationships. For example, putative DNA methyltransferase homologues were found in diatoms while none could be identified in the brown algal genome. On the other hand, no canonical DICER-like protein was found in diatoms in contrast to what is observed in brown algae. A key interest relies in understanding the adaptive nature of epigenetics and its inheritability. In contrast to yeast that lack DNA methylation, homogeneous cultures of diatoms constitute an attractive system to study epigenetic changes in response to environmental conditions such as nutrient-rich to nutrient-poor transitions which is especially relevant because of their ecological importance. P. tricornutum is also of outstanding interest because it is observed as three different morphotypes and thus constitutes a simple and promising model for the study of the epigenetic phenomena that accompany cellular differentiation. In this review we focus on the insights obtained from MAS comparative genomics and epigenomic analyses.     

Phylogenetic aspects of the sulfate assimilation genes from Thalassiosira pseudonana

Diatoms are unicellular algae responsible for approximately 20 % of global carbon fixation. Their evolution by secondary endocytobiosis resulted in a complex cellular structure and metabolism compared to algae with primary plastids. In the last years the interest on unicellular algae increased. On the one hand assessments suggest that diatom-mediated export production can influence climate change through uptake and sequestration of atmospheric CO₂. On the other hand diatoms are in focus because they are discussed as potential producer of biofuels. To follow the one or other idea it is necessary to investigate the diatoms biochemistry in order to understand the cellular regulatory mechanisms. The sulfur assimilation and methionine synthesis pathways provide S-containing amino acids for the synthesis of proteins and a range of metabolites such as dimethylsulfoniopropionate (DMSP) in order to provide basic metabolic precursors needed for the diatoms metabolism. To obtain an insight into the localization and organization of the sulfur metabolism pathways, the genome of Thalassiosira pseudonana—a model organism for diatom research—might help to understand the fundamental questions on adaptive responses of diatoms to dynamic environmental conditions such as nutrient availability in a broader context.     

DIATOM PRODUCTIVITY COMPARED TO OTHER ALGAE IN NATURAL MARINE PHYTOPLANKTON ASSEMBLAGES1,2

Total productivity rates and the component attributed to the diatom proportion in the crop were measured in natural water samples taken from the Scripts Pier (La Jolla, California) at ca, monthly intervals in 1975–76. Two other measurements were done in water samples taken from Saanich Intel, Vancouver Islands, British Columbia (Canada). The percent of photosynthesis by diatoms was compared with the proportion of diatom carbon in the crop. In all instances, percent diatom photosynthesis was higher than percent diatom carbon for dialoms were higher than those of other algae. It is hypothesized that, in the sea, diatoms can outcompete other algae. However, the assemblages consist of some tens of species coexisting, and no portion of the crop out competes the others to exclusion.     

Novel fluorescent dyes based on oligopropylamines for the in vivo staining of eukaryotic unicellular algae

Weakly basic fluorescent dyes are used to visualize organelles within live cells due to their affinity to acidic subcellular organelles. In particular, they are used to stain the silica deposited in the silica deposition vesicles (SDVs) of diatoms during the course of their frustule synthesis. This study involved the synthesis of fluorescent dyes derived from oligopropylamines, compounds similar to those found in diatoms. The dyes were obtained by reacting oligopropylamines with 4-chloro-7-nitrobenzo-2-oxa-1,3-diazole. The reaction was realized using methylated oligopropylamines with two or three nitrogen atoms and yielded two novel fluorescent dyes: NBD-N2 and NBD-N3. The dyes appeared to be highly efficient in the in vivo staining of growing siliceous frustules of diatoms at concentrations at least 10 times lower than those required for staining with HCK-123. NBD-N3 also efficiently stained other subcellular vesicles of eukaryotic unicellular algae. NBD-N2 stained only growing diatom frustules, whereas NBD-N3 also stained various subcellular organelles of different eukaryotic unicellular algae. NBD-N2 and NBD-N3 were not removed from stained diatom frustules by drastic treatments with H₂SO₄ and H₂O₂. Fluorescent silica can also be obtained by its chemical precipitation in the presence of NBD-N2 and NBD-N3.     

Molecular identification of sequestered diatom chloroplasts and kleptoplastidy in foraminifera

Kleptoplastidy is the ability of heterotrophic organisms to preserve chloroplasts of algal preys they eat and partially digest. As the sequestered chloroplasts stay functional for months, the "host" becomes photosynthetically active. Although remaining a marginal process, kleptoplastidy was observed in different protist lineages, including foraminifera. Previous studies showed at least eight species of the foraminiferal genera Haynesina and Elphidium grazing on diatoms and husbanding their chloroplasts. In order to characterize more precisely the origin of kleptochloroplasts in these genera, we obtained 1027 chloroplastic 16S rDNA sequences from 13 specimens of two Haynesina and five Elphidium species. We identified the foraminiferal kleptochloroplasts using a reference phylogeny made of 87 chloroplastic sequences of known species of diatoms and brown algae. All the analyzed specimens were performing kleptoplastidy and according to our phylogenetic analyses they seem to retain exclusively chloroplasts of diatom origin. There is no apparent specificity for the type of diatom from which chloroplasts originated, however some foraminiferal species seem to accept a wider range of diatoms than others. Possibly the diversity of kleptochloroplasts depends on the type of diatoms the foraminiferans feed on.     

Dinoflagellates,diatoms, and their viruses

Since the first discovery of the very high virus abundance in marine environments, a number of researchers were fascinated with the world of "marine viruses", which had previously been mostly overlooked in studies on marine ecosystems. In the present paper, the possible role of viruses infecting marine eukaryotic microalgae is enlightened, especially summarizing the most up-to-the-minute information of marine viruses infecting bloom-forming dinoflagellates and diatoms. To author's knowledge, approximately 40 viruses infecting marine eukaryotic algae have been isolated and characterized to different extents. Among them, a double-stranded DNA (dsDNA) virus "HcV" and a single-stranded RNA (ssRNA) virus "HcRNAV" are the only dinoflagellate-infecting (lytic) viruses that were made into culture; their hosts are a bivalve-killing dinoflagellate Heterocapsa circularisquama. In this article, ecological relationship between H. circularisquama and its viruses is focused. On the other hand, several diatom-infecting viruses were recently isolated and partially characterized; among them, one is infectious to a pen-shaped bloom-forming diatom species Rhizosolenia setigera; some viruses are infectious to genus Chaetoceros which is one of the most abundant and diverse diatom group. Although the ecological relationships between diatoms and their viruses have not been sufficiently elucidated, viral infection is considered to be one of the significant factors affecting dynamics of diatoms in nature. Besides, both the dinoflagellate-infecting viruses and diatom-infecting viruses are so unique from the viewpoint of virus taxonomy; they are remarkably different from any other viruses ever reported. Studies on these viruses lead to an idea that ocean may be a treasury of novel viruses equipped with fascinating functions and ecological roles.     

Complex repeat structures and novel features in the mitochondrial genomes of the diatoms Phaeodactylum tricornutum and Thalassiosira pseudonana

The mitochondrial genome of the raphid pennate diatom Phaeodactylum tricornutum has several novel features compared with the mitochondrial genomes of the centric diatom Thalassiosira pseudonana and the araphid pennate diatom Synedra acus. It is almost double the size (77,356 bp) due to a 35,454 bp sequence block consisting of an elaborate combination of direct repeats, making it the largest stramenopile (heterokont) mitochondrial genome known. In addition, the cox1 gene has a +1 translational frameshift involving Pro codons CCC and CCT, the first translational frameshift to be detected in an algal mitochondrial genome. The nad9 and rps14 genes are fused by the insertion of an in-frame sequence and cotranscribed. The nad11 gene is split into two parts corresponding to the FeS and molybdate-binding domains, but both parts are still on the mitochondrial genome, in contrast to the brown algae where the second domain appears to have been transferred to the nucleus. In contrast to P. tricornutum, the repeat region of T. pseudonana consists of a much smaller 4790 bp string of almost identical double-hairpin elements, evidence of slipped-strand mispairing and active gene conversion. The diatom mitochondrial genomes have undergone considerable gene rearrangement since the three lineages of diatoms diverged, but all three have kept their repeat regions segregated from their relatively compact coding regions.     

CHARACTERIZATION OF A cDNA ENCODING GLUTAMINE SYNTHETASE FROM THE MARINE DIATOM SKELETONEMA COSTATUM (BACILLARIOPHYCEAE)

A cDNA-encoding glutamine synthetase (GS) was isolated from the marine diatom Skeletonema costatum (Greville) Cleve by PCR amplification. Nucleic acid and deduced amino acid sequences of the diatom GS were greater than 50% identical to GS from green algae and vascular plants, and phylogenetic analysis established the diatom GS as a member of the GSII gene family. The presence of an N-terminus signal sequence, identified on the basis of sequence similarity with other chloroplast-localized proteins from diatoms, suggests that the encoded GS isoenzyme is localized to the chloroplast. The GS mRNA was present in log-phase cells grown with either nitrate or ammonium as the sole added nitrogen source. Results from Southern blot analysis of genomic DNA suggested that the cDNA isolated in this study was either a member of a small, highly conserved gene family or that there was allelic variation within the region examined. Phylogenetic analyses further indicated that genes encoding GS from the diatom and two species of green algae diverged prior to the gene duplication, to the isoenzymes in vascular plants, supporting the hypothesis that GS isoenzymes in diatoms, green algae, and vascular plants arose through independent evolutionary events.     

Supramolecular organization of fucoxanthin–chlorophyll proteins in centric and pennate diatoms

Fucoxanthin–chlorophyll proteins (FCP) are the major light-harvesting proteins of diatom algae, a major contributor to marine carbon fixation. FCP complexes from representatives of centric (Cyclotella meneghiniana) and pennate (Phaeodactylum tricornutum) diatoms were prepared by sucrose gradient centrifugation and studied by means of electron microscopy followed by single particle analysis. The oligomeric FCP from a centric diatom were observed to take the form of unusual chain-like or circular shapes, a very unique supramolecular assembly for such antennas. The existence of the often disputed oligomeric form of FCP in pennate diatoms has been confirmed. Contrary to the centric diatom FCP, pennate diatom FCP oligomers are very similar to oligomeric antennas from related heterokont (Stramenopila) algae. Evolutionary aspects of the presence of novel light-harvesting protein arrangement in centric diatoms are discussed.     

Phytoplankton aggregate formation: observations of patterns and mechanisms of cell sticking and the significance of exopolymeric material

Flocculation of ‘sticky’ phytoplankton cells intorapidly sinking aggregates has been invoked as a mechanism explainingmass sedimentation of phytoplankton blooms in the ocean. Phytoplanktonstickiness, defined as the probability of adhesion upon collision,is one key factor determining the potential for aggregate formation.In the laboratory, we examined variation in stickiness in fivespecies of diatoms and two species of flagellates grown in batchcultures. We also investigated the production of paniculatemucus by phytoplankton cells and its role in aggregate formation,and we studied the effects of solute exudates on cell stickiness.Four of the five diatoms investigated were significantly sticky,while one diatom and both of the flagellates were not sticky.Stickiness varied considerably within species. In the diatomSkeletonema costatum, the typical but not entirely consistentpattern was that stickiness decreased with age of the batchcultures. We were otherwise unable to establish consistent relationshipsbetween cell stickiness and the growth stage of the algae, environmentalconcentrations of inorganic nutrients, and abundances of suspendedand epiphytic bacteria. We showed that the diatom S.costatumat times excretes a solute substance that depresses flocculation.This may reduce cell losses from the euphotic zone during thegrowth phase due to flocculation and sedimentation. We demonstratedtwo different mechanisms of phytoplankton aggregate formation.In the diatom S.costatum, the cells are sticky in themselves,and coagulation depends on cell-cell sticking and does not involvemucus. Aggregates are composed solely of cells. Cells of thediatom Chaetoceros affinis, on the other hand, are not in themselvessticky. Transparent exopolymeric particles (TEP), produced bythe diatom, cause the cells to aggregate and coagulation dependson TEP-cell rather than cell-cell sticking. Aggregates are formedof a mixture of mucus and cells. We found several species ofdiatoms and one flagellate species to produce copious amountsof TEP. TEP from some species (e.g. Coscinodiscus sp.) is stickyand may cause other, non-sticky particles to coagulate. Thisemphasizes the potential importance of diatom-derived paniculatemucus for particle flocculation in the ocean.     

Possible role of horizontal gene transfer in the colonization of sea ice by algae

Diatoms and other algae not only survive, but thrive in sea ice. Among sea ice diatoms, all species examined so far produce ice-binding proteins (IBPs), whereas no such proteins are found in non-ice-associated diatoms, which strongly suggests that IBPs are essential for survival in ice. The restricted occurrence also raises the question of how the IBP genes were acquired. Proteins with similar sequences and ice-binding activities are produced by ice-associated bacteria, and so it has previously been speculated that the genes were acquired by horizontal transfer (HGT) from bacteria. Here we report several new IBP sequences from three types of ice algae, which together with previously determined sequences reveal a phylogeny that is completely incongruent with algal phylogeny, and that can be most easily explained by HGT. HGT is also supported by the finding that the closest matches to the algal IBP genes are all bacterial genes and that the algal IBP genes lack introns. We also describe a highly freeze-tolerant bacterium from the bottom layer of Antarctic sea ice that produces an IBP with 47% amino acid identity to a diatom IBP from the same layer, demonstrating at least an opportunity for gene transfer. Together, these results suggest that the success of diatoms and other algae in sea ice can be at least partly attributed to their acquisition of prokaryotic IBP genes.     

Spectral fluorescence signatures in the characterization of phytoplankton community composition

Fluorescence spectral signatures from 28 algal cultures aredescribed.The cultures are split into four groups accordingto their accessory pigments. Phycocyanin and phycoerythrin,characteristic pigments of cyanobacteria, form groups I andII. The characteristic pigment found in group III is chlorophyllb (green and rasinophyte algae) and in group IV it is chlorophyllc (diatoms, dinophytes and some other algae).This preliminarycatalogue of spectral signatures was used to characterize fivenatural phytoplankton communities from brackish water environmentsas a comparison with phytoplankton species found in the samples.Accessory pigments such as phycocyanin and phycoerythrin, characterizinggroups I and II, can be used for identification without confusion.Distinguishing between groups III and IV is more complicated,because their accessory pigments do not have their own fluorescence.These groups can be characterized by increased fluorescenceof chlorophyll a induced by energy excited through chlorophyllb and c. The possibilities of applying the spectral fluorescencesignatures approach to the characterization of natural communitiesare discussed.     

Mitochondrial genome of a tertiary endosymbiont retains genes for electron transport proteins

Mitochondria and plastids originated through endosymbiosis, and subsequently became reduced and integrated with the host in similar ways. Plastids spread between lineages through further secondary or even tertiary endosymbioses, but mitochondria appear to have originated once and have not spread between lineages. Mitochondria are also generally lost in secondary and tertiary endosymbionts, with the single exception of the diatom tertiary endosymbiont of dinoflagellates like Kryptoperidinium foliaceum, where both host and endosymbiont are reported to contain mitochondria. Here we describe the first mitochondrial genes from this system: cytochrome c oxidase 1 (cox1), cytochrome oxidase 3 (cox3), and cytochrome b (cob). Phylogenetic analyses demonstrated that all characterized genes were derived from the pennate diatom endosymbiont, and not the host. We also demonstrated that all three genes are expressed, that cox1 contains spliced group II introns, and that cob and cox3 form an operon, all like their diatom relatives. The endosymbiont mitochondria not only retain a genome, but also express their genes, and are therefore likely involved in electron transport. Ultrastructural examination confirmed the endosymbiont mitochondria retain normal tubular cristae. Overall, these data suggest the endosymbiont mitochondria have not reduced at the genomic or functional level.     

西江下游浮游植物群落周年变化模式

西江肇庆段是珠江干流汇入珠三角河网水域的咽喉通道,对2009年该江段的浮游植物群落组成及变化进行系统阐析.调查期间共发现浮游植物7门,245种(包括变种、变型),其中硅藻和绿藻是浮游植物群落组成的最主要类群,分别占总种数的42.44％和34.69％,其次是裸藻和蓝藻.PCA分析结果显示,浮游植物各类群的种类丰富度和生物量的周年变化趋势主要受水温和径流量的影响.浮游植物种类丰富度的周年变化呈现明显的高温季节高,低温季节低的特征,除因大多数藻类物种直接喜好高温之外,径流量的增大有助于真浮游绿藻种类的外源注入及半浮游和偶然性浮游硅藻种类的增加.浮游植物生物量的周年变化呈现明显的双峰型,峰值分别出现在8月和11月.然而8月份第1个高峰出现之前,生物量波动不大,尽管水温的升高有助于生物量的增加,但是径流量增大所带来的稀释作用掩盖了水温上升对浮游植物生长的促进作用.真浮游植物生物量的变化趋势和数值与总浮游植物极其接近,主要得益于真浮游硅藻物种颗粒直链藻在总种群中的绝对优势地位.综上,水温升高对浮游植物种类丰富度和生物量的增长均有促进,径流量的增大虽然有助于种类丰富度的增加,但不利于生物量的增加.     

Metabolomic Profiling of 13 Diatom Cultures and Their Adaptation to Nitrate-Limited Growth Conditions

Diatoms are very efficient in their use of available nutrients. Changes in nutrient availability influence the metabolism and the composition of the cell constituents. Since diatoms are valuable candidates to search for oil producing algae, measurements of diatom-produced compounds can be very useful for biotechnology. In order to explore the diversity of lipophilic compounds produced by diatoms, we describe the results from an analysis of 13 diatom strains. With the help of a lipidomics platform, which combines an UPLC separation with a high resolution/high mass accuracy mass spectrometer, we were able to measure and annotate 142 lipid species. Out of these, 32 were present in all 13 cultures. The annotated lipid features belong to six classes of glycerolipids. The data obtained from the measurements were used to create lipidomic profiles. The metabolomic overview of analysed cultures is amended by the measurement of 96 polar compounds. To further increase the lipid diversity and gain insight into metabolomic adaptation to nitrogen limitation, diatoms were cultured in media with high and low concentrations of nitrate. The growth in nitrogen-deplete or nitrogen-replete conditions affects metabolite accumulation but has no major influence on the species-specific metabolomic profile. Thus, the genetic component is stronger in determining metabolic patterns than nitrogen levels. Therefore, lipid profiling is powerful enough to be used as a molecular fingerprint for diatom cultures. Furthermore, an increase of triacylglycerol (TAG) accumulation was observed in low nitrogen samples, although this trend was not consistent across all 13 diatom strains. Overall, our results expand the current understanding of metabolomics diversity in diatoms and confirm their potential value for producing lipids for either bioenergy or as feed stock.     

Changes in diatom-dominated biofilms during simulated improvements in water quality: implications for diatom-based monitoring in rivers

Although benthic diatoms are used to assess river water quality, there are few data on the rate at which diatom assemblages react to changes in water quality. The aim of this study was to assess the reaction time of diatoms and to discuss the changes occurring during water quality improvement on the basis of their autecological characteristics. In order to simulate this improvement, diatom-dominated biofilms grown on artificial sandstone substrata were transferred from several polluted rivers to an unpolluted river. They were sampled three times: before transfer and 1 and 2 months after transfer. The ecology and growth-forms of the taxa explained most of the changes in species composition observed during the experiment. Adnate diatoms gradually replaced motile and stalked taxa. Gomphonema parvulum, a stalked diatom positioned vertically in the biofilm, is adapted for light and space competition in high-density algal biofilms. When transferred to an unpolluted site, this growth-form is less competitive and does not tolerate the high grazing pressure. Fistulifera saprophila is a single celled motile diatom, living in organic matrices. When the artificial substrata were transferred to the unpolluted site, this particular ecological niche disappeared quickly. On the other hand, Achnanthidium minutissimum, which is considered to be cosmopolitan and an early colonizer, increased during the first month of transfer and then decreased. It was gradually replaced by A. biasolettianum, which was the taxon best suited to this pristine stream. The changes observed differed between treatments depending on the species composition and architecture of the biofilms. In particular, biofilms dominated by stalked and motile diatoms were more quickly modified than those dominated by small motile diatoms. The diatom index reflects these changes, and its values showed that about 60 days following a water quality improvement were necessary for transferred diatom assemblages to reach diatom index values similar as those at the unpolluted river.