GENETIC DIFFERENTIATION AMONG POPULATIONS OF THE PLANKTONIC MARINE DIATOM DITYLUM BRIGHTWELLII (BACILLARIOPHYCEAE)1

Population genetic structure was determined for the planktonic diatom Ditylum brightwellii (West) Grunow in two connected estuaries—Puget Sound and the Strait of Juan de Fuca (WA, USA). Three genetically distinct populations were detected that were characterized by different microsatellite allele distributions and unique alleles. Isolates from the two most genetically diverged populations displayed identical full‐length 18S rDNA sequences suggesting that either a single or two recently diverged species were sampled. The extent of genetic differentiation between populations was not correlated with distance between water samples or time between sampling. Instead, distinct populations were associated with different estuaries. In Puget Sound waters, one population was detected three times over the course of 28 months. Cells from this population were likely maintained inside Puget Sound over long periods through water recirculation within the Sound. In Strait of Juan de Fuca waters, two additional populations were detected. Maximum growth rates of Puget Sound isolates were significantly different from Strait of Juan de Fuca isolates, indicating that populations were composed of cells with different physiological capabilities. The genetic and physiological differentiation observed between populations from intermixing estuaries suggested that genetic exchange between populations was restricted through differential selection. Despite the potential for widespread dispersal in planktonic organisms, it appears that populations with distinct genetic and physiological characteristics can be maintained over long time periods through a combination of hydrology and differential selection.     

Copper toxicity and adaptation in the marine diatom Ditylum brightwellii

• 1.1. The planktonic diatom Ditylum brightwellii, grown in light-limited resp. nitrogen-limited continuous culture, has been exposed to Cu levels, comparable with those in the Scheldt estuary.
• 2.2. At increasing levels D. brightwellii initially detoxified Cu, producing metal-binding ligands (amino acids), and increasing its cell volume.
• 3.3. In light-limited D. brightwellii, photosynthesis could be adjusted to increasing Cu stress, division rates remained constant, and cells proved to be adaptable to 200 nM dissolved Cu.
• 4.4. Nitrogen-limited D. brightwellii detoxified Cu inadequately: it stored large amounts of Cu (30–60 μM) that inhibited cell division.

     

Complexation of iron by microbial siderophores and effects of iron chelates on the growth of marine microalgae causing red tides

The growth rates of 13 species of abundant red tide algae in media with different iron species complexed with microbial siderophores (Ferrichrome and Ferrioxamine) and Fe‐Catechol were investigated. Our study demonstrated that the Fe‐chelates (at molar ratios = 1:1) were bioavailable to some red tide species. In Fe‐Catechol medium, growth was observed for the raphidophyte Heterosigma akashiwo, the dinoflagellates Heterocapsa circularisquama and Heterocapsa triquetra, the diatom Ditylum brightwellii, the cryptophyte Rhodomonas ovalis, the chlorophyte Oltmannsiellopsis viridis, and the haptophyte Cricosphaera roscoffensis. In Ferrioxamine medium, we found the growth of the dinoflagellate Karenia mikimotoi, the diatom Ditylum brightwellii, and the cryptophyte Rhodomonas ovalis. But, the existence of higher ligand concentrations (molar ratios ≥ 1:10) decreased the growth rates of most red tide species that were examined. Furthermore, all red tide species examined were not able to grow in Ferrichrome medium. In particular, the Chattonella species examined did not grow in the presence of Fe‐chelates. These results suggest that bioavailability of iron depends not only on ligand species, but also on the concentration of the ligands; moreover, microbial siderophores may play an important role in controlling the uptake of iron complexed with organic materials that exist in coastal water and the formation of red tides in coastal areas.     

DIEL OSCILLATIONS IN THE PHOTOSYNTHESIS-IRRADIANCE RELATIONSHIP OF A PLANKTONIC MARINE DIATOM1

The amplitude of diel oscillations in photosynthesis as a function of irradiance varied with the growth phase in a marine phytoplankton species. The common centric diatom (Bacillariophyta), Ditylum brightwellii (West) Grun., showed strong periodicity in the photosynthesis-irradiance (P-I) relationship, which damped progressively from early to late exponential and stationary phase. These findings suggest that short-term temporal characteristics of phytoplankton production depend on factors which affect growth, and that the amplitude is most enhanced at maximal growth rates likely to be encountered in the natural environment.     

AUTOLYSIS KINETICS OF THE MARINE DIATOM DITYLUM BRIGHTWELLII (BACILLARIOPHYCEAE) UNDER NITROGEN AND PHOSPHORUS LIMITATION AND STARVATION1

Autolysis kinetics in axenic cultures of the diatom Ditylum brightwellii (West) Grunow were studied under nutrient limitation in continuous cultures and under nutrient starvation in batch-mode cultures obtained by switching off nutrient supply in the continuous cultures. Under N limitation, the specific algal autolysis rates (δ, day^?1) were found constant at 0.014 ± 0.002 day^?1over a broad range of specific dilution rates (D, day^?1) (0.09–0.56 day^?1), implying an intrinsic death factor independent of the physiologzc state of the algal cells. Under P limitation, 8 was inversely related to D and ranged between 0.067 and 0.005 day^?1 at D = 0.17–0.44 day^?1. Under conditions of nutrient stamation, the degree of algal nutrient deficiency prior to stamation affected autolysis rates (δ_b, day^?1) and subsequently survival of the algal cultures. Nitrogen-starved D. brightwellii showed highest δ_b (maximum, 0.10 day^?1) when precultured at the higher growth rates. Phosphorus stamation led to highest δ_b (maximum, 0.21 day^?1) in the cultures preconditioned at the lower steady state growth rates. The lower death rates for D. brightwellii under limitation and starvation of N compared to P suggest that D. brightwellii was better equipped to handle N than P deficiency. The present results showed that cell lysis induced by nutrient stress was a significant cause of mortality in D. brightwellii and provided more insight into the field distribution of this neritic diatom.     

The cyanobacterial endosymbiont of the unicellular algae <Emphasis Type="Italic">Rhopalodia gibba</Emphasis> shows reductive genome evolution

Background  

Bacteria occur in facultative association and intracellular symbiosis with a diversity of eukaryotic hosts. Recently, we have helped to characterise an intracellular nitrogen fixing bacterium, the so-called spheroid body, located within the diatom Rhopalodia gibba. Spheroid bodies are of cyanobacterial origin and exhibit features that suggest physiological adaptation to their intracellular life style. To investigate the genome modifications that have accompanied the process of endosymbiosis, here we compare gene structure, content and organisation in spheroid body and cyanobacterial genomes.     

Pattern of Proteins after Heat Shock and UV-B Radiation of some Temperate Marine Diatoms and the Antarctic Odontella weissflogii

Synthesis of stress proteins after heat shock and different periods of UV-B radiation were investigated with marine diatom species from the North Sea Ditylum brightwellii, Lithodesmium variabile, Odontella sinensis, Thalassiosira rotula and the Antarctic diatom Odontella weissfloggii from the Weddell Sea. Algae were grown in an artifical sea-water medium under controlled laboratory conditions: light/dark regime of 12:12 h (7.2 W m^?2), normal air (0.035 vol.% CO₂) and 18° or 4 °C. All the tested diatom species can produce heat shock proteins (HSPS) of the 70 kDa family by in vivo labelling with [³⁵S]-methionine. The same results were obtained for Odontella sinensis, Ditylum brightwellii and Odontella weissflogii by estimation of the in vitro translation products with poly-A-mRNA isolated from these organisms. However, Odontella weissflogii, a species relatively insensitive to UV-B irradiance, did not synthesize UV-induced HSPS, whereas the UV-sensitive diatom Odontella sinensis, as well as Lithodesmium variabile, produced all the observed HSPS after UV-B exposure. In addition, a protein of 43 kDa was found after UV-B irradiance of the temperate Odontella sinensis. The temperate marine diatom Thalassiosira rotula synthesized 70 kDa and 5 7 kDa proteins after a heat shock and a UV-B exposure of 2 h, but a 40 kDa protein could not be detected, whereas a 60 kDa protein was found after 2 h UV-B exposure. The results are discussed in view of a possible adaptation of O. weissflogii to an enhanced UV dose.     

"Dinoflagellate Sterols" in marine diatoms

Sterol compositions for three diatom species, recently shown to contain sterols with side chains typically found in dinoflagellates, were determined by HPLC and ¹H NMR spectroscopic analyses. The centric diatom Triceratium dubium (= Biddulphia sp., CCMP 147) contained the highest percentage of 23-methylated sterols (37.2% (24R)-23-methylergosta-5,22-dienol), whereas the pennate diatom Delphineis sp. (CCMP 1095) contained the cyclopropyl sterol gorgosterol, as well as the 27-norsterol occelasterol. The sterol composition of Ditylum brightwellii (CCMP 358) was the most complex, containing Δ⁰- and Δ⁷-sterols, in addition to the predominant Δ⁵-sterols. A pair of previously unknown sterols, stigmasta-5,24,28-trienol and stigmasta-24,28-dienol, were detected in D. brightwellii and their structures were determined by NMR spectroscopic analysis and by synthesis of the former sterol from saringosterol. Also detected in D. brightwellii was the previously unknown 23-methylcholesta-7,22-dienol.     

Spatial heterogeneity of diatom silicification and growth in a eutrophic reservoir

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Life cycle size dynamics in Didymosphenia geminata (Bacillariophyceae)

Didymosphenia geminata has received a great deal of attention in the last 25 years, and considerable effort has gone into determining the origin, ecological impact, and economic consequences of its invasive behavior. While environmental conditions are a controlling influence in distribution, the extreme success of the species may be tied to its basic biology and life history. Little is known, however, about population dynamics, size restoration and reproduction of D. geminata. The objective of this study was to determine the temporal patterns in cell size frequency, size restoration strategy, and synchronization of life cycles between populations in close proximity. We implemented FlowCam technology to measure the length of more than 100,000 D. geminata cells from two sites in South Boulder Creek, Colorado over 1 year. We applied finite mixture modeling to uncover temporal patterns in size distribution. Our results show that collections of D. geminata exhibited a complex, multimodal size distribution, almost always containing four overlapping age cohorts. We failed to observe direct visual evidence of the sexual phase. Multiple abrupt and directional shifts in size distribution, however, were documented providing conclusive evidence of cell size restoration. Lastly, nodules in close proximity were asynchronous with respect to size frequency profiles and size diminution, highlighting the relevance of spatial heterogeneity in in situ diatom size dynamics. This study is the first to document the complexity of diatom cell size distribution in a lotic system, size restoration in D. geminata, and the variability in rates of size reduction at microhabitat spatial scales.     

Influence of Cell Size and DNA Content on Growth Rate and Photosystem II Function in Cryptic Species of Ditylum brightwellii

DNA content and cell volume have both been hypothesized as controls on metabolic rate and other physiological traits. We use cultures of two cryptic species of Ditylum brightwellii (West) Grunow with an approximately two-fold difference in genome size and a small and large culture of each clone obtained by isolating small and large cells to compare the physiological consequences of size changes due to differences in DNA content and reduction in cell size following many generations of asexual reproduction. We quantified the growth rate, the functional absorption cross-section of photosystem II (PSII), susceptibility of PSII to photoinactivation, PSII repair capacity, and PSII reaction center proteins D1 (PsbA) and D2 (PsbD) for each culture at a range of irradiances. The species with the smaller genome has a higher growth rate and, when acclimated to growth-limiting irradiance, has higher PSII repair rate capacity, PSII functional optical absorption cross-section, and PsbA per unit protein, relative to the species with the larger genome. By contrast, cell division rates vary little within clonal cultures of the same species despite significant differences in average cell volume. Given the similarity in cell division rates within species, larger cells within species have a higher demand for biosynthetic reductant. As a consequence, larger cells within species have higher numbers of PSII per unit protein (PsbA), since PSII photochemically generates the reductant to support biosynthesis. These results suggest that DNA content, as opposed to cell volume, has a key role in setting the differences in maximum growth rate across diatom species of different size while PSII content and related photophysiological traits are influenced by both growth rate and cell size.     

Rapid evolution in response to introduced predators I: rates and patterns of morphological and life-history trait divergence

Background  

Introduced species can have profound effects on native species, communities, and ecosystems, and have caused extinctions or declines in native species globally. We examined the evolutionary response of native zooplankton populations to the introduction of non-native salmonids in alpine lakes in the Sierra Nevada of California, USA. We compared morphological and life-history traits in populations of Daphnia with a known history of introduced salmonids and populations that have no history of salmonid introductions.     

Quantification of toxic effects of the herbicide metolachlor on marine microalgae Ditylum brightwellii (Bacillariophyceae), Prorocentrum minimum (Dinophyceae), and Tetraselmis suecica (Chlorophyceae)

Toxic effects of the herbicide metolachlor (MC) were evaluated for three marine microalgae, Tetraselmis suecica (chlorophyte), Ditylum brightwellii (diatom), and Prorocentrum minimum (dinoflagellate). MC showed a significant reduction in cell counts and chlorophyll a levels. Median effective concentration (EC₅₀) was calculated based on chlorophyll a levels after a 72-h MC exposure. EC₅₀ values for T. suecica, D. brightwellii, and P. minimum were 21.3, 0.423, and 0.07 mg/L, respectively. These values showed that the dinoflagellate was most sensitive when exposed to the herbicide, at a concentration comparable to freshwater algae, suggesting its potential as an appropriate model organism for ecotoxicity assessments in marine environments.     

DOES CARBOHYDRATE CONTENT AFFECT THE SINKING RATES OF MARINE DIATOMS?1

We tested the hypothesis that positive relationships between sinking rate and irradiance were due to increases in cell density caused by accumulations of carbohydrate. In semicontinuous batch cultures of Thalassiosira weissflogii (Gru.) Fryxell el Hasle and Ditylum brightwellii (t. West) Grunow in Van Huerk, carbohydrate content was varied by growing cells under diel cycles of high or low light. Sinking rate was measured at the end of the light period and the end of the dark period, on live and heat-killed cells. No positive correlations were found between sinking rate (which varied from – 0.060 to 0.13 m·d^?1) and carbohydrate content (which varied from 10 to 950 pg · cell^?1), indicating that accumulations of carbohydrate did not significantly affect sinking rate. There were no large diel variations in the sinking rate of T. weissflogii, but sinking rates of D. brightwellii grown under high light ranged from being negative (i.e. cells were floating) at the end of the light period to positive at the end of the dark period. This is the first report of positive buoyancy in vegetative D. brightwellii, a phenomenon that may only occur in D. brightwellii grown under diel cycles.     

Characterizing the silica deposition vesicle of diatoms

Summary The present study on a centric diatom,Ditylum brightwellii, includes two parts: detection of sugars in the silica deposition vesicle (SDV) with lectins and labeling the developing siliceous cell wall in the SDV with rhodamine 123. Cells with developing valves are treated with SDS to remove all the cytoplasmic contents, then either stained with fluorescein labeled lectins or thin-sectioned and stained with colloidal gold labeled lectins. The results show that mannose is part of the organic matrix in the SDV. Rhodamine 123, a non-toxic fluorescent laser dye, enters the cell immediately and is trapped in the SDV probably by the high reducing potential of the SDV. Silica is co-deposited with rhodamine 123 in the SDV, and the resulting valves and girdle bands become fluorescent. Implications of this study for the mechanism of silicification are discussed.Abbreviation SDV Silica deposition vesicle     

Marine Plankton Algae Grown with Light-Dark Cycles.

The diatoms Ditylum brightwellii and Nitzschia turgidula were grown in semi-continuous culture under various combinations of light intensity, temperature and daylength (photoperiod). Growth was strongly limited by light intensities below 0.03 cal/em². min in both species. Above this intensity, light saturation of growth was rapidly approached in Nitzschia but only gradually so in Ditylum. The growth rate in continuous light was never significantly higher than with 16 hours of light plus 8 hours of dark. In Ditylum, continuous light above 0.03 cal/cm². min caused a strong inhibition of growth at all temperatures. The chlorophyll concentration in the cells was greater the shorter the photopceriod. In cultures synchronised by different combinations of light intensity and photoperiod, cell division generally took place in the light. Synchrony was best under short photoperiods of bright light. Time courses are shown for chlorophyll synthesis and photosynthesis in synchronised cultures.     

Genetic variation of two species with different life‐history traits in the endangered renosterveld of South Africa – a comparative analysis of Eriocephalus africanus and Hemimeris racemosa

We tested the effects of life‐history traits on genetic variation and conducted a comparative analysis of two plant species with differing life‐history traits co‐occurring in the highly endangered renosterveld of South Africa. We selected eighteen renosterveld remnants with varying degrees of size and isolation where populations of the herbaceous, annual and insect‐pollinated Hemimeris racemosa and the shrubby perennial and both wind‐ and insect‐pollinated Eriocephalus africanus occurred. We postulated a lower genetic variation within populations and increased genetic variation between populations in the annual than in the perennial species. Genetic variation was lower within populations of H. racemosa than within E. africanus, as is typical for annual compared to perennial species. Variation within populations was, however, not correlated with fragment size or distance in either of the two species and genetic variation between populations of the two species was comparable (Φ_ST = 0.10, 0.09).