LOCALIZATION OF IRON WITHIN CENTRIC DIATOMS OF THE GENUS THALASSIOSIRA1

The cellular iron (Fe) quota of centric diatoms has been shown to vary in response to the ambient dissolved Fe concentration; however, it is not known how centric diatoms store excess intracellular Fe. Here, we use synchrotron X‐ray fluorescence (SXRF) element mapping to identify Fe storage features in cells of Thalassiosira pseudonana Hasle et Heimdal and Thalassiosira weissflogii G. A. Fryxell et Hasle grown at low and high Fe concentrations. Localized intracellular Fe storage features, defined as anomalously high Fe concentrations in regions of relatively low phosphorus (P), sulfur (S), silicon (Si), and zinc (Zn), were twice as common in T. weissflogii cells grown at high Fe compared to low‐Fe cells. Cellular Fe quotas of this strain increased 2.9‐fold, the spatial extent of the features increased 4.6‐fold, and the Fe content of the features increased 14‐fold under high‐Fe conditions, consistent with a vacuole storage mechanism. The element stoichiometry of the Fe features is consistent with polyphosphate‐bound Fe as a potential vacuolar Fe storage pool. Iron quotas increased 2.5‐fold in T. pseudonana grown at high Fe, but storage features contained only 2‐fold more Fe and did not increase in size compared to low‐Fe cells. The differences in Fe storage observed between T. pseudonana and T. weissflogii may have been due to differences in the growth states of the cultures.     

Response of the diatom flora in Jiaozhou Bay, China to environmental changes during the last century

The diatom flora in a 164 cm long sediment core obtained from Jiaozhou Bay (Yellow Sea, China) was analyzed in order to trace the response of diatoms to environmental changes over the past 100 years. The sediment core was dated by ²¹⁰Pb and ¹³⁷Cs and represented approximately 100 years (1899–2001 A.D.). The flora was mainly composed of centric diatoms (59–96%). The concentration of diatoms declined sharply above 30 cm (after ~ 1981 A.D.), while the dominant species changed from Thalassiosira anguste-lineatus, Thalassiosira eccentria, Coscinodiscus excentricus, Coscinodiscus concinnus and Diploneis gorjanovici to Cyclotella stylorum and Paralia sulcata. Species richness decreased slightly, and the cell abundance of warm-water species increased. We argue that these floral changes were probably caused by climate change in combination with eutrophication resulting from aquaculture and sewage discharge.     

THE EVOLUTION OF ELONGATE SHAPE IN DIATOMS1

Diatoms have been classified historically as either centric or pennate based on a number of features, cell outline foremost among them. The consensus among nearly every estimate of the diatom phylogeny is that the traditional pennate diatoms (Pennales) constitute a well‐supported clade, whereas centric diatoms do not. The problem with the centric–pennate classification was highlighted by some recent analyses concerning the phylogenetic position of Toxarium, whereby it was concluded that this “centric” diatom independently evolved several pennate‐like characters including an elongate, pennate‐like cell outline. We performed several phylogenetic analyses to test the hypothesis that Toxarium evolved its elongate shape independently from Pennales. First, we reanalyzed the original data set used to infer the phylogenetic position of Toxarium and found that a more thorough heuristic search was necessary to find the optimal tree. Second, we aligned 181 diatom and eight outgroup SSU rDNA sequences to maximize the juxtapositioning of similar primary and secondary structure of the 18S rRNA molecule over a much broader sampling of diatoms. We then performed a number of phylogenetic analyses purposely based on disparate sets of assumptions and found that none of these analyses supported the conclusion that Toxarium acquired its pennate‐like outline independently from Pennales. Our results suggest that elongate outline is congruent with SSU rDNA data and may be synapomorphic for a larger, more inclusive clade than the traditional Pennales.     

Grazing-induced changes in cell wall silicification in a marine diatom

In aquatic environments, diatoms (Bacillariophyceae) constitute a central group of microalgae which contribute to about 40% of the oceanic primary production. Diatoms have an absolute requirement for silicon to build-up their silicified cell wall in the form of two shells (the frustule). To date, changes in diatom cell wall silicification have been only studied in response to changes in the growth environment, with consistent increase in diatom silica content when specific growth rates decrease under nutrient or light limitations. Here, we report the first evidence for grazing-induced changes in cell wall silicification in a marine diatom. Cells grown in preconditioned media that had contained both diatoms and herbivores are significantly more silicified than diatoms grown in media that have contained diatoms alone or starved herbivores. These observations suggest that grazing-induced increase in cell wall silicification can be viewed as an adaptive reaction in habitats with variable grazing pressure, and demonstrate that silicification in diatoms is not only a constitutive mechanical protection for the cell, but also a phenotypically plastic trait modulated by grazing. In turn, our results corroborate the idea that plant-herbivore interactions, beyond grazing sensu stricto, contribute to drive ecosystem structure and biogeochemical cycles in the ocean.     

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.     

Molecular and morphological analyses of solitary forms of brackish Thalassiosiroid diatoms (Coscinodiscophyceae), with emphasis on their phenotypic plasticity

Blooms of centric diatoms are a common feature in the Bilbao estuary during summer when river flow is at its lowest and water temperature is above 20ºC. To gain insight into the specific composition of these diatom blooms, net samples and cultures of estuarine isolates were analysed under the scanning electron microscope (SEM) and by molecular analyses of the Internal Transcribed Spacers 1 and 2 plus the coding region 5.8S (ITS region) and the 28S rRNA gene. Seven species of solitary centric diatoms belonging to four genera were found in the estuary including: Conticribra weissflogii, Cyclotella atomus var. atomus, Cyclotella cryptica, Cyclotella marina, Cyclotella meneghiniana, Discostella pseudostelligera and Thalassiosira pseudonana. Dominant species during blooms were C. meneghiniana and Co. weissflogii in the upper estuary and D. pseudostelligera and T. pseudonana in the middle estuary. The morphological traits used to differentiate between species pairs of similar morphology (C. meneghiniana/C. cryptica or D. pseudostelligera/D. woltereckii) were observed to vary with environmental conditions, denoting a great deal of phenotypic plasticity which would hinder accurate identification of the species when using morphological approaches alone.     

UNEXPECTED RESPONSE TO VITAMIN B12 OF DOMINANT CENTRIC DIATOMS FROM THE SPRING BLOOM IN THE GULF OF MAINE (NORTHEAST ATLANTIC OCEAN)1,2

Twelve clones (seven species) representative of centric diatoms dominant in the spring phytoplankton bloom in the Gulf of Maine were isolated and rendered axenic. Genera included were Thalassiosira, Porosira and Chaetoceros. Unlike most centric diatoms studied previously, none of these has an absolute requirement for vitamin B_12. However, B₁₂ (5 ng.l^-1) stimulated growth of most clones by eliminating or reducing the lag phase and increasing the growth rate. Bloom population densities developed 4–54 days earlier with B₁₂ present. Several clones grown with B₁₂ removed more than 80% of the vitamin from the medium. When grown in vitamin-free medium the cells put 0.01–0.7 ng.l^-1 B₁₂ into the medium. We conclude that vitamin B₁₂ is of ecological significance even though the requirement for it is not absolute.     

Biomineralization by photosynthetic organisms: Evidence of coevolution of the organisms and their environment?

Biomineralization is widespread among photosynthetic organisms in the ocean, in inland waters and on land. The most quantitatively important biogeochemical role of land plants today in biomineralization is silica deposition in vascular plants, especially grasses. Terrestrial plants also increase the rate of weathering, providing the soluble substrates for biomineralization on land and in water bodies, a role that has had global biogeochemical impacts since the Devonian. The dominant photosynthetic biomineralizers in today's ocean are diatoms and radiolarians depositing silica and coccolithophores and foraminifera depositing calcium carbonate. Abiotic precipitation of silica from supersaturated seawater in the Precambrian preceded intracellular silicification dominated by sponges, then radiolarians and finally diatoms, with successive declines in the silicic acid concentration in the surface ocean, resulting in some decreases in the extent of silicification and, probably, increases in the silicic acid affinity of the active influx mechanisms. Calcium and bicarbonate concentrations in the surface ocean have generally been supersaturating with respect to the three common calcium carbonate biominerals through geological time, allowing external calcification as well as calcification in compartments within cells or organisms. The forms of calcium carbonate in biominerals, and presumably the evolution of the organisms that produce them, have been influenced by abiotic variations in calcium and magnesium concentrations in seawater, and calcium carbonate deposition has probably also been influenced by carbon dioxide concentration whose variations are in part biologically determined. Overall, there has been less biological feedback on the availability of substrates for calcification than is the case for silicification.     

Diatom elemental and morphological changes in response to iron limitation: a brief review with potential paleoceanographic applications

Diatoms are a major group of phytoplankton that account for approximately 40% of the ocean carbon fixation and the vast majority of biogenic silica production through the construction of their cell walls (termed frustules). These frustules accumulate and are partially preserved in the ocean sediments. Diatom growth and nutrient utilization in high‐nitrate, low‐chlorophyll regions of the world’s oceans are mostly regulated by iron availability. Diatoms acclimate to iron limitation by decreasing cell size. The associated increase in surface area‐to‐volume ratio and decrease in diffusive boundary layer thickness may improve nutrient uptake kinetics. In parallel, cellular silicon (Si) contents are elevated in iron‐limited diatoms relative to nitrogen (N) and carbon (C). Variations in degree of silicification and nutritional requirements of iron‐limited diatoms have been hypothesized to account for higher cellular Si and/or lower cellular N and C, respectively. However, in some diatoms, frustule silicification does not significantly change when cells are iron‐limited. Instead, changes in the Si‐containing valve surface area relative to volume within these diatoms is hypothesized to be responsible for the variations in the cellular Si : N and Si : C ratios. In particular, some examined iron‐limited pennate diatoms have reduced widths relative to their lengths (i.e. lower length‐normalized widths, LNW) compared to iron‐replete cells. In the pennate diatom Fragilariopsis kerguelensis, the mean LNWs of valves preserved in sediments throughout the Southern Ocean (a well‐characterized iron‐limited region) is positively correlated with satellite‐derived, climatological net primary productivity in the overlying waters. Because of the specific morphological changes in pennate diatom frustules in response to iron availability, the valve morphometerics (e.g. LNWs) can potentially be used as a diagnostic tool for iron‐limited diatom growth and relative changes in the Si : N (and Si : C) ratios in extant diatom assemblages as well as those preserved in the sediments.     

ELEVATED CARBON DIOXIDE DIFFERENTIALLY ALTERS THE PHOTOPHYSIOLOGY OF THALASSIOSIRA PSEUDONANA (BACILLARIOPHYCEAE) AND EMILIANIA HUXLEYI (HAPTOPHYTA)1

Increasing anthropogenic carbon dioxide is causing changes to ocean chemistry, which will continue in a predictable manner. Dissolution of additional atmospheric carbon dioxide leads to increased concentrations of dissolved carbon dioxide and bicarbonate and decreased pH in ocean water. The concomitant effects on phytoplankton ecophysiology, leading potentially to changes in community structure, are now a focus of concern. Therefore, we grew the coccolithophore Emiliania huxleyi (Lohmann) W. W. Hay et H. Mohler and the diatom strains Thalassiosira pseudonana (Hust.) Hasle et Heimdal CCMP 1014 and T. pseudonana CCMP 1335 under low light in turbidostat photobioreactors bubbled with air containing 390 ppmv or 750 ppmv CO₂. Increased pCO₂ led to increased growth rates in all three strains. In addition, protein levels of RUBISCO increased in the coastal strains of both species, showing a larger capacity for CO₂ assimilation at 750 ppmv CO₂. With increased pCO₂, both T. pseudonana strains displayed an increased susceptibility to PSII photoinactivation and, to compensate, an augmented capacity for PSII repair. Consequently, the cost of maintaining PSII function for the diatoms increased at increased pCO₂. In E. huxleyi, PSII photoinactivation and the counter‐acting repair, while both intrinsically larger than in T. pseudonana, did not change between the current and high‐pCO₂ treatments. The content of the photosynthetic electron transport intermediary cytochrome b6/f complex increased significantly in the diatoms under elevated pCO₂, suggesting changes in electron transport function.     

Diatom growth responses to photoperiod and light are predictable from diel reductant generation

Light drives phytoplankton productivity, so phytoplankton must exploit variable intensities and durations of light exposure, depending upon season, latitude, and depth. We analyzed the growth, photophysiology and composition of small, Thalassiosira pseudonana, and large, Thalassiosira punctigera, centric diatoms from temperate, coastal marine habitats, responding to a matrix of photoperiods and growth light intensities. T. pseudonana showed fastest growth rates under long photoperiods and low to moderate light intensities, while the larger T. punctigera showed fastest growth rates under short photoperiods and higher light intensities. Photosystem II function and content responded primarily to instantaneous growth light intensities during the photoperiod, while diel carbon fixation and RUBISCO content responded more to photoperiod duration than to instantaneous light intensity. Changing photoperiods caused species‐specific changes in the responses of photochemical yield (e^?/photon) to growth light intensity. These photophysiological variables showed complex responses to photoperiod and to growth light intensity. Growth rate also showed complex responses to photoperiod and growth light intensity. But these complex responses resolved into a close relation between growth rate and the cumulative daily generation of reductant, across the matrix of photoperiods and light intensities.     

SYNCHRONIZED GROWTH OF THALASSIOSIRA PSEUDONANA (BACILLARIOPHYCEAE) PROVIDES NOVEL INSIGHTS INTO CELL‐WALL SYNTHESIS PROCESSES IN RELATION TO THE CELL CYCLE1

Cell‐cycle effects in phytoplankton have both general and specific influences over a variety of cellular processes. Understanding these effects requires that the majority of cells in a culture are progressing through the same cell‐cycle stage, which requires synchronous growth. We report the development of a silicon starvation–recovery synchrony for the first diatom with a sequenced genome, Thalassiosira pseudonana Hasle et Heimdale, which provides several novel insights into the process of cell‐wall formation. After 24 h of silicate starvation, flow cytometry measurements indicated that 80% of the cells were arrested in the early G1 phase of the cell cycle and then upon silicate replenishment progressed synchronously through the cycle. An early G1‐arrest point was not previously documented in diatoms. After silicate replenishment, girdle‐band synthesis was confined to a particular period in G1, and cells did not lengthen in accordance with each girdle band added, which has implications related to cell growth and separation processes in diatoms. Measurements of silicic acid uptake, intracellular pools, and silica incorporation into the cell wall, coupled with fluorescence visualization of newly synthesized cell‐wall structures, provide the first direct measurements of silica amounts in individual girdle bands and valves in a diatom. Fluorescence imaging indicated why valves in T. pseudonana do not have to reduce in size with each generation and enabled visualization of intermediates in structure formation. The development of a synchrony procedure for T. pseudonana enables correlation of cellular events with the cell cycle, which should facilitate the use of genomic information.     

Miocene methane‐derived carbonates from southwestern Washington,USA and a model for silicification at seeps

Kuechler, R.R., Birgel, D, Kiel, S, Freiwald, A, Goedert, J.L., Thiel, V & Peckmann, J. 2011: Miocene methane‐derived carbonates from southwestern Washington, USA and a model for silicification at seeps. Lethaia, Vol. 45, pp. 259–273. Exotic limestone masses with silicified fossils, enclosed within deep‐water marine siliciclastic sediments of the Early to Middle Miocene Astoria Formation, are exposed along the north shore of the Columbia River in southwestern Washington, USA. Samples from four localities were studied to clarify the origin and diagenesis of these limestone deposits. The bioturbated and reworked limestones contain a faunal assemblage resembling that of modern and Cenozoic deep‐water methane‐seeps. Five phases make up the paragenetic sequence: (1) micrite and microspar; (2) fibrous, banded and botryoidal aragonite cement, partially replaced by silica or recrystallized to calcite; (3) yellow calcite; (4) quartz replacing carbonate phases and quartz cement; and (5) equant calcite spar and pseudospar. Layers of pyrite frequently separate different carbonate phases and generations, indicating periods of corrosion. Negative δ¹³C_carbonate values as low as ?37.6‰ V‐PDB reveal an uptake of methane‐derived carbon. In other cases, δ¹³C_carbonate values as high as 7.1‰ point to a residual, ¹³C‐enriched carbon pool affected by methanogenesis. Lipid biomarkers include ¹³C‐depleted, archaeal 2,6,10,15,19‐pentamethylicosane (PMI; δ¹³C: ?128‰), crocetane and phytane, as well as various iso‐ and anteiso‐carbon chains, most likely derived from sulphate‐reducing bacteria. The biomarker inventory proves that the majority of the carbonates formed as a consequence of sulphate‐dependent anaerobic oxidation of methane. Silicification of fossils and early diagenetic carbonate cements as well as the precipitation of quartz cement – also observed in other methane‐seep limestones enclosed in sediments with abundant diatoms or radiolarians – is a consequence of a preceding increase of alkalinity due to anaerobic oxidation of methane, inducing the dissolution of silica skeletons. Once anaerobic oxidation of methane has ceased, the pH drops again and silica phases can precipitate. □Biomarkers, carbonates, isotopes, methane, Miocene, silicification, Washington.     

New data on the flora of diatom algae (Centrophyceae) in waterbodies of Sakhalin Island

The first studies of phytoplankton in inland waterbodies of Sakhalin Island by electron microscopy have revealed 19 representatives of the class Centrophyceae, including some that are new for the flora of the region (Aulacoseira subarctica, Coscinodiscopsis commutata, Stephanodiscus delicatus, S. makarovae, Thalassiosira baltica, and T. cf. hyalina). Species of centric diatoms that are new for these waterbodies have been documented in lakes Vavaiskiye and Tunaicha, and the species composition of Centrophyceae have been revised. The first complete data on the taxonomical spectrum of Centrophyceae in Lake Sladkoye, lakes of Mount Spamberg, and the Tym River have been obtained. A revision of the species composition of Centrophyceae in the waterbodies of Sakhalin Island has been made, and the list of the class has been broadened to 44 taxa from 17 genera.     

AUXOSPORE FORMATION AND THE MORPHOLOGY OF THE INITIAL CELL OF THE MARINE ARAPHID DIATOM GEPHYRIA MEDIA (BACILLARIOPHYCEAE)1

Recent studies have led to a rapid increase in knowledge of auxospore formation in diatoms. However, these studies have been limited to centric and raphid pennate diatoms, and there is still very little information for the araphid pennate diatoms. Using LM and SEM, we studied the development of the auxospore and the initial cell of the marine epiphytic diatom Gephyria media Arnott. Auxospores were bipolar and curved in side view, as in many other pennate diatoms. SEM revealed many transverse perizonial bands, all of which were incomplete rings. There was an elongate, sprawling, silicified structure beneath the ventral suture of the transverse perizonial bands. This structure is presumably equivalent to the longitudinal perizonial band in other pennate diatoms, although we could not determine the homologous relationship between the two features. Scales were found both in the inner wall of the perizonium and around the primary perizonial bands. The presence or absence of scales may be of phylogenetic significance in diatoms, only during the final stages of auxospore formation because scales are found in early spherical stages. The distinctive finger‐like structures observed throughout all stage of G. media have not been observed before in the other diatom taxa.     

MOLECULAR EVIDENCE CONFIRMS SISTER RELATIONSHIP OF ARDISSONEA,CLIMACOSPHENIA, AND TOXARIUM WITHIN THE BIPOLAR CENTRIC DIATOMS (BACILLARIOPHYTA,MEDIOPHYCEAE), AND CLADISTIC ANALYSES CONFIRM THAT EXTREMELY ELONGATED SHAPE HAS ARISEN TWICE IN THE DIATOMS1

With the use of a new kit from Qiagen to amplify total genome quantity, DNA was bulked up from two diatoms that are difficult to grow (Ardissonea and Climacosphenia), and the nuclear SSU rRNA gene was successfully amplified. Results of Bayesian analyses showed that these diatoms are sister to Toxarium and belong to the bi‐ and multipolar centric diatoms. The results indicate that extremely elongate shape has arisen at least twice in diatoms, in the true pennates, and in the bipolar centrics. The two lateral pattern centers of Ardissonea and Climacosphenia likely represent a modified annulus that subtends ribs internally as well as externally. Studies of sexual reproduction are needed to determine whether Ardissonea, Climacosphenia, and Toxarium achieve their elongate shape by similar means to each other and to true pennates, that is, by controlling the expansion of the auxospores by sequential addition of silicified bands (to form a properizonium or perizonium).     

Long-term survival of marine planktonic diatoms and dinoflagellates in stored sediment samples

Sediment samples from Scottish coastal sites, taken over the last 9 years, were stored in closed containers at 5C. Slurry cultures were used to determine the survival of phytoplankton in these sediments. A range of diatom and dinoflagellate species survived for at least 27 months in these stored samples. A number of species grew for which no resting stage has yet been described: Thalassiosira angulata, T.pacifica, T.punctigera, T.eccentrica, T.minima and T.anguste-lineata. Notable results were survival times of 73 months for Skeletonema costatum, 96 months for Chaetoceros socialis, C.didymus and C.diadema, 109 months for Scrippsiella sp. and 112 months for Lingulodinium polyedrum. A single sample was stored and repeatedly cultured for diatoms over a period of 16 months. The number of species cultured from the sediment declined over this time. Lingulodinium polyedrum cysts isolated from sediments collected at least 18 months previously gave a hatching success of 97%, and cysts isolated from a 9-year-old sample gave a hatching success of 3%. The study indicated the potential importance of coastal sediments as a source of phytoplankton to their overlying waters. The validity of using marine planktonic diatoms and dinoflagellates for modelling geological events is discussed.      

The Role of Extracellular Polymeric Substances in the Silicification of Calothrix: Evidence from Microbial Mat Communities in Hot Springs at Yellowstone National Park,USA

We provide nanoscale evidence of the role of sheath exopolymers in the silicification of the sheathed cyanobacteria Calothrix. Electron microscope observations of silicified Calothrix cells revealed that silica accretes directly onto EPS sheath fibrils to produce an open web of silica particles that could remain permeable to nutrients and waste products. We also found that silicified Calothrix cells from different microhabitats contained morphologically distinct silica particles. Differences in silicification texture suggest that environmental variables may influence silicification at the nanoscale. We develop a framework based on aggregation kinetics to address silicification processes in Calothrix and other sheathed cyanobacteria.