Centric diatom algae of the Selenga River and its delta branches

Different stages of the study of taxonomical composition of centric diatoms in the Selenga River are considered. A revision of the species composition and a general systematic list are made. New for the Selenga River diatom species are discovered. The role of small-sized centric diatoms in different years and vegetation periods is assessed. A comparison of the composition and the level of development of centric diatoms in 1958–1959 and 2002–2005 is made.     

BORON AS A GROWTH REQUIREMENT FOR DIATOMS1

A boron requirement has been shown for 12 species of marine pennate diatoms, 4 species of marine centric diatoms, and S freshwater diatom species. It can be concluded that boron is essential for the growth of most, probably all, diatoms. It is much easier to demonstrate a requirement for the marine species than for the freshwater species. Some species of marine algal flagellates also require boron for growth; others apparently do not.     

Glycerol 1,2-cyclic phosphate in centric diatoms. Observation by 31P NMR in vivo, isolation, and structural determination

We have isolated and identified glycerol 1,2-cyclic phosphate as the compound responsible for a unique prominent resonance at 19.1 ppm in the 31P NMR spectrum in vivo of four species of centric diatoms, where it is responsible for 15 to 30% of the total signals from organophosphates. This appears to be the first observation of this compound as a major metabolite. It was not detectable in nine other species of algae, including four species of pennate diatoms.     

The use of outdoor phytoplankton continuous cultures to analyse factors influencing species selection

Natural phytoplankton populations have been grown in outdoor continuous cultures at three dilution rates (D = 0.5, 0.25, and 0.1 · day^?1) under nitrogen (N) or silicon (Si) limitation and two light intensities. At a high specific nutrient flux (high dilution rate) under N limitation an assemblage of primarily small, fast growing centric diatoms such as Skeletonema costatum (Grev.) Cleve and Chaetoceros spp. dominated with a low percentage of flagellates. At a low specific nutrient flux, a mixture of larger, slower growing centric diatoms, small flagellates, and pennate diatoms was obtained. Similar trends were observed under silicate limitation. Decreasing the light intensity at the lowest dilution rate selected for an assemblage similar to that observed at the high dilution rate and high light intensity.The results of these competition experiments suggest that specific nutrient flux (dilution rate) is an important factor in determining between group dominance (e.g., centric and pennate diatoms and small flagellates). Successful competitors representing broad phytoplankton groups can be arranged along a resource gradient of specific nutrient flux (dilution rate), with groups such as centric and pennate diatoms, represented as high and medium flux species, respectively.     

Size scaling of extracellular carbonic anhydrase activity in centric marine diatoms

Many microalgae have a surface‐associated extracellular carbonic anhydrase (eCA) that converts HCO₃^? to CO₂ for uptake and subsequent photosynthetic fixation. We investigated eCA activity and assessed its importance for photosynthetic CO₂ supply in six centric diatom species spanning nearly the full range of cell sizes for centric diatoms (equivalent spherical radius 3–67 μm). Since larger cells are more susceptible to diffusion limitation, we hypothesized that eCA activity would increase with cell size as would its importance for CO₂ supply. eCA activity did increase with cell size, increasing with cell radius by a size‐scaling exponent of 2.6 ± 0.3. The rapid increase in eCA activity with cell radius keeps the absolute CO₂ concentration difference between bulk seawater and the cell surface very low (<~0.2 μM) allowing high rates of CO₂ uptake even for large diatoms. Although inhibiting eCA did reduce photosynthesis in the diatoms, there was no overall relationship between the extent of inhibition of photosynthesis and cell size. The only indication that eCA may be more important for larger diatoms was that photosynthesis in the smallest diatoms (<4 μm radius) was only affected by eCA inhibition when CO₂ concentrations were very low, while photosynthesis in some larger diatoms was affected even at typical seawater CO₂ concentrations. eCA is ubiquitous in centric marine diatoms, in contrast to other taxa where its presence is irregularly distributed among different species, and plays an important role in supplying CO₂ for photosynthesis across the size spectrum.     

九寨沟长海中心纲硅藻的分类学研究及报道1个新种

研究报道了采自中国四川九寨沟自然保护区长海的4种中心类硅藻种类, 其中新种1个: 长海小环藻(Cyclotella changhai sp. nov.), 中国新记录种1个: 克里特小环藻(C. cretica)和2种较少报道的种类: 可辨小环藻(C. distinguenda)和辐纹琳达藻(Lindavia radiosa)。通过光学显微镜和扫描电子显微镜对这4个种类形态学的特征进行观察, 并与相似种类进行比较。与长海小环藻相似的种类包括C. comensis Grunow, C. pseudocomensis Scheffler和C. costei Druart & Straub, 主要通过壳面中央区的不同形态进行区分-长海小环藻的中央区近平滑且具有不规则的凹陷。     

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.     

Cell size trade-offs govern light exploitation strategies in marine phytoplankton

Marine phytoplankton show complex community structures and biogeographic distributions, the net results of physiological and ecological trade-offs of species responses to fluctuating, heterogeneous environments. We analysed photosynthesis, responses to variable light and macromolecular allocations across a size panel of marine centric diatoms. The diatoms have strong capacities to withstand and exploit fluctuating light, when compared with picophytoplankton. Within marine diatoms, small species show larger effective cross-sections for photochemistry, and fast metabolic repair of photosystem II after photoinactivation. In contrast, large diatoms show lower susceptibility to photoinactivation, and therefore incur lower costs to endure short-term exposures to high light, especially under conditions that limit metabolic rates. This size scaling of key photophysiological parameters thus helps explain the relative competitive advantages of larger versus smaller species under different environmental regimes, with implications for the function of the biogenic carbon pump. These results provide a mechanistic framework to explain and predict shifts in marine phytoplankton community size structure with changes in surface irradiance and mixed layer depth.     

Genotypic and phenotypic variation in diatom silicification under paleo‐oceanographic conditions

Diatoms have co‐evolved with the silicon cycle and are largely responsible for reducing surface concentrations of silicate in the ocean to their present levels. We quantify silicification in marine diatoms at a range of high silicate concentrations representative of environments found over their geological history. The species examined include Stephanopyxis turris, an ancient centric species found throughout the Cenozoic, Thalassiosira pseudonana and Thalassiosira weissflogii, two younger centric species, and two pennate ecotypes of Staurosirella pinnata isolated from different nutrient regimes. Frustule thickness and micromorphological structure are strongly affected by silicate concentration. All species become increasingly silicified with silicate concentrations at concentrations vastly in excess of surface ocean concentrations today. In contrast, the half‐saturation constant for silicate uptake for most modern diatoms is below 2 μm . Based on the results, we hypothesize that silicate uptake is multiphasic in diatoms and that multiple silicate transport systems may have evolved in response to decreases in surface silicate concentration over geological time. The oldest species examined is more heavily silicified than the more modern species, presumably reflecting the conditions under which it originated. Yet diversification in silicification can be rapid, as illustrated by greater silicification in onshore versus the offshore ecotype of the same modern species. This work suggests that silicification of fossil frustules may eventually provide a paleoproxy for surface silicate concentrations over the Cenozoic, although development of species‐specific calibrations will be necessary and the effects of a range of environmental conditions must be investigated.     

Evolution of centric diatoms inferred from patterns of oogenesis and spermatogenesis

Centric diatoms have three patterns of oogenesis and four patterns of spermatogenesis, yielding a combination of seven patterns. The evolution of these combinations was inferred from differences in character states of the meiotic patterns observed in oogenesis and spermatogenesis. Furthermore, we have inferred the evolutionary relationships among centric diatoms based on the evolution of these combinations, the data on occurrence of combinations and a molecular phylogenetic tree. Our results suggested that the last common ancestor of extant diatoms was a bipolar or multipolar centric diatom with the most primitive among extant oogenetic/spermatogenic combinations. Our results also suggested that ancestors of the radial centrics and pennates would have diverged, in that order, from a bipolar or multipolar centric diatom with the most primitive among extant oogenetic/spermatogenic combinations.     

Les assemblages de diatomées du bassin messinien de Boudinar (Maroc nord-oriental)

The Messinian is one of the strongest biogenic silica accumulation periods in the world and more particularly in the Mediterranean, where it is marked by an important diatomitic sedimentation. In the Boudinar basin (Morocco north-Eastern, Western Mediterranean), the section of Sidi Haj Youssef, localised near the volcano of Ras Tarf, has approximately 100 m thickness of clayey-marly series in which 12 diatomitic levels of variable thickness are intercalated. The microfloristic study of diatoms on 86 samples, carried out in detail for the first time, made it possible to recognize 50 genus of diatoms (24 of centric and 26 of pennate) represented by 185 species (75 species of centric and 110 species of pennate). Four hundred individuals were taken from each sample to determine the relative frequency of each taxon within the diatoms assemblages. Thus, several assemblages were defined by the predominance of the following species: Coscinodiscusmarginatus, Actinoptychussenarius, Thalassionemanitzschioides, Actinocycluscurvatulus, Thalassiothrixlongissima, Rhizosoleniastyliformis and Actinocyclusehrenbergii. These diatoms assemblages display a littoral marine environment in communication with the opened sea. The abundance of the cold water species towards the base and the top of the section suggests broad exchanges of the basin with the Atlantic Ocean in Messinian. The predominance of the species Thalassionemanitzschioides and/or Thalassiothrixlongissima indicates periods of strong productivity that can be associated to upwelling systems.     

Comparing radiolarian and diatom diversity and abundance from the northeast Pacific

Particle trap samples (VERTEX) from 4 locations in the northeast Pacific were used to compare centric and pennate diatoms with nassellarian and spumellarian radiolarians. The proportion of pennates and nassellarians were highest at VERTEX 3 from the northeast tropical oceans, off the west coast of Mexico. Colonial radiolarians were characteristic of warm, high saline waters from the southern edge of the north Pacific central gyre, north of Hawaii (VERTEX 4), while pennate diatoms were abundant. The subtropic oceanic Pacific (VERTEX 5A) had more pennate diatoms, while VERTEX (5C), in a coastal cyclonic eddy located west of California, had an even proportion of centric and pennate diatoms. Comparison of major faunas and flora at various oceanic sites of present siliceous sedimentation in conjunction with atmospheric, hydrographic and chemical data, can be helpful in reconstructing paleoceanographic and paleoenvironmental conditions of the past oceans throughout the Phanerozoic.     

The species composition and distribution of diatom algae in sphagnum bogs in European Russia: The Polistovo-Lovatskii land tract

The species composition of diatom algae in the Polistovo-Lovatskii sphagnum tract (Rdeiskoye bog, Novgorod oblast) \are studied. The systematic and ecological-geographical analyses of the flora are performed. A total of 256 species and intraspecific taxa differing in ecological specificities and frequencies of occurrence in samples are revealed. A considerable number of centric diatoms is found. It is shown that primary and secondary diatom communities are formed in large sphagnum tracts. The horizontal heterogeneity of diatom communities in sphagnum bog is revealed. The species diversities of diatom algae in different bogs are compared.     

Dominant diatoms in Tjeukemeer

Diatoma elongatum has been observed in Tjeukemeer (The Netherlands) from time to time. Its intermittent presence is probably a consequence of the local hydrology. In early 1972, besides the usual increase of Melosira spp. and of unicellular centric diatoms, a development of D. elongatum took place. Its maximum was found 5-6 weeks later than those of the centric diatoms. Possible factors causing the decrease in number of cells are discussed.     

Distance and Character-Based Evaluation of the V4 Region of the 18S rRNA Gene for the Identification of Diatoms (Bacillariophyceae)

DNA barcoding is a molecular tool that exploits a unique DNA sequence of a standardized gene or non-coding region for the species identification of unknown individuals. The investigation into a suitable barcode for diatoms is ongoing and there are several promising candidates including mitochondrial, plastidial and nuclear markers. We analyzed 272 sequences from 76 diatoms species in the orders Thalassiosirales, Lithodesmiales and Cymatosirales, using distance and character based approaches, to assess the applicability of a DNA barcode based on the hypervariable V4 region of the nuclear 18S rRNA gene. We show that the proposed V4 barcode separated ca. 97% of all centric diatom taxa tested using a threshold p-distance of 0.02 and that many problem pairs were further separated using a character based approach. The reliability of amplification, extensive reference library and variability seen in the V4 region make it the most promising candidate to date for a barcode marker for diatoms particularly when combined with DNA character analysis.     

PHYLOGENETIC ANALYSIS OF THE MARINE AND FRESHWATER THALAS-SIOSIROID DIATOMS

The thalassiosiroid centric diatoms are distinguished by at least one synapomorphy, the strutted process or fultoportula. Variously classified as a family (Thalassiosiraceae) or an order (Thalassiosirales) among centric diatoms, it is generally conceded that the group of several hundred fossil and living species is monophyletic as a whole. There are two ecological groups of thalassiosiroids, marine and freshwater. It has been hypothesized, based on an ecletic, non-rigorous, evolutionary taxonomy perspective that both the marine and freshwater ecological groups are also monophyletic, but this hypothesis has never been tested in a rigorous framework. Likewise, the freshwater thalassiosiroid species have been grouped into several genera and subgenera using an evolutionary taxonomic approach, but these hypotheses have not fully been tested using cladistic analysis. Focusing mainly on freshwater species, but including at least one representative of each marine genus and one representative from each of several proposed subgeneric groupings of the genus Thalassiosira , we scored morphological characters for fossil and living marine and freshwater Thalassiosiraceae to test these hypotheses. Our cladistic results provide strong support for monophyly for the freshwater group, but it seems unlikely that the marine group is monophyletic. The cladistic results are corroborated to greater or lesser degrees by the fossil record. The implications for evolution in the group and for taxon sampling in molecular studies we are conducting will be discussed.     

Warming promotes cold-adapted phytoplankton in temperate lakes and opens a loophole for Oscillatoriales in spring

The effects of the recent warming trend in many northern temperate lakes on the species composition of spring phytoplankton remain poorly understood, especially because a recent change in nutrients has complicated efforts, and previous studies have defined spring according to the calendar. We analysed data from 1979 to 2004 from Lake Müggelsee (Berlin, Germany), using physical and biological parameters to define the spring period. We show that a change in timing of spring plankton events in warm years led to the paradox of lower mean water temperatures during the growth period, favouring cold-adapted diatoms over cyanobacteria, and within the diatoms, some cold-adapted centric forms over pennate forms. Under high P : Si ratios, the increased time between phytoplankton and cladoceran peaks opened a loophole for filamentous cyanobacteria (Oscillatoriales) in warm years to establish dominance after the diatoms, which are silicate limited. Therefore, the warming trend promotes filamentous cyanobacteria, a well-known nuisance in eutrophic lakes, and surprisingly, cold-adapted diatoms.     

Quantification of diatoms in biofilms: Standardisation of methods

Abstract

Benthic diatoms, which often dominate marine biofilms are mostly pennate along with a few centric species that have an attached mode of life. Even though the range of diatoms in biofilms is diverse, their ecology is poorly understood because of the difficulty in sampling and enumeration. Scraping or brushing are the traditional methods used for removal of diatoms from biofilms developed on solid substrata. The method of removal is the most critical step in enumerating the biofilm diatom community structure. In this study, a nylon brush and ceramic scraper were used as tools for the removal of diatoms from 1 – 4-day-old biofilms developed on fibreglass coupons and glass microscope slides. Standardisation of methods showed that the sample volume used in the analyses had the least influence on the quantification, whereas the method of removal was critical. The nylon brush was more efficient at recovering diatoms compared to a ceramic scraper. Direct microscopic enumeration of the community in the case of glass slides indicated that scraping resulted in between 30–50% underestimation. Heterogeneity in diatom community structure between replicate samples is one possible reason for such underestimation.     

Quantification of diatoms in biofilms: standardisation of methods

Benthic diatoms, which often dominate marine biofilms are mostly pennate along with a few centric species that have an attached mode of life. Even though the range of diatoms in biofilms is diverse, their ecology is poorly understood because of the difficulty in sampling and enumeration. Scraping or brushing are the traditional methods used for removal of diatoms from biofilms developed on solid substrata. The method of removal is the most critical step in enumerating the biofilm diatom community structure. In this study, a nylon brush and ceramic scraper were used as tools for the removal of diatoms from 1 - 4-day-old biofilms developed on fibreglass coupons and glass microscope slides. Standardisation of methods showed that the sample volume used in the analyses had the least influence on the quantification, whereas the method of removal was critical. The nylon brush was more efficient at recovering diatoms compared to a ceramic scraper. Direct microscopic enumeration of the community in the case of glass slides indicated that scraping resulted in between 30-50% underestimation. Heterogeneity in diatom community structure between replicate samples is one possible reason for such underestimation.