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.     

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.     

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.     

SPORE FORMATION IN THE LIFE CYCLES OF THE DIATOMS CHAETOCEROS DIADEMA AND LEPTOCYLINDRUS DANICUS1

A nitrogen limitation technique elicited the entire life cycle of the marine centric diatoms Chaetoceros diadema (Ehr.) Gran and Leptocylindrus danicus Cleve. In C. diadema the sexual cycle followed the same pattern as in the previously investigated C. didymus. Sexuality took place in narrow diameter cells, only at 2 and 5° C, and was seldom seen. Resting spore formation took place in cells of all sizes and at all temperatures at which the species grew vegetatively (2–15° C). The L. danicus life cycle is probably unique among diatoms. Nitrogen depletion induced sexuality in the entire culture at 10 and 15° C if the cell diameter was narrow (3–8 μm). Auxospore formation was followed by resting spore formation directly within the auxospore. In C. diadema, as in most centric diatoms, resting spores are not an obligate part of the life cycle, but they are in L. danicus. Resting spore formation is a versatile adaptive response in C. diadema, depending only on nitrogen depletion, although promoted by low temperatures. In L. danicus the linkage to the sexual process sharply limits conditions under which resting spores can form.     

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).     

A checklist of algal species found in the East Siberian Sea in May 1987

Summary A checklist of microalgae found in sea-ice in the East Siberian Sea in May 1987 has been compiled. A total of 122 taxa have been identified, consisting of 101 taxa in 18 genera of pennate diatoms, 14 taxa in 8 genera of centric diatoms, 4 taxa in 3 genera of dinoflagellates (including Ebria sp.), and 2 taxa in 1 genus of chrysophytes (silicoflagellates); choanoflagellates (Craspedomonadales) were found in one sample.     

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.     

Spatial and temporal dynamics of phytoplankton communities along a regulated river system, the Nakdong River, Korea

The longitudinal distribution and seasonal fluctuation of phytoplankton communities was studied along the middle to lower part of a regulated river system (Nakdong River, Korea). Phytoplankton biomass decreased sharply in the middle part of the river (182 km upward the estuary dam), and then increased downstream reaching a maximum at the last sampling station (27 km upward the estuary dam). In contrast, there was little downstream fluctuation in species composition, irrespective of pronounced differences in nutrient concentrations (TN, TP, NO₃, NH₄, PO₄) as well as in algal biomass. In the main river channel, small centric diatoms (Stephanodiscus hantzschii, Cyclotella meneghiniana) and pennate diatoms (Synedra, Fragilaria, Nitzschia) were dominant from winter to early spring (November–April). A mixed community of cryptomonads, centric and pennate diatoms, and coenobial greens (Pediastrum, Scenedesmus) was dominant in late spring (May–June). Blue-green algae (Anabaena, Microcystis, Oscillatoria) were dominant in the summer (July–September). A mid-summer Microcystis bloom occurred at all study sites during the dry season, when discharge was low, though the nutrient concentration varied in each study site. Nutrients appeared everywhere to be in excess of algal requirement and apparently did not influence markedly the downstream and seasonal phytoplankton compositional differences in this river.     

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.     

Spermatogenesis and auxospore structure in the multipolar centric diatom Hydrosera

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

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.     

Relationships between phytoplankton and periphyton communities in a central Iowa stream

Relationships between phytoplankton and periphyton communities were investigated in a central Iowa stream. Results generally support the hypothesis that the phytoplankton community arises from the epipelic periphyton community. A high correlation existed between the proportion of benthic diatoms composing the epipelon and phytoplankton. One dominant epipelic species (Nitzschia acicularis) showed a greater tendency to become planktonic than the grouped remainder of Nitzschia spp. There was a significant inverse relationship between the proportion of centric diatoms in the plankton and volume of flow. Centric diatoms were important members of the plankton only when volume of flow was less than 60 ft³ / sec (2.1 m³ / sec). Possible mechanisms explaining these phenomena are discussed.This study represents a portion of a dissertation submitted to the Graduate College, Iowa State University in partial fulfillment of requirements for the degree Doctor of Philosophy.     

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.     

Phytoplankton and Nutrients in Goat Island Bay,New Zealand

A quantitative study of the phytoplankton and nutrients in Goat Island Bay, northern New Zealand, was carried out at two stations of depths 7 m and 20 m in 1966-69. There were significant differences in the concentrations of the phytoplankton at the two stations, the deeper station having, in general, higher concentrations than the shallower. Low phytoplankton standing crops were found in winter when the increased run-off from the land resulted in lower salinities, higher turbidity, and higher dissolved silicate levels. The spring growth of diatoms was dominated by small diatoms (Cylindrotheca closterium, Leptocylindrus danicus, Nitzschia pseudoseriata, Phaeodactylum tricornutum, and Rhizosolenia delicatula). Occasionally it was dominated by a population of large centric diatoms (Chaetoceros spp., Coscinodiscus spp., Thalassiosira spp.), but these appeared to be found in a separate water mass which entered the bay and displaced the resident water mass. A third water mass entered the bay in spring and was characterised by the coccolithophore Emiliana huxleyi and a lower temperature. During the summer diatoms dominated the phytoplankton population, but were replaced by dinoflagellates for a short period in autumn.     

OBSERVATIONS ON THE ULTRASTRUCTURE OF THE MALI GAMETES OF BIDDULPHIA LEVIS EHR.1

The marine centric diatom Biddulphia levis produced uniflagellate fusiform male gametes completely within the parent cell frustule. These gametes lacked both a central pair of microtubules in the flagellar axoneme and chloroplasts but did contain a cone of microtubules which passed posteriorly from the base of the kinetosome along the nuclear envelope. The gametes were released through a specialized pore in the girdle band leaving behind a cytoplasmic mass which contained chloroplasts and other cytoplasmic components. Tubules which resembled the flimmer hairs on the gamete flagellum occurred in cisternae of the cytoplasmic reticulum in the residual cytoplasm and in the nuclear envelope of the gametes. Gametogenesis in B. levis is compared with similar processes in other centric diatoms.     

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.     

Cytogenetic and genomic relationships ofThinopyrum elongatum with twoPsathyrostachys species and withLeymus secalinus (Poaceae)

Intergeneric hybridizations were made betweenT. elongatum, and twoPsathyrostachys and fiveLeymus species. The seed set obtained onT. elongatum ×Leymus hybrids ranged from 5.65% to 20.00%, depending onLeymus species. The seed set obtained onT. elongatum ×Psathyrostachys hybrids ranged from 16.07% to 19.70%. Meiotic pairing at metaphase-I in JN diploid hybrids ofT. elongatum ×Psathyrostachys species revealed a very low level homology between the basic J and N genomes, and further demonstrated that the two genomes are quite diverged. Chromosome pairing in theT. elongatum ×Leymus secalinus hybrid averaged 15.19 univalents + 2.62 rod bivalents + 0.26 ring bivalents + 0.02 trivalents, suggesting that the partial J^e chromosomes ofT. elongatum has homology withLeymus secalinus genomes.L. secalinus might have 3–4 chromosomes originating from J^e genome.     

Physiological response of 10 phytoplankton species exposed to macondo oil and the dispersant,Corexit

Culture experiments were conducted on ten phytoplankton species to examine their biological and physiological responses during exposure to oil and a combination of oil and dispersant. The species tested included a range of taxa typically found in the Gulf of Mexico such as cyanobacteria, chlorophytes, and diatoms. Cultures were exposed to Macondo surrogate oil using the water accommodated fraction (WAF), and dispersed oil using a chemically enhanced WAF (CEWAF) and diluted CEWAF, to replicate conditions following the Deepwater Horizon spill in the Gulf of Mexico. A range of responses were observed, that could broadly class the algae as either “robust” or “sensitive” to oil and/or dispersant exposure. Robust algae were identified as Synechococcus elongatus, Dunaliella tertiolecta, two pennate diatoms Phaeodactylum tricornutum and Navicula sp., and Skeletonema grethae CCMP775, and were largely unaffected by any of the treatments (no changes to growth rate or time spent in lag phase relative to controls). The rest of the phytoplankton, all centric diatoms, exhibited at least some combination of reduced growth rates or increased lag time in response to oil and/or dispersant exposure. Photophysiology did not have a strong treatment effect, with significant inhibition of photosynthetic efficiency (F_v/F_m) only observed in the CEWAF, if at all. We found that the effects of oil and dispersants on phytoplankton physiology were species‐dependent, and not always detrimental. This has significant implications on how oil spills might impact phytoplankton community structure and bloom dynamics in the Gulf of Mexico, which in turn impacts higher trophic levels.     

Biofilm diatom community structure: Influence of temporal and substratum variability

Abstract

Diatoms, which are early autotrophic colonisers, are an important constituent of the biofouling community in the marine environment. The effects of substratum and temporal variations on the fouling diatom community structure in a monsoon-influenced tropical estuary were studied. Fibreglass and glass coupons were exposed every month for a period of 4 days and the diatom population sampled at 24 h intervals, over a period of 14 months. The planktonic diatom community structure differed from the biofilm community. Pennate diatoms dominated the biofilms whilst centric diatoms were dominant in the water column. Among the biofilm diatoms, species belonging to the genera Navicula, Amphora, Nitzschia, Pleurosigma and Thalassionema were dominant. On certain occasions, the influence of planktonic blooms was also seen on the biofilm community. A comparative study of biofilms formed on the two substrata revealed significant differences in density and diversity. However species composition was almost constant. In addition to substratum variations, the biofilm diatom community structure also showed significant seasonal variations, which were attributed to physico-chemical and biological changes in both the water and substratum. Temporal variations in the tychopelagic diatoms of the water were also observed to exert an influence on the biofilm diatom community. Variations in diatom communities may determine the functional ecosystem of the benthic environment.     

STICTOCYCLUS STICTODISCUS (BACILLARIOPHYTA): COMMENTS ON ITS ECOLOGY,STRUCTURE AND CLASSIFICATION1,2

The rarely studied large diatom Stictocyclus stictodiscus (Grun.) R. Ross has been investigated by light and electron microscopy. Confusion over the classification of this diatom has been reduced, and it has been assigned to a new family, Stictocyclaceae, though the relationship of this to the overall classification of centric diatoms is still not clear.