Search

As human induced climate change continues to alter the world’s oceans, it becomesincreasingly important to hone the predictive power of models to understand the ecosystemlevel changes and challenges that the coming decades will bring. However, models are onlyas robust as the data upon which they are formulated, and the experimentation required toinform them must be based on an interconnected and concomitantly changing set ofconditions. Phytoplankton, specifically diatoms, are a worthy focus group as they areparticularly ecologically successful—and responsible for approximately 40% of marineprimary production. By scrutinizing the potential effects of climate change onphytoplankton, the base of the marine food web, researchers can obtain crucial informationupon which to build predictions for entire ecosystems.This series of experiments was designed to investigate the combined effects oftemperature and light on the growth and photophysiology of two strains (one coastal and oneopen-ocean) of the diatom Thalassiosira pseudonana. The goal in producing this data-set isto add to the growing body of multi-stressor research which will aid in understanding theresponse of phytoplankton to future ocean conditions. This set of experiments takesadvantage of advances in culturing techniques and utilizes a bioreactor (multicultivatorZ160-OD) containing individual treatment vessels, thus allowing for the easy cultivation ofdiatoms under eight different light regimes at the same temperature. Through the use ofhigher treatment numbers across a gradient of conditions, we exploit the opportunity todetect and quantify potential non-linear response patterns.Our results show that the response of T. pseudonana to simultaneous changes intemperature and irradiance is dependent on the measured response trait, which suggests thatinterpretation of performance curves requires clear identification of all conditions underwhich they were generated. Our data also suggest subtle differences between the

As human induced climate change continues to alter the world’s oceans, it becomesincreasingly important to hone the predictive power of models to understand the ecosystemlevel changes and challenges that the coming decades will bring. However, models are onlyas robust as the data upon which they are formulated, and the experimentation required toinform them must be based on an interconnected and concomitantly changing set ofconditions. Phytoplankton, specifically diatoms, are a worthy focus group as they areparticularly ecologically successful—and responsible for approximately 40% of marineprimary production. By scrutinizing the potential effects of climate change onphytoplankton, the base of the marine food web, researchers can obtain crucial informationupon which to build predictions for entire ecosystems.This series of experiments was designed to investigate the combined effects oftemperature and light on the growth and photophysiology of two strains (one coastal and oneopen-ocean) of the diatom Thalassiosira pseudonana. The goal in producing this data-set isto add to the growing body of multi-stressor research which will aid in understanding theresponse of phytoplankton to future ocean conditions. This set of experiments takesadvantage of advances in culturing techniques and utilizes a bioreactor (multicultivatorZ160-OD) containing individual treatment vessels, thus allowing for the easy cultivation ofdiatoms under eight different light regimes at the same temperature. Through the use ofhigher treatment numbers across a gradient of conditions, we exploit the opportunity todetect and quantify potential non-linear response patterns.Our results show that the response of T. pseudonana to simultaneous changes intemperature and irradiance is dependent on the measured response trait, which suggests thatinterpretation of performance curves requires clear identification of all conditions underwhich they were generated. Our data also suggest subtle differences between the

Chitin is generally considered to be present in centric diatoms but not in pennate species. Many aspects of chitin biosynthetic pathways have not been explored in diatoms. We retrieved chitin metabolic genes from pennate (Phaeodactylum tricornutum) and centric (Thalassiosira pseudonana) diatom genomes. Chitin deacetylase (CDA) genes from each genome (PtCDA and TpCDA) were overexpressed in P. tricornutum. We performed comparative analysis of their sequence structure, phylogeny, transcriptional profiles, localization and enzymatic activities. The chitin relevant proteins show complex subcellular compartmentation. PtCDA was likely acquired by horizontal gene transfer from prokaryotes, whereas TpCDA has closer relationships with sequences in Opisthokonta. Using transgenic P. tricornutum lines expressing CDA-green fluorescent protein (GFP) fusion proteins, PtCDA predominantly localizes to Golgi apparatus whereas TpCDA localizes to endoplasmic reticulum/chloroplast endoplasmic reticulum membrane. CDA-GFP overexpression upregulated the transcription of chitin synthases and potentially enhanced the ability of chitin synthesis. Although both CDAs are active on GlcNAc(5), TpCDA is more active on the highly acetylated chitin polymer DA60. We have addressed the ambiguous characters of CDAs from P. tricornutum and T. pseudonana. Differences in localization, evolution, expression and activities provide explanations underlying the greater potential of centric diatoms for chitin biosynthesis. This study paves the way for in vitro applications of novel CDAs., QC 20190313

The neurotoxins beta-N-methylamino-L-alanine (BMAA) and 2,4-diaminobutyric acid (DAB) are produced by cyanobacteria, diatoms and dinoflagellates and have been detected in seafood worldwide. Our present knowledge of their metabolism or biosynthesis is limited. In this study, the production of BMAA and DAB as a function of time was monitored in five strains representing four species of diatoms, i.e. Phaeodactylum tricornutum, Thalassiosira weissflogii, Thalassiosira pseudonana and Navicula pelliculosa, previously identified as BMAA and DAB producers. Subsequently, three strains were selected and exposed to three nitrogen treatments - starvation, control (the standard concentration in f/2 medium) and enrichment, because BMAA metabolism has been suggested to be closely associated with cellular nitrogen metabolism in both cyanobacteria and diatoms. Chlorophyll a and total protein concentrations were also determined. Our results indicate that BMAA and DAB production in diatoms is species- and strain-specific. However, production might also be affected by stress, particularly as related to nitrogen starvation and cell density. Furthermore, this study shows a significant correlation between the production of the two neurotoxins which might further suggest common steps in the metabolic pathways.

© The Author(s), 2018. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Applied and Environmental Microbiology 84 (2018): e02034-17, doi:10.1128/AEM.02034-17., Phytoplankton replace phosphorus-containing lipids (P-lipids) with non-P analogues, boosting growth in P-limited oceans. In the model diatom Thalassiosira pseudonana, the substitution dynamics of lipid headgroups are well described, but those of the individual lipids, differing in fatty acid composition, are unknown. Moreover, the behavior of lipids outside the common headgroup classes and the relationship between lipid substitution and cellular particulate organic P (POP) have yet to be reported. We investigated these through the mass spectrometric lipidomics of P-replete (P+) and P-depleted (P−) T. pseudonana cultures. Nonlipidic POP was depleted rapidly by the initiation of P stress, followed by the cessation of P-lipid biosynthesis and per-cell reductions in the P-lipid levels of successive generations. Minor P-lipid degradative breakdown was observed, releasing P for other processes, but most P-lipids remained intact. This may confer an advantage on efficient heterotrophic lipid consumers in P-limited oceans. Glycerophosphatidylcholine (PC), the predominant P-lipid, was similar in composition to its betaine substitute lipid. During substitution, PC was less abundant per cell and was more highly unsaturated in composition. This may reflect underlying biosynthetic processes or the regulation of membrane biophysical properties subject to lipid substitution. Finally, levels of several diglycosylceramide lipids increased as much as 10-fold under P stress. These represent novel substitute lipids and potential biomarkers for the study of P limitation in situ, contributing to growing evidence highlighting the importance of sphingolipids in phycology. These findings contribute much to our understanding of P-lipid substitution, a powerful and widespread adaptation to P limitation in the oligotrophic ocean., This work was funded by the University of Southampton Vice Chancellors Scholarship Award.

© The Author(s), 2018. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Applied and Environmental Microbiology 84 (2018): e02034-17, doi:10.1128/AEM.02034-17., Phytoplankton replace phosphorus-containing lipids (P-lipids) with non-P analogues, boosting growth in P-limited oceans. In the model diatom Thalassiosira pseudonana, the substitution dynamics of lipid headgroups are well described, but those of the individual lipids, differing in fatty acid composition, are unknown. Moreover, the behavior of lipids outside the common headgroup classes and the relationship between lipid substitution and cellular particulate organic P (POP) have yet to be reported. We investigated these through the mass spectrometric lipidomics of P-replete (P+) and P-depleted (P−) T. pseudonana cultures. Nonlipidic POP was depleted rapidly by the initiation of P stress, followed by the cessation of P-lipid biosynthesis and per-cell reductions in the P-lipid levels of successive generations. Minor P-lipid degradative breakdown was observed, releasing P for other processes, but most P-lipids remained intact. This may confer an advantage on efficient heterotrophic lipid consumers in P-limited oceans. Glycerophosphatidylcholine (PC), the predominant P-lipid, was similar in composition to its betaine substitute lipid. During substitution, PC was less abundant per cell and was more highly unsaturated in composition. This may reflect underlying biosynthetic processes or the regulation of membrane biophysical properties subject to lipid substitution. Finally, levels of several diglycosylceramide lipids increased as much as 10-fold under P stress. These represent novel substitute lipids and potential biomarkers for the study of P limitation in situ, contributing to growing evidence highlighting the importance of sphingolipids in phycology. These findings contribute much to our understanding of P-lipid substitution, a powerful and widespread adaptation to P limitation in the oligotrophic ocean., This work was funded by the University of Southampton Vice Chancellors Scholarship Award.

Algae are vegetal organisms that have a great distribution around the world, and they are very important to the good maintenance of the biological equilibrium in all the ecosystems of the planet. They have an important chance in atmospherically CO2 fixation and the increase of O2 in the atmosphere. Nowadays, the studies about this organisms are experimenting a great evolution in order to know more about their lifestyle and do better artificial cultures with biotechnological and food industrial finalities. There are lots of algae species which are studied today, but the most important is the diatom Thalassiosira pseudonana. This species presents a great growth rate in high-density cultures which permits, for example, obtain a big quantity of biomass, whose nutritional value is excellent. This issue permits, specially, use this biomass for feed fish, mollusks and another marine organisms which are important in aquaculture. The main objective of this project is to modelate and simulate, with Agent-Based Model's point of view, the poblational dynamic's evolution of the diatom Thalassiosira pseudonana in artificial cultures located in experimentation labs. The conceptual base of the model, the consulted experimental results and protocol operation types developed and referred to the development of cultures in monitorized conditions will be inspirited in various scientific articles which had studied this issues in different perspectives. The implementation of the designed model will be done in freeaccess computational software NetLogo. The obtained simulator will should permit the experimentation on the algae behavior in different scenarios and culture methods which should permit the subsistence and development of microorganisms. In order to analyze the developed model, some simulations have been made. The obtained results can explain that the evolution of population dynamics of Thalassiosira pseudonana have similarities with another plancktonic organisms dynamics in liqui, Les algues son organismes vegetals amb una distribucio molt extensa per tot el planeta i organismes clau en el manteniment de l'equilibri biologic de tots els ecosistemes de la terra. Tenen un rol important en la fixacio de dioxid de carboni i en l'enriquiment en oxigen de l'atmosfera terrestre. Actualment, s'esta aprofundint cada cop mes en la biologia d'aquests microorganismes per comprendre millor el seu cicle de vida i per poder dur a terme cultius artificials d'algues controlats amb condicions especifiques que siguin d'interes biotecnologic, industrial i agroalimentari. D'entre les diferents especies d'algues que avui en dia s'estan utilitzant amb exit en l'ambit de la nutricio, la sanitat i la millora de cultius microbiologics esta l'especie Thalassiosira pseudonana, una de les especies mes destacades en l'ambit cientific. Aquesta alga presenta un creixement molt rapid en densitats de cultiu elevades, la qual cosa, per exemple, permet generar una generosa quantitat de biomassa algal, amb un elevat valor nutritiu que fa que s'utilitzi com aliment per a crustacis i mol·luscs en aquicultura, entre altres possibles aplicacions biotecnologiques. L'objectiu d'aquest treball consisteix en modelitzar i simular, utilitzant la perspectiva propia de la modelitzacio basada en agents, la dinamica de poblacions d'algues de l'especie Thalassiosira pseudonana quan es desenvolupa en condicions de laboratori. La base conceptual del model, les dades experimentals consultades i els tipus de protocols desenvolupats i lligats al desenvolupament del cultiu en condicions controlades s'ha fonamentat en una revisio de literatura cientifica que estudia aquest sistema viu des de diferents perspectives. La implementacio del model computacional dissenyat s'ha fet en la plataforma de programacio multiagent d'acces lliure NetLogo. El simulador obtingut permet explorar el comportament d'aquestes algues sota diferents escenaris o protocols de cultiu controlat que han de permetre la subsistenci, Las algas son organismos vegetales con una amplia distribucion por todo el globo, y son la clave para el mantenimiento del equilibrio biologico de todos los ecosistemas de la tierra. Tienen un importante papel en la fijacion del dioxido de carbono atmosferico y en el posterior enriquecimiento de la atmosfera terrestre en oxigeno. Actualmente se esta profundizando en el estudio de estos organismos para comprender mejor su ciclo de vida y llevar a cabo cultivos artificiales en condiciones controladas con finalidades biotecnologicas y agroalimentarias. De entre las distintas especies de algas que actualmente se utilizan en estudios nutritivos, sanitarios y de mejora de cultivos microbiologicos, la especie Thalassiosira pseudonana es una de las mas destacadas en el ambito de la ciencia. Presenta una tasa de crecimiento muy alta en cultivos de alta densidad poblacional, que permite, por ejemplo, conseguir una generosa cantidad de biomasa algal cuyo valor nutritivo es excelente. Esta caracteristica permite, entre otras cosas, su uso como alimento para peces, moluscos y crustaceos en piscifactorias. El objetivo de este trabajo consiste en modelizar y simular, desde la perspectiva de la modelizacion basada en agentes, la dinamica de poblaciones de Thalassiosira pseudonana en cultivos artificiales y controlados en laboratorio. La base conceptual del modelo, los datos experimentales consultados y los tipos de protocolo desarrollados y ligados al desarrollo del cultivos en condiciones controladas se fundamentara en articulos cientificos que han estudiado este sistema vivo desde distintas perspectivas. La implantacion del modelo computacional diseñado se realizara en la plataforma de programacion multiagente y de acceso libre NetLogo. El simulador obtenido debera permitir la explotacion del comportamiento de estas algas bajo diferentes escenarios y protocolos de cultivo que deben permitir la subsistencia y el crecimiento de estos microorganismos. Con el proposito de analizar el

© The Author(s), 2012. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Frontiers in Microbiology 3 (2012): 385, doi:10.3389/fmicb.2012.00385., Genes that are constitutively expressed across multiple environmental stimuli are crucial to quantifying differentially expressed genes, particularly when employing quantitative reverse transcriptase polymerase chain reaction (RT-qPCR) assays. However, the identification of these potential reference genes in non-model organisms is challenging and is often guided by expression patterns in distantly related organisms. Here, transcriptome datasets from the diatom Thalassiosira pseudonana grown under replete, phosphorus-limited, iron-limited, and phosphorus and iron co-limited nutrient regimes were analyzed through literature-based searches for homologous reference genes, k-means clustering, and analysis of sequence counts (ASC) to identify putative reference genes. A total of 9759 genes were identified and screened for stable expression. Literature-based searches surveyed 18 generally accepted reference genes, revealing 101 homologs in T. pseudonana with variable expression and a wide range of mean tags per million. k-means analysis parsed the whole transcriptome into 15 clusters. The two most stable clusters contained 709 genes, but still had distinct patterns in expression. ASC analyses identified 179 genes that were stably expressed (posterior probability < 0.1 for 1.25 fold change). Genes known to have a stable expression pattern across the test treatments, like actin, were identified in this pool of 179 candidate genes. ASC can be employed on data without biological replicates and was more robust than the k-means approach in isolating genes with stable expression. The intersection of the genes identified through ASC with commonly used reference genes from the literature suggests that actin and ubiquitin ligase may be useful reference genes for T. pseudonana and potentially other diatoms. With the wealth of transcriptome sequence data becoming available, ASC can be easily applied to transcriptome datasets from other phytoplankton to identify reference genes., This research was funded by the National Science Foundation grant #OCE-0723667 (to Sonya T. Dyhrman, Mak A. Saito, Bethany D. Jenkins, and Tatiana A. Rynearson). Harriet Alexander is funded under a National Defense Science and Engineering Graduate (NDSEG) Fellowship.

© The Author(s), 2012. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Frontiers in Microbiology 3 (2012): 385, doi:10.3389/fmicb.2012.00385., Genes that are constitutively expressed across multiple environmental stimuli are crucial to quantifying differentially expressed genes, particularly when employing quantitative reverse transcriptase polymerase chain reaction (RT-qPCR) assays. However, the identification of these potential reference genes in non-model organisms is challenging and is often guided by expression patterns in distantly related organisms. Here, transcriptome datasets from the diatom Thalassiosira pseudonana grown under replete, phosphorus-limited, iron-limited, and phosphorus and iron co-limited nutrient regimes were analyzed through literature-based searches for homologous reference genes, k-means clustering, and analysis of sequence counts (ASC) to identify putative reference genes. A total of 9759 genes were identified and screened for stable expression. Literature-based searches surveyed 18 generally accepted reference genes, revealing 101 homologs in T. pseudonana with variable expression and a wide range of mean tags per million. k-means analysis parsed the whole transcriptome into 15 clusters. The two most stable clusters contained 709 genes, but still had distinct patterns in expression. ASC analyses identified 179 genes that were stably expressed (posterior probability < 0.1 for 1.25 fold change). Genes known to have a stable expression pattern across the test treatments, like actin, were identified in this pool of 179 candidate genes. ASC can be employed on data without biological replicates and was more robust than the k-means approach in isolating genes with stable expression. The intersection of the genes identified through ASC with commonly used reference genes from the literature suggests that actin and ubiquitin ligase may be useful reference genes for T. pseudonana and potentially other diatoms. With the wealth of transcriptome sequence data becoming available, ASC can be easily applied to transcriptome datasets from other phytoplankton to identify reference genes., This research was funded by the National Science Foundation grant #OCE-0723667 (to Sonya T. Dyhrman, Mak A. Saito, Bethany D. Jenkins, and Tatiana A. Rynearson). Harriet Alexander is funded under a National Defense Science and Engineering Graduate (NDSEG) Fellowship.

© The Author(s), 2012. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Frontiers in Microbiology 3 (2012): 385, doi:10.3389/fmicb.2012.00385., Genes that are constitutively expressed across multiple environmental stimuli are crucial to quantifying differentially expressed genes, particularly when employing quantitative reverse transcriptase polymerase chain reaction (RT-qPCR) assays. However, the identification of these potential reference genes in non-model organisms is challenging and is often guided by expression patterns in distantly related organisms. Here, transcriptome datasets from the diatom Thalassiosira pseudonana grown under replete, phosphorus-limited, iron-limited, and phosphorus and iron co-limited nutrient regimes were analyzed through literature-based searches for homologous reference genes, k-means clustering, and analysis of sequence counts (ASC) to identify putative reference genes. A total of 9759 genes were identified and screened for stable expression. Literature-based searches surveyed 18 generally accepted reference genes, revealing 101 homologs in T. pseudonana with variable expression and a wide range of mean tags per million. k-means analysis parsed the whole transcriptome into 15 clusters. The two most stable clusters contained 709 genes, but still had distinct patterns in expression. ASC analyses identified 179 genes that were stably expressed (posterior probability < 0.1 for 1.25 fold change). Genes known to have a stable expression pattern across the test treatments, like actin, were identified in this pool of 179 candidate genes. ASC can be employed on data without biological replicates and was more robust than the k-means approach in isolating genes with stable expression. The intersection of the genes identified through ASC with commonly used reference genes from the literature suggests that actin and ubiquitin ligase may be useful reference genes for T. pseudonana and potentially other diatoms. With the wealth of transcriptome sequence data becoming available, ASC can be easily applied to transcriptome datasets from other phytoplankton to identify reference genes., This research was funded by the National Science Foundation grant #OCE-0723667 (to Sonya T. Dyhrman, Mak A. Saito, Bethany D. Jenkins, and Tatiana A. Rynearson). Harriet Alexander is funded under a National Defense Science and Engineering Graduate (NDSEG) Fellowship.

Emission of trace gases from the marine diatom Thalassiosira pseudonana (CCMP 1335) was continuously monitored with a proton transfer reaction-mass spectrometry (PTR-MS) in an axenic batch culture system under a 13:11-h light:dark cycle. Substantial increases in the signals at m/z 49, 63, and 69, attributable to methanethiol, dimethyl sulfide (DMS), and isoprene, respectively, were observed in response to increases in cell density. Signals at m/z 69 showed diurnal variations throughout the experiment whereas those at m/z 49 were more pronounced at the beginning of the incubation. Interestingly, the signals at m/z 49 and 69 changed immediately following the light-dark and dark-light transitions, suggesting that light plays a crucial role in the production of methanethiol and isoprene. However, in the latter half of the experiment, methanethiol showed negligible diurnal variations regardless of light conditions, suggesting the production of methanethiol from enzymatic cleavage of DMS. The trend ill signals at m/z 63 was similar to that of the abundance of senescent cells plus cell debris rather than vegetative cells. The results suggest that aging or death of phytoplankton cells could also substantially control DMS production in natural waters along with the other microbial processes related to bacteria and zooplankton.

Emission of trace gases from the marine diatom Thalassiosira pseudonana (CCMP 1335) was continuously monitored with a proton transfer reaction-mass spectrometry (PTR-MS) in an axenic batch culture system under a 13:11-h light:dark cycle. Substantial increases in the signals at m/z 49, 63, and 69, attributable to methanethiol, dimethyl sulfide (DMS), and isoprene, respectively, were observed in response to increases in cell density. Signals at m/z 69 showed diurnal variations throughout the experiment whereas those at m/z 49 were more pronounced at the beginning of the incubation. Interestingly, the signals at m/z 49 and 69 changed immediately following the light-dark and dark-light transitions, suggesting that light plays a crucial role in the production of methanethiol and isoprene. However, in the latter half of the experiment, methanethiol showed negligible diurnal variations regardless of light conditions, suggesting the production of methanethiol from enzymatic cleavage of DMS. The trend ill signals at m/z 63 was similar to that of the abundance of senescent cells plus cell debris rather than vegetative cells. The results suggest that aging or death of phytoplankton cells could also substantially control DMS production in natural waters along with the other microbial processes related to bacteria and zooplankton.

Emission of trace gases from the marine diatom Thalassiosira pseudonana (CCMP 1335) was continuously monitored with a proton transfer reaction-mass spectrometry (PTR-MS) in an axenic batch culture system under a 13:11-h light:dark cycle. Substantial increases in the signals at m/z 49, 63, and 69, attributable to methanethiol, dimethyl sulfide (DMS), and isoprene, respectively, were observed in response to increases in cell density. Signals at m/z 69 showed diurnal variations throughout the experiment whereas those at m/z 49 were more pronounced at the beginning of the incubation. Interestingly, the signals at m/z 49 and 69 changed immediately following the light-dark and dark-light transitions, suggesting that light plays a crucial role in the production of methanethiol and isoprene. However, in the latter half of the experiment, methanethiol showed negligible diurnal variations regardless of light conditions, suggesting the production of methanethiol from enzymatic cleavage of DMS. The trend ill signals at m/z 63 was similar to that of the abundance of senescent cells plus cell debris rather than vegetative cells. The results suggest that aging or death of phytoplankton cells could also substantially control DMS production in natural waters along with the other microbial processes related to bacteria and zooplankton.

We examined the influences of different concentrations and species of Se (selenite, selenate, and seleno-L-methionine) in the ambient environment on the accumulation of inorganic Hg(II) and methylmercury (MeHg) by the diatom Thalassiosira pseudonana and the green mussel Perna viridis. At the experimental concentrations tested (<500 μg l(-1)), selenite and selenate did not significantly affect the uptake of either mercury species by the diatoms and the green mussels. The assimilation efficiency of Hg(II) and MeHg by the mussels from ingested diatoms was also independent of the inorganic Se loadings in the food particles. In contrast, selenomethionine significantly inhibited the uptake of MeHg and enhanced the uptake of Hg(II) by the diatoms and the mussels, but it did not affect the assimilation from the ingested diatoms. The influence of tissue body burden of Se in the green mussels following pre-exposure to selenite and selenomethionine for different periods (1-5 weeks) on the accumulation of Hg(H) and MeHg was further investigated. Our results showed that tissue Se concentrations did not significantly affect the dietary assimilation of mercury, but the influences on the aqueous uptake were variable. Our study thus, strongly highlights the specificity of the Se-Hg interaction in marine mussels for different Se and Hg species. Both dissolved and dietary uptake appeared to be equally important in the accumulation of Hg(II) and MeHg in the green mussels. (C) 2004 Elsevier B.V. All rights reserved.

We examined the influences of different concentrations and species of Se (selenite, selenate, and seleno-L-methionine) in the ambient environment on the accumulation of inorganic Hg(II) and methylmercury (MeHg) by the diatom Thalassiosira pseudonana and the green mussel Perna viridis. At the experimental concentrations tested (<500 μg l(-1)), selenite and selenate did not significantly affect the uptake of either mercury species by the diatoms and the green mussels. The assimilation efficiency of Hg(II) and MeHg by the mussels from ingested diatoms was also independent of the inorganic Se loadings in the food particles. In contrast, selenomethionine significantly inhibited the uptake of MeHg and enhanced the uptake of Hg(II) by the diatoms and the mussels, but it did not affect the assimilation from the ingested diatoms. The influence of tissue body burden of Se in the green mussels following pre-exposure to selenite and selenomethionine for different periods (1-5 weeks) on the accumulation of Hg(H) and MeHg was further investigated. Our results showed that tissue Se concentrations did not significantly affect the dietary assimilation of mercury, but the influences on the aqueous uptake were variable. Our study thus, strongly highlights the specificity of the Se-Hg interaction in marine mussels for different Se and Hg species. Both dissolved and dietary uptake appeared to be equally important in the accumulation of Hg(II) and MeHg in the green mussels. (C) 2004 Elsevier B.V. All rights reserved.

We examined the influences of different concentrations and species of Se (selenite, selenate, and seleno-L-methionine) in the ambient environment on the accumulation of inorganic Hg(II) and methylmercury (MeHg) by the diatom Thalassiosira pseudonana and the green mussel Perna viridis. At the experimental concentrations tested (<500 μg l(-1)), selenite and selenate did not significantly affect the uptake of either mercury species by the diatoms and the green mussels. The assimilation efficiency of Hg(II) and MeHg by the mussels from ingested diatoms was also independent of the inorganic Se loadings in the food particles. In contrast, selenomethionine significantly inhibited the uptake of MeHg and enhanced the uptake of Hg(II) by the diatoms and the mussels, but it did not affect the assimilation from the ingested diatoms. The influence of tissue body burden of Se in the green mussels following pre-exposure to selenite and selenomethionine for different periods (1-5 weeks) on the accumulation of Hg(H) and MeHg was further investigated. Our results showed that tissue Se concentrations did not significantly affect the dietary assimilation of mercury, but the influences on the aqueous uptake were variable. Our study thus, strongly highlights the specificity of the Se-Hg interaction in marine mussels for different Se and Hg species. Both dissolved and dietary uptake appeared to be equally important in the accumulation of Hg(II) and MeHg in the green mussels. (C) 2004 Elsevier B.V. All rights reserved.

Optical properties of the marine diatom thalassiosira pseudonana were examined in nitrate-limited semicontinuous cultures at four rates of growth. Two experiments were conducted at each growth rate in which sampling of the culture was done at either the dark-to-light (D-L) or light-to-dark (L-D) transition of a 12 h photoperiod. Optical cross-sections and efficiency factors for absorption, scattering, and attenuation were calculated from measurements of the spectral absorption and beam attenuation coefficients, cell concentration, and the size distribution of cells in suspension. The refractive index of cells was calculated from this data using an inverse method. Nitrogen limitation strongly influences the optical properties of this species. Absorption cross-sections increased more than twofold with increasing growth rate, predominantly caused by increases in the imaginary part of the refractive index, n'. Variability in n(1) was strongly correlated with intracellular chlorophyll alpha concentration. In contrast; to absorption, scattering cross-sections were not a strong function of growth rate but exhibited significant differences between D-L and L-D experiments at any given growth rate. Variability in the real part of the refractive index was correlated with intracellular carbon concentration. The chlorophyll alpha-specific absorption coefficient decreased with increasing growth rate in response to decreases in the carotenoid to chlorophyll alpha ratio and increased pigment packaging. The chlorophyll alpha-specific scattering coefficient also decreased with increasing growth rate, and was correlated with the cellular carbon to chlorophyll alpha ratio. Our results suggest that the optical properties of phytoplankton populations in nitrogen-limited surface waters may be significantly altered from those in deeper portions of the water column.