Growth limitation of three Arctic sea ice algal species: effects of salinity, pH, and inorganic carbon availability

The effect of salinity, pH, and dissolved inorganic carbon (TCO₂) on growth and survival of three Arctic sea ice algal species, two diatoms (Fragilariopsis nana and Fragilariopsis sp.), and one species of chlorophyte (Chlamydomonas sp.) was assessed in controlled laboratory experiments. Our results suggest that the chlorophyte and the two diatoms have different tolerance to fluctuations in salinity and pH. The two species of diatoms exhibited maximum growth rates at a salinity of 33, and growth rates at a salinity of 100 were reduced by 50% compared to at a salinity of 33. Growth ceased at a salinity of 150. The chlorophyte species was more sensitive to high salinities than the two diatom species. Growth rate of the chlorophyte was greatly reduced already at a salinity of 50 and it could not grow at salinities above 100. At salinity 33 and constant TCO₂ concentration, all species exhibited maximal growth rate at pH 8.0 and/or 8.5. The two diatom species stopped growing at pH > 9.5, while the chlorophyte species still was able to grow at a rate which was 1/3 of its maximum growth rate at pH 10. Thus, Chlamydomonas sp. was able to grow at high pH levels in the succession experiment and therefore outcompeted the two diatom species. Complementary experiments indicated that growth was mainly limited by pH, while inorganic carbon limitation only played an important role at very high pH levels and low TCO₂ concentrations.     

The diversity of relationships between Antarctic sponges and diatoms: the case of Mycale acerata Kirkpatrick, 1907 (Porifera,Demospongiae)

The diatom assemblage associated with the Antarctic sponge Mycale acerata was studied through an analysis of the diatom frustule and pigment concentrations in both the sponge ectosome and choanosome. Sponges were sampled weekly from November 2001 to February 2002 at Terra Nova Bay, Antarctica, at a depth of 25–35 m. The most abundant diatoms were Porannulus contentus, Fragilariopsis curta, Thalassiosira cf. gracilis, T. perpusilla and Plagiotropis sp. High abundances of P. contentus were found on the sponge ectosome up to the beginning of November, before the ice melted, while later frustules were incorporated inside, indicating that P. contentus lives epibiontically on M. acerata and represents a potential food source for the sponge. The presence of other diatom species was mainly related to the summer phytoplankton bloom. The sponge incorporates diatoms from the water column and utilises them as a food source, accumulating frustules inside the choanosome. The lack of planktonic diatom frustules at the beginning of the summer indicates that they are expelled or dissolved during the cold season.     

Short-term variations in particulate matter sedimentation off Kapp Norvegia,Weddell Sea,Antarctica: relation to water mass advection,ice cover,plankton biomass and feeding activity

Summary A multi-cup sediment trap was deployed at 250m in the shelf area off Kapp Norvegia, Weddell Sea (630 m water depth) to determine the relative importance of water mass advection, sea ice movement, phytoplankton biomass and plankton feeding. Short-term fluctuations in sedimentation were determined using a sampling frequency of 2.7 days over 54 days during January and February 1988. Three periods of enhanced sedimentation were associated with water mass exchange, settling of diatoms following break-up of ice cover and release of fecal matter by krill feeding on particulate matter derived from phytoplankton and ice algae. An initial sedimentation pulse (28 Jan) was mainly due to sinking pelagic diatoms and krill fecal strings containing algae released from sea ice passing over the trap position. The ¹³C-composition of the sedimented organic carbon was about-24. The isotope ratio decreased sharply by about 5.5 at the end of the first pulse indicating the source of sinking matter becoming pelagic diatoms of the retreating ice-edge. At this time the diatom Corethron criophilum contributed a very high proportion of the organic flux causing an increase of the opal/C_org ratios. The second pulse (6 Feb) was due to empty diatom frustules, minipellets and small planktonic aggregates. Much of the organic carbon was transported by round fecal pellets. During the third pulse (14 Feb), round fecal pellets transported even more; the percentage of C. criophilum to the diatom organic carbon flux was more than 80% (>2mg C m^–2 day^–1).Data presented here were collected during the European Polarstern Study (EPOS) sponsored by the European Science Foundation     

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.     

EVIDENCE OF AN ORGANIC MATRIX FROM DIATOM BIOSILICA1

Most biominerals appear to be composites of organic material and mineral. Whether biosilica is such a composite is unresolved because of a lack of evidence for such organic components. We present evidence that organic material exists within diatom biosilica and can be extracted using HF/NH₄F solutions from frustules isolated from Cyclotella meneghiniana Kütz and diatomaceous earth. To eliminate organic casing on the silicified frustules as a source of organic materials, the casing was removed by oxidation of frustules with NaOCl before extraction. The removal of the casing was confirmed in that oxidized frustules no longer displayed the ability to be stained with ruthenium red and fluorescamine. Frustules examined with EDXA showed an emission peak from sulfur before treatment but no peak following treatment, indicating that oxidation removed organic sulfur. The organic material obtained from extracts of fresh frustules contained both soluble and insoluble components. Only soluble material was evident in extracts from diatomaceous earth. The soluble material appears to contain glycoproteins with relatively high levels of serine and glycine. The soluble proteins from fresh frustules also appear to be phosphorylated. Indirect evidence is presented that suggests the soluble proteins may contain regions of primary structure enriched in anionic amino acids. The soluble extracts differ from general cell contents when the two fractions are compared, suggesting that frustules contain specialized organic material. The identification of silica-specific organic material suggests that mineralization in diatoms may be in part matrix-mediated.     

MORPHOLOGICAL FLEXIBILITY OF COCCONEIS PLACENTULA (BACILLARIOPHYCEAE) NANOSTRUCTURE TO CHANGING SALINITY LEVELS1

Diatoms possess a silica frustule decorated with unique patterns of nanosize features. Here, we show for the first time from in situ samples that the size of the nanopores present at the surface of the diatom Cocconeis placentula Ehrenb. varies with fluctuating salinity levels. The observed reduction in nanopore size with decreasing salinity agrees with previous laboratory experiments. We also uniquely combined our observations with theoretical considerations to demonstrate that the decrease in the diffusive layer thickness is compensated for by the changes in pore size, which maintain a steady diffusive flux toward the diatom’s cell at different salinities. This process allows diatoms to absorb similar amount of nutrients whatever the salinity and as such to increase their ecological competitiveness in fluctuating environments. These results further suggest that the overall ecological success of diatoms, and their ability to react to environmental changes, may be controlled by the flexibility of the morphological characteristics of their frustules.     

Luminescence properties of a nanoporous freshwater diatom

Freshwater diatom frustules show special optical properties. In this paper we observed luminescence properties of the freshwater diatom Cyclotella meneghiniana. To confirm the morphological properties we present scanning electron microscopy (SEM) images. X‐ray diffraction (XRD) studies were carried out to visualize the structural properties of the frustules, confirming that silica present in diatom frustules crystallizes in an α‐quartz structure. Study of the optical properties of the silica frustules of diatoms using ultra‐violet‐visible (UV‐vis) spectroscopy and photoluminescence spectroscopy confirmed that the diatom C. meneghiniana shows luminescence in the blue region of the electromagnetic spectrum when irradiated with UV light. This property of diatoms can be exploited to obtain many applications in day‐to‐day life. Also, using time‐resolved photoluminescence spectroscopy (TRPL) it was confirmed that this species of diatom shows bi‐exponential decay. Copyright © 2011 John Wiley & Sons, Ltd.     

GROWTH OF VITAMIN B12-REQUIRING MARINE DIATOMS IN MIXED LABORATORY CULTURES WITH VITAMIN B12-PRODUCING MARINE BACTERIA12

The vitamin B₁₂ requirement of several marine diatoms can be satisfied in B₁₂^?limited laboratory cultures by heterotrophic marine bacteria isolated from the same waters and from sediments. The bacteria can utilize diatom excretory products, or the remains of dead diatom cells, in the production of the vitamin. The growth of 12 B₁₂¹? requiring diatoms (7 genera) in mixed cultures with 14 different bacteria (without added B₁₂) was compared to the growth of those same diatoms in axenic cultures with excess added B₁₂. Diatom growth was generally rapid in the first few days, followed by sustained, slower growth. The diatom yields in mixed cultures ranged from 0.8 to 84% of the yields in axenic cultures with added B₁₂. In a detailed study of one mixed culture, increases in diatom densities were paralleled by increases in cell densities of the bacterium during the first few days of exponential diatom growth. During the period of slow diatom growth, when diatom densities oscillated but steadily increased, the decreases in diatom densities were associated with increased bacterial growth. This suggests that death of a fraction of the B₁₂-limited diatom population releases sufficient organic matter to stimulate growth of the bacteria and their subsequent excretion of B₁₂; this B₁₂ in turn stimulates further growth of the diatoms. Diatom-bacteria interactions leading to the production of B₁₂ may be important in maintaining viable populations of B₁₂-requiring diatoms in nutrient-poor waters during periods between blooms, when conditions are unfavorable for rapid growth.     

Prevalence of chytrid parasitism among diatom populations in the lower Columbia River (2009–2013)

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DISTRIBUTION OF INTERTIDAL DIATOMS ASSOCIATED WITH SEDIMENTS IN YAQUINA ESTUARY,OREGON1,2

Sediment samples and environmental data were collected from November 1973 to August 1974 to analyze the distribution of sediment-associated diatom assemblages relative to vertical, horizontal, and seasonal gradients in Yaquina Estuary, Oregon. The distribution of diatoms was regulated primarily by mean salinity and characteristics of the sediment, i.e., mean sediment size and percentage of organic carbon. Prominent taxa in the river above Yaquina Bay exhibited overlapping distributions along the salinity gradient to a location in brackish water where the mean salinity was approximately 5%_o. At this salinity, a relatively sharp discontinuity in the diatom flora existed which appeared to represent the biochemical and biophysical mechanisms involved in the osmotic regulation of fresh- and brackish-water diatoms. Relatively large disparities m the structure of diatom assemblages were found within relatively small areas of Yaquina Bay. These differences were attributed to properties of the sediment. Differences in diatom assemblages relative to variations in light intensity, water temperature and exposure to intertidal emergence were not apparent from the analysis of field data.     

The Probable Mechanism for Silicon Capture by Diatom Algae: Assimilation of Polycarbonic Acids with Diatoms—Is Endocytosis a Key Stage in Building of Siliceous Frustules?

Many organisms including unicellular (diatoms, radiolaria, and chrysophytes), higher plants (rice and horsetail) and animals (sponges) use silica as a main part of skeletons. The bioavailable form of silicon is silicic acid and the mechanism of silicic acid penetration into living cells is still an enigma. Macropinocytosis was assumed as a key stage of the silicon capture by diatoms but assimilation of monomeric silicic acid by this way requires enormous amounts of water to be passed through the cell. We hypothesized that silicon can be captured by diatoms via endocytosis in the form of partially condensed silicic acid (oligosilicates) whose formation on the diatom surface was supposed. Oligosilicates are negatively charged nanoparticles and similar to coils of poly(acrylic acid) (PAA). We have synthesized fluorescent tagged PAA as well as several neutral and positively charged polymers. Cultivation of the diatom Ulnaria ferefusiformis in the presence of these polymers showed that only PAA is able to penetrate into siliceous frustules. The presence of PAA in the frustules was confirmed with chromatography and PAA causes various aberrations of the valve morphology. Growth of U. ferefusiformis and two other diatoms in the presence of tri- and tetracarbonic fluorescent tagged acids points to the ability of diatoms to recognize substances that bear four acidic groups and to include them into siliceous frustules. Thus, partial condensation of silicic acid is a plausible first stage of silicon assimilation.     

Nanoscale uniformity of pore architecture in diatomaceous silica: a combined small and wide angle x-ray scattering study

Combined small and wide angle X‐ray scattering (SAXS and WAXS) analysis was applied to purified biogenic silica of cultured diatom frustules and of natural populations sampled on marine tidal flats. The overall WAXS patterns did not reveal crystalline phases (WAXS domain between 0.07 to 0.5 nm) in this biogenic silica, which is in line with previous reports on the amorphous character of the SiO₂ matrix of diatom frustules. One exception was the silica of the pennate species Cylindrotheca fusiformis Reimann et Lewin, which revealed wide peaks in the WAXS spectra. These peaks either indicate the presence of a yet unknown crystalline phase with a repetitive distance (d‐value ≈0.06 nm) or are caused by the ordering of the fibrous silica fragments; numerous girdle bands. The SAXS spectra revealed the size range of pores (diameter d between 3.0 and 65 nm), the presence of distinct pores (slope transitions), and structure factors (oscillation of the spectra). All slopes varied in the range of ?4.0 to ?2.5, with two clear common regions among species: d < 10 nm (slopes –4, denoted as region I and also called the Porod region), and 10.0 < d < 40.0 nm (slopes ?2.9 to ?3.8, denoted as region II). The existence of these common regions suggests the presence of comparable form (region I) and structure (region II) factors, respectively the shape of the primary building units of the silica and the geometry of the pores. Contrast variation experiments using dibromomethane to fill pores in the SiO₂ matrix showed that scattering was caused by pores rather than silica particles. Electron microscopic analysis confirmed the presence of circular, elliptical, and rectangular pores ranging in size from 3 to 65 nm, determining the structure factor. The fine architecture (length/width ratio of pore diameters) and distribution of the pores, however, seemed to be influenced by environmental factors, such as the salinity of and additions of AlCl₃ to the growth medium. The results indicate that diatoms deposit silica with pores <50 nm in size and are highly homologous with respect to geometry. Consequently, it is suggested that in diatoms, whether pennate or centric, the formation of silica at a nanoscale level is a uniform process.     

Organic geochemical studies of modern microbial mats from Shark Bay: Part I: Influence of depth and salinity on lipid biomarkers and their isotopic signatures

The present study investigated the influence of abiotic conditions on microbial mat communities from Shark Bay, a World Heritage area well known for a diverse range of extant mats presenting structural similarities with ancient stromatolites. The distributions and stable carbon isotopic values of lipid biomarkers [aliphatic hydrocarbons and polar lipid fatty acids (PLFAs)] and bulk carbon and nitrogen isotope values of biomass were analysed in four different types of mats along a tidal flat gradient to characterize the microbial communities and systematically investigate the relationship of the above parameters with water depth. Cyanobacteria were dominant in all mats, as demonstrated by the presence of diagnostic hydrocarbons (e.g. n‐C₁₇ and n‐C_17:1). Several subtle but important differences in lipid composition across the littoral gradient were, however, evident. For instance, the shallower mats contained a higher diatom contribution, concordant with previous mat studies from other locations (e.g. Antarctica). Conversely, the organic matter (OM) of the deeper mats showed evidence for a higher seagrass contribution [high C/N, ¹³C‐depleted long‐chain n‐alkanes]. The morphological structure of the mats may have influenced CO₂ diffusion leading to more ¹³C‐enriched lipids in the shallow mats. Alternatively, changes in CO₂ fixation pathways, such as increase in the acetyl COA‐pathway by sulphate‐reducing bacteria, could have also caused the observed shifts in δ¹³C values of the mats. In addition, three smooth mats from different Shark Bay sites were analysed to investigate potential functional relationship of the microbial communities with differing salinity levels. The C_25:1 HBI was identified in the high salinity mat only and a lower abundance of PLFAs associated with diatoms was observed in the less saline mats, suggesting a higher abundance of diatoms at the most saline site. Furthermore, it appeared that the most and least saline mats were dominated by autotrophic biomass using different CO₂ fixation pathways.     

Effect of bacteria from the bottom water layer of lake baikal on degradation of diatoms

Among isolates of the eubacteria Brevundimonas bullata, Sphingomonas rhizogenes, Agrobacterium tumefaciens, Bacillus simplex, Acinetobacter johnsonii, Methylobacterium adhaesivum, and Deinococcus aquaticus isolated from the bottom layer on the medium, where the only source of organic matter was hydrolysate from diatoms, the activity of hydrolytic enzymes, such as proteases, β-xylosidase, β-glucosidase, β-galactosidase, chitobiase, and amylase, is revealed. An algicidal effect and degradation of siliceous diatom frustules are recorded in algal-bacterial cultures. These data show that the bottom water layer of Lake Baikal is inhabited by potential participants of silicon cycle, which degrade siliceous valves of diatoms.     

Ocean acidification stimulates particulate organic carbon accumulation in two Antarctic diatom species under moderate and high natural solar radiation

Impacts of rising atmospheric CO₂ concentrations and increased daily irradiances from enhanced surface water stratification on phytoplankton physiology in the coastal Southern Ocean remain still unclear. Therefore, in the two Antarctic diatoms Fragilariopsis curta and Odontella weissflogii, the effects of moderate and high natural solar radiation combined with either ambient or future pCO₂ on cellular particulate organic carbon (POC) contents and photophysiology were investigated. Results showed that increasing CO₂ concentrations had greater impacts on diatom physiology than exposure to increasing solar radiation. Irrespective of the applied solar radiation regime, cellular POC quotas increased with future pCO₂ in both diatoms. Lowered maximum quantum yields of photochemistry in PSII (F_v/F_m) indicated a higher photosensitivity under these conditions, being counteracted by increased cellular concentrations of functional photosynthetic reaction centers. Overall, our results suggest that both bloom‐forming Antarctic coastal diatoms might increase carbon contents under future pCO₂ conditions despite reduced physiological fitness. This indicates a higher potential for primary productivity by the two diatom species with important implications for the CO₂ sequestration potential of diatom communities in the future coastal Southern Ocean.     

SILICON UPTAKE BY ALGAE WITH NO KNOWN Si REQUIREMENT. I. TRUE CELLULAR UPTAKE AND pH-INDUCED PRECIPITATION BY PHAEODACTYLUM TRICORNUTUM (BACILLARIOPHYCEAE) AND PLATYMONAS SP. (PRASINOPHYCEAE)1

Phaeodactylum tricornutum Bohlin, the one diatom known to lack a silicon requirement for growth, and the prasinophyte Platymonas sp. are two representatives of a taxonomically diverse group of planktonic algae that have been reported to take up Si without a demonstrable requirement for the element. For both species, removal of Si from solution during growth in batch culture has at least two components; true biological uptake throughout the growth of the culture, and spontaneous inorganic precipitation of a solid silicate phase–probably Mg₂Si₃O₈ (sepiolite)–under the elevated pH conditions that prevail late in batch growth. It is not clear to what extent previous observations of Si uptake by algae without siliceous frustules may be influenced by inorganic, non-cellular precipitation. The kinetics of true cellular uptake of Si are similar in Phaeodalylum and Platymonas, and different from those reported for the Si-requiring diatoms. Uptake follows hyperbolic saturation kinetics in both species, with half-saturation concentrations of 97.4 μM in Phaeodactylum and 80.9 μM in Platymonas, as compared to ca. 1–6 μM in diatoms that form siliceous frustules. Uptake by Phaeodactylum and Platymonas is not substrate-saturated until the dissolved Si concentration of the medium exceeds 200 μM. Concentrations this high do not occur in the surface layer of the ocean, and the kinetics suggest that both species deposit much less silica in nature than they can be induced to deposit in culture.     

Diversity and distribution of epibiotic bacteria on Pseudo-nitzschia multiseries (Bacillariophyceae) in culture, and comparison with those on diatoms in native seawater

Previous studies on the interaction between bacteria and harmful algal bloom species have mostly considered the bacteria in the bulk solution. Here, we document the abundance and mode of attachment of bacteria growing on the cell surface of the domoic acid-producing diatom Pseudo-nitzschia multiseries (Hasle) Hasle in culture, compared with diatoms in field samples. The epiphytic bacteria were examined by scanning electron microscopy to visualize their morphology and mode of attachment. Two P. multiseries cultures were studied: clone CLN-1 and sub-clone CLN-1-NRC; the latter had been maintained in another laboratory for 2 years. Each of these P. multiseries cultures exhibited a clearly different assemblage of epibiotic bacteria, even though both originated from the same parent culture. The bacterial diversity was greater in clone CLN-1 (nine distinct morphotypes seen) than in sub-clone CLN-1-NRC (six morphotypes). The former clone also produced more domoic acid than the latter. There was a succession of bacterial morphotypes as well as an increase in the number of epiphytic bacteria per diatom cell during the progression from exponential to stationary phase. The most diverse and common morphotypes were rod-shaped cells (e.g. a Caulobacter-like bacterium attached by a discoid holdfast). Epibionts showed a preference for attachment at specific regions of the host diatom frustule, e.g. the raphe or cingulum, locations where organic matter may be extruding from the diatom cell. Most diatom cells carried only one to five bacteria, and up to ca. 60% of the intact diatom cells (although intact cells themselves were infrequent) were still free of epibiotic bacteria at the end of the 31-day batch culture experiment. Sequencing of the SSU rRNA gene showed that five of the eight bacterial strains isolated from the P. multiseries cultures were members of the Alphaproteobacteria, three of the Gammaproteobacteria and one of the Bacteroidetes. A morphologically diverse assemblage of epibiotic bacteria was also found on both centric and pennate planktonic diatoms in natural coastal waters. Of the eight morphotypes recorded, all but two were also found in the cultures. Relatively fewer wild diatom cells carried bacteria compared to cells in culture. We hypothesize that the diversity and abundance of epiphytic bacteria may explain some of the variability seen in the production of DA by different P. multiseries clones, and should be considered as another important and controllable variable that influences diatom cell physiology.     

CYTOPLASMIC MASSES PRESERVED IN EARLY HOLOCENE DIATOMS: A POSSIBLE TAPHONOMIC PROCESS AND ITS PALEO‐ECOLOGICAL IMPLICATIONS1

In Lake Suigetsu, central Japan, greenish/light‐brown granules identified as cytoplasmic masses had been preserved in siliceous cell walls of freshwater diatoms in annual layers of lacustrine muds since the early Holocene. The lacustrine muds consisted of alternating dark‐colored (rich in diatom valves, clay, and organic matter) and light‐colored (mainly diatom valves) laminae. The greenish/light‐brown granules were predominately preserved in frustules of the genus Aulacoseira preserved in the dark‐colored laminae. The dark‐colored laminae were inferred to have formed annually under stratified water caused by surface water warming in summer that caused the formation of an organic‐rich anoxic layer on the lake bottom that favored granule preservation. The good preservation of cytoplasmic masses in dark‐colored laminae suggested a cause for diatom assemblage periodicity, a phenomenon that was commonly noted in temperate lakes: the cells containing these masses could be potential seed stocks for subsequent spring blooms. Frustules of the most abundant granule‐containing species, Aulacoseira nipponica (Skvortzow) Tuji, in the dark‐colored laminae of the Early Holocene muds were abundant in the overlying light‐colored laminae, suggesting that these species reproduced abundantly in springtime yielding a massive diatom bloom.     

Purification of biosilica from living diatoms by a two-step acid cleaning and baking method

Due to their sustainability, intact cell walls, availability of pure cultures, and others, living diatoms show a lot of promise for the application in various fields in particular for micro/nano-devices. In order to purify the biosilica structures of diatoms called frustules, a two-step acid cleaning and baking method was employed. By this path, organic matter and inorganic impurities can be removed very effectively. In addition, the highest quality of frustules was achieved when the samples were cleaned in an excess of boiling 10~15 % HCl and subsequently heated to 600 °C at a heating rate of 3 °C min^?1 for 6 h. In our operation, the native frustule morphology was maintained completely, and dry frustules with more than 90 % SiO₂ in weight can be obtained, and furthermore, the surface area of them reached a good value of 48.47 m² g^?1.     

Dynamics of diatom colonization process in some rivers influenced by urban pollution (Hanoi, Vietnam)

Periphytic diatom communities’ colonization patterns were studied at three sampling stations of the Red–Nhue–Tolich hydrosystem presenting different urban pollution levels by using artificial substrates for 6 weeks in dry season 2005. Structural characteristics of periphytic diatoms developed on glass substrates at each sampling site were followed and compared. This experiment showed, through various general criteria (total diatom density, dry weight biomass) and specific criteria (relative diatom abundances, indices) that the structure of benthic diatoms developed on these substrates was strongly affected by pollution as early as the second week of colonization. Communities exposed to heavily and moderately polluted sites contained species which are known to be saprophilous or tolerant to organic pollution such as Nitzschia umbonata, Nitzschia palea, Cyclotella meneghiniana, Eolimna minima. Growth inhibition of diatom communities at the heavily polluted site was mostly related to a strong increase of organic load rather than to low metallic input, though metallic (Cd and Zn) burdens have been successfully quantified in the biofilms developed at the three studied sites. Nevertheless, no significant difference in species richness and diversity index between colonization duration times was observed. Based on values of diatom indices Indice de Polluosensibilité (IPS) and diatom assemblage index to organic water pollution (DAIPo), water quality could be classified as medium quality at Red site, polluted at NT₂ and heavily polluted at TL. Thus, the use of diatoms as a tool for water assessment appears suitable for monitoring rivers in Vietnam, as it is in several other countries.