Marine Diatom Thalassiosira Pseudonana Research Articles

Induction of photosynthesis under anoxic condition in Thalassiosira pseudonana and Euglena gracilis: interactions between fermentation and photosynthesis.

In their natural environment, microalgae can be transiently exposed to hypoxic or anoxic environments. Whereas fermentative pathways and their interactions with photosynthesis are relatively well characterized in the green alga model Chlamydomonas reinhardtii, little information is available in other groups of photosynthetic micro-eukaryotes. In C. reinhardtii cyclic electron flow (CEF) around photosystem (PS) I, and light-dependent oxygen-sensitive hydrogenase activity both contribute to restoring photosynthetic linear electron flow (LEF) in anoxic conditions. Here we analyzed photosynthetic electron transfer after incubation in dark anoxic conditions (up to 24h) in two secondary microalgae: the marine diatom Thalassiosira pseudonana and the excavate Euglena gracilis. Both species showed sustained abilities to prevent over-reduction of photosynthetic electron carriers and to restore LEF. A high and transient CEF around PSI was also observed specifically in anoxic conditions at light onset in both species. In contrast, at variance with C. reinhardtii, no sustained hydrogenase activity was detected in anoxic conditions in both species. Altogether our results suggest that another fermentative pathway might contribute, along with CEF around PSI, to restore photosynthetic activity in anoxic conditions in E. gracilis and T. pseudonana. We discuss the possible implication of the dissimilatory nitrate reduction to ammonium (DNRA) in T. pseudonana and the wax ester fermentation in E. gracilis.

Common environmental stress responses in a model marine diatom.

Marine planktonic diatoms are among the most important contributors to phytoplankton blooms and marine net primary production. Their ecological success has been attributed to their ability to rapidly respond to changing environmental conditions. Here, we report common molecular mechanisms used by the model marine diatom Thalassiosira pseudonana to respond to 10 diverse environmental stressors using RNA-Seq analysis. We identify a specific subset of 1076 genes that are differentially expressed in response to stressors that induce an imbalance between energy or resource supply and metabolic capacity, which we termed the diatom environmental stress response (d-ESR). The d-ESR is primarily composed of genes that maintain proteome homeostasis and primary metabolism. Photosynthesis is strongly regulated in response to environmental stressors but chloroplast-encoded genes were predominantly upregulated while the nuclear-encoded genes were mostly downregulated in response to low light and high temperature. In aggregate, these results provide insight into the molecular mechanisms used by diatoms to respond to a range of environmental perturbations and the unique role of the chloroplast in managing environmental stress in diatoms. This study facilitates our understanding of the molecular mechanisms underpinning the ecological success of diatoms in the ocean.

UV-B radiation alleviated detrimental effects of polymethyl methacrylate microplastics on marine diatom Thalassiosira pseudonana

Microplastics (MPs) in marine environments simultaneously affect microalgae with UV-B radiation, while their joint effecting mechanisms remain largely unknown. To fill this research gap, the joint effects of polymethyl methacrylate (PMMA) MPs and UV-B radiation (natural environments intensity) on the model marine diatom Thalassiosira pseudonana were investigated. Antagonism was found between the two factors with regards to population growth. Furthermore, we found more inhibited population growth and photosynthetic parameters when pre-treated with PMMA MPs compared to pre-treated with UV-B radiation before joint-treated by the two factors. Transcriptional analysis elucidated that UV-B radiation could alleviate the down-regulation of photosynthetic (PSII, cyt b6/f complex and photosynthetic electron transport) and chlorophyll biosynthesis genes caused by PMMA MPs. Besides, the genes encoding carbon fixation and metabolisms was up-regulated under UV-B radiation, which could provide extra energy for the enhanced anti-oxidative activities and DNA replication-repair processes. These consequences showed that the toxicity of PMMA MPs was comprehensively alleviated when T. pseudonana was jointed treated by UV-B radiation. Our results reveled the underlying molecular mechanisms of antagonistic effects between PMMA MPs and UV-B radiation. This study provides important information that environmental factors like UV-B radiation should be considered when accessing the ecological risks of MPs on marine organisms.

Improving the genome and proteome annotations of the marine model diatom Thalassiosira pseudonana using a proteogenomics strategy

Diatoms are unicellular eukaryotic phytoplankton that account for approximately 20% of global carbon fixation and 40% of marine primary productivity; thus, they are essential for global carbon biogeochemical cycling and climate. The availability of ten diatom genome sequences has facilitated evolutionary, biological and ecological research over the past decade; however, a complimentary map of the diatom proteome with direct measurements of proteins and peptides is still lacking. Here, we present a proteome map of the model marine diatom Thalassiosira pseudonana using high-resolution mass spectrometry combined with a proteogenomic strategy. In-depth proteomic profiling of three different growth phases and three nutrient-deficient samples identified 9526 proteins, accounting for ~ 81% of the predicted protein-coding genes. Proteogenomic analysis identified 1235 novel genes, 975 revised genes, 104 splice variants and 234 single amino acid variants. Furthermore, our quantitative proteomic analysis experimentally demonstrated that a considerable number of novel genes were differentially translated under different nutrient conditions. These findings substantially improve the genome annotation of T. pseudonana and provide insights into new biological functions of diatoms. This relatively comprehensive diatom proteome catalog will complement available diatom genome and transcriptome data to advance biological and ecological research of marine diatoms.

Characterizing Ag uptake and storage in the marine diatom Thalassiosira pseudonana: Implications for Ag biogeochemical cycling

Silver is a non-nutrient element that is readily taken up by diatoms. It exists in the ocean in pM concentrations and is known to accumulate in marine sediments underlying surface waters that support diatom blooms. Laboratory studies and open-ocean water column profiles of Ag versus Si have led to the hypothesis that diatoms accumulate Ag within their silica frustules, subsequently delivering Ag to underlying sediments as the organisms die and sink to the seafloor. Through this delivery mechanism, sedimentary Ag concentrations might serve as a useful record of past export production. However, the actual partitioning of Ag between diatom organic and frustule parts is unknown, and as such represents a considerable source of uncertainty in the development of Ag as a proxy for diatom paleoproductivity and in fully describing Ag biogeochemical cycling. In this study, we use a synchrotron XRF microprobe to map the location of Ag within the laboratory-grown diatom Thalassiosira pseudonana and show that diatoms primarily store Ag in their soft tissue, not in their frustules. Intracellular Ag appears to be concentrated in vacuoles, although the majority of Ag is widely distributed and may be associated with the cell membrane and/or cytoplasm. These results imply an alternate mechanism for Ag delivery to sediments whereby Ag is associated with decomposing organic particles rather than skeletal remains. Silver therefore continues to show promise as a qualitative paleoproductivity proxy. Quantitative use is unlikely, however, given variable uptake rates by diatoms in response to environmental conditions and the significant potential for remineralization with organic matter in the water column.

Flavobacterial exudates disrupt cell cycle progression and metabolism of the diatom Thalassiosira pseudonana

Phytoplankton and bacteria form the base of marine ecosystems and their interactions drive global biogeochemical cycles. The effects of bacteria and bacteria-produced compounds on diatoms range from synergistic to pathogenic and can affect the physiology and transcriptional patterns of the interacting diatom. Here, we investigate physiological and transcriptional changes in the marine diatom Thalassiosira pseudonana induced by extracellular metabolites of a known antagonistic bacterium Croceibacter atlanticus. Mono-cultures of C. atlanticus released compounds that inhibited diatom cell division and elicited a distinctive morphology of enlarged cells with increased chloroplast content and enlarged nuclei, similar to what was previously observed when the diatom was co-cultured with live bacteria. The extracellular C. atlanticus metabolites induced transcriptional changes in diatom pathways that include recognition and signaling pathways, cell cycle regulation, carbohydrate and amino acid production, as well as cell wall stability. Phenotypic analysis showed a disruption in the diatom cell cycle progression and an increase in both intra- and extracellular carbohydrates in diatom cultures after bacterial exudate treatment. The transcriptional changes and corresponding phenotypes suggest that extracellular bacterial metabolites, produced independently of direct bacterial-diatom interaction, may modulate diatom metabolism in ways that support bacterial growth.

Predictability of thermal fluctuations influences functional traits of a cosmopolitan marine diatom.

Evolutionary theory predicts that organismal plasticity should evolve in environments that fluctuate regularly. However, in environments that fluctuate less predictably, plasticity may be constrained because environmental cues become less reliable for expressing the optimum phenotype. Here, we examine how the predictability of +5°C temperature fluctuations impacts the phenotype of the marine diatom Thalassiosira pseudonana. Thermal regimes were informed by temperatures experienced by microbes in an ocean simulation and featured regular or irregular temporal sequences of fluctuations that induced mild physiological stress. Physiological traits (growth, cell size, complexity and pigmentation) were quantified at the individual cell level using flow cytometry. Changes in cellular complexity emerged as the first impact of predictability after only 8–11 days, followed by deleterious impacts on growth on days 13–16. Specifically, cells with a history of irregular fluctuation exposure exhibited a 50% reduction in growth compared with the stable reference environment, while growth was 3–18 times higher when fluctuations were regular. We observed no evidence of heat hardening (increasingly positive growth) with recurrent fluctuations. This study demonstrates that unpredictable temperature fluctuations impact this cosmopolitan diatom under ecologically relevant time frames, suggesting shifts in environmental stochasticity under a changing climate could have widespread consequences among ocean primary producers.

Conservation and architecture of housekeeping genes in the model marine diatom Thalassiosira pseudonana.

Housekeeping genes (HKGs) are constitutively expressed with low variation across tissues/conditions. They are thought to be highly conserved and fundamental to cellular maintenance, with distinctive genomic features. Here, we identify 1505 HKGs in the unicellular marine diatom Thalassiosira pseudonana based on an RNA-seq analysis of 232 samples taken under 12 experimental conditions over 0-72 h. We identify promising internal reference genes (IRGs) for T. pseudonana from the most stably expressed HKGs. A comparative analysis indicates < 18% of HKGs in T. pseudonana have orthologs in other eukaryotes, including other diatom species. Contrary to work on human tissues, T. pseudonana HKGs are longer than non-HKGs, due to elongated introns. More ancient HKGs tend to be shorter than more recent HKGs, and expression levels of HKGs decrease more rapidly with gene length relative to non-HKGs. Our results indicate that HKGs are highly variable across the tree of life and thus unlikely to be universally fundamental for cellular maintenance. We hypothesize that the distinct genomic features of HKGs of T. pseudonana may be a consequence of selection pressures associated with high expression and low variance across conditions.

Temperature‐dependent evolution of cell morphology and carbon and nutrient content in a marine diatom

AbstractPhytoplankton are key players in global biogeochemical cycles, and the effects of ocean warming on their carbon–nitrogen–phosphorus (CNP) stoichiometry, photosynthesis, size, morphology, growth rates, and other traits are of great ecological consequence. The physiological mechanisms of adaptation to temperature in phytoplankton are poorly understood, as are the consequences of the evolution of these processes (e.g., nutrient uptake, photosynthesis) for global biogeochemistry. In general, high temperatures favor smaller cells with higher surface area‐to‐volume ratios, but repeatable patterns in cellular CNP stoichiometry across temperature remain elusive. Here, we compared thermal reaction norms for cellular C, N, P, and chlorophyll a (Chl a) content and for carbon assimilation rate in replicate populations of the marine diatom Thalassiosira pseudonana evolved for 500 generations at 16°C and 31°C. We also examined the thermal reaction norms for cell volume and morphological traits. T. pseudonana has a cylindrical frustule and likely primarily exchanges materials with the environment through the round valve faces. We found that the 31°C‐selected T. pseudonana populations had smaller cells and higher per‐biovolume densities of nutrients and Chl a than the 16°C‐selected populations across assay temperatures but there were no detectable patterns in CNP stoichiometry. The 31°C‐selected populations also had higher valve surface area‐to‐cell volume ratio that increased more with temperature, suggesting better nutrient uptake capabilities than in the 16°C‐selected populations. Our study demonstrates that temperature‐dependent physiological plasticity may evolve differently at different temperatures and suggests that future phytoplankton communities will consist of smaller, more nutrient‐dense cells.

Structural Contour Map of the Iota Carbonic Anhydrase from the Diatom Thalassiosira pseudonana Using a Multiprong Approach.

Carbonic anhydrases (CAs) are a family of ubiquitous enzymes that catalyze the interconversion of CO2 and HCO3−. The “iota” class (ι-CA) was first found in the marine diatom Thalassiosira pseudonana (tpι-CA) and is widespread among photosynthetic microalgae and prokaryotes. The ι-CA has a domain COG4875 (or COG4337) that can be repeated from one to several times and resembles a calcium–calmodulin protein kinase II association domain (CaMKII-AD). The crystal structure of this domain in the ι-CA from a cyanobacterium and a chlorarachniophyte has been recently determined. However, the three-dimensional organization of the four domain-containing tpι-CA is unknown. Using biophysical techniques and 3-D modeling, we show that the homotetrameric tpι-CA in solution has a flat “drone-like” shape with a core formed by the association of the first two domains of each monomer, and four protruding arms formed by domains 3 and 4. We also observe that the short linker between domains 3 and 4 in each monomer confers high flexibility, allowing for different conformations to be adopted. We propose the possible 3-D structure of a truncated tpι-CA containing fewer domain repeats using experimental data and discuss the implications of this atypical shape on the activity and metal coordination of the ι-CA.

A new type of flexible CP12 protein in the marine diatom Thalassiosira pseudonana

BackgroundCP12 is a small chloroplast protein that is widespread in various photosynthetic organisms and is an actor of the redox signaling pathway involved in the regulation of the Calvin Benson Bassham (CBB) cycle. The gene encoding this protein is conserved in many diatoms, but the protein has been overlooked in these organisms, despite their ecological importance and their complex and still enigmatic evolutionary background.MethodsA combination of biochemical, bioinformatics and biophysical methods including electrospray ionization-mass spectrometry, circular dichroism, nuclear magnetic resonance spectroscopy and small X ray scattering, was used to characterize a diatom CP12.ResultsHere, we demonstrate that CP12 is expressed in the marine diatom Thalassiosira pseudonana constitutively in dark-treated and in continuous light-treated cells as well as in all growth phases. This CP12 similarly to its homologues in other species has some features of intrinsically disorder protein family: it behaves abnormally under gel electrophoresis and size exclusion chromatography, has a high net charge and a bias amino acid composition. By contrast, unlike other known CP12 proteins that are monomers, this protein is a dimer as suggested by native electrospray ionization-mass spectrometry and small angle X-ray scattering. In addition, small angle X-ray scattering revealed that this CP12 is an elongated cylinder with kinks. Circular dichroism spectra indicated that CP12 has a high content of α-helices, and nuclear magnetic resonance spectroscopy suggested that these helices are unstable and dynamic within a millisecond timescale. Together with in silico predictions, these results suggest that T. pseudonana CP12 has both coiled coil and disordered regions.ConclusionsThese findings bring new insights into the large family of dynamic proteins containing disordered regions, thus increasing the diversity of known CP12 proteins. As it is a protein that is more abundant in many stresses, it is not devoted to one metabolism and in particular, it is not specific to carbon metabolism. This raises questions about the role of this protein in addition to the well-established regulation of the CBB cycle.Choregraphy of metabolism by CP12 proteins in Viridiplantae and Heterokonta.While the monomeric CP12 in Viridiplantae is involved in carbon assimilation, regulating phosphoribulokinase (PRK) and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) through the formation of a ternary complex, in Heterokonta studied so far, the dimeric CP12 is associated with Ferredoxin-NADP reductase (FNR) and GAPDH. The Viridiplantae CP12 can bind metal ions and can be a chaperone, the Heterokonta CP12 is more abundant in all stresses (C, N, Si, P limited conditions) and is not specific to a metabolism.5h3nzM1M4bjv4RCQxTYJACVideo

Mixotrophic cultivation of Thalassiosira pseudonana with pure and crude glycerol: Impact on lipid profile

The mixotrophic cultivation of microalgae often couples high biomass production with lipid accumulation, and thus it is considered a useful approach to obtain high quantities of added value microalgal biomass. In this study, the marine diatom Thalassiosira pseudonana was supplied with glycerol to determine the potential of the mixotrophic alga for the accumulation of lipids. In preliminarily experiments, T. pseudonana was cultivated in the presence of increasing concentrations of glucose, acetate and glycerol in order to select the best organic carbon source for growth promotion, and glycerol was selected for further experiments. The best glycerol dose for both biomass productivity and lipid production was 2.5 g L−1. To overcome the costs of the pure glycerol supply, the crude glycerol derived from a biodiesel production plant was also tested, as a low-cost alternative. Beside growth and photosynthetic parameters useful to monitor the algal conditions, lipid amount and profile were compared between autotrophic and pure/crude glycerol-treated algae. Interestingly, an almost doubled lipid content was accumulated in mixotrophic algae compared to the autotrophic ones. Moreover, an evident increase in a relatively short-chain fatty acid (C14:0, myristic acid), potentially useful for bio-jet fuels, and in a long-chain omega3-polyunsaturated fatty acid (C18:4; stearidonic acid), interesting for nutritional purposes, was highlighted in mixotrophic samples, especially when cultivated with crude glycerol. Differently, the fatty acid profile obtained from the autotrophic cultures suggested a preferable employment of the algal biomass for biodiesel production, owing to the prevalence of palmitic and palmitoleic acids. This study highlights that the mixotrophic cultivation of T. pseudonana drives the algal metabolism towards the production of different lipids, suitable for diversified applications. Moreover, the use of biodiesel-derived crude glycerol could make mixotrophy economically advantageous, in particular considering a bio-refinery approach.

Temperatures above thermal optimum reduce cell growth and silica production while increasing cell volume and protein content in the diatom Thalassiosira pseudonana

Temperature plays a fundamental role in determining phytoplankton community structure, distribution, and abundance. With climate models predicting increases in ocean surface temperatures of up to 3.2°C by 2100, there is a genuine need to acquire data on the phenotypic plasticity, and thus performance, of phytoplankton in relation to temperature. We investigated the effects of temperature (14–28°C) on the growth, morphology, productivity, silicification and macromolecular composition of the marine diatom Thalassiosira pseudonana. Optimum growth rate and maximum P:R ratio were obtained around 21°C. Cell volume and chlorophyll a increased with temperature, as did lipids and proteins. One of the strongest temperature-induced shifts was the higher silicification rates at low temperature. Our results reveal temperature-driven responses in physiological, morphological and biochemical traits in T. pseudonana; whereby at supra-optimal temperatures cells grew slower, were larger, had higher chlorophyll and protein content but reduced silicification, while those exposed to sub-optimal temperatures were smaller, heavily silicified with lower lipid and chlorophyll content. If these conserved across species, our findings indicate that as oceans warm, we may see shifts in diatom phenotypes and community structure, with potential biogeochemical consequences of higher remineralisation and declines in carbon and silicon export to the ocean interior.

A Genome-Scale Metabolic Model of Thalassiosira pseudonana CCMP 1335 for a Systems-Level Understanding of Its Metabolism and Biotechnological Potential.

Thalassiosira pseudonana is a transformable and biotechnologically promising model diatom with an ability to synthesise nutraceuticals such as fucoxanthin and store a significant amount of polyglucans and lipids including omega-3 fatty acids. While it was the first diatom to be sequenced, a systems-level analysis of its metabolism has not been done yet. This work presents first comprehensive, compartmentalized, and functional genome-scale metabolic model of the marine diatom Thalassiosira pseudonana CCMP 1335, which we have termed iThaps987. The model includes 987 genes, 2477 reactions, and 2456 metabolites. Comparison with the model of another diatom Phaeodactylum tricornutum revealed presence of 183 unique enzymes (belonging primarily to amino acid, carbohydrate, and lipid metabolism) in iThaps987. Model simulations showed a typical C3-type photosynthetic carbon fixation and suggested a preference of violaxanthin–diadinoxanthin pathway over violaxanthin–neoxanthin pathway for the production of fucoxanthin. Linear electron flow was found be active and cyclic electron flow was inactive under normal phototrophic conditions (unlike green algae and plants), validating the model predictions with previous reports. Investigation of the model for the potential of Thalassiosira pseudonana CCMP 1335 to produce other industrially useful compounds suggest iso-butanol as a foreign compound that can be synthesized by a single-gene addition. This work provides novel insights about the metabolism and potential of the organism and will be helpful to further investigate its metabolism and devise metabolic engineering strategies for the production of various compounds.

Interactive Effects of CO2, Temperature, Irradiance, and Nutrient Limitation on the Growth and Physiology of the Marine Diatom Thalassiosira pseudonana (Coscinodiscophyceae)

The marine diatom Thalassiosira pseudonana was grown in continuous culture systems to study the interactive effects of temperature, irradiance, nutrient limitation, and the partial pressure of CO2 (pCO2 ) on its growth and physiological characteristics. The cells were able to grow at all combinations of low and high irradiance (50 and 300μmolphotons·m-2 ·s-1 , respectively, of visible light), low and high pCO2 (400 and 1,000μatm, respectively), nutrient limitation (nitrate-limited and nutrient-replete conditions), and temperatures of 10-32°C. Under nutrient-replete conditions, there was no adverse effect of high pCO2 on growth rates at temperatures of 10-25°C. The response of the cells to high pCO2 was similar at low and high irradiance. At supraoptimal temperatures of 30°C or higher, high pCO2 depressed growth rates at both low and high irradiance. Under nitrate-limited conditions, cells were grown at 38±2.4% of their nutrient-saturated rates at the same temperature, irradiance, and pCO2 . Dark respiration rates consistently removed a higher percentage of production under nitrate-limited versus nutrient-replete conditions. The percentages of production lost to dark respiration were positively correlated with temperature under nitrate-limited conditions, but there was no analogous correlation under nutrient-replete conditions. The results suggest that warmer temperatures and associated more intense thermal stratification of ocean surface waters could lower net photosynthetic rates if the stratification leads to a reduction in the relative growth rates of marine phytoplankton, and at truly supraoptimal temperatures there would likely be a synergistic interaction between the stresses from temperature and high pCO2 (lower pH).

Rapid and precise genome editing in a marine diatom, Thalassiosira pseudonana by Cas9 nickase (D10A)

Diatoms are dominant phytoplankton accounting for up to 20% of annual global primary production. Diatoms are also one of the first model organisms amongst secondary endosymbionts, which have a highly chimeric genome structure accommodating genes from multiple endosymbiosis partners in their evolutionary history. Despite their ecological importance and evolutionary uniqueness, molecular-level studies have lagged behind other model organisms, hindered by difficulties in controlling their ploidy and homologous recombination system. Recently, genome editing tools, such as clustered regularly interspaced short palindromic repeat (CRISPR)-Cas9 and transcription activator-like effector nucleases (TALEN), have been established in some diatoms. However, the improvement of CRISPR/Cas9 precision technology in diatoms is still underway, and the experimental complexity of TALEN limits their application in diatom molecular genetics. Cas9 nickase, a mutated version of Cas9 with a paired-single guide RNAs (sgRNAs), was recently developed to introduce a precise double cut of target DNA, suppressing the off-target effects of native Cas9 nuclease. In this study, we developed the Cas9 nickase (D10A) system for genome editing of the marine diatom Thalassiosira pseudonana. Using this system, we successfully introduced mutations into a gene encoding a putative θ-type carbonic anhydrase, a recently discovered class of carbonic anhydrase widespread in Stramenopiles. The results showed that this system introduced precise and relatively short biallelic indels to the diatom genome with minimal off-target effects.

A disposable bulk-acoustic-wave microalga trapping device for real-time water monitoring

Recently, there has been an increase in the use of algal physiology as a convenient and useful indicator for monitoring of water quality in aquatic ecosystems. However, current methods for monitoring algal physiology are high-cost, power intensive, and have limited sensitivity for practical samples in which algal concentration is low. In order to alleviate these problems, we developed a simple-to-fabricate acoustophoretic particle trapping device that can effectively enrich various types of microalgae such as chlorella and diatoms for real-time monitoring of water quality. This microalgae trap is fabricated by embedding a piezoelectric resonator in a single channel. In comparison to previously reported acoustophoretic particle traps, this device has a simple structure and does not require sheath flows, which makes the device low cost and simple to fabricate and operate. Using Chlorella kessleri and the marine diatom Thalassiosira pseudonana as model algae, the microalgae trap has demonstrated notable particle trapping efficiencies between 82 % to 74 % for Chlorella with a through-flow of 0.5 μl/min to 2.7 μl/min and 85 % to 79 % for diatoms with a 0.5 μl/min to 4 μl/min through-flow. The trap has also shown to simultaneously trap Chlorella and diatoms at different heights of the microchannel. This device has high promise for trapping, separating and manipulating microalgae.

Toxicities of Irgarol 1051 derivatives, M2 and M3, to two marine diatom species

Irgarol 1051 is highly toxic to marine autotrophs and has been widely used as an antifouling booster biocide. This study tested the toxicities of two s-triazine derivatives of Irgarol, namely M2 (3-[4-tert-butylamino-6-methylthiol-s-triazin-2-ylamino]propionaldehyde) and M3 (2-methylthio-4,6-bis-tert-butylamino-s-triazine) to two marine diatom species, Skeletonema costatum and Thalassiosira pseudonana through standard acute (96h) and chronic (7d) growth inhibition tests. Results showed that both of the two chemicals significantly inhibited the growth of S. costatum (M2: 96h-EC50 = 6789.7 μg L−1, 7d-EC50 = 3503.7 μg L−1; M3: 96h-EC50 = 45193.9 μg L−1, 7d-EC50 = 5330.0 μg L−1) and T. pseudonana (M2: 96h-EC50 = 366.2 μg L−1, 7d-EC50 = 312.5 μg L−1; M3: 96h-EC50 = 2633.4 μg L−1, 7d-EC50 = 710.5 μg L−1), while their toxicity effects were much milder than Irgarol and its major degradation product M1. By comparing with previous findings, the susceptibilities of these s-triazine compounds to two tested species were ranked as: Irgarol > M1 ≫ M2 > M3. This study promotes future research efforts on better understanding of the ecotoxicities of M2 and M3, and incorporating such information to improve the current monitoring, risk assessment and regulation of the use of Irgarol.

A new widespread subclass of carbonic anhydrase in marine phytoplankton

Most aquatic photoautotrophs depend on CO2-concentrating mechanisms (CCMs) to maintain productivity at ambient concentrations of CO2, and carbonic anhydrase (CA) plays a key role in these processes. Here we present different lines of evidence showing that the protein LCIP63, identified in the marine diatom Thalassiosira pseudonana, is a CA. However, sequence analysis showed that it has a low identity with any known CA and therefore belongs to a new subclass that we designate as iota-CA. Moreover, LCIP63 unusually prefers Mn2+ to Zn2+ as a cofactor, which is potentially of ecological relevance since Mn2+ is more abundant than Zn2+ in the ocean. LCIP63 is located in the chloroplast and only expressed at low concentrations of CO2. When overexpressed using biolistic transformation, the rate of photosynthesis at limiting concentrations of dissolved inorganic carbon increased, confirming its role in the CCM. LCIP63 homologs are present in the five other sequenced diatoms and in other algae, bacteria, and archaea. Thus LCIP63 is phylogenetically widespread but overlooked. Analysis of the Tara Oceans database confirmed this and showed that LCIP63 is widely distributed in marine environments and is therefore likely to play an important role in global biogeochemical carbon cycling.

Experimental evolution of phytoplankton fatty acid thermal reaction norms.

Temperature effects on the fatty acid (FA) profiles of phytoplankton, major primary producers in the ocean, have been widely studied due to their importance as industrial feedstocks and to their indispensable role as global producers of long‐chain, polyunsaturated FA (PUFA), including omega‐3 (ω3) FA required by organisms at higher trophic levels. The latter is of global ecological concern for marine food webs, as some evidence suggests an ongoing decline in global marine‐derived ω3 FA due to both a global decline in phytoplankton abundance and to a physiological reduction in ω3 production by phytoplankton as temperatures rise. Here, we examined both short‐term (physiological) and long‐term (evolutionary) responses of FA profiles to temperature by comparing FA thermal reaction norms of the marine diatom Thalassiosira pseudonana after ~500 generations (ca. 2.5 years) of experimental evolution at low (16°C) and high (31°C) temperatures. We showed that thermal reaction norms for some key FA classes evolved rapidly in response to temperature selection, often in ways contrary to our predictions based on prior research. Notably, 31°C‐selected populations showed higher PUFA percentages (including ω3 FA) than 16°C‐selected populations at the highest assay temperature (31°C, above T. pseudonana's optimum temperature for population growth), suggesting that high‐temperature selection led to an evolved ability to sustain high PUFA production at high temperatures. Rapid evolution may therefore mitigate some of the decline in global phytoplankton‐derived ω3 FA production predicted by recent studies. Beyond its implications for marine food webs, knowledge of the effects of temperature on fatty acid profiles is of fundamental importance to our understanding of the mechanistic causes and consequences of thermal adaptation.